Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 171 Effective Description of Anisotropic Wave Dispersion in Mechanical Band-Gap Metamaterials via the Relaxed Micromorphic Model
    d’Agostino, M.V. and Barbagallo, G. and Ghiba, I.-D. and Eidel, B. and Neff, P. and Madeo, A.
    Journal of Elasticity 139 299-329 (2020)
    In this paper the relaxed micromorphic material model for anisotropic elasticity is used to describe the dynamical behavior of a band-gap metamaterial with tetragonal symmetry. Unlike other continuum models (Cauchy, Cosserat, second gradient, classical Mindlin–Eringen micromorphic etc.), the relaxed micromorphic model is endowed to capture the main microscopic and macroscopic characteristics of the targeted metamaterial, namely, stiffness, anisotropy, dispersion and band-gaps. The simple structure of our material model, which simultaneously lives on a micro-, a meso- and a macroscopic scale, requires only the identification of a limited number of frequency-independent and thus truly constitutive parameters, valid for both static and wave-propagation analyses in the plane. The static macro- and micro-parameters are identified by numerical homogenization in static tests on the unit-cell level in Neff et al. (J. Elast.,, 2019, in this volume). The remaining inertia parameters for dynamical analyses are calibrated on the dispersion curves of the same metamaterial as obtained by a classical Bloch–Floquet analysis for two wave directions. We demonstrate via polar plots that the obtained material parameters describe very well the response of the structural material for all wave directions in the plane, thus covering the complete panorama of anisotropy of the targeted metamaterial. © 2019, Springer Nature B.V.
    view abstractdoi: 10.1007/s10659-019-09753-9
  • 2020 • 170 Biomimetic scaffold fabricated with a mammalian trabecular bone template
    Bulygina, I. and Senatov, F. and Choudhary, R. and Kolesnikov, E. and Kaloshkin, S. and Scholz, R. and Knyazeva, M. and Walther, F. and Anisimova, N. and Kiselevskiy, M.
    Polymer Degradation and Stability 172 (2020)
    This study proposes the method of ultra-high molecular weight polyethylene (UHMWPE) biomimetic scaffold fabrication. Anisotropy is considered to be a distinctive feature of native bone but basically only a 3D-fabricated scaffold structure may be anisotropic, while 3D-printing is not applicable to UHMWPE. We proposed a novel method that suggested a template of native mammalian bone to be used as a negative for UHMWPE scaffold fabrication. This method allows direct replication of the bone's structural features on the micro- and macro-scale. Bone scaffolds obtained using the specified method showed anisotropic structure; the pores' average proportions for scaffold and bone were 770 and 470, and 700 and 500 μm, respectively. According to SEM and CT investigations, the scaffolds' macro- and microstructure mimicked the native bone architecture; this feature distinguishes the proposed method from the other UHMWPE scaffold fabrication techniques. The combination of the hydrophilic surface and the nanorelief affected the adhesion and proliferation of cells: the adhesion of multipotent mesenchymal stromal cells (MMSC) amounted to 40% after 4 h; the proliferation of MMSC was 75% after 48 h. The proposed novel method of fabricating biomimetic scaffolds can be used to obtain bone implants of the complex microstructure and anisotropy from high-melt viscosity polymers which cannot be 3D-printed to be further applied in bone reconstruction. The FT-IR analysis confirmed the occurrence of carboxyl oxidation when the surface of UHMWPE sample was treated with chromic acid. The oxidation index (OI) of the samples was found in the order of etching in chromic acid > sterilization > hot moulding respectively. It can be suggested that the oxidative degradation of UHMWPE can be reduced by optimizing manufacturing conditions and further selection of an appropriate processing method. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.polymdegradstab.2020.109076
  • 2020 • 169 Using spectral-based representative volume element crystal plasticity simulations to predict yield surface evolution during large scale forming simulations
    Han, F. and Diehl, M. and Roters, F. and Raabe, D.
    Journal of Materials Processing Technology 277 (2020)
    We present a new approach to predict the evolution of anisotropic yield functions by coupling large scale forming simulations with crystal plasticity-spectral based virtual experiments, realizing a multi-scale model for metal forming. Employing a fast spectral method solver enables us to conduct on-the-fly full-field virtual experiments to evolve the yield surface at each integration point of the macroscopic finite element model. As illustrative example, two advanced anisotropic yield functions, namely Yld2000-2D and Yld2004-18p, are used in finite element simulations of deep drawing for a 2090-T3 aluminum alloy sheet. The simulated earing profiles are compared to the experimental ones as well as to simulations with non-evolving yield functions. It is found that the prediction of the earing is improved for the case of the evolving Yld2000-2D yield function. The evolution of the plastic anisotropy during cup drawing is systematically analyzed, showing that the evolution of anisotropy can have considerable effect on the prediction accuracy of the macroscopic simulations. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116449
  • 2020 • 168 Data-oriented constitutive modeling of plasticity in metals
    Hartmaier, A.
    Materials 13 (2020)
    Constitutive models for plastic deformation of metals are typically based on flow rules determining the transition from elastic to plastic response of a material as function of the applied mechanical load. These flow rules are commonly formulated as a yield function, based on the equivalent stress and the yield strength of the material, and its derivatives. In this work, a novel mathematical formulation is developed that allows the efficient use of machine learning algorithms describing the elastic-plastic deformation of a solid under arbitrary mechanical loads and that can replace the standard yield functions with more flexible algorithms. By exploiting basic physical principles of elastic-plastic deformation, the dimensionality of the problem is reduced without loss of generality. The data-oriented approach inherently offers a great flexibility to handle different kinds of material anisotropy without the need for explicitly calculating a large number of model parameters. The applicability of this formulation in finite element analysis is demonstrated, and the results are compared to formulations based on Hill-like anisotropic plasticity as reference model. In future applications, the machine learning algorithm can be trained by hybrid experimental and numerical data, as for example obtained from fundamental micromechanical simulations based on crystal plasticity models. In this way, data-oriented constitutive modeling will also provide a new way to homogenize numerical results in a scale-bridging approach. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13071600
  • 2020 • 167 Thermoelastic anisotropy in NdScO3 and NdGaO3 perovskites
    Hirschle, C. and Schreuer, J. and Ganschow, S. and Peters, L.
    Materials Chemistry and Physics 254 (2020)
    Single crystal thermal expansion and elastic stiffness of NdGaO3 and NdScO3 were investigated by inductive gauge dilatometry and resonant ultrasound spectroscopy between 103K and 1673K, as they are used extensively as perovskite-type substrates for epitaxial crystal growth. Thermal expansion of NdGaO3 is in agreement with literature data and has very similar magnitude and anisotropy compared to NdScO3. The anisotropy of the elastic stiffness of NdGaO3 is more pronounced and qualitatively different from what is found for NdScO3. It is explained in terms of structural instabilities, which lead to known phase transitions in other perovskites. The anisotropy of the elastic stiffness of NdGaO3 is compatible with what is found for other orthorhombic perovskites that undergo a transition to a rhombohedral structure at high temperatures. The elastic properties of NdScO3 directly follow from the properties of other REScO3. The samples were characterized with regards to their compositions and lattice parameters using electron probe microanalysis and X-ray powder diffraction. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2020.123528
  • 2020 • 166 A generalized micromorphic approach accounting for variation and dispersion of preferred material directions
    von Hoegen, M. and Skatulla, S. and Schröder, J.
    Computers and Structures 232 (2020)
    Materials exhibiting a heterogeneous and non-uniform composition in terms of elastic and anisotropic properties such as biological tissues require special efforts to accurately describe their constitutive behavior. In contrast to classical models, micromorphic formulations can predict the macroscopically observable material response as originated from distinct scale-dependent micro-structural deformation mechanisms. This is facilitated by additional independent degrees of freedom and associated additional strain and stress quantities. Here, a generalized continuum is mathematically constructed from a macro-continuum and a micro-continuum which are both adequately coupled on kinematics and constitutive levels as well as by micro-boundary conditions. In view of biomechanical modeling, the potential of the formulation is studied for a number of academic examples characterized by an anisotropic material composition to elucidate the micromorphic material response as compared with the one obtained using a classical continuum mechanics approach. The results demonstrate the ability of the generalized continuum approach to address non-affine elastic reorientation of the preferred material direction in the macro-space and its dispersion in the micro-space as affecting deformation, strain and stress on the macroscopic level. In particular, if the anisotropy in the micromorphic formulation is solely linked to the extra degrees of freedom and associated strain and stress measures, the deformation for small and large strains is shown to be distinctly different to the classical response. Together with the ability to implicitly account for scale-dependent higher-order deformation effects in the constitutive law the proposed generalized micromorphic formulation provides an advanced description, especially for fibrous biological materials. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruc.2017.11.013
  • 2020 • 165 Regularized, parameter free scale similarity type models for Large Eddy Simulation
    Klein, M. and Ketterl, S. and Engelmann, L. and Kempf, A. and Kobayashi, H.
    International Journal of Heat and Fluid Flow 81 (2020)
    The fidelity of Large Eddy Simulations (LES) depends strongly on the closures of the sub-grid scale (SGS) stress tensor. Although it is well known that the SGS stresses in LES are not aligned with the strain rate tensor, the most widely used models are still of eddy viscosity type, due to their robust behavior in LES and reasonable performance in a posteriori testing. The unstable behavior of more advanced anisotropic models, that is typically found in LES, has been attributed to either the fact that these models provide backscatter or to the fact that they do not provide a sufficient amount of dissipation. Based on recent advances in the field, an alternative modeling strategy is suggested, which can be used to regularize an arbitrary anisotropic (e.g. scale similarity type) model. The resulting model is easy to implement, can be written in compact form and is free of model parameters. The model has been tested a-posteriori and results are presented for a Taylor-Green-Vortex, a free plane jet and a turbulent channel flow of friction Reynolds numbers 395, 590 and 934. The results are compared to well-known eddy viscosity models and when applicable, to simulations without explicit LES model. The new model exhibits good performance for a variety of mesh resolutions and for all configurations. Furthermore, a-priori analysis results in the context of liquid atomization indicate that the model might be suitable as well in more complex physical scenarios. The a-priori analysis performance of the model is found to be nearly equivalent to the underlying structural anisotropic model in terms of its correlation coefficient, but the model is free of backscatter and provides good stability in LES. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.ijheatfluidflow.2019.108496
  • 2020 • 164 Single-layer Janus black arsenic-phosphorus (b-AsP): Optical dichroism, anisotropic vibrational, thermal, and elastic properties
    Li, L.L. and Bacaksiz, C. and Nakhaee, M. and Pentcheva, R. and Peeters, F.M. and Yagmurcukardes, M.
    Physical Review B 101 (2020)
    By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.134102
  • 2020 • 163 Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot
    Lorenz, S. and Erickson, C.S. and Riesner, M. and Gamelin, D.R. and Fainblat, R. and Bacher, G.
    Nano Letters 20 1896-1906 (2020)
    One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn2+:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects - hypothesized for more than a decade - dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b05136
  • 2020 • 162 Identification of Scale-Independent Material Parameters in the Relaxed Micromorphic Model Through Model-Adapted First Order Homogenization
    Neff, P. and Eidel, B. and d’Agostino, M.V. and Madeo, A.
    Journal of Elasticity 139 269-298 (2020)
    We rigorously determine the scale-independent short range elastic parameters in the relaxed micromorphic generalized continuum model for a given periodic microstructure. This is done using both classical periodic homogenization and a new procedure involving the concept of apparent material stiffness of a unit-cell under affine Dirichlet boundary conditions and Neumann’s principle on the overall representation of anisotropy. We explain our idea of “maximal” stiffness of the unit-cell and use state of the art first order numerical homogenization methods to obtain the needed parameters for a given tetragonal unit-cell. These results are used in the accompanying paper (d’Agostino et al. in J. Elast. 2019. Accepted in this volume) to describe the wave propagation including band-gaps in the same tetragonal metamaterial. © 2019, Springer Nature B.V.
    view abstractdoi: 10.1007/s10659-019-09752-w
  • 2020 • 161 Role of inclination dependence of grain boundary energy on the microstructure evolution during grain growth
    Salama, H. and Kundin, J. and Shchyglo, O. and Mohles, V. and Marquardt, K. and Steinbach, I.
    Acta Materialia 188 641-651 (2020)
    The role of inclination dependence of grain boundary energy on the microstructure evolution and the orientation distribution of grain boundary planes during grain growth in polycrystalline materials is investigated by three-dimensional phase-field simulations. The anisotropic grain boundary energy model uses the description of the faceted surface structure of the individual crystals and constructs an anisotropic energy of solid-solid interface. The energy minimization occurs by the faceting of the grain boundary due to inclination dependence of the grain boundary energy. The simulation results for a single grain show the development of equilibrium shapes (faceted grain morphologies) with different families of facets which agrees well with the theoretical predictions. The results of grain growth simulations with isotropic and anisotropic grain boundary energy for cubic symmetry show that inclination dependence of grain boundary energy has a significant influence on the grain boundary migration, grain growth kinetics and the grain boundary plane distribution. It has been shown that the model essentially reproduces the experimental studies reported for NaCl and MgO polycrystalline systems where the anisotropic distribution of grain boundary planes has a peak for the low-index {100} type boundaries. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.043
  • 2020 • 160 Analytical and numerical study of the out-of-equilibrium current through a helical edge coupled to a magnetic impurity
    Vinkler-Aviv, Y. and May, D. and Anders, F.B.
    Physical Review B 101 (2020)
    We study the conductance of a time-reversal-symmetric helical electronic edge coupled antiferromagnetically to a magnetic impurity, employing analytical and numerical approaches. The impurity can reduce the perfect conductance G0 of a noninteracting helical edge by generating a backscattered current. The backscattered steady-state current tends to vanish below the Kondo temperature TK for time-reversal-symmetric setups. We show that the central role in maintaining the perfect conductance is played by a global U(1) symmetry. This symmetry can be broken by an anisotropic exchange coupling of the helical modes to the local impurity. Such anisotropy, in general, dynamically vanishes during the renormalization group (RG) flow to the strong-coupling limit at low temperatures. The role of the anisotropic exchange coupling is further studied using the time-dependent numerical renormalization group method, uniquely suitable for calculating out-of-equilibrium observables of strongly correlated setups. We investigate the role of finite-bias voltage and temperature in cutting the RG flow before the isotropic strong-coupling fixed point is reached, and extract the relevant energy scales and the manner in which the crossover from the weakly interacting regime to the strong-coupling backscattering-free screened regime is manifested. Most notably, we find that at low temperatures the conductance of the backscattering current follows a power-law behavior G∼(T/TK)2, which we understand as a strong nonlinear effect due to time-reversal-symmetry breaking by the finite bias. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.165112
  • 2020 • 159 Crystal anisotropy-dependent shear angle variation in orthogonal cutting of single crystalline copper
    Wang, Z. and Zhang, J. and Xu, Z. and Zhang, J. and Li, G. and Zhang, H. and Li, Z. and Hassan, H.U. and Fang, F. and Hartmaier, A. and Yan, Y. and Sun, T.
    Precision Engineering 63 41-48 (2020)
    Shear deformation that dominates elementary chip formation in metal cutting greatly relies on crystal anisotropy. In the present work we investigate the influence of crystallographic orientation on shear angle in ultra-precision orthogonal diamond cutting of single crystalline copper by joint crystal plasticity finite element simulations and in-situ experiments integrated in scanning electron microscope. In particular, the experimental cutting conditions including a straight cutting edge are the same with that used in the 2D finite element simulations. Both simulations and experiments demonstrate a well agreement in chip profile and shear angle, as well as their dependence on crystallography. A series of finite element simulations of orthogonal cutting along different cutting directions for a specific crystallographic orientation are further performed, and predicated values of shear angle are used to calibrate an extended analytical model of shear angle based on the Ernst–Merchant relationship. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.precisioneng.2020.01.006
  • 2020 • 158 Three-dimensional numerical simulation of soft-tissue wound healing using constrained-mixture anisotropic hyperelasticity and gradient-enhanced damage mechanics
    Zuo, D. and Avril, S. and Yang, H. and Jamaleddin Mousavi, S. and Hackl, K. and He, Y.
    Journal of the Royal Society Interface 17 (2020)
    Healing of soft biological tissues is the process of self-recovery or self-repair after injury or damage to the extracellular matrix (ECM). In this work, we assume that healing is a stress-driven process, which works at recovering a homeostatic stress metric in the tissue by replacing the damaged ECM with a new undamaged one. For that, a gradient-enhanced continuum healing model is developed for three-dimensional anisotropic tissues using the modified anisotropic Holzapfel-Gasser-Ogden constitutive model. An adaptive stress-driven approach is proposed for the deposition of new collagen fibres during healing with orientations assigned depending on the principal stress direction. The intrinsic length scales of soft tissues are considered through the gradient-enhanced term, and growth and remodelling are simulated by a constrained-mixture model with temporal homogenization. The proposed model is implemented in the finite-element package Abaqus by means of a user subroutine UEL. Three numerical examples have been achieved to illustrate the performance of the proposed model in simulating the healing process with various damage situations, converging towards stress homeostasis. The orientations of newly deposited collagen fibres and the sensitivity to intrinsic length scales are studied through these examples, showing that both have a significant impact on temporal evolutions of the stress distribution and on the size of the damage region. Applications of the approach to carry out in silico experiments of wound healing are promising and show good agreement with existing experiment results. © 2020 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsif.2019.0708
  • 2019 • 157 Relaxed micromorphic model of transient wave propagation in anisotropic band-gap metastructures
    Barbagallo, G. and Tallarico, D. and D'Agostino, M.V. and Aivaliotis, A. and Neff, P. and Madeo, A.
    International Journal of Solids and Structures 162 148-163 (2019)
    In this paper, we show that the transient waveforms arising from several localised pulses in a micro-structured material can be reproduced by a corresponding generalised continuum of the relaxed micromorphic type. Specifically, we compare the dynamic response of a bounded micro-structured material to that of bounded continua with special kinematic properties: (i) the relaxed micromorphic continuum and (ii) an equivalent Cauchy linear elastic continuum. We show that, while the Cauchy theory is able to describe the overall behaviour of the metastructure only at low frequencies, the relaxed micromorphic model goes far beyond by giving a correct description of the pulse propagation in the frequency band-gap and at frequencies intersecting the optical branches. In addition, we observe a computational time reduction associated with the use of the relaxed micromorphic continuum, compared to the sensible computational time needed to perform a transient computation in a micro-structured domain. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2018.11.033
  • 2019 • 156 Influence of Microstructural Features on the Strain Hardening Behavior of Additively Manufactured Metallic Components
    Biswas, A. and Prasad, M.R.G. and Vajragupta, N. and ul Hassan, H. and Brenne, F. and Niendorf, T. and Hartmaier, A.
    Advanced Engineering Materials 21 (2019)
    Additive manufacturing (AM) has recently become one of the key manufacturing processes in the era of Industry 4.0 because of its highly flexible production scheme. Due to complex thermal cycles during the manufacturing process itself and special solidification conditions, the microstructure of AM components often exhibits elongated grains together with a pronounced texture. These microstructural features significantly contribute to an anisotropic mechanical behavior. In this work, the microstructure and mechanical properties of additively manufactured samples of 316L stainless steel are characterized experimentally and a micromechanical modeling approach is employed to predict the macroscopic properties. The objective of this work is to study the effects of texture and microstructural morphology on yield strength and strain hardening behavior of face-centered cubic additively manufactured metallic components. To incorporate the texture in synthetic representative volume elements (RVE), the proposed approach considers both the crystallographic and grain boundary textures. The mechanical behavior of these RVEs is modeled using crystal plasticity finite element method, which incorporates size effects through the implementation of strain gradients. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201900275
  • 2019 • 155 Direct measurement of anisotropic conductivity in a nanolaminated (Mn0.5Cr0.5)2GaC thin film
    Flatten, T. and Matthes, F. and Petruhins, A. and Salikhov, R. and Wiedwald, U. and Farle, M. and Rosen, J. and Bürgler, D.E. and Schneider, C.M.
    Applied Physics Letters 115 (2019)
    The direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic Mn+1AXn (MAX) phase film is presented. A multitip scanning tunneling microscope is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities ρab and ρc. A (Cr0.5Mn0.5)2GaC MAX phase film shows a large anisotropy ratio ρ c / ρ ab = 525 ± 49. This is a consequence of the complex bonding scheme of MAX phases with covalent M-X and metallic M-M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5115347
  • 2019 • 154 Tension/compression anisotropy enhanced topology design
    Gaganelis, G. and Jantos, D.R. and Mark, P. and Junker, P.
    Structural and Multidisciplinary Optimization 59 2227-2255 (2019)
    A strategy for tension/compression anisotropy enhancement of topology optimization approaches is presented. To this end, a spectral decomposition of stresses and strains into tension and compression contributions allows for a multi-material optimization that favors tension or compression affine materials, dependent on the predominant local state. Numerical computations hence yield the topology of a construction part with maximum stiffness at constraint design volume. Additionally, the spatial distribution of a tension affine and a compression affine material is optimized, which is motivated by concrete engineering: financially cheap material, for example concrete, is applied in compression dominant regions in favor of stiffer but more expensive material, which is applied only in tension dominant regions, for example steel. The enhancement is applied both to a classical (mathematical) optimization method and the thermodynamic topology optimization. Several numerical examples are investigated and yield design suggestions for tension/compression sensitive construction parts, e.g., for future lightweight structures made of reinforced concrete. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00158-018-02189-0
  • 2019 • 153 Profilin and formin constitute a pacemaker system for robust actin filament growth
    Funk, J. and Merino, F. and Venkova, L. and Heydenreich, L. and Kierfeld, J. and Vargas, P. and Raunser, S. and Piel, M. and Bieling, P.
    eLife 8 (2019)
    The actin cytoskeleton drives many essential biological processes, from cell morphogenesis to motility. Assembly of functional actin networks requires control over the speed at which actin filaments grow. How this can be achieved at the high and variable levels of soluble actin subunits found in cells is unclear. Here we reconstitute assembly of mammalian, non-muscle actin filaments from physiological concentrations of profilin-actin. We discover that under these conditions, filament growth is limited by profilin dissociating from the filament end and the speed of elongation becomes insensitive to the concentration of soluble subunits. Profilin release can be directly promoted by formin actin polymerases even at saturating profilin-actin concentrations. We demonstrate that mammalian cells indeed operate at the limit to actin filament growth imposed by profilin and formins. Our results reveal how synergy between profilin and formins generates robust filament growth rates that are resilient to changes in the soluble subunit concentration. © 2019, eLife Sciences Publications Ltd. All rights reserved.
    view abstractdoi: 10.7554/eLife.50963
  • 2019 • 152 Investigation of evolving yield surfaces of dual-phase steels
    Hou, Y. and Min, J. and Guo, N. and Lin, J. and Carsley, J.E. and Stoughton, T.B. and Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology (2019)
    The aim of this paper is to describe the evolving yield behavior of dual-phase steels during plastic deformation characterized for ten loading paths using a series of mechanical tests including uniaxial tension, uniaxial compression, in-plane torsion and cruciform biaxial tension with the aid of digital image correlation techniques for strain measurement. Large plastic strains in the gauge area of cruciform specimens tested were enabled by a laser deposition process to strengthen the arms in order to measure deformation behavior of the sheet without arbitrarily thinning the gauge section. Experimental yield loci were determined for three dual phase steels with different strength levels up to equivalent plastic strains of ˜0.11 for DP590, ˜0.07 for DP780, ˜0.05 for DP980, respectively. Several existing anisotropic yield criteria under both associated flow rule (AFR) and non-associated flow rule (non-AFR) were applied to describe the anisotropic yield behavior of these DP steels. A comparative study was preformed to validate prediction accuracy of yield criteria with experimental measurements including yield loci, yield stresses and rφ -values under uniaxial tension in seven orientations as well as yield stresses and rb -value under equi-biaxial tension. The results show that non-AFR significantly improved prediction accuracy of both stresses and r-values simultaneously. Under non-AFR, an order of two in the yield stress function is sufficient to accurately predict flow stresses. The evolution of both yield stress and plastic potential surfaces of DP steels were illustrated by changing parameters in the yield criterion as functions of equivalent compliance λ¯. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116314
  • 2019 • 151 Low-order locking-free mixed finite element formulation with approximation of the minors of the deformation gradient
    Kraus, A. and Wriggers, P. and Viebahn, N. and Schröder, J.
    International Journal for Numerical Methods in Engineering 120 1011-1026 (2019)
    In this work, a low-order mixed finite element formulation for three-dimensional nonlinear elastic problems is presented. The main goal of this paper is to develop a robust and efficient element formulation to overcome locking arising in the cases of hyperelastic bending, quasi-incompressibility, and anisotropy. For this, a low-order discretisation of a five-field Hu-Washizu functional written in terms of the minors of the Cauchy-Green tensor is used. For the tested boundary value problems, the proposed element formulation is more accurate and computational efficient than comparable element formulations. © 2019 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.6168
  • 2019 • 150 Comparative study of different anisotropy and potential formulations of phase-field models for dendritic solidification
    Kundin, J. and Steinbach, I.
    Computational Materials Science 170 (2019)
    Phase-field model formulations with double well and double obstacle potentials, and different anisotropy models are investigated with respect to their potential to simulate (i) tip growth on a quantitative level, (ii) well resolved side-branching. The dilute binary alloy Al-4 at%Cu is used as a model alloy. The effects of the numerical resolution (the ratio of the capillary length to the grid spacing) on the growth velocity are studied by means of convergence tests for isothermal and directional solidification in comparison to the theoretical values calculated by the Green-function method (A. Karma, W.J. Rappel, Phys. Rev. E 57 (1998) 4323). An interface stability parameter is introduced as a measure for the estimation of the maximum value of the grid spacing for effective simulations. We show that predominantly the side-branching occurs at numerical resolution lower than the limit value needed to produce correct results in accordance to the convergence analysis. The best results for dendrite growth at a relevant numerical resolution are obtained for the double well potential. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2019.109197
  • 2019 • 149 A multiscale and multiphase model for the description of function-perfusion processes in the human liver
    Lambers, L. and Ricken, T. and König, M.
    Advances in Engineering Materials, Structures and Systems: Innovations, Mechanics and Applications - Proceedings of the 7th International Conference on Structural Engineering, Mechanics and Computation, 2019 304-308 (2019)
    Numerical simulations of biological systems have become more and more important in recent years. In order to understand and predict hepatic function in health and disease, we developed a multiscale and multiphase model for the description of function-perfusion processes in the liver. With respect to the different scales of the hierarchically structured liver, processes like glucose homeostasis or detoxification of paracetamol as well as the influence of hepatic disease like the non-alcoholic fatty liver disease (NAFLD) can be investigated. On the tissue scale the liver consists of hexagonal liver lobules, containing anisotropically oriented capillaries, called sinusoids, which lead macroscopically to an anisotropic blood flow as well as a nonlinear, anisotropic and poro-elastic response. This structure is described using a homogenization method based on the Theory of Porous Media (TPM). This leads to a coupled set of partial differential equations (PDE) describing the tissue deformation as well as the transport of blood and metabolites like nutrients or xenobiotics. The lobule scale is coupled to the cellular scale where hepatic metabolism takes place. With the use of embedded ODE equations we can simulate metabolic processes depending on various nutrients or substances. The performance of the developed theory is demonstrated by a numerical example. The results provide an overview of possible applications of our approach using the NAFLD as a showcase. During the development of fatty liver, fat is stored in the liver cells resulting in swelling of the hepatocytes. This accumulation of fat drives tissue growth, which has an impact on the blood perfusion in the liver lobules. The results also clarify the spatial distribution of flow and fat accumulation since many hepatic processes proceed zonated. The processes in one single lobule are then expanded to a group of lobules to investigate the mutual liver lobe interaction. © 2019 Taylor & Francis Group, London, UK.
    view abstractdoi: 10.1201/9780429426506-52
  • 2019 • 148 Ultrafast spin-lasers
    Lindemann, M. and Xu, G. and Pusch, T. and Michalzik, R. and Hofmann, M.R. and Žutić, I. and Gerhardt, N.C.
    Nature 568 212-215 (2019)
    Lasers have both ubiquitous applications and roles as model systems in which non-equilibrium and cooperative phenomena can be elucidated 1 . The introduction of novel concepts in laser operation thus has potential to lead to both new applications and fundamental insights 2 . Spintronics 3 , in which both the spin and the charge of the electron are used, has led to the development of spin-lasers, in which charge-carrier spin and photon spin are exploited. Here we show experimentally that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 gigahertz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation of the resultant spin-laser relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both of which are commonly viewed as detrimental in spintronics 3 and conventional lasers 4 . Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-019-1073-y
  • 2019 • 147 Template-Free Synthesis and Selective Filling of Janus Nanocups
    Qiang, X. and Steinhaus, A. and Chen, C. and Chakroun, R. and Gröschel, A.H.
    Angewandte Chemie - International Edition 58 7122-7126 (2019)
    We report on the formation of shape- and surface-anisotropic Janus nanocups (JNCs) by evaporation-induced confinement assembly (EICA) of ABC triblock terpolymers. During microphase separation in spherical confinement, the triblock terpolymer spontaneously adopted a hemispherical shape with an inner concentric lamella–lamella (ll) morphology. Cross-linking and disassembly of the microparticles resulted in well-defined JNCs with different chemistry on the inside and outside. By synthesizing polymers with increasing length of the cross-linkable block, we tuned the mechanical stability of the nanocups, which is relevant to control opening and closing of the cup cavity. We utilize the Janus properties for selective uptake of cargo exemplified by the filling of JNCs with polymer or gold nanoparticles. The directional properties of JNCs suggest applications in locomotion, oil-spill recovery, storage and release, templating, and as nanoreactors with attoliter volume. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201814014
  • 2019 • 146 Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
    Rautert, J. and Shamirzaev, T.S. and Nekrasov, S.V. and Yakovlev, D.R. and Klenovský, P. and Kusrayev, Y.G. and Bayer, M.
    Physical Review B 99 (2019)
    The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a Γ-valley electron and the excitons in the indirect QDs contributed by an X-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.195411
  • 2019 • 145 Forming properties of additively manufactured monolithic Hastelloy X sheets
    Rosenthal, S. and Platt, S. and Hölker-Jäger, R. and Gies, S. and Kleszczynski, S. and Tekkaya, A.E. and Witt, G.
    Materials Science and Engineering A 753 300-316 (2019)
    Additive manufacturing (AM) of semi-finished sheets for a subsequent forming operation has not been investigated yet. The potentials in resource efficiency and effective use of build-chamber-volumes, by combining laser powder bed fusion of metals and forming technology are demonstrated. The overarching aim of this process chain are time savings of up to 50% and the benefit of material strengthening by work hardening. The scope of this paper is the understanding and characterization of the flow behavior of additively manufactured semi-finished parts for the use in a subsequent forming application of the nickel-based superalloy, Hastelloy X. Characterization methods used in sheet metal forming are applied to monolithic additively manufactured tensile, compression and in-plane torsion specimens. The resulting characterization and yield criteria can be used to predict the forming behavior of additively manufactured semi finished parts with integrated functions like cooling channels that are formed in its final geometry. A correction function is introduced to consider the surface roughness in the stress-strain diagrams. The material shows a high anisotropic yield stress with a nearly isotropic hardening behavior in the as-build condition. The heat treatment reveals a homogenization of the material accompanied with an isotropic initial yield stress but anisotropic yield behavior. To numerically model those effects, different yield surfaces based on the preceding material characterization are discussed. It turns out, that the additively manufactured Hastelloy X shows high potential in terms of formability combined with high tensile strengths. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.03.035
  • 2019 • 144 Intrinsic and magnetic-field-induced linear polarization of excitons in ultrathin indirect-gap type-II GaAs/AlAs quantum wells
    Shamirzaev, T.S. and Rautert, J. and Yakovlev, D.R. and Glazov, M.M. and Bayer, M.
    Physical Review B 99 (2019)
    The exciton dynamics in transverse magnetic field is investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The observed linear polarization of the quantum well photoluminescence has two contributions, one of which arises from the crystalline structure of the quantum well. It does not depend on temperature and demonstrates a strong spectral dependence across the emission band. The other one is induced by a transverse magnetic field. It strongly decreases with increasing temperature, has no spectral dependence, and demonstrates an unexpectedly long-time dynamics. The experimental findings can be explained in the framework of the developed theoretical model which accounts for the quantum well anisotropy, the Zeeman effect of electrons and holes in the transverse magnetic field, and the redistribution of excitons over the spin sublevels. It provides quantitative agreement with the experiment and allows us to evaluate, for the studied structure, the heavy-hole in-plane g-factor tensor, which turns out to be extremely anisotropic with principal values of opposite signs and the same magnitude of 0.25. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.155301
  • 2019 • 143 Investigation of the anisotropic cyclic damage behavior of selective laser melted AISI 316L stainless steel
    Stern, F. and Kleinhorst, J. and Tenkamp, J. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 42 2422-2430 (2019)
    doi: 10.1111/ffe.13029
  • 2019 • 142 Crystal plasticity finite element modeling and simulation of diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Xu, Z. and Zhang, J. and Hassan, H.U. and Li, G. and Zhang, H. and Hartmaier, A. and Fang, F. and Yan, Y. and Sun, T.
    Journal of Manufacturing Processes 38 187-195 (2019)
    Microstructural-related deformation behavior leads to anisotropic machining characteristics of polycrystalline materials. In the present work, we develop a crystal plasticity finite element model of ultra-precision diamond cutting of polycrystalline copper, aiming to evaluate the influence of grain boundaries on the correlation between microscopic deformation behavior of the material and macroscopic machining results. The crystal plasticity dealing with the anisotropy of polycrystalline copper is implemented in a user subroutine (UMAT), and an efficient element deletion technique based on the Johnson-Cook damage model is adopted to describe material removal and chip formation. The effectiveness of as-established crystal plasticity finite element model is verified by experiments of nanoindentation, nanoscratching and in-situ diamond microcutting. Subsequent crystal plasticity finite element simulation of diamond cutting across a high angle grain boundary demonstrates significant anisotropic machining characteristics in terms of machined surface quality, chip profile and cutting force, due to heterogeneous plastic deformation behavior in the grain level. © 2019
    view abstractdoi: 10.1016/j.jmapro.2019.01.007
  • 2019 • 141 Adjustment of isotropic part properties in laser sintering based on adapted double laser exposure strategies
    Wegner, A. and Witt, G.
    Optics and Laser Technology 109 381-388 (2019)
    Laser sintering of polymers gets more and more importance for small series production. However, laser sintered parts have often an explicit anisotropy of mechanical properties. Due to the layer-wise production, parts show no homogenous morphological structure as known from injection molded parts. Voids appear within the part and are concentrated in the area between two consecutive layers, resulting in reduced bonding. Therefore, mechanical properties and in particular the elongation at break show significantly lower values in the direction of build compared to the properties within the build plane. Sometimes, these effects are an obstacle in the usage of laser sintering for series production of parts. The aim of the experiments was to investigate to what extent an improvement of the layer-to-layer bonding and thus of the characteristic values in the building direction can be achieved by alternative exposure strategies. For this purpose, the influence of adaptive double laser exposure strategies on layer-to-layer bonding and anisotropy was investigated. The influence of different parameter settings for the first and second exposure was investigated. In a further step it was examined to what extent the influence of disturbances on the process, such as the inhomogeneous temperature distribution or cycle time variations can be reduced by applying the developed double laser exposure strategies. For this purpose, a comparison was made between an optimized double laser exposure parameter set and the standard parameter set. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.optlastec.2018.08.017
  • 2019 • 140 Large thermopower anisotropy in PdCo O2 thin films
    Yordanov, P. and Sigle, W. and Kaya, P. and Gruner, M.E. and Pentcheva, R. and Keimer, B. and Habermeier, H.-U.
    Physical Review Materials 3 (2019)
    Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite PdCoO2, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut Al2O3 substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300-815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.085403
  • 2019 • 139 A photosystem i monolayer with anisotropic electron flow enables Z-scheme like photosynthetic water splitting
    Zhao, F. and Wang, P. and Ruff, A. and Hartmann, V. and Zacarias, S. and Pereira, I.A.C. and Nowaczyk, M.M. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Energy and Environmental Science 12 3133-3143 (2019)
    Photosynthetic protein complexes are attractive building blocks for the fabrication of semi-artificial energy conversion devices. However, limitations in the efficiency of the implemented biophotovoltaic systems prevent their use in practical applications. In particular, the effective minimization of recombination processes in photosystem I (PSI) modified bioelectrodes is crucial for enabling a unidirectional electron flow allowing the true potential of the large charge separation at PSI being exploited. Here, we present controlled immobilization of PSI monolayers with a predefined preferential orientation that translates into anisotropic electron flow upon irradiation. Further interface of the oriented PSI monolayer with redox polymers allows an efficient electron transfer and minimization of possible short-circuiting pathways. To complete the functional biophotocathode, the PSI monolayer is coupled to a hydrogenase (H2ase) to realize light-induced H2 evolution. The PSI/H2ase biocathode is then combined with a redox polymer/photosystem II-based bioanode demonstrating a fully light-driven Z-scheme mimic biophotovoltaic cell for bias-free water splitting. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee01901d
  • 2018 • 138 Two-site jumps in dimethyl sulfone studied by one- and two-dimensional 17O NMR spectroscopy
    Beerwerth, J. and Storek, M. and Greim, D. and Lueg, J. and Siegel, R. and Cetinkaya, B. and Hiller, W. and Zimmermann, H. and Senker, J. and Böhmer, R.
    Journal of Magnetic Resonance 288 84-94 (2018)
    Polycrystalline dimethyl sulfone is studied using central-transition oxygen-17 exchange NMR. The quadrupolar and chemical shift tensors are determined by combining quantum chemical calculations with line shape analyses of rigid-lattice spectra measured for stationary and rotating samples at several external magnetic fields. Quantum chemical computations predict that the largest principal axes of the chemical shift anisotropy and electrical field gradient tensors enclose an angle of about 73°. This prediction is successfully tested by comparison with absorption spectra recorded at three different external magnetic fields. The experimental one-dimensional motionally narrowed spectra and the two-dimensional exchange spectrum are compatible with model calculations involving jumps of the molecules about their two-fold symmetry axis. This motion is additionally investigated by means of two-time stimulated-echo spectroscopy which allows for a determination of motional correlation functions over a wider temperature range than previously reported using carbon and deuteron NMR. On the basis of suitable second-order quadrupolar frequency distributions, sin-sin stimulated-echo amplitudes are calculated for a two-site model in the limit of vanishing evolution time and compared with experimental findings. The present study thus establishes oxygen-17 NMR as a powerful method that will be particularly useful for the study of solids and liquids devoid of nuclei governed by first-order anisotropies. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.jmr.2018.01.016
  • 2018 • 137 Influence of anisotropic Si(111)-(4 × 1)-In surface on growth of nanoscale in islands
    Chandola, S. and Esser, N.
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 36 (2018)
    Reflectance anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) are used to study the growth of indium (In) on the anisotropic Si(111)-(4 × 1)-In surface at room temperature. RAS shows that epitaxial growth of In is accompanied by the disappearance of the surface optical anisotropy at 1.9 eV which is the fingerprint for the (4 × 1)-In surface reconstruction and the appearance of a large minimum at 1.4 eV which is at the same energy as interband transitions observed on bulk crystalline In. Subsequent spectra taken on the surface, over 3 h, show that this minimum decreases and eventually disappears along with the reappearance of the original RAS signature of the (4 × 1)-In surface. STM of this surface shows elongated, anisotropic In crystal islands on top of a (4 × 1)-In reconstructed surface. Upon annealing the surface to 720 K, the surface reconstruction changes with STM showing regions covered with a phase that resembles the (√7 × √3)-In reconstruction and RAS showing a large positive anisotropy at about 1.5 eV. The epitaxial In islands now show a hexagonal shape, unlike on the (4 × 1) surface. Thus, the growth morphology of the islands is shown to be dependent on the initial surface reconstruction. The authors attribute these findings to Ostwald ripening of the In islands mediated by diffusion, which is dependent on the structurally different In wetting layers on the Si substrate. © 2018 Author(s).
    view abstractdoi: 10.1116/1.5031228
  • 2018 • 136 Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: Role of the dipolar field
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Klyatskaya, S. and Ruben, M. and Velez-Fort, E. and Kummer, K. and Brookes, N.B. and Gargiani, P...
    Nanoscale 10 277-283 (2018)
    Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7nr06610d
  • 2018 • 135 Energy Structure of an Individual Mn Acceptor in GaAs : Mn
    Dimitriev, G.S. and Krainov, I.V. and Sapega, V.F. and Averkiev, N.S. and Debus, J. and Lähderanta, E.
    Physics of the Solid State 60 1568-1577 (2018)
    The energy structure of the Mn acceptor, which is a complex of Mn2+ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn2+ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor. © 2018, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S106378341808005X
  • 2018 • 134 Size limits of magnetic-domain engineering in continuous in-plane exchange-bias prototype films
    Gaul, A. and Emmrich, D. and Ueltzhöffer, T. and Huckfeldt, H. and Doganay, H. and Hackl, J. and Khan, M.I. and Gottlob, D.M. and Hartmann, G. and Beyer, A. and Holzinger, D. and Nemšák, S. and Schneider, C.M. and Gölzhäuser,...
    Beilstein Journal of Nanotechnology 9 2968-2979 (2018)
    Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chip applications crucially depends on the ability to control their motion. One approach for magnetic-particle motion control is the superposition of static magnetic stray field landscapes (MFLs) with dynamically varying external fields. These MFLs may emerge from magnetic domains engineered both in shape and in their local anisotropies. Motion control of smaller beads does necessarily need smaller magnetic patterns, i.e., MFLs varying on smaller lateral scales. The achievable size limit of engineered magnetic domains depends on the magnetic patterning method and on the magnetic anisotropies of the material system. Smallest patterns are expected to be in the range of the domain wall width of the particular material system. To explore these limits a patterning technology is needed with a spatial resolution significantly smaller than the domain wall width. Results: We demonstrate the application of a helium ion microscope with a beam diameter of 8 nm as a mask-less method for local domain patterning of magnetic thin-film systems. For a prototypical in-plane exchange-bias system the domain wall width has been investigated as a function of the angle between unidirectional anisotropy and domain wall. By shrinking the domain size of peri odic domain stripes, we analyzed the influence of domain wall overlap on the domain stability. Finally, by changing the geometry of artificial two-dimensional domains, the influence of domain wall overlap and domain wall geometry on the ultimate domain size in the chosen system was analyzed. Conclusion: The application of a helium ion microscope for magnetic patterning has been shown. It allowed for exploring the fundamental limits of domain engineering in an in-plane exchange-bias thin film as a prototypical system. For two-dimensional domains the limit depends on the domain geometry. The relative orientation between domain wall and anisotropy axes is a crucial parameter and therefore influences the achievable minimum domain size dramatically. © 2018 Gaul et al.
    view abstractdoi: 10.3762/bjnano.9.276
  • 2018 • 133 Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: A path to induce magnetic anisotropy in Fe-Co films over large thicknesses
    Giannopoulos, G. and Salikhov, R. and Varvaro, G. and Psycharis, V. and Testa, A.M. and Farle, M. and Niarchos, D.
    Journal of Physics D: Applied Physics 51 (2018)
    Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaa41c
  • 2018 • 132 Experimental setup to characterize flow-induced anisotropy of sheet metals
    Gutknecht, F. and Gerstein, G. and Traphöner, H. and Clausmeyer, T. and Nürnberger, F.
    IOP Conference Series: Materials Science and Engineering 418 (2018)
    For many metals, a transient variation of the yield stress can be observed when changing the orientation of a load-path. Such behavior affects the manufacturing process itself, e.g. by increasing forming forces, altered material properties or springback of the manufactured components. Hence, the aim of this work is to develop a novel experimental setup to characterize hardening effects due to flow-induced anisotropy for sheet metals. The proposed experiment consists of two subsequent forming operations. Initially, a hydraulic bulge test is conducted, followed by torsion of the hemispherical preformed sheet. Such approach captures the effects of flow-induced anisotropy like cross hardening as could be proved for the example of the conventional deep-drawing steel DC04. The benefits of the presented setup are (i) high plastic strains in the pre-loading step and (ii) determination of several combinations of pre- and subsequent loading. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/418/1/012085
  • 2018 • 131 Computationally-efficient modeling of inelastic single crystal responses via anisotropic yield surfaces: Applications to shape memory alloys
    Hartl, D.J. and Kiefer, B. and Schulte, R. and Menzel, A.
    International Journal of Solids and Structures 136-137 38-59 (2018)
    Phenomenological constitutive models of inelastic responses based on the methods of classical plasticity provide several advantages, especially in terms of computational efficiency. For this reason, they are attractive for the analysis of complex boundary value problems comprising large computational domains. However, for the analysis of problems dominated by single crystal behavior (e.g., inclusion, granular interaction problems or inter-granular fracture), such approaches are often limited by the symmetry assumptions inherent in the stress invariants used to form yield-type criteria. On the other hand, the high computational effort associated with micro-mechanical or crystal plasticity-type models usually prevents their use in large structural simulations, multi-scale analyses, or design and property optimization computations. The goal of the present work is to establish a modeling strategy that captures micro-scale single-crystalline sma responses with sufficient fidelity at the computational cost of a phenomenological macro-scale model. Its central idea is to employ an anisotropic transformation yield criterion with sufficiently rich symmetry class—which can directly be adopted from the literature on plasticity theory—at the single crystal level. This approach is conceptually fundamentally different from the common use of anisotropic yield functions to capture tension-compression asymmetry and texture-induced anisotropy in poly-crystalline SMAs. In our model, the required anisotropy parameters are calibrated either from experimental data for single crystal responses, theoretical considerations or micro-scale computations. The model thus efficiently predicts single crystal behaviors and can be applied to the analysis of complex boundary value problems. In this work we consider the application of this approach to the modeling of shape memory alloys (SMAs), though its potential utility is much broader. Example analyses of SMA single crystals that include non-transforming precipitates and poly-crystalline aggregates are considered and the effects of both elastic and transformation anisotropy in these materials are demonstrated. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2017.12.002
  • 2018 • 130 Interplay of cation ordering and thermoelastic properties of spinel structure MgGa2O4
    Hirschle, C. and Schreuer, J. and Galazka, Z.
    Journal of Applied Physics 124 (2018)
    The coefficient of thermal expansion and elastic stiffnesses of spinel structure MgGa2O4 were determined from 103 K to 1673 K using dilatometry and resonant ultrasound spectroscopy. The state of cation order was investigated on specimens quenched from temperatures up to 1473 K via single-crystal X-ray diffraction. Even at room-temperature, the material is stiffer than what was expected from DFT simulations at 0 K, however, the stiffness falls within the predicted range based on the stiffness of the constituent oxides of MgGa2O4. The anisotropy of its longitudinal elastic stiffness is low, whereas there is a high anisotropy of the shear resistance compared to other cubic materials. At about 820 K-860 K, the temperature dependences of both thermal expansion and elastic properties change rapidly. Cation reordering also starts in this temperature range; the state of order is static at lower temperatures. Thus, MgGa2O4 undergoes a glass-like transition when heated above 820 K-860 K, where the state of cation order starts relaxing towards equilibrium in laboratory timescales. Landau-theory for nonconvergent cation ordering can describe the observed cation order at elevated temperatures well. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5037786
  • 2018 • 129 Effective Diffusivity of Porous Materials with Microcracks: Self-Similar Mean-Field Homogenization and Pixel Finite Element Simulations
    Timothy, J.J. and Meschke, G.
    Transport in Porous Media 125 413-434 (2018)
    We investigate the influence of distributed microcracks on the overall diffusion properties of a porous material using the self-similar cascade continuum micromechanics model within the framework of mean-field homogenization and computational homogenization of diffusion simulations using a high-resolution pixel finite element method. In addition to isotropic, also anisotropic crack distributions are considered. The comparison of the results from the cascade continuum micromechanics model and the numerical simulations provides a deeper insight into the qualitative transport characteristics such as the influence of the crack density on the complexity and connectivity of crack networks. The analysis shows that the effective diffusivity for a disordered microcrack distribution is independent of the absolute length scale of the cracks. It is observed that the overall effective diffusivity of a microcracked material with the microcracks oriented in the direction of transport is not necessarily higher than that of a material with a random orientation of microcracks, independent of the microcrack density. © 2018, Springer Nature B.V.
    view abstractdoi: 10.1007/s11242-018-1126-y
  • 2018 • 128 Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Arras, R. and Ramshaw, B.J. and Pentcheva, R. and Suzuki, Y.
    Nature Communications 9 (2018)
    The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields. However, there is another promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting. Here we report evidence for a giant Rashba-type splitting at the interface of LaTiO3 and SrTiO3. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO3/SrTiO3 interface, but it is promising for the development of a new kind of oxide-based spintronics. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-04014-0
  • 2018 • 127 Finite and virtual element formulations for large strain anisotropic material with inextensive fibers
    Wriggers, P. and Hudobivnik, B. and Schröder, J.
    Lecture Notes in Applied and Computational Mechanics 86 205-231 (2018)
    Anisotropic material with inextensive or nearly inextensible fibers introduce constraints in the mathematical formulations of the underlying differential equations from mechanics. This is always the case when fibers with high stiffness in a certain direction are present and a relatively weak matrix material is supporting these fibers. In numerical solution schemes like the finite element method or the virtual element method the presence of constraints—in this case associated to a possible fiber inextensibility compared to a matrix—lead to so called locking-phenomena. This can be overcome by special interpolation schemes as has been discussed extensively for volume constraints like incompressibility as well as contact constraints. For anisotropic material behaviour the most severe case is related to inextensible fibers. In this paper a mixed method is developed for finite elements and virtual elements that can handle anisotropic materials with inextensive and nearly inextensive fibers. For this purpose a classical ansatz, known from the modeling of volume constraint is adopted leading stable elements that can be used in the finite strain regime. © Springer International Publishing AG 2018.
    view abstractdoi: 10.1007/978-3-319-65463-8_11
  • 2017 • 126 Anisotropic slope limiting for discontinuous Galerkin methods
    Aizinger, V. and Kosík, A. and Kuzmin, D. and Reuter, B.
    International Journal for Numerical Methods in Fluids 84 543-565 (2017)
    In this paper, we present an anisotropic version of a vertex-based slope limiter for discontinuous Galerkin methods. The limiting procedure is carried out locally on each mesh element utilizing the bounds defined at each vertex by the largest and smallest mean value from all elements containing the vertex. The application of this slope limiter guarantees the preservation of monotonicity. Unnecessary limiting of smooth directional derivatives is prevented by constraining the x and y components of the gradient separately. As an inexpensive alternative to optimization-based methods based on solving small linear programming problems, we propose a simple operator splitting technique for calculating the correction factors for the x and y derivatives. We also provide the necessary generalizations for using the anisotropic limiting strategy in an arbitrary rotated frame of reference and in the vicinity of exterior boundaries with no Dirichlet information. The limiting procedure can be extended to elements of arbitrary polygonal shape and three dimensions in a straightforward fashion. The performance of the new anisotropic slope limiter is illustrated by two-dimensional numerical examples that employ piecewise linear discontinuous Galerkin approximations. © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/fld.4360
  • 2017 • 125 Transparent anisotropy for the relaxed micromorphic model: Macroscopic consistency conditions and long wave length asymptotics
    Barbagallo, G. and Madeo, A. and d'Agostino, M.V. and Abreu, R. and Ghiba, I.-D. and Neff, P.
    International Journal of Solids and Structures 120 7-30 (2017)
    In this paper, we study the anisotropy classes of the fourth order elastic tensors of the relaxed micromorphic model, also introducing their second order counterpart by using a Voigt-type vector notation. In strong contrast with the usual micromorphic theories, in our relaxed micromorphic model only classical elasticity-tensors with at most 21 independent components are studied together with rotational coupling tensors with at most 6 independent components. We show that in the limit case Lc → 0 (which corresponds to considering very large specimens of a microstructured metamaterial) the meso- and micro-coefficients of the relaxed model can be put in direct relation with the macroscopic stiffness of the medium via a fundamental homogenization formula. We also show that a similar homogenization formula is not possible in the case of the standard Mindlin-Eringen-format of the anisotropic micromorphic model. Our results allow us to forecast the successful short term application of the relaxed micromorphic model to the characterization of anisotropic mechanical metamaterials. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2017.01.030
  • 2017 • 124 Electron transport in stepped Bi2Se3 thin films
    Bauer, S. and Bobisch, C.A.
    Journal of Physics Condensed Matter 29 (2017)
    We analyse the electron transport in a 16 quintuple layer thick stepped Bi2Se3 film grown on Si(1 1 1) by means of scanning tunnelling potentiometry (STP) and multi-point probe measurements. Scanning tunnelling microscopy images reveal that the local structure of the Bi2Se3 film is dominated by terrace steps and domain boundaries. From a microscopic study on the nm scale by STP, we find a mostly linear gradient of the voltage on the Bi2Se3 terraces which is interrupted by voltage drops at the position of the domain boundaries. The voltage drops indicate that the domain boundaries are scatterers for the electron transport. Macroscopic resistance measurements (2PP and in-line 4PP measurement) on the μm scale support the microscopic results. An additional rotational square 4PP measurement shows an electrical anisotropy of the sheet conductance parallel and perpendicular to the Bi2Se3 steps of about 10%. This is a result of the anisotropic step distribution at the stepped Bi2Se3 surface while domain boundaries are distributed isotropically. The determined value of the conductivity of the Bi2Se3 steps of about 1000 S cm-1 verifies the value of an earlier STP study. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7a3c
  • 2017 • 123 Stress intensity factor dependence on anisotropy and geometry during micro-fracture experiments
    Brinckmann, S. and Kirchlechner, C. and Dehm, G.
    Scripta Materialia 127 76-78 (2017)
    Miniaturized fracture beam experiments are often used to identify the fracture toughness of single phases and particular grain boundaries because large-scale experiments reveal only homogenized material properties. The evaluation of the microscale toughness is based on isotropic 2D models although the majority of materials are anisotropic. Moreover, the thickness influences the fracture toughness because the crack driving force is maximum in the beam center. This study quantifies the influence of anisotropy, Poisson's ratio and beam geometry using thousands of 3D simulations. We give guidelines for micro-cantilever design and quantify the changes in fracture toughness, if the guidelines cannot be fulfilled. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2016.08.027
  • 2017 • 122 Optical anisotropy of quasi-1D rare-earth silicide nanostructures on Si(001)
    Chandola, S. and Speiser, E. and Esser, N. and Appelfeller, S. and Franz, M. and Dähne, M.
    Applied Surface Science 399 648-653 (2017)
    Rare earth metals are known to interact strongly with Si(001) surfaces to form different types of silicide nanostructures. Using STM to structurally characterize Dy and Tb silicide nanostructures on vicinal Si(001), it will be shown that reflectance anisotropy spectroscopy (RAS) can be used as an optical fingerprint technique to clearly distinguish between the formation of a semiconducting two-dimensional wetting layer and the metallic one-dimensional nanowires. Moreover, the distinctive spectral features can be related to structural units of the nanostructures. RAS spectra of Tb and Dy nanostructures are found to show similar features. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2016.12.044
  • 2017 • 121 Inverse Ripening and Rearrangement of Precipitates under Chemomechanical Coupling
    Darvishi Kamachali, R. and Schwarze, C.
    Computational Materials Science 130 292-296 (2017)
    A coupling between diffusional and mechanical relaxation raised from composition-dependent elastic constants, and its effects on the evolution of precipitates with finite misfit strain are investigated. Inverse ripening has been observed where smaller precipitate grows at the expense of a larger one. This occurs due to fluxes generated under elastically-strained solute gradients around precipitates that scales with Rr6 where R and r are the precipitate radius and the radial coordinate, respectively. Both isotropic and anisotropic dependency of elastic constants on the composition were considered. The latter leads to the emergence of new patterns of elastic anisotropy and rearrangement of precipitates in the matrix. © 2017
    view abstractdoi: 10.1016/j.commatsci.2017.01.024
  • 2017 • 120 Scanning tunnelling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets
    Gastiasoro, M.N. and Eremin, I. and Fernandes, R.M. and Andersen, B.M.
    Nature Communications 8 (2017)
    The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-Q magnetic ground states, hosting unusual states such as chiral density waves and quantum Hall insulators. Distinguishing single-Q and multi-Q magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states (LDOS) near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors - one single-Q and two double-Q phases. Our results open a promising avenue to investigate the complex magnetic configurations in itinerant systems via standard scanning tunnelling spectroscopy, without requiring spin-resolved capability. © The Author(s) 2017.
    view abstractdoi: 10.1038/ncomms14317
  • 2017 • 119 Experimental analysis of anisotropic damage in dual-phase steel by resonance measurement
    Gerstein, G. and Clausmeyer, T. and Isik, K. and Nürnberger, F. and Tekkaya, A.E. and Bruchanov, A.A. and Maier, H.J.
    International Journal of Damage Mechanics 26 1147-1169 (2017)
    The ductile damage in deformed dual-phase steel sheets (DP600) was investigated based on measurements of the degradation of the direction-dependent Young's modulus. The study focuses on the material-induced damage anisotropy in such advanced high-strength steel. The elastic properties in the direction of applied loading of the deformed sheets were determined by measuring the resonance frequency of rectangular samples. The material was investigated in the as-delivered condition and after annealing at 220 for 48 h. Tensile strains of up to 10% were applied after annealing. Tensile tests were performed in different directions with respect to the rolling direction to determine the evolution of damage in different directions. The comparison of the obtained results with the electron micrographs shows that the damage in the steel sheets occurs in the form of nano and micro damages near the grain boundary and interfaces of phases. The maximum decrease of the Young's modulus in the transverse direction was observed for the largest applied deformation of 10% tensile strain in the transverse direction. An efficient calculation method to obtain information on the distribution of anisotropy in the plane of the sheet was applied. This calculation method relies on an efficient representation of the material's texture. In order to assess the influence of texture, the texture was determined experimentally. © 2017 SAGE Publications.
    view abstractdoi: 10.1177/1056789516650245
  • 2017 • 118 Characterization of anisotropically shaped silver nanoparticle arrays via spectroscopic ellipsometry supported by numerical optical modeling
    Gkogkou, D. and Shaykhutdinov, T. and Oates, T.W.H. and Gernert, U. and Schreiber, B. and Facsko, S. and Hildebrandt, P. and Weidinger, I.M. and Esser, N. and Hinrichs, K.
    Applied Surface Science 421 460-464 (2017)
    The present investigation aims to study the optical response of anisotropic Ag nanoparticle arrays deposited on rippled silicon substrates by performing a qualitative comparison between experimental and theoretical results. Spectroscopic ellipsometry was used along with numerical calculations using finite-difference time-domain (FDTD) method and rigorous coupled wave analysis (RCWA) to reveal trends in the optical and geometrical properties of the nanoparticle array. Ellipsometric data show two resonances, in the orthogonal x and y directions, that originate from localized plasmon resonances as demonstrated by the calculated near-fields from FDTD calculations. The far-field calculations by RCWA point to decoupled resonances in x direction and possible coupling effects in y direction, corresponding to the short and long axis of the anisotropic nanoparticles, respectively. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2016.10.105
  • 2017 • 117 A generalized micromorphic approach accounting for variation and dispersion of preferred material directions
    Horn-Von Hoegen, M. and Skatulla, S. and Schröder, J.
    Computers and Structures (2017)
    Materials exhibiting a heterogeneous and non-uniform composition in terms of elastic and anisotropic properties such as biological tissues require special efforts to accurately describe their constitutive behavior. In contrast to classical models, micromorphic formulations can predict the macroscopically observable material response as originated from distinct scale-dependent micro-structural deformation mechanisms. This is facilitated by additional independent degrees of freedom and associated additional strain and stress quantities. Here, a generalized continuum is mathematically constructed from a macro-continuum and a micro-continuum which are both adequately coupled on kinematics and constitutive levels as well as by micro-boundary conditions. In view of biomechanical modeling, the potential of the formulation is studied for a number of academic examples characterized by an anisotropic material composition to elucidate the micromorphic material response as compared with the one obtained using a classical continuum mechanics approach. The results demonstrate the ability of the generalized continuum approach to address non-affine elastic reorientation of the preferred material direction in the macro-space and its dispersion in the micro-space as affecting deformation, strain and stress on the macroscopic level. In particular, if the anisotropy in the micromorphic formulation is solely linked to the extra degrees of freedom and associated strain and stress measures, the deformation for small and large strains is shown to be distinctly different to the classical response. Together with the ability to implicitly account for scale-dependent higher-order deformation effects in the constitutive law the proposed generalized micromorphic formulation provides an advanced description, especially for fibrous biological materials. © 2017 Elsevier Ltd.
    view abstractdoi: 10.1016/j.compstruc.2017.11.013
  • 2017 • 116 Optimized growth and reorientation of anisotropic material based on evolution equations
    Jantos, D.R. and Junker, P. and Hackl, K.
    Computational Mechanics 1-20 (2017)
    Modern high-performance materials have inherent anisotropic elastic properties. The local material orientation can thus be considered to be an additional design variable for the topology optimization of structures containing such materials. In our previous work, we introduced a variational growth approach to topology optimization for isotropic, linear-elastic materials. We solved the optimization problem purely by application of Hamilton’s principle. In this way, we were able to determine an evolution equation for the spatial distribution of density mass, which can be evaluated in an iterative process within a solitary finite element environment. We now add the local material orientation described by a set of three Euler angles as additional design variables into the three-dimensional model. This leads to three additional evolution equations that can be separately evaluated for each (material) point. Thus, no additional field unknown within the finite element approach is needed, and the evolution of the spatial distribution of density mass and the evolution of the Euler angles can be evaluated simultaneously. © 2017 Springer-Verlag GmbH Germany
    view abstractdoi: 10.1007/s00466-017-1483-3
  • 2017 • 115 Gradient-based nodal limiters for artificial diffusion operators in finite element schemes for transport equations
    Kuzmin, D. and Shadid, J.N.
    International Journal for Numerical Methods in Fluids (2017)
    This paper presents new linearity-preserving nodal limiters for enforcing discrete maximum principles in continuous (linear or bilinear) finite element approximations to transport problems with steep fronts. In the process of algebraic flux correction, the oscillatory antidiffusive part of a high-order base discretization is decomposed into a set of internodal fluxes and constrained to be local extremum dim inishing. The proposed nodal limiter functions are designed to be continuous and satisfy the principle of linearity preservation that implies the preservation of second-order accuracy in smooth regions. The use of limited nodal gradients makes it possible to circumvent angle conditions and guarantee that the discrete maximum principle holds on arbitrary meshes. A numerical study is performed for linear convection and anisotropic diffusion problems on uniform and distorted meshes in two space dimensions. © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/fld.4365
  • 2017 • 114 The effect of dynamical compressive and shear strain on magnetic anisotropy in a low symmetry ferromagnetic film
    Linnik, T.L. and Kats, V.N. and Jager, J. and Salasyuk, A.S. and Yakovlev, D.R. and Rushforth, A.W. and Akimov, A.V. and Kalashnikova, A.M. and Bayer, M. and Scherbakov, A.V.
    Physica Scripta 92 (2017)
    Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of the magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here, we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga), either by injecting a picosecond strain pulse into it from a substrate, or by generating dynamical strain of a complex temporal profile in the film directly. In the former case, we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case, optically-generated strain emerging abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession any more. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on the mixed, compressive, and shear character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study. © 2017 The Royal Swedish Academy of Sciences.
    view abstractdoi: 10.1088/1402-4896/aa6943
  • 2017 • 113 A computational framework for modelling damage-induced softening in fibre-reinforced materials – Application to balloon angioplasty
    Polindara, C. and Waffenschmidt, T. and Menzel, A.
    International Journal of Solids and Structures 118-119 235-256 (2017)
    A computational framework for modelling damage-induced softening in fibre-reinforced materials is presented. The main aspect of this framework is the proposed non-local gradient-enhanced continuum damage formulation. At the material level, the elastic constitutive behaviour is defined by a hyperelastic functional including a volumetric and an isochoric contribution. The isochoric contribution is subdivided into three contributions associated to three different phases i=0,1,2. Phase 0 is represented by an incompressible neo-Hookean material, whereas phases 1 and 2 are represented by an exponential format that accounts for the stretching along two preferred anisotropy directions, i.e. two fibre families. Furthermore, a 1−di–type damage function, is introduced to reproduce the loss of stiffness in each phase i. Following the ideas discussed in (Dimitrijević and Hackl, 2008; Waffenschmidt et al. 2014) and references cited therein, the model is built around the enhancement of the local free energy function by means of terms that contain the referential gradients of the non-local damage variables ϕi. The inclusion of these terms ensures an implicit regularisation of the finite element implementation. A finite element implementation of the non-local gradient-enhanced continuum damage model is presented. To this end we develop an 8-noded Q1Q1P0 hexahedral element following a variational approach, in order to efficiently model the quasi-incompressible behaviour of the hyperelastic material. This element is implemented in Abaqus by means of a user subroutine UEL. Three boundary value problems are studied: an anisotropic plate with a hole, a balloon angioplasty and a full-3D artery-like tube. These computational experiments serve to illustrate the main capabilities of the proposed model. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2017.02.010
  • 2017 • 112 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 111 Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7498
  • 2017 • 110 Elasto-viscoplastic phase field modelling of anisotropic cleavage fracture
    Shanthraj, P. and Svendsen, B. and Sharma, L. and Roters, F. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 99 19-34 (2017)
    A finite-strain anisotropic phase field method is developed to model the localisation of damage on a defined family of crystallographic planes, characteristic of cleavage fracture in metals. The approach is based on the introduction of an undamaged configuration, and the inelastic deformation gradient mapping this configuration to a damaged configuration is microstructurally represented by the opening of a set of cleavage planes in the three fracture modes. Crack opening is modelled as a dissipative process, and its evolution is thermodynamically derived. To couple this approach with a physically-based phase field method for brittle fracture, a scalar measure of the overall local damage is introduced, whose evolution is determined by the crack opening rates, and weakly coupled with the non-local phase field energy representing the crack opening resistance in the classical sense of Griffith. A finite-element implementation of the proposed model is employed to simulate the crack propagation path in a laminate and a polycrystalline microstructure. As shown in this work, it is able to predict the localisation of damage on the set of pre-defined cleavage planes, as well as the kinking and branching of the crack resulting from the crystallographic misorientation across the laminate boundary and the grain boundaries respectively. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2016.10.012
  • 2017 • 109 A simple triangular finite element for nonlinear thin shells: statics, dynamics and anisotropy
    Viebahn, N. and Pimenta, P.M. and Schröder, J.
    Computational Mechanics 59 281-297 (2017)
    This work presents a simple finite element implementation of a geometrically exact and fully nonlinear Kirchhoff–Love shell model. Thus, the kinematics are based on a deformation gradient written in terms of the first- and second-order derivatives of the displacements. The resulting finite element formulation provides C1-continuity using a penalty approach, which penalizes the kinking at the edges of neighboring elements. This approach enables the application of well-known C0-continuous interpolations for the displacements, which leads to a simple finite element formulation, where the only unknowns are the nodal displacements. On the basis of polyconvex strain energy functions, the numerical framework for the simulation of isotropic and anisotropic thin shells is presented. A consistent plane stress condition is incorporated at the constitutive level of the model. A triangular finite element, with a quadratic interpolation for the displacements and a one-point integration for the enforcement of the C1-continuity at the element interfaces leads to a robust shell element. Due to the simple nature of the element, even complex geometries can be meshed easily, which include folded and branched shells. The reliability and flexibility of the element formulation is shown in a couple of numerical examples, including also time dependent boundary value problems. A plane reference configuration is assumed for the shell mid-surface, but initially curved shells can be accomplished if one regards the initial configuration as a stress-free deformed state from the plane position, as done in previous works. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00466-016-1343-6
  • 2016 • 108 Numerical modeling of fluid–structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains
    Balzani, D. and Deparis, S. and Fausten, S. and Forti, D. and Heinlein, A. and Klawonn, A. and Quarteroni, A. and Rheinbach, O. and Schröder, J.
    International Journal for Numerical Methods in Biomedical Engineering 32 (2016)
    The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid–structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid–structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid–structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple – but nonsymmetric – curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid–structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid–structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/cnm.2756
  • 2016 • 107 Dispersion of the electron g factor anisotropy in InAs/InP self-assembled quantum dots
    Belykh, V.V. and Yakovlev, D.R. and Schindler, J.J. and Van Bree, J. and Koenraad, P.M. and Averkiev, N.S. and Bayer, M. and Silov, A.Y.
    Journal of Applied Physics 120 (2016)
    The electron g factor in an ensemble of InAs/InP quantum dots with emission wavelengths around 1.4 μm is measured using time-resolved pump-probe Faraday rotation spectroscopy in different magnetic field orientations. Thereby, we can extend recent single dot photoluminescence measurements significantly towards lower optical transition energies through 0.86 eV. This allows us to obtain detailed insight into the dispersion of the recently discovered g factor anisotropy in these infrared emitting quantum dots. We find with decreasing transition energy over a range of 50 meV a strong enhancement of the g factor difference between magnetic field normal and along the dot growth axis, namely, from 1 to 1.7. We argue that the g factor cannot be solely determined by the confinement energy, but the dot asymmetry underlying this anisotropy therefore has to increase with increasing dot size. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4961201
  • 2016 • 106 Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films
    Fu, Y. and Barsukov, I. and Li, J. and Gonçalves, A.M. and Kuo, C.C. and Farle, M. and Krivorotov, I.N.
    Applied Physics Letters 108 (2016)
    We study perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2-1.35 nm. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945682
  • 2016 • 105 Domain wall dynamics of periodic magnetic domain patterns in Co2MnGe-Heusler microstripes
    Gross, K. and Westerholt, K. and Zabel, H.
    New Journal of Physics 18 (2016)
    Highly symmetric periodic domain patterns were obtained in Co2MnGe-Heusler microstripes as a result of the competition between growth-induced in-plane magnetic anisotropy and shape anisotropy. Zero field magnetic configurations and magnetic field-induced domain wall (DW) motion were studied by magnetic force microscopy-image technique for two different cases: dominant uniaxial- and dominant cubic in-plane anisotropy. We implemented a magneto-optical Kerr effect susceptometer to investigate the DW dynamics of periodic domain structures by measuring the in-phase and out-of-phase components of the Kerr signal as a function of magnetic field frequency and amplitude. The DW dynamics for fields applied transversally to the long stripe axis was found to be dominated by viscous slide motion. We used the inherent symmetry/periodicity properties of the magnetic domain structure to fit the experimental results with a theoretical model allowing to extract the DW mobility for the case of transverse DWs (μ TDW = 1.1 m s-1 Oe-1) as well as for vortex-like DWs (μ VDW = 8.7 m s-1 Oe-1). Internal spin structure transformations may cause a reduction of DW mobility in TDWs as observed by OMMFF simulations. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/3/033007
  • 2016 • 104 Magnetoresponsive Poly(ether sulfone)-Based Iron Oxide cum Hydrogel Mixed Matrix Composite Membranes for Switchable Molecular Sieving
    Lin, X. and Nguyen Quoc, B. and Ulbricht, M.
    ACS Applied Materials and Interfaces 8 29001-29014 (2016)
    Stimuli-responsive membranes that can adjust mass transfer and interfacial properties "on demand" have drawn large interest over the last few decades. Here, we designed and prepared a novel magnetoresponsive separation membrane with remote switchable molecular sieving effect by simple one-step and scalable nonsolvent induced phase separation (NIPS) process. Specifically, poly(ether sulfone) (PES) as matrix for an anisotropic membrane, prefabricated poly(N-isopropylacrylamide) (PNIPAAm) nanogel (NG) particles as functional gates, and iron oxide magnetic nanoparticles (MNP) as localized heaters were combined in a synergistic way. Before membrane casting, the properties of the building blocks, including swelling property and size distribution for NG, and magnetic property and heating efficiency for MNP, were investigated. Further, to identify optimal film casting conditions for membrane preparation by NIPS, in-depth rheological study of the effects of composition and temperature on blend dope solutions was performed. At last, a composite membrane with 10% MNP and 10% NG blended in a porous PES matrix was obtained, which showed a large, reversible, and stable magneto-responsivity. It had 9 times higher water permeability at the "on" state of alternating magnetic field (AMF) than at the "off"-state. Moreover, the molecular weight cutoff of such membrane could be reversibly shifted from ∼70 to 1750 kDa by switching off or on the external AMF, as demonstrated in dextran ultrafiltration tests. Overall, it has been proved that the molecular sieving performance of the novel mixed matrix composite membrane can be controlled by the swollen/shrunken state of PNIPAAm NG embedded in the nanoporous barrier layer of a PES-based anisotropic porous matrix, via the heat generation of nearby MNP. And the structure of such membrane can be tailored by the NIPS process conditions. Such membrane has potential as enabling material for remote-controlled drug release systems or devices for tunable fractionations of biomacromolecule/-particle mixtures. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b09369
  • 2016 • 103 Anisotropic n-Type Bi2Te3-In2Te3 Thermoelectric Material Produced by Seeding Zone Melting and Solid State Transformation
    Liu, D. and Stötzel, J. and Seyring, M. and Drüe, M. and Li, X. and Schmechel, R. and Rettenmayr, M.
    Crystal Growth and Design 16 617-624 (2016)
    Seeding zone melting is applied to produce bulk Bi1.625In0.375Te3 with 7.5 atom % In in solid solution. The concentration distribution is markedly homogeneous and exhibits pronounced anisotropic electrical and thermal conductivity. Subsequent precipitation from the solid solution leads to the formation of a highly anisotropic composite thermoelectric material consisting of aligned microscaled Bi2Te3 and extended micro- to nanoscaled In2Te3 plates. By the precipitation, an increase of zT by a factor of 6 compared with the parent supersaturated solid solution crystal is achieved. This is attributed to the combination of a decrease of In concentration from 7.5 to 3 atom % in the Bi2Te3 layer and an increasing interface density due to the precipitation of In2Te3. The Bi2Te3/In2Te3 interface is determined as coherent, and the crystallographic orientation between the two phases is determined as «2¯11»In2Te3//«11¯00»Bi2Te3, {111}In2Te3//{0001}Bi2Te3. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.5b01015
  • 2016 • 102 Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet
    Mlynczak, E. and Eschbach, M. and Borek, S. and Minár, J. and Braun, J. and Aguilera, I. and Bihlmayer, G. and Döring, S. and Gehlmann, M. and Gospodaric, P. and Suga, S. and Plucinski, L. and Blügel, S. and Ebert, H. and Schne...
    Physical Review X 6 (2016)
    We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of ΔE = 100 meV and Δk = 0.1 Å-1. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spinorbit interaction even for elements as light as 3d ferromagnets. These results set a new paradigm for the investigations of spin-orbit effects in the spintronic materials. The same methodology could be used in the bottom-up design of the devices based on the switching of spin-orbit gaps such as electric-field control of magnetic anisotropy or tunneling anisotropic magnetoresistance.
    view abstractdoi: 10.1103/PhysRevX.6.041048
  • 2016 • 101 Enhanced magnetocrystalline anisotropy of Fe30Co70 nanowires by Cu additives and annealing
    Palmero, E.M. and Salikhov, R. and Wiedwald, U. and Bran, C. and Spasova, M. and Vázquez, M. and Farle, M.
    Nanotechnology 27 (2016)
    The use of 3d transition metal-based magnetic nanowires (NWs) for permanent magnet applications requires large magnetocrystalline anisotropy energy (MAE), which in combination with the NWs' magnetic shape anisotropy yields magnetic hardening and an enhancement of the magnetic energy product. Here, we report on the significant increase in MAE by 125 kJ m-3 in Fe30Co70 NWs with diameters of 20-150 nm embedded in anodic aluminum oxide templates by adding 5 at.% Cu and subsequent annealing at 900 K. Ferromagnetic resonance (FMR) reveals that this enhancement of MAE is twice as large as the enhancement of MAE in annealed, but undoped NWs. X-ray diffraction (XRD) analysis suggests that upon annealing the immiscible Cu in FeCo NWs causes a crystal reorientation with respect to the NW axis with a considerable distortion of the bcc FeCo lattice. This strain is most likely the origin of the strongly enhanced MAE. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/36/365704
  • 2016 • 100 A microstructurally based continuum model of cartilage viscoelasticity and permeability incorporating measured statistical fiber orientations
    Pierce, D.M. and Unterberger, M.J. and Trobin, W. and Ricken, T. and Holzapfel, G.A.
    Biomechanics and Modeling in Mechanobiology 15 229-244 (2016)
    The remarkable mechanical properties of cartilage derive from an interplay of isotropically distributed, densely packed and negatively charged proteoglycans; a highly anisotropic and inhomogeneously oriented fiber network of collagens; and an interstitial electrolytic fluid. We propose a new 3D finite strain constitutive model capable of simultaneously addressing both solid (reinforcement) and fluid (permeability) dependence of the tissue’s mechanical response on the patient-specific collagen fiber network. To represent fiber reinforcement, we integrate the strain energies of single collagen fibers—weighted by an orientation distribution function (ODF) defined over a unit sphere—over the distributed fiber orientations in 3D. We define the anisotropic intrinsic permeability of the tissue with a structure tensor based again on the integration of the local ODF over all spatial fiber orientations. By design, our modeling formulation accepts structural data on patient-specific collagen fiber networks as determined via diffusion tensor MRI. We implement our new model in 3D large strain finite elements and study the distributions of interstitial fluid pressure, fluid pressure load support and shear stress within a cartilage sample under indentation. Results show that the fiber network dramatically increases interstitial fluid pressure and focuses it near the surface. Inhomogeneity in the tissue’s composition also increases fluid pressure and reduces shear stress in the solid. Finally, a biphasic neo-Hookean material model, as is available in commercial finite element codes, does not capture important features of the intra-tissue response, e.g., distributions of interstitial fluid pressure and principal shear stress. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10237-015-0685-x
  • 2016 • 99 Thermoelectric properties of Ge/Si heterostructures: A combined theoretical and experimental study
    Reith, H. and Nielsch, K. and Fiedler, G. and Nausner, L. and Hu, Y. and Chen, P. and Rastelli, A. and Kratzer, P.
    Physica Status Solidi (A) Applications and Materials Science 213 524-532 (2016)
    We present a combined experimental and theoretical investigation of the thermoelectric properties of p-doped Ge/Si superlattices grown on Si(001) substrates by molecular beam epitaxy. Electrical conductivity is measured both in the direction parallel and perpendicular to the interfaces by means of a modified transfer length method. Electronic transport is strongly anisotropic, with the cross-plane conductivity being about five times lower than in plane. This result is in very good agreement with the theoretical predictions based on the tight-binding method combined with the Boltzmann equation applied to the experimentally investigated structure. The cross-plane thermal conductivity of doped superlattices is measured with the differential 3ω method and compared with that of undoped superlattices and alloys with similar average Ge content. The comparison reveals that superlattices have strongly reduced thermal conduction compared to alloys, and that doping increases their thermal conductivity by about 50%. Considering the used doping level, this increase appears surprising. The Seebeck coefficient of the structures is addressed theoretically and displays a less pronounced anisotropy compared to the electric conductivity. Combined with the knowledge of the other thermoelectric parameters, we conclude that, while p-doped Si/Ge superlattices may be used as model systems for the investigation of thermoelectric transport in nanostructured materials, their relevance for application is limited. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532486
  • 2016 • 98 Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): Effect of thermal treatments and influence of substrate and particle size
    Schilling, M. and Ziemann, P. and Zhang, Z. and Biskupek, J. and Kaiser, U. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 7 591-604 (2016)
    Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min. © 2016 Schilling et al.
    view abstractdoi: 10.3762/bjnano.7.52
  • 2016 • 97 Relaxed incremental variational approach for the modeling of damage-induced stress hysteresis in arterial walls
    Schmidt, T. and Balzani, D.
    Journal of the Mechanical Behavior of Biomedical Materials 58 149-162 (2016)
    In this paper, a three-dimensional relaxed incremental variational damage model is proposed, which enables the description of complex softening hysteresis as observed in supra-physiologically loaded arterial tissues, and which thereby avoids a loss of convexity of the underlying formulation. The proposed model extends the relaxed formulation of Balzani and Ortiz [2012. Relaxed incremental variational formulation for damage at large strains with application to fiber-reinforced materials and materials with truss-like microstructures. Int. J. Numer. Methods Eng. 92, 551-570], such that the typical stress-hysteresis observed in arterial tissues under cyclic loading can be described. This is mainly achieved by constructing a modified one-dimensional model accounting for cyclic loading in the individual fiber direction and numerically homogenizing the response taking into account a fiber orientation distribution function. A new solution strategy for the identification of the convexified stress potential is proposed based on an evolutionary algorithm which leads to an improved robustness compared to solely Newton-based optimization schemes. In order to enable an efficient adjustment of the new model to experimentally observed softening hysteresis, an adjustment scheme using a surrogate model is proposed. Therewith, the relaxed formulation is adjusted to experimental data in the supra-physiological domain of the media and adventitia of a human carotid artery. The performance of the model is then demonstrated in a finite element example of an overstretched artery. Although here three-dimensional thick-walled atherosclerotic arteries are considered, it is emphasized that the formulation can also directly be applied to thin-walled simulations of arteries using shell elements or other fiber-reinforced biomembranes. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2015.08.005
  • 2016 • 96 Direction-dependent mechanical characterization of cellulose-based composite vulcanized fiber
    Scholz, R. and Mittendorf, R. M. and Engels, J. K. and Hartmaier, A. and Kunne, B. and Walther, F.
    Materials Testing 58 813--817 (2016)
    Vulcanized fiber is a macromolecular cellulose-based composite material manufactured using the parchmentizing process. The cellulose is produced from the chemical digestion of plant-based raw materials (wood, cotton) or textile waste. Chemical additives used during manufacturing are completely removed. After the process, vulcanized fiber possesses improved properties concerning mechanical strength and abrasion as well as corrosion resistance in comparison to its raw materials. Concerning its economic life cycle assessment, low density, electrical insulating capability and balanced properties, vulcanized fiber has a potential, up to now unused, as a light and renewable structural material for applications in automotive or civil engineering industries. Research activities concerning the mechanical properties are insufficient and existing standards are out-of-date. In this work, for the first time a direction-dependent characterization of the process-related anisotropic mechanical properties of the material is realized with the aim to formulate an adequate material model for numerical simulations in the next step.
    view abstractdoi: 10.3139/120.110929
  • 2016 • 95 A novel mixed finite element for finite anisotropic elasticity; the SKA-element Simplified Kinematics for Anisotropy
    Schröder, J. and Viebahn, N. and Balzani, D. and Wriggers, P.
    Computer Methods in Applied Mechanics and Engineering 310 475-494 (2016)
    A variety of numerical approximation schemes for boundary value problems suffer from so-called locking-phenomena. It is well known that in such cases several finite element formulations exhibit poor convergence rates in the basic variables. A serious locking phenomenon can be observed in the case of anisotropic elasticity, due to high stiffness in preferred directions. The main goal of this paper is to overcome this locking problem in anisotropic hyperelasticity by introducing a novel mixed variational framework. Therefore we split the strain energy into two main parts, an isotropic and an anisotropic part. For the isotropic part we can apply different well-established approximation schemes and for the anisotropic part we apply a constant approximation of the deformation gradient or the right Cauchy–Green tensor. This additional constraint is attached to the strain energy function by a second-order tensorial Lagrange-multiplier, governed by a Simplified Kinematic for the Anisotropic part. As a matter of fact, for the tested boundary value problems the SKA-element based on quadratic ansatz functions for the displacements, performs excellent and behaves more robust than competitive formulations. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2016.06.029
  • 2016 • 94 Magnetic anisotropy and relaxation of single Fe/FexOy core/shell- nanocubes: A ferromagnetic resonance investigation
    Terwey, A. and Meckenstock, R. and Zingsem, B.W. and Masur, S. and Derricks, C. and Römer, F.M. and Farle, M.
    AIP Advances 6 (2016)
    In this work a full angle dependent Ferromagnetic Resonance (FMR) investigation on a system of 20 separated Fe/FexOy nanocubes without dipolar coupling is reported. The angular magnetic field dependence of FMR spectra of 20 single particles and 2 dimers were recorded using a microresonator setup with a sensitivity of 106 μB at X-band frequencies. We determine an effective magnetocrystalline anisotropy field of 2K4,eff/M = 50 mT ± 5 mT for selected particles, which is smaller than the one of bulk Fe due to the core shell morphology of the particles. The FMR resonances have a linewidth of 4 mT ± 1 mT, corresponding to a magnetic effective damping parameter α = 0.0045 ± 0.0005 matching the values of high quality iron thin films. Numerical calculations taking into account the different angular orientations of the 24 particles with respect to the external magnetic field yield a good agreement to the experiment. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4944399
  • 2016 • 93 A virtual laboratory using high resolution crystal plasticity simulations to determine the initial yield surface for sheet metal forming operations
    Zhang, H. and Diehl, M. and Roters, F. and Raabe, D.
    International Journal of Plasticity 80 111-138 (2016)
    We present a virtual laboratory to investigate the anisotropic yield behavior of polycrystalline materials by using high resolution crystal plasticity simulations. Employing a fast spectral method solver enables us to conduct a large number of full-field virtual experiments with different stress states to accurately identify the yield surface of the probed materials. Based on the simulated yield stress points, the parameters for many commonly used yield functions are acquired simultaneously with a nonlinear least square fitting procedure. Exemplarily, the parameters of four yield functions frequently used in sheet metal forming, namely Yld91, Yld2000-2D, Yld2004-18p, and Yld2004-27p are adjusted to accurately describe the yield behavior of an AA3014 aluminum alloy at two material states, namely with a recrystallization texture and a cold rolling texture. The comparison to experimental results proves that the methodology presented, combining accuracy with efficiency, is a promising micromechanics-based tool for probing the mechanical anisotropy of polycrystalline metals and for identifying the parameters of advanced yield functions. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2016.01.002
  • 2015 • 92 Correlation between structure and magnetic properties in CoxFe100-x nanowires: The roles of composition and wire diameter
    Bran, C. and Palmero, E.M. and Li, Z.-A. and Del Real, R.P. and Spasova, M. and Farle, M. and Vázquez, M.
    Journal of Physics D: Applied Physics 48 (2015)
    The structural and magnetic characteristics of CoxFe100-x (0 ≤ x ≤ 100) cylindrical nanowire arrays are investigated for two series of nanowires with diameters of 20 and 40 nm, respectively. The crystalline structure evolves with Co content from bcc Fe through a mixed bcc-fcc phase to a final polycrystalline fcc or hcp phase for 40 and 20 nm diameter Co nanowires, respectively. A monocrystalline structure is found only in a few nanowires with a 40 nm diameter. The magnetic characterization under axial magnetic field reveals an increase in coercivity and remanence for increasing Co content as the crystalline structure evolves from bcc Fe to fcc Co. These parameters decrease when hcp Co with a stronger magnetocrystalline anisotropy and nearly perpendicular 'c' axis is formed. Overall higher values are observed in nanowires when the nanowire diameter decreases from 40 to 20 nm. An increase of the total magnetic anisotropy energy density is found with decreasing temperature, especially for Co wires where the strong magnetocrystalline anisotropy plays the most significant role. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/14/145304
  • 2015 • 91 Comparison of two models for anisotropic hardening and yield surface evolution in bcc sheet steels
    Clausmeyer, T. and Svendsen, B.
    European Journal of Mechanics, A/Solids 54 120-131 (2015)
    The purpose of the current work is the investigation and comparison of aspects of the material behavior predicted by two models for anisotropic, and in particular cross, hardening in bcc sheet steels subject to non-proportional loading. The first model is the modified form (Wang et al., 2008) of that due to Teodosiu and Hu (1995, 1998). In this (modified) Teodosiu-Hu model (THM), cross hardening is assumed to affect the yield stress and the saturation value of the back stress. The second model is due to Levkovitch and Svendsen (2007) and Noman et al. (2010). In the Levkovitch-Svendsen model (LSM), cross hardening is assumed to affect the flow anisotropy. As clearly demonstrated in a number of works applying the THM (e.g., Boers et al., 2010; Bouvier et al., 2005, 2003; Hiwatashi et al., 1997; Li et al., 2003; Thuillier et al., 2010; Wang et al., 2008) and the LSM (e.g., Clausmeyer et al., 2014, 2011b; Noman et al., 2010), both of these are capable of predicting the effect of cross hardening on the stress-deformation behavior observed experimentally in sheet steels. As shown in the current work, however, these two models differ significantly in other aspects, in particular with respect to the development of the yield stress, the back stress, and the yield surface. For example, the THM predicts no change in the shape of the yield surface upon change of loading path, in contrast to the LSM and crystal plasticity modeling of bcc sheet steels (Peeters et al., 2002). On the other hand, the LSM predicts no hardening stagnation after cross hardening as observed in experiments, in contrast to the THM. Examples are given. © 2015 Elsevier Masson SAS. All rights reserved.
    view abstractdoi: 10.1016/j.euromechsol.2015.05.016
  • 2015 • 90 How Atomic Steps Modify Diffusion and Inter-adsorbate Forces: Empirical Evidence from Hopping Dynamics in Na/Cu(115)
    Godsi, O. and Corem, G. and Kravchuk, T. and Bertram, C. and Morgenstern, K. and Hedgeland, H. and Jardine, A.P. and Allison, W. and Ellis, J. and Alexandrowicz, G.
    Journal of Physical Chemistry Letters 6 4165-4170 (2015)
    We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01939
  • 2015 • 89 Microscopic analysis of the composition driven spin-reorientation transition in NixPd1-x/Cu(001)
    Gottlob, D.M. and Doğanay, H. and Nickel, F. and Cramm, S. and Krug, I.P. and Nemšák, S. and Schneider, C.M.
    Ultramicroscopy 159 503-507 (2015)
    The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.022
  • 2015 • 88 Domain Structure and Magnetoresistance in Co2MnGe Zigzag Structures
    Gross, K. and Westerholt, K. and Gómez, M.E. and Zabel, H.
    Physics Procedia 75 1072-1079 (2015)
    We report a clear manifestation of the negative contribution to the magnetoresistance due to domain walls in Co2MnGe-Heusler submicron zigzag wires in which the domain structure, domain size and domain wall density can be well controlled. The magnetic behavior of these systems results from the interplay between the intrinsic magneto-crystalline (K4) anisotropy, growth induced uniaxial (KU = 4.7x103 J/m3) anisotropy and shape anisotropy (KS), as observed by magnetic-force microscopy (MFM) and longitudinal Kerr hysteresis loop measurements. Magnetoresistance measurements were performed by the four-point method under a field applied in the plane of the wires at a temperature of 300 K. In these structures, domain wall-creation and annihilation occur in a coherent way. As a result, clear jumps of the resistance are detected during the transition from single-domain- to multi-domain states. At room temperature a value RDW = -2.5 mΩ was obtained; this result is the same order of magnitude as other experimental and theoretical findings. The negative resistive contribution due to the domain wall is also discussed and compared with the existing theoretical models. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.177
  • 2015 • 87 Influence of the Nuclear Electric Quadrupolar Interaction on the Coherence Time of Hole and Electron Spins Confined in Semiconductor Quantum Dots
    Hackmann, J. and Glasenapp, Ph. and Greilich, A. and Bayer, M. and Anders, F.B.
    Physical Review Letters 115 (2015)
    The real-time spin dynamics and the spin noise spectra are calculated for p and n-charged quantum dots within an anisotropic central spin model extended by additional nuclear electric quadrupolar interactions and augmented by experimental data. Using realistic estimates for the distribution of coupling constants including an anisotropy parameter, we show that the characteristic long time scale is of the same order for electron and hole spins strongly determined by the quadrupolar interactions even though the analytical form of the spin decay differs significantly consistent with our measurements. The low frequency part of the electron spin noise spectrum is approximately 1/3 smaller than those for hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by our experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.207401
  • 2015 • 86 Magnetic hardening of Fe30Co70nanowires
    Liébana Viñas, S. and Salikhov, R. and Bran, C. and Palmero, E.M. and Vazquez, M. and Arvan, B. and Yao, X. and Toson, P. and Fidler, J. and Spasova, M. and Wiedwald, U. and Farle, M.
    Nanotechnology 26 (2015)
    3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of Fe30Co70nanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/26/41/415704
  • 2015 • 85 Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods
    Liébana-Viñas, S. and Wiedwald, U. and Elsukova, A. and Perl, J. and Zingsem, B. and Semisalova, A.S. and Salgueiriño, V. and Spasova, M. and Farle, M.
    Chemistry of Materials 27 4015-4022 (2015)
    Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co<inf>80</inf>Ni<inf>20</inf> nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co<inf>80</inf>Ni<inf>20</inf> alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b00976
  • 2015 • 84 Analytical bounds of in-plane Young's modulus and full-field simulations of two-dimensional monocrystalline stochastic honeycomb structures
    Ma, D. and Eisenlohr, P. and Shanthraj, P. and Diehl, M. and Roters, F. and Raabe, D.
    Computational Materials Science 109 323-329 (2015)
    Abstract In this study, we focus on the interplay between the honeycomb structure and the crystallographic orientation. Specifically, the in-plane Young's moduli of monocrystalline stochastic honeycombs are calculated by a numerical and an analytical approach. The in-plane Young's moduli of the honeycombs were calculated numerically using a solution scheme for the full-field mechanical equilibrium based on spectral methods and anisotropic crystal elasticity. The analytical approach formulates two alternative assumptions, i.e. uniform force and uniform strain per strut, considers the elastic anisotropy of the base material, and depends on the two-variable distribution of the strut length and inclination angle as the structural parameters characterizing the stochastic honeycombs. The uniform strain assumption agrees closely with the numerical simulation results and constitutes an improvement compared to analytical solutions proposed in previous studies. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2015.07.041
  • 2015 • 83 The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys
    Nörtershäuser, P. and Frenzel, J. and Ludwig, Al. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 626 305-312 (2015)
    In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.12.030
  • 2015 • 82 Rotational and translational dynamics of CO2 adsorbed in MOF Zn2(bdc)2(dabco)
    Peksa, M. and Burrekaew, S. and Schmid, R. and Lang, J. and Stallmach, F.
    Microporous and Mesoporous Materials 216 75-81 (2015)
    The dynamics of adsorbed CO2 in the metal-organic framework Zn2(bdc)2 dabco (DMOF-1) was investigated using molecular dynamics (MD) simulations and 13C NMR spectroscopy. The statistical analysis of the MD trajectories suggest a preferred localization of the CO2 molecules in the Zn2(bdc)4 corners of the DMOF-1 lattice. The adsorbed molecules retain a high but anisotropic rotational and translational mobility in the channel system. Based on these MD-results, the residual chemical shift anisotropy 〈Δδ〉MD = -114 ppm and the diffusion anisotropy (D∥D⊥)MD = 9.8 ± 0.5 were calculated. They are found to be in reasonable agreement with the experimental NMR data of 〈Δδ〉NMR=-(55 ± 2) ppm and (D∥D⊥)NMR = 3 respectively. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2015.02.043
  • 2015 • 81 Experimental and numerical investigations of the non-linear rheological properties of viscoelastic surfactant solutions: application and failing of the one-mode Giesekus model
    Rehage, H. and Fuchs, R.
    Colloid and Polymer Science 293 3249-3265 (2015)
    In a series of experiments, we investigated the non-linear rheological properties of aqueous solutions of entangled wormlike micelles (WLMs) in steady-state shear flow and in large amplitude oscillating shear (LAOS) experiments. On grounds of their monoexponential stress relaxation properties, we studied semi-dilute solutions of the cationic surfactants cetylpyridinium chloride (CPyCl) or cetyltrimethylammonium bromide (CTAB) after addition of different amounts of sodium salicylate. The rheological data of these networks of WLMs were systematically compared with the numerically calculated results of the one-mode Giesekus constitutive equation. It turned out that the viscous resistance and the first normal stress difference, measured in steady-state shear flow, start-up, and relaxation experiments, were accurately predicted by the one-mode Giesekus model. In rheological tests, where we applied large oscillating shear amplitudes (LAOS), the transient shear stress could also approximately be described by means of the Giesekus model. The non-linear oscillating first normal stress difference, however, showed large deviations in respect to the theoretical predictions. These discrepancies between different rheological experiments, which we observed in oscillating and stationary flow, pointed to the existence of flow instabilities, which occurred in the LAOS regime. These, more complicated rheological processes, were induced by shear-banding and/or the presence of flow-induced phase transitions, which can occur in oscillatory and stationary shear. The non-linear phenomena, discussed in this article, are of general importance, and they can be equally observed in entangled solutions of flexible macromolecules. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00396-015-3689-2
  • 2015 • 80 Influence of isotropic and anisotropic material models on the mechanical response in arterial walls as a result of supra-physiological loadings
    Schmidt, T. and Pandya, D. and Balzani, D.
    Mechanics Research Communications 64 29-37 (2015)
    As accepted in the literature, arterial tissues have in principle anisotropic material properties. Although some very special situations in arteries exist where isotropic constitutive models may approximate the real material behavior with sufficient accuracy, the larger part of analyses requires an anisotropic model. In particular for overstretched arteries, as e.g. a result of a balloon angioplasty, an accurate representation of the complex softening phenomena is important and then the consideration of anisotropy may be necessary. However, a variety of publications found in the literature, where such supra-physiological loading situations are analyzed to optimize e.g. stent designs, consider isotropic models. Therefore, in this contribution, the response of an isotropic and an anisotropic material model is compared in numerical calculations where arteries are subjected to supra-physiological loading. The constitutive formulations include the typical nonlinear stiffening of the fiber response as well as softening due to microscopic damage. In detail, the isotropic and the anisotropic model are adjusted to the same experimental stress-stretch curves of different arterial layers and then both models are applied to finite element simulations of overstretched arterial walls. As it turns out a significant difference is obtained for both calculations showing the importance of anisotropic models for these loading situations. © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.mechrescom.2014.12.008
  • 2015 • 79 Selective enzymatic removal of elastin and collagen from human abdominal aortas: Uniaxial mechanical response and constitutive modeling
    Schriefl, A.J. and Schmidt, T. and Balzani, D. and Sommer, G. and Holzapfel, G.A.
    Acta Biomaterialia 17 125-136 (2015)
    The ability to selectively remove the structurally most relevant components of arterial wall tissues such as collagen and elastin enables ex vivo biomechanical testing of the remaining tissues, with the aim of assessing their individual mechanical contributions. Resulting passive material parameters can be utilized in mathematical models of the cardiovascular system. Using eighteen wall specimens fromnon-atherosclerotic human abdominal aortas (55±11 years; 9 female, 9 male), we tested enzymatic approaches for the selective digestion of collagen and elastin, focusing on their application to human abdominal aortic wall tissues from different patients with varying sample morphologies. The study resulted in an improved protocol for elastin removal, showing how the enzymatic process is affected by inadequate addition of trypsin inhibitor. We applied the resulting protocol to circumferential and axial specimens from the media and the adventitia, and performed cyclic uniaxial extension tests in the physiological and supra-physiological loading domain. The collagenase-treated samples showed a (linear) response without distinct softening behavior, while the elastase-treated samples exhibited a nonlinear, anisotropic response with pronounced remanent deformations (continuous softening), presumably caused by some sliding of collagen fibers within the damaged regions of the collagen network. In addition, our data showed that the stiffness in the initial linear stress-stretch regime at low loads is lower in elastin-free tissue compared to control samples (i.e. collagen uncrimping requires less force than the stretching of elastin), experimentally confirming that elastin is responsible for the initial stiffness in elastic arteries. Utilizing a continuum mechanical description to mathematically capture the experimental results we concluded that the inclusion of a damage model for the non-collagenous matrix material is, in general, not necessary. To model the softening behavior, continuous damage was included in the fibers by adding a damage variable which led to remanent strains through the consideration of damage. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2015.01.003
  • 2015 • 78 Mechanical Properties of a Calcium Dietary Supplement, Calcium Fumarate Trihydrate
    Sun, S. and Henke, S. and Wharmby, M.T. and Yeung, H.H.-M. and Li, W. and Cheetham, A.K.
    Inorganic Chemistry 54 11186-11192 (2015)
    The mechanical properties of calcium fumarate trihydrate, a 1D coordination polymer considered for use as a calcium source for food and beverage enrichment, have been determined via nanoindentation and high-pressure X-ray diffraction with single crystals. The nanoindentation studies reveal that the elastic modulus (16.7-33.4 GPa, depending on crystallographic orientation), hardness (1.05-1.36 GPa), yield stress (0.70-0.90 GPa), and creep behavior (0.8-5.8 nm/s) can be rationalized in view of the anisotropic crystal structure; factors include the directionality of the inorganic Ca-O-Ca chain and hydrogen bonding, as well as the orientation of the fumarate ligands. High-pressure single-crystal X-ray diffraction studies show a bulk modulus of ∼20 GPa, which is indicative of elastic recovery intermediate between small molecule drug crystals and inorganic pharmaceutical ingredients. The combined use of nanoindentation and high-pressure X-ray diffraction techniques provides a complementary experimental approach for probing the critical mechanical properties related to tableting of these dietary supplements. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.inorgchem.5b01466
  • 2015 • 77 Visualization of nanocrystal breathing modes at extreme strains
    Szilagyi, E. and Wittenberg, J.S. and Miller, T.A. and Lutker, K. and Quirin, F. and Lemke, H. and Zhu, D. and Chollet, M. and Robinson, J. and Wen, H. and Sokolowski-Tinten, K. and Lindenberg, A.M.
    Nature Communications 6 (2015)
    Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. These functional properties often involve large-amplitude strains and structural modifications, and thus require an understanding of the dynamics of these processes. Here we use femtosecond X-ray scattering techniques to visualize, in real time and with atomic-scale resolution, light-induced anisotropic strains in nanocrystal spheres and rods. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrier-induced stress. These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. This work represents the first direct real-time probe of the dynamics of these large-amplitude strains and shape changes in few-nanometre-scale particles. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms7577
  • 2015 • 76 Modeling of anisotropic wound healing
    Valero, C. and Javierre, E. and García-Aznar, J.M. and Gómez-Benito, M.J. and Menzel, A.
    Journal of the Mechanics and Physics of Solids 79 80-91 (2015)
    Biological soft tissues exhibit non-linear complex properties, the quantification of which presents a challenge. Nevertheless, these properties, such as skin anisotropy, highly influence different processes that occur in soft tissues, for instance wound healing, and thus its correct identification and quantification is crucial to understand them. Experimental and computational works are required in order to find the most precise model to replicate the tissues' properties. In this work, we present a wound healing model focused on the proliferative stage that includes angiogenesis and wound contraction in three dimensions and which relies on the accurate representation of the mechanical behavior of the skin. Thus, an anisotropic hyperelastic model has been considered to analyze the effect of collagen fibers on the healing evolution of an ellipsoidal wound. The implemented model accounts for the contribution of the ground matrix and two mechanically equivalent families of fibers. Simulation results show the evolution of the cellular and chemical species in the wound and the wound volume evolution. Moreover, the local strain directions depend on the relative wound orientation with respect to the fibers. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2015.03.009
  • 2015 • 75 Identification of fully coupled anisotropic plasticity and damage constitutive equations using a hybrid experimental-numerical methodology with various triaxialities
    Yue, Z.M. and Soyarslan, C. and Badreddine, H. and Saanouni, K. and Tekkaya, A.E.
    International Journal of Damage Mechanics 24 683-710 (2015)
    A hybrid experimental-numerical methodology is presented for the parameter identification of a mixed nonlinear hardening anisotropic plasticity model fully coupled with isotropic ductile damage accounting for microcracks closure effects. In this study, three test materials are chosen: DP1000, CP1200, and AL7020. The experiments involve the tensile tests with smooth and notched specimens and two types of shear tests. The tensile tests with smooth specimens are conducted in different directions with respect to the rolling direction. This helps to determine the plastic anisotropy parameters of the material when the ductile damage is still negligible. Also, in-plane torsion tests with a single loading cycle are used to determine separately the isotropic and kinematic hardening parameters. Finally, tensile tests with notched specimens and Shouler and Allwood shear tests are used for the damage parameters identification. These are conducted until the final fracture with the triaxiality ratio• lying between 0 and 1 / 3 (i.e. 0• 1/3). The classical force-displacement curves are chosen as the experimental responses. However, for the tensile test with notched specimens, the distribution of displacement components is measured using a full field measurement technique (ARAMIS system). These experimental results are directly used by the identification methodology in order to determine the values of material parameters involved in the constitutive equations. The inverse identification methodology combines an optimization algorithm which is coded within MATLAB together with the finite element (FE) code ABAQUS/Explicit. After optimization, good agreement between experimental and numerically predicted results in terms of force-displacement curves is obtained for the three studied materials. Finally, the applicability and validity of the determined material parameters are proved with additional validation tests. © 2014 The Author(s) Reprints and permissions.
    view abstractdoi: 10.1177/1056789514546578
  • 2015 • 74 Orientation dependent deformation by slip and twinning in magnesium during single crystal indentation
    Zambaldi, C. and Zehnder, C. and Raabe, D.
    Acta Materialia 91 267-288 (2015)
    We present the orientation dependent indentation response of pure magnesium during single grain indentation. A conical indenter and maximum loads between 50 mN and 900 mN were employed. Indent topographies were acquired by confocal microscopy. The indents were also characterized by electron backscatter orientation microscopy for their microstructures. Pronounced activation of specific twinning systems was observed around the impressions. The resulting data were compiled into the inverse pole figure presentation of indent microstructures and topographies after Zambaldi and Raabe, Acta Mater. (2010). Three-dimensional crystal plasticity finite element simulation of the indentation deformation supports the interpretation of the orientation dependent slip and twinning patterns around the indents. The match between the activation of observed and simulated twinning variants is discussed with respect to the conditions for nucleation and growth of extension twins. Furthermore, the compatibility of the twinning strains with the imposed deformation is discussed based on the expanding cavity model of indentation. The orientation dependent response of magnesium during indentation is compared to the literature data for indentation of alpha-titanium and beryllium. Recommendations are given on how to exploit the characteristic nature of the observed indentation patterns to rapidly assess the relative activity of deformation mechanisms and their critical shear stresses during alloy development. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.01.046
  • 2014 • 73 On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Somsen, C. and Dlouhý, A. and Eggeler, G.
    Acta Materialia 69 246-264 (2014)
    The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type <1 1 0> jointly shear the γ′ phase. The resulting <2 2 0>-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different <1 1 0>-type Burgers vectors must combine for γ′ phase cutting. The resulting <2 0 0>-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [1 1 0] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (<2 2 0>glide) in the γ′ phase. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.021
  • 2014 • 72 Plastic anisotropy of electro-deposited pure α-iron with sharp crystallographic <1 1 1>// texture in normal direction: Analysis by an explicitly dislocation-based crystal plasticity model
    Alankar, A. and Field, D.P. and Raabe, D.
    International Journal of Plasticity 52 18-32 (2014)
    We present a single crystal plasticity model based on edge and screw dislocation densities for body centered cubic (bcc) crystals. In a bcc crystal screw dislocations experience high lattice friction due to their non-planar core. Hence, they have much slower velocity compared to edge dislocations. This phenomenon is modeled by accounting for the motion of screw dislocations via nucleation and expansion of kink-pairs. The model, embedded as a constitutive law into a crystal plasticity framework, is able to predict the crystallographic texture of a bcc polycrystal subjected to 70%, 80% and 90% thickness reduction. We perform a parametric study based on the velocities of edge and screw dislocations to analyze the effect on plastic anisotropy of electro-deposited pure iron with long needle-shaped grains having sharp crystallographic <1 1 1>//ND texture (ND: normal direction). The model shows a large change in the r-value (Lankford value, planar anisotropy ratio) for pure iron when the texture changes from random to <1 1 1>//ND. For different simulated cases where the crystallites have an orientation deviation of 1, 3 and 5, respectively, from the ideal <1 1 1>//ND axis, the simulations predict r-values between 4.0 and 7.0 which is in excellent agreement with data observed in experiments by Yoshinaga et al. (ISIJ Intern.; 48 (2008) 667-670). For these specific orientations of grains, we also model the effect of long needle shaped grains via a procedure that excludes dislocation annihilation. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2013.03.006
  • 2014 • 71 Numerical modelling of wave propagation in anisotropic soil using a displacement unit-impulse-response-based formulation of the scaled boundary finite element method
    Chen, X. and Birk, C. and Song, C.
    Soil Dynamics and Earthquake Engineering 65 243-255 (2014)
    An efficient method for modelling the propagation of elastic waves in unbounded domains is developed. It is applicable to soil-structure interaction problems involving scalar and vector waves, unbounded domains of arbitrary geometry and anisotropic soil. The scaled boundary finite element method is employed to derive a novel equation for the displacement unit-impulse response matrix on the soil-structure interface. The proposed method is based on a piecewise linear approximation of the first derivative of the displacement unit-impulse response matrix and on the introduction of an extrapolation parameter in order to improve the numerical stability. In combination, these two ideas allow for the choice of significantly larger time steps compared to conventional methods, and thus lead to increased efficiency. As the displacement unit-impulse response approaches zero, the convolution integral representing the force-displacement relationship can be truncated. After the truncation the computational effort only increases linearly with time. Thus, a considerable reduction of computational effort is achieved in a time domain analysis. Numerical examples demonstrate the accuracy and high efficiency of the new method for two-dimensional soil-structure interaction problems. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.soildyn.2014.06.019
  • 2014 • 70 Molecular spintronics: Topology communicates
    Cinchetti, M.
    9 965-966 (2014)
    doi: 10.1038/nnano.2014.284
  • 2014 • 69 Modeling and finite element simulation of loading-path-dependent hardening in sheet metals during forming
    Clausmeyer, T. and Güner, A. and Tekkaya, A.E. and Levkovitch, V. and Svendsen, B.
    International Journal of Plasticity 63 64-93 (2014)
    A recent material model considering the evolution of plastic anisotropy in interstitial free steels is validated for the forming process of the channel die, a complex part. In the model the evolution of the intra-granular microstructure is represented by tensor-valued internal variables. The model accounts for the cross hardening behavior observed in rheological tests of interstitial free steels. A novel cross hardening indicator which is directly derived from the constitutive model is proposed. This cross hardening indicator is a quantitative measure for the occurrence of cross hardening in the forming process of complex parts. A correlation between the occurrence of cross hardening and larger values of the stored (elastic) energy is observed. The influence of cross hardening on the forming process is investigated, in particular, the drawing forces and the geometric deviations due to springback. The influence of cross hardening on the forming process of the channel die geometry is small. The influence of cross hardening on the more complex S-Rail geometry is larger due to larger plastic deformation and more severe loading path changes. The concept of the proposed transient hardening indicator should be applicable to other models for the evolution of plastic anisotropy. A possible use of the cross hardening indicator would be the efficient choice of the material model in the context of sheet metal forming simulations. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2014.01.011
  • 2014 • 68 Ultrastructural organization and micromechanical properties of shark tooth enameloid
    Enax, J. and Janus, A.M. and Raabe, D. and Epple, M. and Fabritius, H.-O.
    Acta Biomaterialia 10 3959-3968 (2014)
    The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80 nm) and long (>1 μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.04.028
  • 2014 • 67 Room-temperature ferromagnetism in antiferromagnetic cobalt oxide nanooctahedra
    Fontaíña-Troitiño, N. and Liébana-Viñas, S. and Rodríguez-González, B. and Li, Z.-A. and Spasova, M. and Farle, M. and Salgueiriño, V.
    Nano Letters 14 640-647 (2014)
    Cobalt oxide octahedra were synthesized by thermal decomposition. Each octahedron-shaped nanoparticle consists of an antiferromagnetic CoO core enclosed by eight {111} facets interfaced to a thin (∼4 nm) surface layer of strained Co3O4. The nearly perfectly octahedral shaped particles with 20, 40, and 85 nm edge length show a weak room-temperature ferromagnetism that can be attributed to ferromagnetic correlations appearing due to strained lattice configurations at the CoO/Co3O4 interface. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nl4038533
  • 2014 • 66 Uniaxial anisotropy and its manipulation in amorphous Co 68Fe24Zr8 thin films (invited)
    Fu, Y. and Barsukov, I. and Meckenstock, R. and Lindner, J. and Zhai, Y. and Hjörvarsson, B. and Farle, M.
    Journal of Applied Physics 115 (2014)
    We have proven that the growth of Co68Fe24Zr 8 layers under external field yields a uniaxial anisotropy, defined by the direction of the field. No magnetic coupling is present between Co 68Fe24Zr8 layers when separated by a 3nm of Al70Zr30. The anisotropy axis can therefore be manipulated at will and the direction can be tailored, layer by layer in multilayers, by the choice of the direction of the applied field during growth. The g-factor (2.13) and the anisotropy constant, obtained from ferromagnetic resonance, support the existence of short-range order. The relation between the temperature dependences of magnetic anisotropy and magnetization are partially captured by Callen-Callen power law. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4870591
  • 2014 • 65 In-situ measurement of loading stresses with X-ray diffraction for yield locus determination
    Güner, A. and Zillmann, B. and Lampke, T. and Tekkaya, A.E.
    International Journal of Automotive Technology 15 303-316 (2014)
    The application of the X-ray diffraction method is introduced to solve the problem of inhomogeneous deformation fields in the specimens used for sheet metal characterization. In this method, strains are measured on one side of a specimen with optical measurement systems. On the other side, loading stresses on a specimen are captured with an X-ray diffractometer mounted on a universal testing machine. By this way, the whole stress-strain history of a material point is tracked during testing. The method was first applied to uniaxial tension tests, whereby the applicability of the theory of stress factors and effective X-ray elastic constants were tested. The relaxation behavior of a sheet material which shows itself as stress drops during in-situ experimentation was characterized and compensated by a visco-plastic material model for different stress states. The proposed method was applied to characterize aluminum alloy AA5182 under plane strain tension and shear conditions and the results were compared with the conventionally obtained yield locus. Numerical analyses of a workpiece with the Vegter and Yld2000-2D material models show that the enriched yield locus definition with accurate plane strain tension and shear stresses captures the experimentally obtained surface strains more precisely. © 2014 The Korean Society of Automotive Engineers and Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s12239-014-0031-9
  • 2014 • 64 Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates
    Guarise, M. and Dalla Piazza, B. and Berger, H. and Giannini, E. and Schmitt, T. and Rønnow, H.M. and Sawatzky, G.A. and Van Den Brink, J. and Altenfeld, D. and Eremin, I. and Grioni, M.
    Nature Communications 5 (2014)
    The high-Tc cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin waves - magnons - of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi2Sr2CaCu2O8+δ (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle-hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms6760
  • 2014 • 63 Guest-dependent mechanical anisotropy in pillared-layered soft porous crystals - a nanoindentation study
    Henke, S. and Li, W. and Cheetham, A. K.
    Chemical Science 5 2392--2397 (2014)
    Soft porous crystals (SPCs) of the type [Zn-2(L)(2)(dabco)](n) (L _ linear dicarboxylate linker, dabco _ 1,4-diazabicyclo[2.2.2]octane) show exceptional mechanical anisotropy. Single-crystal nanoindentation experiments reveal very large changes of the elastic modulus and hardness triggered by exchange of the guests adsorbed in the porous metal-organic framework. The substantial variations of the mechanical properties as a function of the guest molecules can be explained by the responsive nature of these SPCs. Based on non-specific guest-framework interactions, crucial changes of the network geometry induce a complex and dynamical mechanical behaviour.
    view abstractdoi: 10.1039/c4sc00497c
  • 2014 • 62 Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy
    Himmerlich, M. and Eisenhardt, A. and Shokhovets, S. and Krischok, S. and Speiser, E. and Neumann, M.D. and Navarro-Quezada, A. and Esser, N.
    Applied Physics Letters 104 (2014)
    The electronic structure of GaN(1-100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3eV, i.e., below the bulk band gap. These results indicate that the GaN(1-100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4873376
  • 2014 • 61 Tailoring the morphology of mesoporous titania thin films through biotemplating with nanocrystalline cellulose
    Ivanova, A. and Fattakhova-Rohlfing, D. and Kayaalp, B.E. and Rathouský, J. and Bein, T.
    Journal of the American Chemical Society 136 5930-5937 (2014)
    The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja411292u
  • 2014 • 60 Phase-field modeling for 3D grain growth based on a grain boundary energy database
    Kim, H.-K. and Kim, S.G. and Dong, W. and Steinbach, I. and Lee, B.-J.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A 3D phase-field model for grain growth combined with a grain boundary (GB) energy database is proposed. The phase-field model is applied to a grain growth simulation of polycrystalline bcc Fe to investigate the effect of anisotropic GB energy on the microstructural evolution and its kinetics. It is found that the anisotropy in the GB energy results in different microstructures and slower kinetics, especially when the portion of low-angle, low-energy GBs is large. We discuss the applicability of the proposed phase-field simulation technique, based on the GB or interfacial energy database to simulations for microstructural evolution, including abnormal grain growth, phase transformations, etc., in a wider range of polycrystalline materials. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034004
  • 2014 • 59 Strain response of thermal barrier coatings captured under extreme engine environments through synchrotron X-ray diffraction
    Knipe, K. and Manero II, A. and Siddiqui, S.F. and Meid, C. and Wischek, J. and Okasinski, J. and Almer, J. and Karlsson, A.M. and Bartsch, M. and Raghavan, S.
    Nature Communications 5 (2014)
    The mechanical behaviour of thermal barrier coatings in operation holds the key to understanding durability of jet engine turbine blades. Here we report the results from experiments that monitor strains in the layers of a coating subjected to thermal gradients and mechanical loads representing extreme engine environments. Hollow cylindrical specimens, with electron beam physical vapour deposited coatings, were tested with internal cooling and external heating under various controlled conditions. High-energy synchrotron X-ray measurements captured the in situ strain response through the depth of each layer, revealing the link between these conditions and the evolution of local strains. Results of this study demonstrate that variations in these conditions create corresponding trends in depth-resolved strains with the largest effects displayed at or near the interface with the bond coat. With larger temperature drops across the coating, significant strain gradients are seen, which can contribute to failure modes occurring within the layer adjacent to the interface. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5559
  • 2014 • 58 Orientation dependence of stress-induced martensite formation during nanoindentation in NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 68 19-31 (2014)
    In the present work we used nanoindentation with a spherical indenter tip to study the formation of stress-induced martensite in NiTi shape memory alloys. Prior to nanoindentation, orientation imaging was performed to select austenite grains with specific crystallographic orientations, including the principal crystallographic directions [0 0 1], [1 0 1] and [1 1 1]. We studied a material where stress-induced martensite is stable at room temperature and found surface patterns with four-, two- and threefold symmetries for the [0 0 1], [1 0 1] and [1 1 1] crystallographic indentation directions, respectively. Atomic force microscopy investigations of the topography showed that the surface patterns were associated with sink-ins. The crystallographic sink-in patterns disappeared during heating, which proved their martensitic origin. Our results provide clear experimental evidence which shows that the crystallographic anisotropy of nanoindentation is governed by the crystallographic anisotropy of the stress-induced formation of martensite.©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.006
  • 2014 • 57 Tool design induced anisotropic flow behavior of hot extruded aluminum profiles
    Ossenkemper, S. and Haase, M. and Soyarslan, C. and Jäger, A. and Tekkaya, A.E.
    Key Engineering Materials 585 131-138 (2014)
    The hot extrusion process may lead to frequently observed textures in the profiles, like fiber structures in longitudinal direction. Aluminum profiles (AA6060) were extruded with different tool types (flat-face dies and modified porthole dies). Compression tests of cylindrical specimens, which were machined out of these profiles likewise in extrusion direction, were conducted to examine possible effects of the in-plane anisotropy in lateral direction. A hardness distribution over the cross section of the specimens was measured. It was found that dependent on tool design and profile geometry, the specimens developed preferred lateral flow directions during upsetting. Simulations of the upsetting test, with assigned Hill parameters to consider anisotropy of the material, showed, that this anisotropy, not the local hardness nonuniformity, is the main reason for the detected plastic flow properties. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 56 Chemically induced fracturing in alkali feldspar
    Scheidl, K.S. and Schaeffer, A.-K. and Petrishcheva, E. and Habler, G. and Fischer, F.D. and Schreuer, J. and Abart, R.
    Physics and Chemistry of Minerals 41 1-16 (2014)
    Fracturing in alkali feldspar during Na+-K+ cation exchange with a NaCl-KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, KIc, is estimated as 2.30-2.72 MPa m1/2 (73-86 MPa mm1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00269-013-0617-1
  • 2014 • 55 Point-by-point compositional analysis for atom probe tomography
    Stephenson, L.T. and Ceguerra, A.V. and Li, T. and Rojhirunsakool, T. and Nag, S. and Banerjee, R. and Cairney, J.M. and Ringer, S.P.
    MethodsX 1 12-18 (2014)
    This new alternate approach to data processing for analyses that traditionally employed grid-based counting methods is necessary because it removes a user-imposed coordinate system that not only limits an analysis but also may introduce errors. We have modified the widely used "binomial" analysis for APT data by replacing grid-based counting with coordinate-independent nearest neighbour identification, improving the measurements and the statistics obtained, allowing quantitative analysis of smaller datasets, and datasets from non-dilute solid solutions. It also allows better visualisation of compositional fluctuations in the data. Our modifications include:.using spherical k-atom blocks identified by each detected atom's first k nearest neighbours.3D data visualisation of block composition and nearest neighbour anisotropy.using z-statistics to directly compare experimental and expected composition curves. Similar modifications may be made to other grid-based counting analyses (contingency table, Langer-Bar-on-Miller, sinusoidal model) and could be instrumental in developing novel data visualisation options. Crown Copyright © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.mex.2014.02.001
  • 2014 • 54 Approach to structural anisotropy in compacted cohesive powder
    Strege, S. and Weuster, A. and Zetzener, H. and Brendel, L. and Kwade, A. and Wolf, D.E.
    Granular Matter 16 401-409 (2014)
    We investigate the mesoscopic regime between microscopic particle properties and macroscopic bulk behavior and present a complementary approach of physical experiments and discrete element method simulations to explore the development of the microstructure of cohesive powders during compaction. On the experimental side, a precise micro shear tester (μ ST) for very small powder samples has been developed and integrated into a high resolution X-ray microtomography (XMT) system. The combination of μ ST and XMT provides the unique possibility to access the 3D microstructure and the particle network inside manipulated powder samples experimentally. In simulations we explore the structural changes resulting from compaction: a Hertzian contact model is utilized for compaction of an isotropic initial configuration created by a geometrical algorithm. As a first result of this approach we present the analysis of the compaction of slightly cohesive SiO2 particles with special regard to bulk density, heterogeneity, compaction law and structural anisotropy. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10035-013-0454-4
  • 2014 • 53 A gradient-enhanced large-deformation continuum damage model for fibre-reinforced materials
    Waffenschmidt, T. and Polindara, C. and Menzel, A. and Blanco, S.
    Computer Methods in Applied Mechanics and Engineering 268 801-842 (2014)
    A non-local gradient-based damage formulation within a geometrically non-linear setting is presented. The hyperelastic constitutive response at local material point level is governed by a strain energy which is additively composed of an isotropic matrix and of an anisotropic fibre-reinforced material, respectively. The inelastic constitutive response is governed by a scalar [1d]-type damage formulation, where only the anisotropic elastic part is assumed to be affected by the damage. Following the concept in Dimitrijević and Hackl [28], the local free energy function is enhanced by a gradient-term. This term essentially contains the gradient of the non-local damage variable which, itself, is introduced as an additional independent variable. In order to guarantee the equivalence between the local and non-local damage variable, a penalisation term is incorporated within the free energy function. Based on the principle of minimum total potential energy, a coupled system of Euler-Lagrange equations, i.e., the balance of linear momentum and the balance of the non-local damage field, is obtained and solved in weak form. The resulting coupled, highly non-linear system of equations is symmetric and can conveniently be solved by a standard incremental-iterative Newton-Raphson-type solution scheme. Several three-dimensional displacement- and force-driven boundary value problems-partially motivated by biomechanical application-highlight the mesh-objective characteristics and constitutive properties of the model and illustratively underline the capabilities of the formulation proposed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2013.10.013
  • 2014 • 52 A gradient-enhanced continuum damage model for residually stressed fibre-reinforced materials at finite strains
    Waffenschmidt, T. and Polindara, C. and Menzel, A.
    Lecture Notes in Applied and Computational Mechanics 74 19-40 (2014)
    Themodelling of damage effects inmaterials constitutes a major challenge in various engineering-related disciplines. However, the assumption of purely local continuum damage formulations may lead to ill-posed boundary value problems and—with regard to numerical methods such as the finite element method—to meshdependent solutions, a vanishing localised damage zone upon mesh refinement, and hence physically questionable results. In order to circumvent these deficiencies, we present a non-local gradient-enhanced damage model at finite strains. We additively compose the hyperelastic constitutive response at local material point level of an isotropic matrix and of an anisotropic fibre-reinforced material. The inelastic constitutive response is characterised by a scalar [1– d]-damage model, where we assume only the anisotropic elastic part to damage. Furthermore, we enhance the local free energy by a gradient-term. This term essentially contains the gradient of the non-local damage variable which we introduce as an additional global field variable. In order to guarantee the equivalence between the local and non-local damage variable, we incorporate a penalisation term within the free energy. Based on the principle of minimum total potential energy, we obtain a coupled system of variational equations. The associated non-linear system of equations is symmetric and can conveniently be solved by standard incremental-iterative Newton-Raphson schemes or arc-length-based solution methods. As a further key aspect, we incorporate residual stresses by means of a multiplicative composition of the deformation gradient. As a three-dimensional finite element example, we study the material degradation of a fibre-reinforced tube subjected to internal pressure. This highlights the meshobjective and constitutive properties of the model and illustratively underlines the capabilities of the formulation with regard to biomechanical application such as the simulation of arteries. © Springer International Publishing Switzerland 2014.
    view abstractdoi: 10.1007/978-3-319-10981-7_2
  • 2013 • 51 High-temperature and low-stress creep anisotropy of single-crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Heyer, J.-K. and Lahni, A. and Frenzel, J. and Dlouhy, A. and Somsen, C. and Eggeler, G.
    Acta Materialia 61 2926-2943 (2013)
    The high-temperature and low-stress creep (1293 K, 160 MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [0 0 1] and [1 1 0] directions. In the early stages of creep, the [0 0 1]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [1 1 0] tensile loading. This may be related to the nature of rafting during [1 1 0] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [1 0 0] tensile creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.01.052
  • 2013 • 50 Propagation of sound waves in poroelastic media with anisotropic permeability
    Albers, B. and Wilmanski, K.
    Poromechanics V - Proceedings of the 5th Biot Conference on Poromechanics 83-91 (2013)
    By means of a Biot-like model with isotropic stress-strain relations but anisotropic permeability monochromatic waves in a two-component poroelastic medium are analyzed. The anisotropy is induced by the anisotropy of the tortuosity which is introduced by a second order symmetric tensor. The model describes for a certain choice of orientation of the propagation direction four modes of propagation: a decoupled transversal S1-wave, a pseudo transversal mode S2 and two pseudo longitudinal modes P1 and P2. Phase speeds and attenuations of these waves are shown in dependence on the orientation of the principal directions of the tortuosity. © 2013 American Society of Civil Engineers.
    view abstractdoi: 10.1061/9780784412992.010
  • 2013 • 49 Guided hierarchical co-assembly of soft patchy nanoparticles
    Gröschel, A.H. and Walther, A. and Löbling, T.I. and Schacher, F.H. and Schmalz, H. and Müller, A.H.E.
    Nature 503 247-251 (2013)
    The concept of hierarchical bottom-up structuring commonly encountered in natural materials provides inspiration for the design of complex artificial materials with advanced functionalities. Natural processes have achieved the orchestration of multicomponent systems across many length scales with very high precision, but man-made self-assemblies still face obstacles in realizing well-defined hierarchical structures. In particle-based self-assembly, the challenge is to program symmetries and periodicities of superstructures by providing monodisperse building blocks with suitable shape anisotropy or anisotropic interaction patterns ('patches'). Irregularities in particle architecture are intolerable because they generate defects that amplify throughout the hierarchical levels. For patchy microscopic hard colloids, this challenge has been approached by using top-down methods (such as metal shading or microcontact printing), enabling molecule-like directionality during aggregation. However, both top-down procedures and particulate systems based on molecular assembly struggle to fabricate patchy particles controllably in the desired size regime (10-100 nm). Here we introduce the co-assembly of dynamic patchy nanoparticles-that is, soft patchy nanoparticles that are intrinsically self-assembled and monodisperse-as a modular approach for producing well-ordered binary and ternary supracolloidal hierarchical assemblies. We bridge up to three hierarchical levels by guiding triblock terpolymers (length scale ∼10 nm) to form soft patchy nanoparticles (20-50 nm) of different symmetries that, in combination, co-assemble into substructured, compartmentalized materials (>10 μm) with predictable and tunable nanoscale periodicities. We establish how molecular control over polymer composition programs the building block symmetries and regulates particle positioning, offering a route to well-ordered mixed mesostructures of high complexity. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nature12610
  • 2013 • 48 Trends in spin and orbital magnetism of free and encapsulated FePt nanoparticles
    Gruner, M.E.
    Physica Status Solidi (A) Applications and Materials Science 210 1282-1297 (2013)
    Owed to the large magneto-crystalline anisotropy (MCA) of the bulk FePt alloys, nanostructures with a few nm in diameter are considered for ultra-high density recording applications. First principles calculations in the framework of density functional theory (DFT) permit insight into the close interrelation between particle composition, morphology, and magnetism with access to the electronic level. The present survey will systematically highlight the impact of an additional encapsulation with Cu, Au, Al, and further main group elements on spin- and orbital magnetism and MCA with special emphasis on the role of the interface. Site resolved orbital moment anisotropy (OMA) of an uncovered 147 atom FePt nanoparticle. Large-scale first principles calculations in the framework of density functional theory offer detailed insight into the close interrelation between particle composition, morphology and magnetism with electronic resolution. Exploiting the power of contemporary supercomputers, one can identify systematic trends in spin and orbital magnetism of nanometer-sized hard magnetic particles related to their structure or chemical environment. This Feature Article concentrates on Fe-Pt nanoparticles, which are considered as promising candidates for ultra-high density recording media. Special emphasis is made on the role of the surfaces and the impact of a protective encapsulation with Cu, Au, Al or further main group elements on the hard magnetic properties. The anisotropy of the orbital moments turns out to be a valuable quantity characterizing the particular contribution of surfaces and interfaces on the atomic scale. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201329048
  • 2013 • 47 Massive anisotropic thermal expansion and thermo-responsive breathing in metal-organic frameworks modulated by linker functionalization
    Henke, S. and Schneemann, A. and Fischer, R.A.
    Advanced Functional Materials 23 5990-5996 (2013)
    Functionalized metal-organic frameworks (fu-MOFs) of general formula [Zn2(fu-L)2dabco]n show unprecedentedly large uniaxial positive and negative thermal expansion (fu-L = alkoxy functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. The alkoxy side chains of fu-L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo-responsive properties of these hybrid solid-liquid materials are precisely controlled by the choice and combination of fu-Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo-mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state. Extremely large thermal expansion is shown by pillared-layered metal-organic frameworks (MOFs) exhibiting alkoxy-functionalized 1,4-benzenedicarboxylate linkers. At a certain threshold temperature the materials reversibly switch from a narrow pore to large pore form. This unprecedented thermo-mechanical behavior is an intrinsic property of the materials and can be modulated substantially by mixing differently functionalized linkers to obtain mixed linker MOF solid solutions. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201301256
  • 2013 • 46 Determination of the geometry of the RVE for cancellous bone by using the effective complex shear modulus
    Klinge, S.
    Biomechanics and Modeling in Mechanobiology 12 401-412 (2013)
    This contribution deals with the application of the inverse homogenization method to the determination of geometrical properties of cancellous bone. The approach represents a combination of an extended version of the Marquardt-Levenberg method with the multiscale finite element method. The former belongs to the group of gradient-based optimization strategies, while the latter is a numerical homogenization method, suitable for the modeling of materials with a highly heterogeneous microstructure. The extension of the Marquardt-Levenberg method is concerned with the selection strategy for distinguishing the global minimum from the plethora of local minima. Within the numerical examples, the bone is modeled as a biphasic viscoelastic medium and three different representative volume elements are taken into consideration. Different models enable the simulation of the bone either as a purely isotropic or as a transversally anisotropic medium. Main geometrical properties of trabeculae are determined from data on effective shear modulus but alternative schemes are also possible. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-012-0408-5
  • 2013 • 45 Experimental and numerical investigation of Mg alloy sheet formability
    Mekonen, M.N. and Steglich, D. and Bohlen, J. and Stutz, L. and Letzig, D. and Mosler, J.
    Materials Science and Engineering A 586 204-214 (2013)
    The current paper explores experimentally and numerically obtained mechanical responses of the Nakazima-type sheet forming for the magnesium alloys ZE10 and AZ31 at elevated temperature (200. °C). The results from the experiments revealed sufficient ductility allowing sheet forming processes at the prescribed test temperature. The material's anisotropy recorded in previous experiments was confirmed. Differences in the mechanical response between the two materials in terms of strain paths during the forming experiments were quantified. The corresponding numerical responses were obtained employing a suitable constitutive model taking into account the characteristic anisotropy in deformation. In addition, for predicting limit conditions of the forming process, the localization criterion by Marciniak and Kuczynski was adopted. The constitutive model together with the localization criterion was implemented in a finite element framework based on a fully implicit time integration scheme. The reasonably good agreement between the responses of the model and the respective experiments indicated the predictive capabilities of the implemented model for the considered magnesium alloys. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.07.088
  • 2013 • 44 Surface morphology and atomic structure of thin layers of Fe3Si on GaAs(001) and their magnetic properties
    Noor, S. and Barsukov, I. and Özkan, M.S. and Elbers, L. and Melnichak, N. and Lindner, J. and Farle, M. and Köhler, U.
    Journal of Applied Physics 113 (2013)
    The structural and magnetic properties of ultrathin near-stoichiometric Fe3Si layers on GaAs(001) are investigated after using scanning tunneling microscopy (STM) analysis to optimize the deposition process. This includes atomic resolution imaging of the surface as measured by STM revealing the atomic ordering and characteristic defects in the topmost layers. Emphasis is laid on connections between the layer morphology and its magnetic properties, which are analysed by in situ MOKE, FMR, and SQUID magnetometry. Upon nucleation, the Fe3Si islands behave like superparamagnetic nanoparticles where we find a quantitative agreement between the size of the nanoparticles and their superspin. At higher coverage, the Fe3Si layers show ferromagnetic behaviour. Here, we investigate the superposition of the magnetocrystalline and the uniaxial anisotropies where the latter can be excluded to be caused by shape anisotropy. Furthermore, an unexpected increase of the magnetic moment towards low coverage can be observed which apart from an increased orbital moment can be attributed to an increased step density. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4795163
  • 2013 • 43 On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi
    Pfetzing-Micklich, J. and Somsen, C. and Dlouhy, A. and Begau, C. and Hartmaier, A. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 61 602-616 (2013)
    We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.081
  • 2013 • 42 A hyperelastic biphasic fibre-reinforced model of articular cartilage considering distributed collagen fibre orientations: Continuum basis, computational aspects and applications
    Pierce, D.M. and Ricken, T. and Holzapfel, G.A.
    Computer Methods in Biomechanics and Biomedical Engineering 16 1344-1361 (2013)
    Cartilage is a multi-phase material composed of fluid and electrolytes (68-85% by wet weight), proteoglycans (5-10% by wet weight), chondrocytes, collagen fibres and other glycoproteins. The solid phase constitutes an isotropic proteoglycan gel and a fibre network of predominantly type II collagen, which provides tensile strength and mechanical stiffness. The same two components control diffusion of the fluid phase, e.g. as visualised by diffusion tensor MRI: (i) the proteoglycan gel (giving a baseline isotropic diffusivity) and (ii) the highly anisotropic collagenous fibre network. We propose a new constitutive model and finite element implementation that focus on the essential load-bearing morphology: an incompressible, poroelastic solid matrix reinforced by an inhomogeneous, dispersed fibre fabric, which is saturated with an incompressible fluid residing in strain-dependent pores of the collagen-proteoglycan solid matrix. The inhomogeneous, dispersed fibre fabric of the solid further influences the fluid permeability, as well as an intrafibrillar portion that cannot be 'squeezed out' from the tissue. Using representative numerical examples on the mechanical response of cartilage, we reproduce several features that have been demonstrated experimentally in the cartilage mechanics literature. © 2013 © 2013 Taylor & Francis.
    view abstractdoi: 10.1080/10255842.2012.670854
  • 2013 • 41 Electric in-plane polarization in multiferroic CoFe2O 4/BaTiO3 nanocomposite tuned by magnetic fields
    Schmitz-Antoniak, C. and Schmitz, D. and Borisov, P. and De Groot, F.M.F. and Stienen, S. and Warland, A. and Krumme, B. and Feyerherm, R. and Dudzik, E. and Kleemann, W. and Wende, H.
    Nature Communications 4 (2013)
    Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3051
  • 2013 • 40 Anisotropic elasticity in sheared packings of frictional disks
    Shaebani, M.R. and Boberski, J. and Wolf, D.E.
    Traffic and Granular Flow 2011 339-347 (2013)
    We study the effect of unilaterality of the interparticle interactions on the elastic response of granular materials. The anisotropy of the contact network is related to the opening of contacts during quasi-static shear deformations.As a result, local incremental constitutive relations describing the evolution of stress in terms of shear and volumetric strains are proposed, and it is shown that the macroscopic elastic behavior of anisotropic granular assemblies under biaxial deformations can be described by three independent elastic moduli: bulk, shear, and anisotropy modulus. We show that the bulk and shear moduli are independent of the imposed shear deformation if scaled by the contact density, and the magnitude of the anisotropy modulus is proportional to the ratio between shear and volumetric strain. The theoretical predictions are qualitatively in agreement with MD simulation results far from the jamming transition © Springer-Verlag Berlin Heidelberg 2013.
    view abstractdoi: 10.1007/978-3-642-39669-4__33
  • 2013 • 39 Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations
    Zhang, J.J. and Hartmaier, A. and Wei, Y.J. and Yan, Y.D. and Sun, T.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6° gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/6/065001
  • 2013 • 38 Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 296-304 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.01.030
  • 2012 • 37 Acoustics of two-component porous materials with anisotropic tortuosity
    Albers, B. and Wilmanski, K.
    Continuum Mechanics and Thermodynamics 24 403-416 (2012)
    The paper is devoted to the analysis of monochromatic waves in two-component poroelastic materials described by a Biot-likemodelwhose stress-strain relations are isotropic but the permeability is anisotropic. This anisotropy is induced by the anisotropy of the tortuosity which is given by a second order symmetric tensor. This is a new feature of the model while in earlier papers only isotropic permeabilities were considered. We show that this new model describes four modes of propagation. For our special choice of orientation of the direction of propagation these are two pseudo longitudinal modes P1 and P2, one pseudo transversal mode S2 and one transversal mode S1. The latter becomes also pseudo transversal in the general case of anisotropy. We analyze the speeds of propagation and the attenuation of these waves as well as the polarization properties in dependence on the orientation of the principal directions of the tortuosity.We indicate the practical importance of different shear (transversal)modes of propagation in a possible new nondestructive test of geophysical materials. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0218-5
  • 2012 • 36 Light-induced magnetization reversal of high-anisotropy TbCo alloy films
    Alebrand, S. and Gottwald, M. and Hehn, M. and Steil, D. and Cinchetti, M. and Lacour, D. and Fullerton, E.E. and Aeschlimann, M. and Mangin, S.
    101 (2012)
    Magnetization reversal using circularly polarized light provides a way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different Tb xCo 1-x ferrimagnetic alloy compositions using fs- and ps-laser pulses and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3 × 10 6 ergs/cm 3. Optical magnetization switching is observed only for alloy compositions where the compensation temperature can be reached through sample heating. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4759109
  • 2012 • 35 Strongly anisotropic nonequilibrium phase transition in Ising models with friction
    Angst, S. and Hucht, A. and Wolf, D.E.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 85 (2012)
    The nonequilibrium phase transition in driven two-dimensional Ising models with two different geometries is investigated using Monte Carlo methods as well as analytical calculations. The models show dissipation through fluctuation induced friction near the critical point. We first consider high driving velocities and demonstrate that both systems are in the same universality class and undergo a strongly anisotropic nonequilibrium phase transition, with anisotropy exponent θ=3. Within a field theoretical ansatz the simulation results are confirmed. The crossover from Ising to mean field behavior in dependency of system size and driving velocity is analyzed using crossover scaling. It turns out that for all finite velocities the phase transition becomes strongly anisotropic in the thermodynamic limit. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.85.051120
  • 2012 • 34 Characterization of anisotropy of sheet metals employing inhomogeneous strain fields for Yld2000-2D yield function
    Güner, A. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Solids and Structures 49 3517-3527 (2012)
    A method to include the distribution of strains in the identification of the planar anisotropy of sheet metals is proposed. The method includes the optical measurement of strains on a flat specimen with a varying cross-section and an inverse parameter identification scheme which minimizes the differences between the numerical simulation results and the experimental measurements by using Levenberg-Marquardt algorithm. The main advantage is the reduction of the needed number of material tests especially for complex material models, under the assumption of negligible kinematic hardening. The utilized specimen geometry covers a deformation state between uniaxial tension and plane strain tension cases. In order to supply additional information to the inverse scheme, the equi-biaxial stress state obtained from layer compression test is also included in the definition of the objective function. The anisotropy of the sheet is modeled with the Yld2000-2D model which is implemented as a VUMAT subroutine for ABAQUS-Explicit. Numerical tests point out that the orientation of the specimen defines the quality of the found yield loci. The proposed method is applied to characterize the commercial aluminum alloy AA6016-T4 and the obtained material parameters are used to analyze a deep drawn car hood geometry. The results show that the use of the strain distribution is a crucial point in identification of the planar anisotropy. The yield loci obtained with the proposed method are in accordance with the conventionally obtained yield stresses and r-values. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2012.05.001
  • 2012 • 33 Thermal expansion and elastic properties of mullite-type Bi 2Ga 4O 9 and Bi 2Fe 4O 9 single crystals
    Krenzel, T.F. and Schreuer, J. and Gesing, T.M. and Burianek, M. and Mühlberg, M. and Schneider, H.
    International Journal of Materials Research 103 438-448 (2012)
    Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi 2Ga 4O 9 and Bi 2Fe 4O 9 between room temperature and about 1200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1 280 K using high-resolution dilatometry and X-ray powder diffraction. The elastic constants at 295 K are in GPa c 11 = 143.4(1), c 22 = 161.9(1), c 33 = 224.5(1), c 44 = 68.4(1), c 55 = 49.3(1), c 66 = 76.6(1), c 12 = 74.2(1), c 13 = 62.2(1), c 23 = 70.5(1) for Bi 2Ga 4O 9, and c 11 = 106.7(1), c 22 = 141.2(1), c 33 = 183.7(2), c 44 = 53.7(1), c 55 = 41.9(1), c 66 = 63.8(1), c 12 = 63.5(1), c 13 = 59.8(1), c 23 = 63.4(2) for Bi 2Fe 4O 9. In both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the c ij shows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge- and corner- linked coordination polyhedra. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110718
  • 2012 • 32 Mechanical characterization and constitutive modeling of Mg alloy sheets
    Mekonen, M.N. and Steglich, D. and Bohlen, J. and Letzig, D. and Mosler, J.
    Materials Science and Engineering A 540 174-186 (2012)
    In this paper, an experimental mechanical characterization of the magnesium alloys ZE10 and AZ31 is performed and a suitable constitutive model is established. The mechanical characterization is based on uniaxial tensile tests. In order to avoid poor formability at room temperature, the tests were conducted at elevated temperature (200. °C). The uniaxial tensile tests reveal sufficient ductility allowing sheet forming processes at this temperature. The differences in yield stresses and plastic strain ratios (r-values) confirm the anisotropic response of the materials under study. The constitutive model is established so that the characteristic mechanical features observed in magnesium alloys such as anisotropy and compression-tension asymmetry can be accommodated. This model is thermodynamically consistent, incorporates rate effect, is formulated based on finite strain plasticity theory and is applicable in sheet forming simulations of magnesium alloys. More precisely, a model originally proposed by Cazacu and Barlat in 2004 and later modified to account for the evolution of the material anisotropy is rewritten in a thermodynamically consistent framework. The calibrated constitutive model is shown to capture the characteristic mechanical features observed in magnesium alloy sheets. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.122
  • 2012 • 31 Orientation dependence of stress-induced phase transformation and dislocation plasticity in NiTi shape memory alloys on the micro scale
    Pfetzing-Micklich, J. and Ghisleni, R. and Simon, T. and Somsen, C. and Michler, J. and Eggeler, G.
    Materials Science and Engineering A 538 265-271 (2012)
    NiTi shape memory alloys can be used as micro actuators and small scale pseudoelastic components. Therefore there is a need to characterize their mechanical properties on the micro scale. In several previous studies, such tests (nanoindentation, pillar compression) were performed for different NiTi alloys. However, no consistent results concerning the coupling between plastic deformation and martensitic transformation were obtained. Moreover it is unclear whether the material's response to loading on the micro scale reflects its large scale mechanical anisotropy. In this study, we investigate a binary, solution annealed precipitate free NiTi alloy and compress small pillars in <0. 0. 1>-, <1. 0. 1>- and <1. 1. 1>-directions. Mechanical results are analyzed in the light of SEM and post-mortem TEM investigations. We identify deformation mechanisms and show that there is deformation anisotropy. We show that micro pillar testing yields results which are in good qualitative agreement with previous work from macroscopic investigations. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.042
  • 2012 • 30 Unilateral interactions in granular packings: A model for the anisotropy modulus
    Shaebani, M.R. and Boberski, J. and Wolf, D.E.
    Granular Matter 14 265-270 (2012)
    Unilateral interparticle interactions have an effect on the elastic response of granular materials due to the opening and closing of contacts during quasi-static shear deformations. A simplified model is presented, for which constitutive relations can be derived. For biaxial deformations the elastic behavior in this model involves three independent elastic moduli: bulk, shear, and anisotropy modulus. The bulk and the shear modulus, when scaled by the contact density, are independent of the deformation. However, the magnitude of the anisotropy modulus is proportional to the ratio between shear and volumetric strain. Sufficiently far from the jamming transition, when corrections due to non-affine motion become weak, the theoretical predictions are qualitatively in agreement with simulation results. © Springer-Verlag 2012.
    view abstractdoi: 10.1007/s10035-012-0329-0
  • 2012 • 29 Anisotropic density growth of bone - A computational micro-sphere approach
    Waffenschmidt, T. and Menzel, A. and Kuhl, E.
    International Journal of Solids and Structures 49 1928-1946 (2012)
    Bones are able to adapt their local density when exposed to mechanical loading. Such growth processes result in densification of the bone in regions of high loading levels and in resorption of the material in regions of low loading levels. This evolution and optimisation process generates heterogeneous distributions of bone density accompanied by pronounced anisotropic mechanical properties. While several constitutive models reported in the literature assume the growth process to be purely isotropic, only few studies focus on the modelling and simulation of anisotropic functional adaptation we can observe in vivo. Some of these few computational models for anisotropic growth characterise the evolution of anisotropy by analogy to anisotropic continuum damage mechanics while others include anisotropic growth but assume isotropic elastic properties. The objective of this work is to generalise a well-established framework of energy-driven isotropic functional adaptation to anisotropic microstructural growth and density evolution. We adopt the so-called micro-sphere concept, which proves to be extremely versatile and flexible to extend sophisticated one-dimensional constitutive relations to the three-dimensional case. In this work we apply this framework to the modelling and simulation of anisotropic functional adaptation by means of a directional density distribution, which evolves in time and in response to the mechanical loading condition. Several numerical studies highlight the characteristics and properties of the anisotropic growth model we establish. The formulation is embedded into an iterative finite element algorithm to solve complex boundary value problems. In particular, we consider the finite-element-simulation of a subject-specific proximal tibia bone and a comparison to experimental measurements. The proposed model is able to appropriately represent the heterogeneous bone density distribution. As an advantage over several other computational growth models proposed in the literature, a pronounced local anisotropy evolution is identified and illustrated by means of orientation-distribution-type density plots. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2012.03.035
  • 2012 • 28 Application of an anisotropic growth and remodelling formulation to computational structural design
    Waffenschmidt, T. and Menzel, A.
    Mechanics Research Communications 42 77-86 (2012)
    A classical structural optimisation problem consists of a problem-specific objective function which has to be minimised in consideration of particular constraints with respect to design and state variables. In this contribution we adopt a conceptually different approach for the design of a structure which is not based on a classical optimisation technique. Instead, we establish a constitutive micro-sphere-framework in combination with an energy-driven anisotropic microstructural growth formulation, which was originally proposed for the simulation of adaptation and remodelling phenomena in hard biological tissues such as bones. The goal of this contribution is to investigate this anisotropic growth formulation with a special emphasis on its application to structural design problems. To this end, four illustrative three-dimensional benchmark-type boundary value problems are discussed and compared qualitatively with the results obtained by classical structural optimisation strategies. The simulation results capture the densification effects and clearly identify the main load bearing regions. It turns out, that even though making use of this conceptually different growth formulation as compared to the procedures used in a classical structural optimisation context, we identify qualitatively very similar structures or rather regions of densification. Moreover, in contrast to common structural optimisation strategies, which mostly aim to optimise merely the size, shape or topology, our formulation also contains the improvement of the material itself, which - apart from the structural improvement - results in the generation of problem-specific local material anisotropy and textured evolution. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mechrescom.2011.12.004
  • 2012 • 27 Transient anisotropy in the electron diffraction of femtosecond laser-excited bismuth
    Zhou, P. and Streubühr, C. and Ligges, M. and Brazda, T. and Payer, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M. and Von Der Linde, D.
    New Journal of Physics 14 (2012)
    Laser excitation of thin bismuth films leads to a reduction in the diffraction intensity, which exhibits a characteristic angular anisotropy. The anisotropy depends on the polarization of the laser pulse and persists for approximately 150 ps. The effect clearly indicates coherent atomic motion in a preferential direction that we tentatively attribute to a transient shear deformation due to the photoelastic stress induced by the laser pulse. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/14/10/103031
  • 2011 • 26 A guideline for atomistic design and understanding of ultrahard nanomagnets
    Antoniak, C. and Gruner, M.E. and Spasova, M. and Trunova, A.V. and Römer, F.M. and Warland, A. and Krumme, B. and Fauth, K. and Sun, S. and Entel, P. and Farle, M. and Wende, H.
    Nature Communications 2 (2011)
    Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1538
  • 2011 • 25 In-situ optical spectroscopy and electronic properties of pyrrole sub-monolayers on Ga-rich GaAs(001)
    Bruhn, T. and Ewald, M. and Fimland, B.-O. and Kneissl, M. and Esser, N. and Vogt, P.
    Journal of Nanoparticle Research 13 5847-5853 (2011)
    We report on the characterization of submonolayers of pyrrole adsorbed on Ga-rich GaAs(001) surfaces. The interfaces were characterized by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and reflectance anisotropy spectroscopy (RAS) in a spectral range between 1.5 and 8 eV. The adsorption of pyrrole on Ga-rich GaAs(001) modifies the RAS spectrum of the clean GaAs surface significantly at the surface transitions at 2.2 and 3.5 eV indicating a chemisorption of the molecules. By the help of transients at these surface transitions during the adsorption process, we were able to prepare different molecular coverages from a sub-monolayer up to a complete molecular layer. The different coverages of pyrrole were imaged by STM and electronically characterized by STS. The measurements reveal that the adsorbed molecules electronically insulate the surface and indicate the formation of new interface states around-3.5 and +4.2 eV. The RAS measurements in the UV region show new anisotropies in the spectral range of the optical transitions of the adsorbed pyrrole molecules. Our measurements demonstrate the potential of optical and electronic spectroscopy methods for the characterization of atomically thin molecular layers on semiconductor surfaces allowing a direct access to the properties of single adsorbed molecules. © Springer Science+Business Media B.V. 2011.
    view abstractdoi: 10.1007/s11051-011-0340-0
  • 2011 • 24 Metal ion release kinetics from nanoparticle silicone composites
    Hahn, A. and Brandes, G. and Wagener, P. and Barcikowski, S.
    Journal of Controlled Release 154 164-170 (2011)
    Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jconrel.2011.05.023
  • 2011 • 23 Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals
    Li, T.L. and Gao, Y.F. and Bei, H. and George, E.P.
    Journal of the Mechanics and Physics of Solids 59 1147-1162 (2011)
    Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elasticplastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured loaddisplacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallography and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {1 1 0}〈0 0 1〉 slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to 〈1 1 1〉 and highest for those close to 〈0 0 1〉. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to 〈0 0 1〉. Because very high pop-in loads are measured for orientations close to 〈0 0 1〉, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near 〈0 0 1〉. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.04.003
  • 2011 • 22 A thermodynamically and variationally consistent class of damage-type cohesive models
    Mosler, J. and Scheider, I.
    Journal of the Mechanics and Physics of Solids 59 1647-1668 (2011)
    A novel class of cohesive constitutive models suitable for the analysis of material separation such as that related to cracks, shear bands or delamination processes is presented. The proposed framework is based on a geometrically exact description (finite deformation) and it naturally accounts for material anisotropies. For that purpose, a Helmholtz energy depending on evolving structural tensors is introduced. In sharp contrast to previously published anisotropic cohesive models with finite strain kinematics based on a spatial description, all models belonging to the advocated class are thermodynamically consistent, i.e., they are rigorously derived by applying the Coleman and Noll procedure. Although this procedure seems nowadays to be standard for stressstrain-type constitutive laws, this is not the case for cohesive models at finite strains. An interesting new finding from the Coleman and Noll procedure is the striking analogy between cohesive models and boundary potential energies. This analogy gives rise to the introduction of additional stress tensors which can be interpreted as deformational surface shear. To the best knowledge of the authors, those stresses which are required for thermodynamical consistency at finite strains, have not been taken into account in existing models yet. Furthermore, the additional stress tensors can result in an effective traction-separation law showing a non-trivial stress-free configuration consistent with the underlying Helmholtz energy. This configuration is not predicted by previous models. Finally, the analogy between cohesive models and boundary potential energies leads to a unique definition of the controversially discussed fictitious intermediate configuration. More precisely, traction continuity requires that the interface geometry with respect to the deformed configuration has to be taken as the average of both sides. It will be shown that the novel class of interface models does not only fulfill the second law of thermodynamics, but also it shows an even stronger variational structure, i.e., the admissible states implied by the novel model can be interpreted as stable energy minimizers. This variational structure is used for deriving a variationally consistent numerical implementation. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.04.012
  • 2011 • 21 Anisotropic mechanical behavior of ultrafine eutectic TiFe cast under non-equilibrium conditions
    Schlieter, A. and Kühn, U. and Eckert, J. and Löser, W. and Gemming, T. and Friák, M. and Neugebauer, J.
    Intermetallics 19 327-335 (2011)
    The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.10.012
  • 2011 • 20 Anisotropy of electron and hole g-factors in (In,Ga)As quantum dots
    Schwan, A. and Meiners, B.-M. and Greilich, A. and Yakovlev, D.R. and Bayer, M. and Maia, A.D.B. and Quivy, A.A. and Henriques, A.B.
    Applied Physics Letters 99 (2011)
    The g-factor tensors of electron and hole in self-assembled (In,Ga)As/GaAs quantum dots are studied by time-resolved ellipticity measurements in a three dimensional vector magnet system. Both g-factor tensors show considerable deviations from isotropy. These deviations are much more pronounced for the hole than for the electron and are described by different anisotropy factors, which can even have opposite signs. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3665634
  • 2011 • 19 Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
    Xu, J. and Yamazaki, D. and Katsura, T. and Wu, X. and Remmert, P. and Yurimoto, H. and Chakraborty, S.
    Journal of Geophysical Research: Solid Earth 116 (2011)
    Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO<inf>3</inf> perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D=D<inf>0</inf> exp (-H/RT) at 25GPa, with D <inf>0</inf>=5.10×10-11m2/s for Si and 4.99×10-11m2/s for Mg, H=308kJ/mol for Si, and 305kJ/mol for Mg. Mg diffusivity in MgSiO<inf>3</inf> perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect-based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO<inf>3</inf> perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle. Copyright 2011 by the American Geophysical Union.
    view abstractdoi: 10.1029/2011JB008444
  • 2011 • 18 Liquid-phase epitaxy of multicomponent layer-based porous coordination polymer thin films of [M(L)(P)0.5] type: Importance of deposition sequence on the oriented growth
    Zacher, D. and Yusenko, K. and Bétard, A. and Henke, S. and Molon, M. and Ladnorg, T. and Shekhah, O. and Schüpbach, B. and Dea Losa Arcos, T. and Krasnopolski, M. and Meilikhov, M. and Winter, J. and Terfort, A. and Wöll, C. a...
    Chemistry - A European Journal 17 1448-1455 (2011)
    The progressive liquid-phase layer-by-layer (LbL) growth of anisotropic multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P)0.5] (M: Cu2+, Zn2+; L: dicarboxylate linker; P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers (SAMs) on gold substrates as templates. It was found that the deposition of smooth, highly crystalline, and oriented multilayer films of these PCPs depends on the conditions at the early growth cycles. In the case of a two-step process with an equimolar mixture of L and P, growth along the [001] direction is strongly preferred. However, employing a three-step scheme with full separation of all components allows deposition along the [100] direction on carboxyl-terminated SAMs. Interestingly, the growth of additional layers on top of previously grown oriented seeding layers proved to be insensitive to the particular growth scheme and full retention of the initial orientation, either along the [001] or [100] direction, was observed. This homo- and heteroepitaxial LbL growth allows full control over the orientation and the layer sequence, including introduction of functionalized linkers and pillars. One layer at a time: The stepwise liquid-phase layer-by-layer growth of anisotropic, multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P) 0.5] (M: Cu2+, Zn2+; L: dicarboxylate linker, P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers as templates. Highly oriented PCP multilayers were selectively grown along the [100] and [001] directions (see figure). © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201002381
  • 2010 • 17 X-ray absorption measurements on nanoparticle systems: Self-assembled arrays and dispersions
    Antoniak, C. and Warland, A. and Darbandi, M. and Spasova, M. and Trunova, A. and Fauth, K. and Aziz, E.F. and Farle, M. and Wende, H.
    Journal of Physics D: Applied Physics 43 (2010)
    X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material. As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/43/47/474007
  • 2010 • 16 On the mechanical modeling of anisotropic biological soft tissue and iterative parallel solution strategies
    Balzani, D. and Brands, D. and Klawonn, A. and Rheinbach, O. and Schröder, J.
    Archive of Applied Mechanics 80 479-488 (2010)
    Biological soft tissues appearing in arterial walls are characterized by a nearly incompressible, anisotropic, hyperelastic material behavior in the physiological range of deformations. For the representation of such materials we apply a polyconvex strain energy function in order to ensure the existence of minimizers and in order to satisfy the Legendre-Hadamard condition automatically. The 3D discretization results in a large system of equations; therefore, a parallel algorithm is applied to solve the equilibrium problem. Domain decomposition methods like the Dual-Primal Finite Element Tearing and Interconnecting (FETI-DP) method are designed to solve large linear systems of equations, that arise from the discretization of partial differential equations, on parallel computers. Their numerical and parallel scalability, as well as their robustness, also in the incompressible limit, has been shown theoretically and in numerical simulations. We are using a dual-primal FETI method to solve nonlinear, anisotropic elasticity problems for 3D models of arterial walls and present some preliminary numerical results. © 2009 Springer-Verlag.
    view abstractdoi: 10.1007/s00419-009-0379-x
  • 2010 • 15 Structural, static and dynamic magnetic properties of Co2MnGe thin films on a sapphire a-plane substrate
    Belmeguenai, M. and Zighem, F. and Chauveau, T. and Faurie, D. and Roussigń, Y. and Ch́rif, S.M. and Moch, P. and Westerholt, K. and Monod, P.
    Journal of Applied Physics 108 (2010)
    Magnetic properties of Co2 MnGe thin films of different thicknesses (13, 34, 55, 83, 100, and 200 nm), grown by rf sputtering at 400 °C on single crystal sapphire substrates, were studied using vibrating sample magnetometry and conventional or microstrip line ferromagnetic resonance. Their behavior is described assuming a magnetic energy density showing twofold and fourfold in-plane anisotropies with some misalignment between their principal directions. For all the samples, the easy axis of the fourfold anisotropy is parallel to the c -axis of the substrate while the direction of the twofold anisotropy easy axis varies from sample to sample and seems to be strongly influenced by the growth conditions. Its direction is most probably monitored by the slight unavoidable miscut angle of the Al2 O 3 substrate. The twofold in-plane anisotropy field Hu is almost temperature independent, in contrast with the fourfold field H 4 which is a decreasing function of the temperature. Finally, we study the frequency dependence of the observed line-width of the resonant mode and we conclude to a typical Gilbert damping constant α value of 0.0065 for the 55-nm-thick film. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3475501
  • 2010 • 14 Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    Proceedings of the National Academy of Sciences of the United States of America 107 17503-17508 (2010)
    In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.
    view abstractdoi: 10.1073/pnas.1011295107
  • 2010 • 13 A computational remodeling approach to predict the physiological architecture of the collagen fibril network in corneo-scleral shells
    Grytz, R. and Meschke, G.
    Biomechanics and Modeling in Mechanobiology 9 225-235 (2010)
    Organized collagen fibrils form complex networks that introduce strong anisotropic and highly nonlinear attributes into the constitutive response of human eye tissues. Physiological adaptation of the collagen network and the mechanical condition within biological tissues are complex and mutually dependent phenomena. In this contribution, a computational model is presented to investigate the interaction between the collagen fibril architecture and mechanical loading conditions in the corneo-scleral shell. The biomechanical properties of eye tissues are derived from the single crimped fibril at the micro-scale via the collagen network of distributed fibrils at themeso-scale to the incompressible and anisotropic soft tissue at the macro-scale. Biomechanically induced remodeling of the collagen network is captured on the meso-scale by allowing for a continuous re-orientation of preferred fibril orientations and a continuous adaptation of the fibril dispersion. The presented approach is applied to a numerical human eye model considering the cornea and sclera. The predicted fibril morphology correlates well with experimental observations from X-ray scattering data. © Springer-Verlag 2009.
    view abstractdoi: 10.1007/s10237-009-0173-2
  • 2010 • 12 Framework for deformation induced anisotropy in glassy polymers
    Harrysson, M. and Ristinmaa, M. and Wallin, M. and Menzel, A.
    Acta Mechanica 211 195-213 (2010)
    In this paper a constitutive model for glassy polymers is developed. Glassy polymers consist of a number of polymer chains that at a microscopic level form a network. If the distribution of the polymer chains shows some preferred direction, the mechanical response at a global macroscopic level will be anisotropic. To incorporate the orientational distribution of the polymer chains, a homogenization procedure involving a chain orientation distribution function was undertaken. When polymers are exposed to external loading, the chains at the microscopic level orient in a certain manner, leading to an evolution of the macroscopic anisotropic properties. This phenomenon was modeled by use of evolution equations for the chains at a microscopic level and are then-by using the orientation distribution function-transformed to the macroscopic level. The theories involved are developed in a large strain setting in which a multiplicative split of the deformation gradient for the elastic-viscoplastic response is adopted. Various numerical experiments were conducted to evaluate the model that was developed. © 2009 Springer-Verlag.
    view abstractdoi: 10.1007/s00707-009-0232-x
  • 2010 • 11 EBSD study of substructure and texture formation in dual-phase steel sheets for semi-finished products
    Masimov, M. and Peranio, N. and Springub, B. and Roters, F. and Raabe, D.
    Solid State Phenomena 160 251-256 (2010)
    Using SEM/EBSD the substructure and texture evolution in dual phase steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized. In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 10 On the implementation of rate-independent standard dissipative solids at finite strain - Variational constitutive updates
    Mosler, J. and Bruhns, O.T.
    Computer Methods in Applied Mechanics and Engineering 199 417-429 (2010)
    This paper is concerned with an efficient, variationally consistent, implementation for rate-independent dissipative solids at finite strain. More precisely, focus is on finite strain plasticity theory based on a multiplicative decomposition of the deformation gradient. Adopting the formalism of standard dissipative solids which allows to describe constitutive models by means of only two potentials being the Helmholtz energy and the yield function (or equivalently, a dissipation functional), finite strain plasticity is recast into an equivalent minimization problem. In contrast to previous models, the presented framework covers isotropic and kinematic hardening as well as isotropic and anisotropic elasticity and yield functions. Based on this approach a novel numerical implementation representing the main contribution of the paper is given. In sharp contrast to by now classical approaches such as the return-mapping algorithm and analogously to the theoretical part, the numerical formulation is variationally consistent, i.e., all unknown variables follow naturally from minimizing the energy of the considered system. Consequently, several different numerically efficient and robust optimization schemes can be directly employed for solving the resulting minimization problem. Extending previously published works on variational constitutive updates, the advocated model does not rely on any material symmetry and therefore, it can be applied to a broad range of different plasticity theories. As two examples, an anisotropic Hill-type and a Barlat-type model are implemented. Numerical examples demonstrate the applicability and the performance of the proposed implementation. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cma.2009.07.006
  • 2010 • 9 Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator
    Piegdon, K.A. and Offer, M. and Lorke, A. and Urbanski, M. and Hoischen, A. and Kitzerow, H.-S. and Declair, S. and Frstner, J. and Meier, T. and Reuter, D. and Wieck, A.D. and Meier, C.
    Physica E: Low-Dimensional Systems and Nanostructures 42 2552-2555 (2010)
    GaAs-based semiconductor microdisks with high quality whispering gallery modes (Q>4000) have been fabricated. A layer of self-organized InAs quantum dots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4-cyano-4′-pentylbiphenyl (5CB), a liquid crystal (LC) with a nematic phase below the clearing temperature of TC≈34°C. We have studied the device performance in the temperature range of T=2050°C, in order to investigate the influence of the nematicisotropic phase transition on the optical modes. Moreover, we have applied an AC electric field to the device, which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal. This electrical anisotropy can be used to achieve electrical tunability of the optical modes. Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.physe.2009.12.051
  • 2010 • 8 Morphological analysis of articular cartilage using multiphoton microscopy as input for constitutive modeling: Experiment and mathematical implementation
    Pierce, D.M. and Lilledahl, M.B. and Ricken, T. and De Lange Davies, C. and Holzapfel, G.A.
    IFMBE Proceedings 31 IFMBE 895-898 (2010)
    The 3D structure of collagen fibers in chicken cartilage was quantified using multiphoton microscopy. Samples of fresh chicken cartilage were sectioned in three orthogonal planes using a vibratome. The sections were imaged using multiphoton microscopy, specifically imaging the collagen fibers using the second harmonic signal. Employing image analysis techniques based on Fourier analysis, the primary direction and anisotropy of the collagen fibers were extracted for the superficial layer resulting in a 3D map of the collagen fiber fabric. In the middle layer, image analysis using objective thresholding techniques was employed to extract the volume fraction occupied by extracellular matrix, the rest being occupied by the lacunae and residing chondrocytes. To implement these imaging data in a computational setting, we propose a new, 3D large strain constitutive model for articular cartilage, focused on the essential load-bearing morphology: an inhomogeneous, visco-poroelastic solid matrix reinforced by an anisotropic, viscoelastic dispersed fiber fabric which is saturated by an incompressible fluid residing in strain-dependent pores of the collagen-proteoglycan solid matrix. High-fidelity models, combining advanced imaging and computational biomechanics, will allow us to consider complex problems in structure-function relationships and provide insight to microphysical (mechanobiological) cellular stimuli. © 2010 International Federation for Medical and Biological Engineering.
    view abstractdoi: 10.1007/978-3-642-14515-5_228
  • 2010 • 7 The exoskeleton of the American lobster- From texture to anisotropic properties
    Raue, L. and Klein, H. and Raabe, D.
    Solid State Phenomena 160 287-294 (2010)
    The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 6 Overview of constitutive laws, kinematics, homogenization and multiscale methods in crystal plasticity finite-element modeling: Theory, experiments, applications
    Roters, F. and Eisenlohr, P. and Hantcherli, L. and Tjahjanto, D.D. and Bieler, T.R. and Raabe, D.
    Acta Materialia 58 1152-1211 (2010)
    This article reviews continuum-based variational formulations for describing the elastic-plastic deformation of anisotropic heterogeneous crystalline matter. These approaches, commonly referred to as crystal plasticity finite-element models, are important both for basic microstructure-based mechanical predictions as well as for engineering design and performance simulations involving anisotropic media. Besides the discussion of the constitutive laws, kinematics, homogenization schemes and multiscale approaches behind these methods, we also present some examples, including, in particular, comparisons of the predictions with experiments. The applications stem from such diverse fields as orientation stability, microbeam bending, single-crystal and bicrystal deformation, nanoindentation, recrystallization, multiphase steel (TRIP) deformation, and damage prediction for the microscopic and mesoscopic scales and multiscale predictions of rolling textures, cup drawing, Lankfort (r) values and stamping simulations for the macroscopic scale. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.10.058
  • 2010 • 5 Anisotropy of Ag diffusion on vicinal Si surfaces
    Sindermann, S. and Wall, D. and Roos, K.R. and Horn-Von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    e-Journal of Surface Science and Nanotechnology 8 372-376 (2010)
    Photoemission electron microscopy (PEEM) is used to study Ag surface diffusion on vicinal Si surfaces. The diffusion field is represented by Iso-Coverage Zones around Ag islands during desorption. By analyzing the shape and radius of the Iso-Coverage Zone we can determine diffusion parameters. For anisotropic diffusion the zone has an elliptical shape and the aspect ratio gives a measure for the anisotropy. Using this technique, we study the degree of anisotropy of Ag diffusion on vicinal Si(001) and Si(111). With increasing miscut angles, starting from Si(001) as well as from Si(111), we find a gradually increasing anisotropy, caused by the higher step density. On higher index surfaces, like Si(119), Si(115) and Si(113), we find isotropic diffusion for surfaces with comparable dimer and (double) step structure as on Si(001)-4°, where diffusion is strongly anisotropic. © 2010 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2010.372
  • 2010 • 4 Fast, physically-based algorithms for online calculations of texture and anisotropy during fabrication of steel sheets
    Winning, M. and Raabe, D.
    Advanced Engineering Materials 12 1206-1211 (2010)
    Because of the complex microstructures of crystalline materials exposed to commercial manufacturing processes it is up to now not possible to obtain fast and on-line simulations of crystallographic texture and anisotropy in the course of multiple deformation- and heat treatment procedures. In the present paper a hybrid approach for the on-line texture and anisotropy prediction will be developed for the fabrication of low alloyed ferritic steel sheets during cold rolling and subsequent annealing procedures. Our approach is based on two consecutive models: The first one is an artificial neuronal network (ANN) for the description of the rolling texture evolution. The second one is an analytical, Avrami-based texture component approach for the recrystallization. First results on low carbon steels will be presented. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000206
  • 2010 • 3 Optical anisotropy of Si(111)- (4×1)/(8×2)-In nanowires calculated from first-principles
    Wippermann, S. and Schmidt, W.G. and Bechstedt, F. and Chandola, S. and Hinrichs, K. and Gensch, M. and Esser, N. and Fleischer, K. and McGilp, J.F.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 133-136 (2010)
    First-principles calculations of the anisotropic optical response of Si(111)-(4×1)(8×2)-In from the mid-infrared to the visible region are compared with recent experimental data. The experimental data show that the anisotropic Drude tail of the (4×1) room-temperature phase is replaced by two peaks at 0.50eV and 0.72eV after the phase transition to the low-temperature (8×2) structure took place. The spectrum calculated from both intraband and interband transitions for the metallic zig-zag chain model of the (4×1) phase accounts well for the room-temperature data. The low-temperature data are well explained on the basis of the semiconducting hexagon (8×2) model by interband transitions mainly from a region close the XM high-symmetry line of the Brillouin zone. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.200982413
  • 2010 • 2 Crystal plasticity modelling and experiments for deriving microstructure-property relationships in γ-TiAl based alloys
    Zambaldi, C. and Raabe, D.
    Journal of Physics: Conference Series 240 (2010)
    Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α 2-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012140
  • 2010 • 1 Influence of crystal anisotropy on elastic deformation and onset of plasticity in nanoindentation: A simulational study
    Ziegenhain, G. and Urbassek, H.M. and Hartmaier, A.
    Journal of Applied Physics 107 (2010)
    Using molecular-dynamics simulation we simulate nanoindentation into the three principal surfaces-the (100), (110), and (111) surface-of Cu and Al. In the elastic regime, the simulation data agree fairly well with the linear elastic theory of indentation into an elastically anisotropic substrate. With increasing indentation depth, the effect of pressure hardening becomes visible. When the critical stress for dislocation nucleation is reached, even the elastically isotropic Al shows a strong dependence of the force-displacement curves on the surface orientation. After the load drop, when plasticity has set in, the influence of the surface orientation is lost, and the contact pressure (hardness) becomes independent of the surface orientation. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3340523