Prof. Dr. Günther Meschke

Computational Engineering
Ruhr-Universität Bochum

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  • Comparison of Hilbert Transform and Complex Demodulation for Defect Identification in Cutting Discs using Vibration-Based Feature Extraction
    Priebe, S. and Brackmann, L. and Alabd-Allah, A. and Butt, S. and Röttger, A. and Meschke, G. and Mueller, I.
    Lecture Notes in Civil Engineering 127 (2021)
    This paper presents a novel concept for vibration-based feature extraction to identify damages in cutting discs of Tunnel Boring Machines (TBM). Defect frequencies resulting from repeated interaction of rock and disc defects are analysed. The data set is represented by the normal force acting on the edge of a cutting disc and the rock. Two different methods, the Hilbert transform and the complex demodulation, are used to generate the envelope of the time series, which was used to analyse whether the signal shows a feature representing an existing defect in the frequency domain. For the first proof of concept two numerical models were used - a multi-body system and a peridynamics 3D model simulating time series of normal forces. With both models, the linear motion of the disc on a rock sample with constant velocity was simulated. An experimental setup, mechanically similar to the simulations, was used in two experiments for further comparison. All implemented defects could be detected using vibration data of forces and one of the proposed data analysis techniques. © 2021, Springer Nature Switzerland AG.
    view abstract10.1007/978-3-030-64594-6_55
  • From digital models to numerical analysis for mechanised tunnelling: A fully automated design-through-analysis workflow
    Ninic, J. and Alsahly, A. and Vonthron, A. and Bui, H.-G. and Koch, C. and König, M. and Meschke, G.
    Tunnelling and Underground Space Technology 107 (2021)
    Large infrastructure projects involving the construction of tunnels in urban areas constitute complex, integrated and multi-disciplinary systems, which require building and construction information modelling as well as computational design assessment tools for decision making during all project phases and during their complete life cycle. Even if the underlying information needed for computational analysis is stored in an information model, the translation to computational models is still cumbersome and requires significant manual work for model generation and set-up as well as excessive computing resources and time. To address these shortcomings, this paper presents a systematic summary of concepts for integrated information modelling, numerical analysis and visualisation for urban mechanized tunnelling. Our first approach “BIM-to-FEM” is characterised by a fully automated link for error-free data exchange between a standalone Tunnelling Information Model and the process-oriented simulation model for mechanized tunnelling “ekate”. In the second approach “SATBIM”, a fully automated data exchange workflow is established between a parametric multi-level information model for tunnelling and multi-level numerical models based on both Finite Element and Isogeometric Analysis, where meta models are employed for real-time design assessment. We discuss the different applications of these concepts, such as scenario-based exploration of design alternatives, real-time design assessment within a TIM based on meta-models, and the potentials of using these models for the process control during construction. Furthermore, we present two case studies where real project data has been used for the integration of information and numerical modelling. The examples in this paper indicate clear advantages of this approach compared to traditional approaches in terms of efficiency of modelling achieved by reduced user interactions and error-free information exchange, and show the benefits of multi-level model representation and real-time analysis tasks. © 2020 Elsevier Ltd
    view abstract10.1016/j.tust.2020.103622
  • Object-oriented framework for 3D bending and free vibration analysis of multilayer plates: Application to cross-laminated timber and soft-core sandwich panels
    Marjanović, M. and Meschke, G. and Damnjanović, E.
    Composite Structures 255 (2021)
    In the paper, the main steps involved in the development of an object-oriented computational framework for the 3D bending and free vibration analysis of multilayer plates are presented. The mathematical formulation for layered finite elements is based on Reddy's plate theory for laminated composites. The analysis model has been implemented into Matlab, and the pre- and post-processing phases are performed using GiD. The proposed solver is characterized by a fast assembly procedure of sparse matrices using matrix vectorization, and a novel algorithm for the evaluation of interlaminar stresses satisfying continuity at layer interfaces. The performance, efficiency and accuracy of the computational framework are demonstrated through a number of validation examples by comparing the obtained results against the exact solution. Results from both static and dynamic analyses of multilayer panels are shown. © 2020 Elsevier Ltd
    view abstract10.1016/j.compstruct.2020.112859
  • Structural forces in segmental linings: process-oriented tunnel advance simulations vs. conventional structural analysis
    Marwan, A. and Gall, V.E. and Alsahly, A. and Meschke, G.
    Tunnelling and Underground Space Technology 111 (2021)
    Tunnel linings are designed to permanently fulfill basic structural, serviceability and durability requirements throughout the lifetime of a tunnel. In order to ensure structural stability, it is important to correctly assess the response of tunnel linings with respect to the loading from the ground and process loads to which lining structures are subjected. For the design of segmental tunnel linings, precise structural models are needed, as the segmentation imbues the lining system with non-trivial kinematics. In this contribution, a technique for modeling the segment-wise installation of tunnel linings in the context of a 3D tunnel advance simulation is proposed in order to better predict the time dependent structural forces that develop in segmental lining systems during tunnel advance. The segments of the lining ring are explicitly modeled as separate bodies, and the interactions between segments at the longitudinal and ring joints are modeled by means of a surface-to-surface frictional contact algorithm. In order to examine the 3D stress distribution in the segmental concrete lining under realistic, time-dependent process loadings, the lining model is integrated into the process oriented finite element simulation model ekate. The influence of the joint arrangement and the segmentation is investigated through comparison with simulations in which a standard, continuous lining modeling technique is employed and with standard structural beam models used in engineering practice. It is shown that the magnitude of structural forces obtained by the explicit modelling of segmental lining joints and their time-dependent installation process within a 3D structural model diverges significantly from those obtained using standard methods, i.e. bedded beam models. © 2021 Elsevier Ltd
    view abstract10.1016/j.tust.2021.103836
  • Variational interface element model for 2D and 3D hydraulic fracturing simulations
    Khisamitov, I. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 373 (2021)
    This paper presents an extension of the variational interface fracture model proposed in Khisamitov and Meschke (2018) to model fluid driven fracture in porous materials. The fluid saturated material is described by the theory of poroelasticity and the effective stress concept, while the coupling between fluid flow and fracture initiation and propagation is accomplished via a variational interface element formulation, using a damage variable c as an additional degree of freedom in conjunction with linear momentum and mass balance equations along the interface zones. Introducing the damage variable c, fracture propagates according to the values of the minimizers of the total potential energy expressed in terms of effective stresses. Hence, neither local crack propagation criteria nor tracking algorithms are required. Taking into account that a fracture grows quasi-statically, the discretized system of PDEs is solved by the backward Euler-scheme, ignoring the contribution from inertia forces. In the Newton–Raphson iterative solution procedure, an operator splitting algorithm is employed to solve first the poroelastic equations, and updating subsequently the damage variable c. The proposed model for hydraulic fracturing is validated first by means of one-dimensional analytical solutions in toughness as well as in viscosity dominated regimes. The performance and the applicability of the model to simulate interactions between propagating and existing fractures is demonstrated by means of 2D and 3D numerical examples. © 2020 Elsevier B.V.
    view abstract10.1016/j.cma.2020.113450
  • A 3D particle finite element model for the simulation of soft soil excavation using hypoplasticity
    Bal, A.R.L. and Dang, T.S. and Meschke, G.
    Computational Particle Mechanics 7 (2020)
    A numerical model based on the particle finite element method (PFEM) combined with a hypoplastic constitutive model is proposed for the analysis of soft soil excavations by means of single and multiple excavation tools. The PFEM allows to efficiently account for large deformations of the excavated soil material and free surfaces characterizing the simulation of tool–soil interaction during excavation. The utilization of a hypoplastic model, formulated in rate form, allows for a straightforward coupling with the standard velocity-based PFEM. Furthermore, effects such as pressure and density dependency of the soil stiffness are consistently incorporated into the formulation. The solution of the governing equations is performed implicitly, while an adaptive sub-stepping scheme is employed for the explicit time integration of the constitutive equation. Thus, the accuracy of the solution is improved at both global and local (constitutive) levels. The performance of the method is evaluated by means of the numerical re-analysis of selected geotechnical benchmark examples and soft soil excavations in 2D and 3D setups. © 2019, OWZ.
    view abstract10.1007/s40571-019-00271-y
  • A parallelization strategy for hydro-mechanically coupled mechanized tunneling simulations
    Bui, H.-G. and Meschke, G.
    Computers and Geotechnics 120 (2020)
    3D computational simulations of the tunnel advancement in mechanized tunneling usually demand high computational resources. This paper addresses strategies to enable large scale simulations of the advancement process by means of iterative, parallel solvers, considering frictional contact between the tunnel boring machine and the soil, fully saturated soil with different permeabilities and a number of additional features inherently connected with tunnel simulations, which require special attention in regards to robustness. To improve the computational performance of mechanized tunnel simulations a solution strategy based on domain decomposition is proposed to solve the resulting discretized linear system in parallel using an appropriately accelerated iterative solver. The interaction between the TBM and the ground is modeled by means of robust contact algorithm based on penalty method. The domain decomposition scheme accounts for contact treatment in parallel, which becomes relevant when the TBM traverses through different domains in the mesh. Different variants of block preconditioners are investigated in regards to their performance to speed-up the convergence of the iterative solver for the block linear system arising from simulations of tunnel advancement in saturated soft soil, considering different permeabilities. A validation of the computational model and selected numerical examples to showcase the efficiency of the proposed method and to verify the applicability of this strategy to high performance tunneling simulations are presented. © 2019 Elsevier Ltd
    view abstract10.1016/j.compgeo.2019.103378
  • Artificial neural network surrogate modelling for real-time predictions and control of building damage during mechanised tunnelling
    Cao, B.T. and Obel, M. and Freitag, S. and Mark, P. and Meschke, G.
    Advances in Engineering Software 149 (2020)
    Tunnelling induced surface settlements can cause damage in buildings located in the vicinity of the tunnel. Currently, surface settlements and associated building damage risks usually are estimated based on empirical equations, e.g. by assuming Gaussian curves for the settlement trough and by applying the Limit Tensile Strain Method or the tilt-based method to evaluate and categorise the expected building damage. In this paper, finite element simulations are used to predict the soil-structure interaction in mechanised tunnelling during the tunnel advancement. The time variant surface settlement field and the corresponding tunnelling induced strains in the facade of a building are computed by two independent finite element models. Coupling both models allows predicting the expected category of damage (cod) for the building, given the operational parameters of the tunnel drive. Based upon this coupled approach, a method is proposed in the paper, which provides optimised operational parameters (e.g. tail void grouting pressure and face support pressure) during the advancement of tunnel boring machines below vulnerable buildings, such that the risk of damage for existing buildings is minimised. For real-time applicability of this method two different types of Artificial Neural Networks in combination with the Proper Orthogonal Decomposition approach are generated as surrogate models of the finite element simulations. The surrogate models are finally linked and implemented into a user-friendly application, which can be used as an assistant tool to adjust the operational parameters of the tunnel boring machine at the construction site. © 2020 Elsevier Ltd
    view abstract10.1016/j.advengsoft.2020.102869
  • BIM-to-IGA: A fully automatic design-through-analysis workflow for segmented tunnel linings
    Ninić, J. and Bui, H.G. and Meschke, G.
    Advanced Engineering Informatics 46 (2020)
    Both planning and design phase of large infrastructural project require analysis, modelling, visualization, and numerical analysis. To perform these tasks, different tools such as Building Information Modelling (BIM) and numerical analysis software are commonly employed. However, in current tunnel engineering practice, there are no systematic solutions for the exchange between design and analysis models, and these tasks usually involve manual and error-prone model generation, setup and update. In this paper, focussing on tunnelling engineering, we demonstrate a systematic and versatile approach to efficiently generate a tunnel design and analyse the lining in different practical scenarios. To this end, a BIM-based approach is developed, which connects a user-friendly industry-standard BIM software with effective simulation tools for high-performance computing. A fully automatized design-through-analysis workflow solution for segmented tunnel lining is developed based on a fully parametric design model and an isogeometric analysis software, connected through an interface implemented with a Revit plugin. The IGA-Revit interface implements a reconstruction algorithm based on sweeping teachnique to construct trivariate NURBS lining segment geometry, which avoids the burden to deal with trimmed geometries. © 2020 Elsevier Ltd
    view abstract10.1016/j.aei.2020.101137
  • Blind competition on the numerical simulation of steel-fiber-reinforced concrete beams failing in shear
    Barros, J. and Sanz, B. and Kabele, P. and Yu, R.C. and Meschke, G. and Planas, J. and Cunha, V. and Caggiano, A. and Ozyurt, N. and Gouveia, V. and van den Bos, A. and Poveda, E. and Gal, E. and Cervenka, J. and Neu, G.E. and Rossi, P. and Dias-da-Costa, D. and Juhasz, P.K. and Cendon, D. and Ruiz, G. and Valente, T.
    Structural Concrete (2020)
    Experimental research has shown the extraordinary potential of the addition of short fibers to cement-based materials by improving significantly the behavior of concrete structures for serviceability and ultimate limit states. Software based on the finite element method has been used for the simulation of the material nonlinear behavior of fiber-reinforced concrete (FRC) structures. The applicability of the existing approaches has often been assessed by simulating experimental tests with structural elements, in general of a small scale, where the parameter values of the material constitutive laws are adjusted for the aimed predicting level, which constitutes an inverse technique of arguable utility for structural design practice. For assessing the predictive performance of these approaches, a blind simulation competition was organized. Two twin T-cross section steel FRC beams, flexurally reinforced with steel bars and without conventional shear reinforcement in the critical shear span, were experimentally tested up to failure. Despite the experimental data provided for the definition of the relevant model parameters, inaccuracies on the load capacity, deflection, and strain at peak load attained 40, 113, and 600%, respectively. Inadequate failure modes and highly different results were estimated with the same commercial software, indicating the need for deeper analysis and understanding of the models and influence of their parameters on their predictive performance. © 2020 fib. International Federation for Structural Concrete
    view abstract10.1002/suco.202000345
  • Cementitious composites with high compaction potential: Modeling and calibration
    Vu, G. and Iskhakov, T. and Timothy, J.J. and Schulte-Schrepping, C. and Breitenbücher, R. and Meschke, G.
    Materials 13 (2020)
    There is an increasing need for the development of novel technologies for tunnel construction in difficult geological conditions to protect segmental linings from unexpected large deformations. In the context of mechanized tunneling, one method to increase the damage tolerance of tunnel linings in such conditions is the integration of a compressible two-component grout for the annular gap between the segmental linings and the deformable ground. In this regard, expanded polystyrene (EPS) lightweight concrete/mortar has received increasing interest as a potential “candidate material” for the aforementioned application. In particular, the behavior of the EPS lightweight composites can be customized by modifying their pore structure to accommodate deformations due to specific geological conditions such as squeezing rocks. To this end, novel compressible cementitious EPS-based composite materials with high compaction potential have been developed. Specimens prepared from these composites have been subjected to compressive loads with and without lateral confinement. Based on these experimental data a computational model based on the Discrete Element Method (DEM) has been calibrated and validated. The proposed calibration procedure allows for modeling and prognosis of a wide variety of composite materials with a high compaction potential. The calibration procedure is characterized by the identification of physically quantifiable parameters and the use of phenomenological submodels. Model prognoses show excellent agreement with new experimental measurements that were not incorporated in the calibration procedure. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma13214989
  • Efficient cut-cell quadrature based on moment fitting for materially nonlinear analysis
    Bui, H.-G. and Schillinger, D. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 366 (2020)
    Cut-cell quadrature based on the moment fitting scheme generates an accurate numerical integration rule for each cut element with the same small number of point evaluations as a standard Gauss quadrature rule. It therefore significantly increases the efficiency of unfitted finite element schemes such as the finite cell method that have often relied on cut-cell integration with prohibitively many quadrature points. Moment fitting, however, does not directly apply to inhomogeneous integrands as they result from nonlinear material behavior. In this article, we describe a novel modification of moment fitting approach that opens the door for its application in materially nonlinear analysis. The basic idea is the decomposition of each cut cell into material subdomains, each of which can be assigned a physically valid location where constitutive integration and the update of local history variables can be performed. We formulate a moment fitting scheme for each material subdomain using the same quadrature points, such that the resulting weights from all material subdomains can be added and the total number of point evaluations remains the same as in standard Gauss quadrature. We discuss numerical details of the modified scheme, including its ramifications for consistent linearization, and demonstrate its optimal performance in the context of the finite cell method and elastoplasticity. © 2020 Elsevier B.V.
    view abstract10.1016/j.cma.2020.113050
  • Elasto-plastic large deformation analysis of multi-patch thin shells by isogeometric approach
    Huynh, G.D. and Zhuang, X. and Bui, H.G. and Meschke, G. and Nguyen-Xuan, H.
    Finite Elements in Analysis and Design 173 (2020)
    This paper studies elasto-plastic large deformation behaviour of thin shell structures using the isogeometric computational approach with the main focus on the efficiency in modelling the multi-patches and arbitrary material formulation. In terms of modelling, we employ the bending strip method to connect the patches in the structure. The incorporation of bending strips allows to eliminate the strict demand of the C1 continuity condition, which is postulated in the Kirchhoff-Love theory for thin shell, and therefore it enables us to use the standard multi-patch structure even with C0 continuity along the patch boundaries. Furthermore, arbitrary nonlinear material models such as hyperelasticity and finite strain plasticity are embedded in the shell formulation, from which a unified thin shell formulation can be achieved. In terms of analysis, the Bézier decomposition concept is used to retain the local support of the traditional finite element. The performance of the presented approach is verified through several numerical benchmarks. © 2020
    view abstract10.1016/j.finel.2020.103389
  • Influence of muck properties and chamber design on pressure distribution in EPB pressure chambers – Insights from computational flow simulations
    Dang, T.S. and Meschke, G.
    Tunnelling and Underground Space Technology 99 (2020)
    Earth Pressure Balance (EPB) shield machines are widely used in mechanized tunneling operations in soft grounds. In contrast to slurry shield machines, the pressure distribution in EPB excavation chambers is not well undersood, as it is influenced by the muck properties and the chamber design. A computational model specifically developed in Dang and Meschke (2018) for the numerical simulation of material transport in EPB pressure chambers is employed to investigate the pressure distribution in two different types of EPB chambers, characterized by a different design of the mixing components, the rotators and the screw conveyor. The numerical model allows for the description of all rotating and fixed components of the EPB chambers including the screw conveyor and considers adequately the compressibility and the consistency of the soil paste. The spatio-temporal pressure distribution is investigated with respect to the influence of chamber design and the properties of the soil paste. Essential characteristics of the pressure distribution, such as the pressure unbalance between the left and right side and pressure fluctuations observed in in situ measurements are captured by the numerical analyses. The key factors influencing the pressure unbalance and pressure fluctuations are identified by parametric studies for two different EPB chamber designs. © 2020 Elsevier Ltd
    view abstract10.1016/j.tust.2020.103333
  • Integrated BIM-to-FEM approach in mechanised tunnelling
    Alsahly, A. and Hegemann, F. and König, M. and Meschke, G.
    Geomechanik und Tunnelbau 13 (2020)
    In current tunnelling practice, Finite Element (FE) simulations form an integral element in the planning and the design phase of mechanised tunnelling projects. The generation of adequate computational models is often time consuming and requires data from many different sources, in particular, when manually generated using 2D-CAD drawings. Incorporating Building Information Modelling (BIM) concepts offers opportunities to simplify this process by using geometrical BIM sub-models as a basis for structural analyses. This paper presents a Tunnel Information Model (TIM) as a BIM specifically tailored to fit the needs of mechanised tunnelling projects and a ”BIM-to-FEM“ technology, that automatically extracts relevant information (geology, alignment, lining, material and process parameters) needed for FE simulations from BIM sub-models and subsequently performs FE analysis of the tunnel drive. The results of the analysis are stored centrally on a data server to which the user has continuous access. A case study from the Wehrhahn-Metro line project in Düsseldorf, Germany, is presented and discussed to demonstrate the efficiency and the applicability of the proposed BIM-to-FEM workflow. © 2020, Ernst und Sohn. All rights reserved.
    view abstract10.1002/geot.202000002
  • Multilevel surrogate modeling approach for optimization problems with polymorphic uncertain parameters
    Freitag, S. and Edler, P. and Kremer, K. and Meschke, G.
    International Journal of Approximate Reasoning 119 (2020)
    The solution of optimization problems with polymorphic uncertain data requires combining stochastic and non-stochastic approaches. The concept of uncertain a priori parameters and uncertain design parameters quantified by random variables and intervals is presented in this paper. Multiple runs of the nonlinear finite element model solving the structural mechanics with varying a priori and design parameters are needed to obtain a solution by means of iterative optimization algorithms (e.g. particle swarm optimization). The combination of interval analysis and Monte Carlo simulation is required for each design to be optimized. This can only be realized by substituting the nonlinear finite element model by numerically efficient surrogate models. In this paper, a multilevel strategy for neural network based surrogate modeling is presented. The deterministic finite element simulation, the stochastic analysis as well as the interval analysis are approximated by sequentially trained artificial neural networks. The approach is verified and applied to optimize the concrete cover of a reinforced concrete structure, taking the variability of material parameters and the structural load as well as construction imprecision into account. © 2019 Elsevier Inc.
    view abstract10.1016/j.ijar.2019.12.015
  • Numerical multi-level model for fiber-reinforced concrete - Multi-level validation based on an experimental study on high-strength concrete [Numerisches Mehrebenen-Modell für Stahlfaserbeton: Von der Faser- zur Bauteilebene: Mehrstufige Validierung anhand einer experimentellen Studie an hochfestem Faserbeton]
    Gudžulic, V. and Neu, G.E. and Gebuhr, G. and Anders, S. and Meschke, G.
    Beton- und Stahlbetonbau 115 (2020)
    Numerical multi-level model for fiber-reinforced concrete – Multi-level validation based on an experimental study on high-strength concrete. The contribution systematically examines the predictive capability of a numerical multi-level model for steel fiber reinforced concrete made of high-strength concrete by means of test series performed on the fiber as well as the structure level. The experimental study comprises pull-out tests of Dramix 3D fibers in high-strength concrete with different embedment lengths and inclinations to the crack surface as well as three-point bending tests on notched beams with three significantly different fiber contents. The numerical model is designed to directly track the influence of design parameters such as fiber type, fiber orientation, fiber content and concrete strength on the structural response. For this purpose, submodels on the single fiber level are combined into a crack bridging model, considering the fiber orientation and the fiber content, and are integrated into a finite element model for the purpose of numerical structural analysis. The validation of the models for hooked-end steel fibers shows that the interaction mechanisms between fiber and high-strength concrete are realistically captured for all investigated cases (fiber inclinations, embedment lengths). On the structural level, the load-displacement diagrams from the numerical simulations show a good agreement of the peak load as well as the post-peak behavior for all fiber contents. © 2020, Ernst und Sohn. All rights reserved.
    view abstract10.1002/best.201900067
  • Parallel finite cell method with adaptive geometric multigrid
    Saberi, S. and Vogel, A. and Meschke, G.
    Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) 12247 LNCS (2020)
    Generation of appropriate computational meshes in the context of numerical methods for partial differential equations is technical and laborious and has motivated a class of advanced discretization methods commonly referred to as unfitted finite element methods. To this end, the finite cell method (FCM) combines high-order FEM, adaptive quadrature integration and weak imposition of boundary conditions to embed a physical domain into a structured background mesh. While unfortunate cut configurations in unfitted finite element methods lead to severely ill-conditioned system matrices that pose challenges to iterative solvers, such methods permit the use of optimized algorithms and data patterns in order to obtain a scalable implementation. In this work, we employ linear octrees for handling the finite cell discretization that allow for parallel scalability, adaptive refinement and efficient computation on the commonly regular background grid. We present a parallel adaptive geometric multigrid with Schwarz smoothers for the solution of the resultant system of the Laplace operator. We focus on exploiting the hierarchical nature of space tree data structures for the generation of the required multigrid spaces and discuss the scalable and robust extension of the methods across process interfaces. We present both the weak and strong scaling of our implementation up to more than a billion degrees of freedom on distributed-memory clusters. © Springer Nature Switzerland AG 2020.
    view abstract10.1007/978-3-030-57675-2_36
  • Structural reliability and durability assessment of reinforced concrete structures
    Freitag, S. and Kremer, K. and Edler, P. and Hofmann, M. and Meschke, G.
    Proceedings of the 29th European Safety and Reliability Conference, ESREL 2019 (2020)
    In this paper, a concept for safety and durability assessment of reinforced concrete structures is presented. It is based on a multilevel modeling approach, where three different models are combined using simple linear elastic beam models for the reinforcement design, low fidelity nonlinear finite element models to compute the deformation at the full structural scale and finally high fidelity nonlinear finite element models to compute the crack widths at critical hot spots of the structure. The uncertainty of structural parameters is quantified by means of polymorphic uncertainty models combining stochastic and non-stochastic approaches. The multilevel approach is applied to the safety assessment and the crack widths prediction of a reinforced concrete beam structure. Copyright © 2019 European Safety and Reliability Association.
    view abstract10.3850/978-981-11-2724-30934-cd
  • Active learning for accurate settlement prediction using numerical simulations in mechanized tunneling
    Saadallah, A. and Egorov, A. and Cao, B.-T. and Freitag, S. and Morik, K. and Meschke, G.
    Procedia CIRP 81 (2019)
    Finite Element simulation is a possible tool to investigate interactions between the Tunnel Boring Machine and the surrounding soil. Surface settlements can be predicted in real-time based on simulation results by machine learning surrogate models. However, to train such models, large amounts of computationally intensive simulations are required. To accomplish this step with minimal costs, we propose a hybrid active learning approach to select the minimal amount of simulations necessary to build an accurate model. During the tunnel construction, the real-time settlements prediction model will be used to analyze associated risks to ensure safe and sustainable constructions in urban areas. © 2019 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.procir.2019.03.250
  • Big data and simulation – a new approach for real-time TBM steering
    Meschke, G. and Cao, B.T. and Freitag, S. and Egorov, A. and Saadallah, A. and Morik, K.
    Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art- Proceedings of the WTC 2019 ITA-AITES World Tunnel Congress (2019)
    A new concept, which combines machine and monitoring data analysis with numerical simulation of mechanized tunneling processes, is presented in this work. The main idea is to fuse sensors data collected from the TBM and the settlement measurements with finite element simulations capturing the physical behavior of the tunneling process. Both data and simulation based prediction models are developed to assist the TBM steering with respect to the settlement behavior in real-time. The real-time machine and monitoring data analysis offers the possibility of detecting anomalies, e.g. in the soil layers, which are not considered a priori in the simulation models. First results show that most anomalies created by concrete walls in the ground can be detected up to 1 m ahead of the cutting wheel by using the specific torque (a combination of torque and penetration rate data) together with time series preprocessing and drift detection methods. © 2019 Taylor & Francis Group, London.
    view abstract10.1201/9780429424441-284
  • Computationally Efficient Simulation in Urban Mechanized Tunneling Based on Multilevel BIM Models
    Ninić, J. and Bui, H.-G. and Koch, C. and Meschke, G.
    Journal of Computing in Civil Engineering 33 (2019)
    The design of complex underground infrastructure projects involves various empirical, analytical, or numerical models with different levels of complexity. The use of simulation models in the current state-of-the-art tunnel design process can be cumbersome when significant manual time-consuming preparation, analysis, and excessive computing resources are required. This paper addresses the challenges connected with minimizing the user workload and computational time, as well as enabling real-time computations during construction. To ensure a seamless workflow during design and to minimize the computation time of the analysis, we propose a novel concept for Building Information Modeling (BIM)-based numerical simulations enabling the modeling of the tunnel advance with different levels of detail in terms of geometrical representation, material modeling, and modeling of the advancement process. To ensure computational efficiency, simulation software was developed with special emphasis on efficient implementation, including parallelization strategies for shared and distributed memory systems. For real-time on-demand calculations, simulation-based metamodels were integrated into the software platform. The components of the BIM-based multilevel simulation concept were described and evaluated in detail by means of representative numerical examples. © 2019 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)CP.1943-5487.0000822
  • Configurational-force interface model for brittle fracture propagation
    Khisamitov, I. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 351 (2019)
    This paper proposes a numerical model for the simulation of fracture propagation in brittle materials based on the theory of configurational forces. Configurational forces are adopted as the driving force for fracture in association with a tensile stretch criterion to identify crack tips of propagating cracks. Fracture surfaces are approximated by zero-thickness interface elements embedded into the finite element mesh between the bulk elements in the entire domain. In order to eliminate the mesh bias in regards to the fracture pattern, a new fracture segment is allowed to grow along several interface elements at once. The minimal fracture surface is found using an algorithm based on Graph Theory. The solution strategy is explicit, and the fracture path using this algorithm is identified in a post-processing step. The proposed configurational interface model (CIM) for fracture is validated by the analytical solution for a mode I benchmark and by comparisons with results from a variety of laboratory experiments on cylindrical granite specimens containing one and two initial cracks at different angles, which are characterized by complex curved and partially irregular crack patterns. © 2019 Elsevier B.V.
    view abstract10.1016/j.cma.2019.03.029
  • Expansion and deterioration of concrete due to ASR: Micromechanical modeling and analysis
    Iskhakov, T. and Timothy, J.J. and Meschke, G.
    Cement and Concrete Research 115 (2019)
    A multi-scale micromechanics model is proposed to describe the expansion and deterioration of concrete due to Alkali-Silica Reaction (ASR). The mechanics of ASR induced deterioration of a Representative Elementary Volume (REV) of concrete is modeled through a synthesis of distributed microcracking and mean-field homogenization. At the microscale, ASR-gel-pressure induced microcrack growth in and around the reactive aggregates is modeled using the framework of linear elastic fracture mechanics. Mean-field homogenization across multiple scales is used to obtain the overall expansion and degradation of the material. By specifying the spatial distribution of the pressurizing gel, two different ASR mechanisms associated with “slowly” and “rapidly” reactive aggregates can be modeled. Experimental data for concrete degradation as a function of the macroscopic expansion is found to lie within the theoretical upper and lower bounds that characterize the distribution of the gel in the aggregate or the cement paste. © 2018 Elsevier Ltd
    view abstract10.1016/j.cemconres.2018.08.001
  • Fatigue behavior of HPC and FRC under cyclic tensile loading: Experiments and modeling
    Schäfer, N. and Gudžulić, V. and Timothy, J.J. and Breitenbücher, R. and Meschke, G.
    Structural Concrete 20 (2019)
    Systematic investigations of hardened cement paste, high-performance concrete and mortar with and without microfibers, subjected to static and cyclic tensile loadings, were conducted. The material degradation was investigated by means of microscopic analyses of the microcrack development. Notched specimens were subjected to a predefined number of load cycles. A nonsteady increase of microcracking with increasing load cycles was observed in high-strength concrete, whereas the addition of steel fibers lead to a steady increase of microcracks. High-strength mortar often showed premature failure, while addition of steel micro fibers allowed completion of the cyclic tests. To obtain a deeper insight into physical mechanisms governing fatigue and structural failure, high-performance concrete (HPC) and fiber-reinforced concrete (FRC) under static and cyclic tensile loadings have been modeled using cohesive interface finite elements, micromechanics, and a fiber-bundle model. Analysis of model predictions shows the significance of strength disorder and fiber properties on the structural behavior. © 2019 The Authors. Structural Concrete published by John Wiley & Sons Ltd on behalf of International Federation for Structural Concrete
    view abstract10.1002/suco.201900056
  • Modeling of structures with polymorphic uncertainties at different length scales
    Kremer, K. and Edler, P. and Miska, N. and Leichsenring, F. and Balzani, D. and Freitag, S. and Graf, W. and Kaliske, M. and Meschke, G.
    GAMM Mitteilungen 42 (2019)
    Multiscale analyses require to consider the scale bridging influences of uncertain parameters. In this paper, approaches for polymorphic uncertainty quantification at different length scales are presented. Especially, the effect of uncertain material parameters computed at lower structural scales is investigated with respect to the resulting macroscopic structural behavior. Also the dependencies of uncertain parameters at the macroscale on the structural response evaluated with submodels are discussed. Three examples are shown, where interval and stochastic uncertainty quantification approaches are combined. Two examples deal with material modeling of concrete and the durability of reinforced concrete structures under consideration of polymorphic uncertainties. A mesoscale model of concrete is developed based on a representative volume element containing aggregates, pores, and the cement phase, where the values of the Young's moduli for mortar and the aggregates as well as the volume fraction of the cement phase to aggregates are considered as intervals. Within the durability assessment of reinforced concrete structures, the influence of stochastic distributed loading and concrete material parameters onto the cracking behavior is analyzed by means of a submodeling strategy. In another application, the metal forming process of dual-phase steel sheets is investigated using statistical information of the microscopic material behavior in combination with epistemic uncertainties of the failure criterion and the friction coefficient between the sheet metal and the forming tools. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/gamm.201900006
  • Optimization with constraints considering polymorphic uncertainties
    Mäck, M. and Caylak, I. and Edler, P. and Freitag, S. and Hanss, M. and Mahnken, R. and Meschke, G. and Penner, E.
    GAMM Mitteilungen 42 (2019)
    In this contribution, a numerical design strategy for the optimization under polymorphic uncertainty is introduced and applied to a self-weight minimization of a framework structure. The polymorphic uncertainty, which affects the constraint function of the optimization problem, is thereby modeled in terms of stochastic variables, fuzzy sets, and intervals to account for variability, imprecision and insufficient information. The stochastic quantities are computed using polynomial chaos expansion resulting in a purely fuzzy-valued formulation of the constraint functions which can be computed using α-cut optimization. Afterward, the constraint function can be interpreted in a possibilistic manner, resulting in a flexible formulation to include expert knowledge and to achieve a robust design. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/gamm.201900005
  • Robust segmental lining design combining steel fiber reinforced concrete and traditional reinforcement
    Neu, G.E. and Gall, V.E. and Meschke, G.
    Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art- Proceedings of the WTC 2019 ITA-AITES World Tunnel Congress (2019)
    The improper estimation of design ground loadings as well as the exceedance of installation tolerances during segmental lining construction often results in unwanted segment damage. Incurred damages rarely impact the structural stability of the tunnel ring, but can significantly impact the serviceability state of a finished construction. For this reason, existing design codes often provide limits on allowable crack widths. In this contribution, a newly developed Finite Element modeling scheme for hybrid reinforced segmental tunnel linings is proposed with which the non-linear local cracking response can be predicted. By subjecting this model to various loading scenarios (e.g. jack forces, steady-state) and by modeling the exceedance of installation tolerances during the ring-build phase, the response of the segment can be analyzed. Using the proposed method, various traditionally reinforced and fiber-reinforced designs are investigated. Furthermore, hybrid-reinforced designs combining the strengths of both traditional and fiber reinforced concrete are proposed. © 2019 Taylor & Francis Group, London.
    view abstract10.1201/9780429424441-295
  • Robust segmental lining design – Potentials of advanced numerical simulations for the design of TBM driven tunnels
    Meschke, G. and Neu, G.E. and Marwan, A.
    Geomechanik und Tunnelbau 12 (2019)
    Loading assumptions used for the structural design of segmental linings often improperly reflect the complex load combinations that develop during the construction of a bored tunnel. Therefore segment designs used in practice tend to be on the safe side and often rely on conventional reinforcement methods instead of including other reinforcement concepts, such as steel fibres. In this contribution, a multi-scale computational modelling framework is proposed to investigate the response of steel-fibre reinforced, traditionally reinforced, and hybrid-reinforced lining segments to radial loadings with an emphasis on the longitudinal joints. This modelling approach offers an opportunity to directly investigate the influence of type and content of steel fibres on the performance of segmented linings at the structural scale. Using this framework, a method for robust optimization is applied in order to generate damage-tolerant hybrid segment designs. © 2019 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstract10.1002/geot.201900032
  • Soil-building interaction in mechanized tunneling: A comparison of modeling approaches
    Marwan, A. and Alsahly, A. and Obel, M. and Mark, P. and Meschke, G.
    Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art- Proceedings of the WTC 2019 ITA-AITES World Tunnel Congress (2019)
    During the planning phase of tunneling projects, it is crucial to ensure that existing buildings are protected from damage when tunneling in urban environments. Tunneling inevitably causes ground movements whose impact on above-ground structures must be assessed. Various approaches that differ in precision and complexity may be employed to predict settlements, depending on the magnitude of expected settlements and the vulnerability of the structures with regard to tunneling induced damage. This contribution presents a threestep damage assessment concept adjustable to the level of detail necessary. First, ground movements are predicted using analytical or numerical approaches. Second, the above-ground structures are idealized by means of surrogate beam-, slab-or 3D-models. Lastly, structural damage is assessed according to strain information or tilt. This method enables the evaluation of the potential damage to above ground structures associated with various tunnel alignments during the planning phase. BIM concepts offer opportunities to streamline and simplify this process by using geometrical BIM sub-models as a basis for performing structural calculations. This paper aims to provide recommendations for a sufficient level of detail (LOD) of surface structures for the assessment of the induced damage in computational simulations of mechanized tunneling process. © 2019 Taylor & Francis Group, London.
    view abstract10.1201/9780429424441-281
  • Tbm drive along curved alignments: Model based prognosis of shield movement
    Alsahly, A. and Marwan, A. and Meschke, G.
    Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art- Proceedings of the WTC 2019 ITA-AITES World Tunnel Congress (2019)
    In current mechanized tunneling practice, the position of the shield machine during TBM-advancement is controlled by the shield driver with the aid of monitoring-based guidance systems. This results in an uneven movement of the machine. This contribution proposes a computational model able to support the TBM-steering during tunnel drives along curved alignments. A computational framework is developed to simulate the advancement and excavation processes, and to enable an efficient and realistic 3D-modeling of the advancement process and the shield-soil interactions during tunneling along arbitrarily curved alignments using the finite element method. The proposed framework combines a newly developed steering algorithm to simulate the TBM-movement during the excavation process. The steering algorithm serves as an artificial guidance system to automatically determine the exact position and the driving direction of the TBM in 3D-space. This modeling approach allows for better prediction of the shield behavior and thrusting forces during the advancement process. © 2019 Taylor & Francis Group, London.
    view abstract10.1201/9780429424441-173
  • A fuzzy surrogate modelling approach for real-time predictions in mechanised tunnelling
    Cao, B.T. and Freitag, S. and Meschke, G.
    International Journal of Reliability and Safety 12 (2018)
    In mechanised tunnelling, it is important to perform reliability analyses with respect to the tunnel face collapse and the damage risks of the tunnel lining and existing structures on the ground surface due to the tunnelling induced settlements. The reliability assessment requires to deal with limited information describing the local geology and the soil parameters due to the availability of only a small number of borehole data. In this paper, it is focused on real-time reliability analyses in mechanised tunnelling considering different types of uncertain data, i.e. combining epistemic and aleatoric sources of uncertainty within polymorphic uncertainty models. The system output of interest in these analyses is time variant tunnelling induced surface settlement fields, which are computed by a finite element simulation model. However, for real-time predictions with uncertain data, efficient and reliable surrogate models are required. A new surrogate modelling strategy is developed to predict time variant high dimensional fuzzy settlement fields in real-time. The predicted results of the new surrogate model show similar accuracy compared to the results obtained by optimisation based fuzzy analyses. Meanwhile, the computation time is significantly reduced especially in case of high dimensional outputs and in combination with the p-box approach in the case of polymorphic uncertain data. Copyright © 2018 Inderscience Enterprises Ltd.
    view abstract10.1504/IJRS.2018.092521
  • A multiscale model for high performance FRC
    Timothy, J.J. and Iskhakov, T. and Zhan, Y. and Meschke, G.
    RILEM Bookseries 15 (2018)
    The behavior of high performance fiber reinforced cementitious composites is simulated using semi-analytical and computational sub-models specified at multiple scales. At the scale of a single fiber, a semi-analytical model is developed to characterize the microslip behavior at the interface between the matrix and the fiber. The microcrack bridging and arresting mechanisms of fiber bundles is taken into account within the framework of linear elastic fracture mechanics. Upscaling from the level of distributed microcracks to the macroscopic level is achieved using continuum micromechanics. According to the proposed model, the macroscopic hardening and softening constitutive characteristics is resulting from the microcrack-fiber interaction, the microcrack growth and the evolution of the microcrack density. Model predictions for FRC concrete are validated against experimental data. For the finite element analyses of failure behavior at the structural level, interface solid elements supplemented by a fiber bridging law specified according to the fiber pull-out mechanics are used to represent the cracking process. Selected numerical examples demonstrate that the crack pattern as well as the structural response can be well replicated by the proposed model. © 2018, RILEM.
    view abstract10.1007/978-94-024-1194-2_11
  • A Shear-Slip Mesh Update – Immersed Boundary Finite Element model for computational simulations of material transport in EPB tunnel boring machines
    Dang, T.S. and Meschke, G.
    Finite Elements in Analysis and Design 142 (2018)
    Earth Pressure Balance (EPB) machines are widely used in mechanized tunneling operations in soft soils in a variety of soil conditions. In contrast to slurry shield machines, the pressure distribution at the tunnel face is not well defined. This paper proposes a computational model for the numerical simulation of the material transport inside the pressure chamber of EPB machines. The governing equations of the soil flow are discretized by using the Finite Element (FE) method and the Fractional Step (FS) scheme. A pressure-dependent viscoplastic fluid is adopted to model the behavior of the soil paste, which is assumed to be a compressible homogeneous mixture. The flow behavior along the boundaries of the chamber wall and the cutterhead rotators is described by the Navier slip law. The moving boundaries associated with the rotators and the mixing arms inside the pressure chamber are imposed in the numerical model by means of the Shear-Slip Mesh Update (SSMU) method. The description of the rotating screw in the conveyor system is enabled by using the Immersed Boundary (IB) method. The combination of the two methods enables to exploit the advantages and to mutually compensate for the disadvantages associated with each of the two methods. The numerical components in the model are presented and verified by several numerical benchmark examples. The setup for the chamber model is presented in detail. Selected results from the simulation of a pressure chamber of a tunnel boring machine are shown and some important observations and remarks are discussed. © 2017 Elsevier B.V.
    view abstract10.1016/j.finel.2017.12.008
  • An algorithm based on incompatible modes for the global tracking of strong discontinuities in shear localization analyses
    Alsahly, A. and Callari, C. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 330 (2018)
    Numerical methods for predicting localized shear failure in elasto-plastic solids have experienced considerable advancements in the last decades. Among these approaches, the so-called “Embedded Strong Discontinuity (ESD)” method is often successfully used to accurately simulate the post-localization response with negligible dependence on the finite element discretization. However, it was observed that the employed discontinuity tracking strategy plays a crucial role in the successful localization analysis. In this contribution, we propose a novel strategy for the global tracking of discontinuity surfaces. It is based on exploiting information obtained from the enhanced parameters employed in Enhanced Assumed Strain (EAS) formulations. It is well known, that enhanced strain element formulations are able to better capture localized shear deformations as compared to standard finite elements. This can be explained as a consequence of the improved performance in bending. We observed, that the approximation of the strain jumps delimiting the shear band is connected with a deformation field characterized by opposite bending curvatures across these two discontinuities. Hence, in view of the relations existing between the kinematics of strong and weak discontinuities, we formulate a proper scalar function of the enhanced parameters to identify potential strong discontinuity surfaces, which are evaluated in each step of the analysis with negligible computational cost. This proposed approach has a global character, as it is based upon evaluating discontinuity surfaces defined in the complete analysis domain that are, by construction, continuous across elements. We demonstrate that the tracking algorithm correctly identifies the potential strong discontinuity surface already in early loading stages, even before a localization condition is fulfilled. In those elements which are crossed by the potential failure surface and which also satisfy the localization condition, the kinematics of embedded strong discontinuities is activated to capture the shear failure surface. The performance of the new tracking algorithm is demonstrated by means of several numerical shear localization analyses using associative and non-associative Drucker–Prager elastoplastic models to simulate 2-D and 3-D benchmarkanalyses. © 2017 Elsevier B.V.
    view abstract10.1016/j.cma.2017.10.014
  • Building Information Modelling in mechanised shield tunnelling – A practitioner's outlook to the near future
    Stascheit, J. and Ninić, J. and Meschke, G. and Hegemann, F. and Maidl, U.
    Geomechanik und Tunnelbau 11 (2018)
    The contribution takes a glance at the application of BIM technologies in the design and construction phases of shield tunnelling projects. The intention is to show how Building Information Modelling can be translated into actual benefit, not only in the operation phase but also in the design and construction phases of bored tunnels. Emphasising the integrative character of referencing data uniformly in space and time, examples are given of seamless communication between 3D geometrical modelling, efficient numerical simulation and model adaptation based on measured data acquired during the boring process. The article covers the complete range from predesign through structural analysis and detailed design until the actual excavation process including its interactions with the environment. Special emphasis is given to data management, which is the key to transforming a mere 3D visualisation into a Building Information Model. The article therefore presents a concept for database-supported, web-based integration of software modules for geometrical modelling in various levels of detail, efficient numerical simulation tools that are based upon this representation, and process controlling that manages all data acquired during the construction process in a spatially and temporally coordinated reference system. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstract10.1002/geot.201700060
  • Computational modeling of fiber flow during casting of fresh concrete
    Gudžulić, V. and Dang, T.S. and Meschke, G.
    Computational Mechanics (2018)
    The Folgar–Tucker fiber orientation model coupled with weakly compressible Smoothed Particle Hydrodynamics is used to simulate the process of casting of fiber reinforced concrete and to predict the spatial-temporal evolution of the probability density function of fiber orientation. The flowable concrete-fiber mix is modeled as a viscous Bingham-type fluid. Model predictions qualitatively agree with fiber orientations observed in an L-box test with fibers suspended in transparent gel. Important factors and assumptions regarding the fiber flow are reviewed and conclusions are drawn based on numerical experiments. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00466-018-1639-9
  • 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 (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 abstract10.1007/s11242-018-1126-y
  • Variational approach to interface element modeling of brittle fracture propagation
    Khisamitov, I. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 328 (2018)
    The paper proposes a variational approach to model brittle fracture propagation based on zero-thickness finite elements. Similar to the phase-field model for fracture, the problem of a fractured structure is variationally formulated by considering a minimization problem involving bulk and fracture surface energies. With the help of a damage variable used as an additional degree of freedom, the fracture propagates according to the values of the minimizers of the total potential energy. This damage variable is restricted to dimensionally reduced interface elements inserted between element boundaries. Crack opening is predicted when the elastic energy within the interface surface exceeds the critical energy release rate. The solution of the discretized system of equations is performed in a staggered scheme, solving first for the displacement field and then searching for the solution for the updated nodal damage variables. Selected numerical examples, including re-analyses of laboratory tests characterized by rather complex crack paths, are presented to demonstrate the performance of the proposed variational interface model. © 2017 Elsevier B.V.
    view abstract10.1016/j.cma.2017.08.031
  • A coupled computational approach for the simulation of soil excavation and transport in earth-pressure balance shield machines
    Dang, T.S. and Wessels, N. and Nguyen, N.-S. and Hackl, K. and Meschke, G.
    International Journal for Multiscale Computational Engineering 15 (2017)
    A prototype modeling framework for the coupled simulation of excavation processes at the tunnel face and the subsequent transport of the foam-soil mixture within the pressure chamber of EPB shield machines is proposed. The discrete element method is used for the modeling of soil excavation and the stabilized finite element method, using a non-Newtonian fluid model, is employed for the modeling of fluid transport. A variational approach is applied to directly obtain interparticle parameters of the DEM from a macroscopic strength criterion. A 2D numerical simulation model for a simplified representation of the cutting process at the tunnel face and the transport of the excavated soil-foam mixture is used to demonstrate the proposed coupled excavation-transport modeling approach. According to the proposed coupled DEM-FEM model, the mass flow obtained from the excavation simulation by means of the DEM serves as the input for the finite element flow simulation to generate the pressure distribution within the excavation chamber. It is shown that the proposed approach helps to obtain insight into the coupled excavation and transport processes at the tunnel face and the spatiotemporal distribution of the face pressure. © 2017 by Begell House, Inc.
    view abstract10.1615/IntJMultCompEng.2017020271
  • A hybrid finite element and surrogate modelling approach for simulation and monitoring supported TBM steering
    Ninić, J. and Freitag, S. and Meschke, G.
    Tunnelling and Underground Space Technology 63 (2017)
    The paper proposes a novel computational method for real-time simulation and monitoring-based predictions during the construction of machine-driven tunnels to support decisions concerning the steering of tunnel boring machines (TBMs). The proposed technique combines the capacity of a process-oriented 3D simulation model for mechanized tunnelling to accurately describe the complex geological and mechanical interactions of the tunnelling process with the computational efficiency of surrogate (or meta) models based on artificial neural networks. The process-oriented 3D simulation model with updated model parameters based on acquired monitoring data during the advancement process is used in combination with surrogate models to determine optimal tunnel machine-related parameters such that tunnelling-induced settlements are kept below a tolerated level within the forthcoming process steps. The performance of the proposed strategy is applied to the Wehrhahn-line metro project in Düsseldorf, Germany and compared with a recently developed approach for real-time steering of TBMs, in which only surrogate models are used. © 2016 Elsevier Ltd
    view abstract10.1016/j.tust.2016.12.004
  • A multiscale homogenization model for strength predictions of fully and partially frozen soils
    Zhou, M.-M. and Meschke, G.
    Acta Geotechnica (2017)
    Soil freezing is often used to provide temporary support of soft soils in geotechnical interventions. During the freezing process, the strength properties of the soil–water–ice mixture change from the original properties of the water-saturated soil to the properties of fully frozen soils. In the paper, a multiscale homogenization model for the upscaling of the macroscopic strength of freezing soil based upon information on three individual material phases—the solid particle phase (S), the crystal ice phase (C) and the liquid water phase (L)—is proposed. The homogenization procedure for the partially frozen soil–water–ice composite is based upon an extension of the linear comparison composite (LCC) method for a two-phase matrix–inclusion composite, using a two-step homogenization procedure. In each step, the LCC methodology is implemented by estimating the strength criterion of a two-phase nonlinear matrix–inclusion composite in terms of an optimally chosen linear elastic comparison composite with a similar underlying microstructure. The solid particle phase (S) and the crystal ice phase (C) are assumed to be characterized by two different Drucker–Prager strength criteria, and the liquid water phase (L) is assumed to have zero strength capacity under drained conditions. For the validation of the proposed upscaling strategy, the predicted strength properties for fully and partially frozen fine sands are compared with experimental results, focussing on the investigation of the influence of the porosity and the degree of ice saturation on the predicted failure envelope. © 2017 Springer-Verlag Berlin Heidelberg
    view abstract10.1007/s11440-017-0538-0
  • Adaptive crack modeling with interface solid elements for plain and fiber reinforced concrete structures
    Zhan, Y. and Meschke, G.
    Materials 10 (2017)
    The effective analysis of the nonlinear behavior of cement-based engineering structures not only demands physically-reliable models, but also computationally-efficient algorithms. Based on a continuum interface element formulation that is suitable to capture complex cracking phenomena in concrete materials and structures, an adaptive mesh processing technique is proposed for computational simulations of plain and fiber-reinforced concrete structures to progressively disintegrate the initial finite element mesh and to add degenerated solid elements into the interfacial gaps. In comparison with the implementation where the entire mesh is processed prior to the computation, the proposed adaptive cracking model allows simulating the failure behavior of plain and fiber-reinforced concrete structures with remarkably reduced computational expense. © 2017 by the authors.
    view abstract10.3390/ma10070771
  • Cascade Continuum Micromechanics model for the effective permeability of solids with distributed microcracks: Comparison with numerical homogenization
    Leonhart, D. and Timothy, J.J. and Meschke, G.
    Mechanics of Materials 115 (2017)
    The effective permeability of microcracked heterogeneous materials such as rocks, ceramics and concrete can be determined using analytical and computational homogenization methods. While in the companion paper (Timothy and Meschke, 2017) a semi-analytical Cascade Continuum Micromechanics (CCM) model is proposed to predict the effective permeability and the percolation threshold for porous materials with microcracks, the focus of this paper is to validate the CCM model by means of direct numerical meso-scale simulations of representative elementary volumes concerned with water flow through porous materials. Algorithms are developed to generate overlapping and non-overlapping microcrack morphologies within the framework of the finite element method to analyse the effective permeability as a function of the microcrack volume fraction. The numerical results confirm the predictions from the CCM model for different scenarios, including a low and high permeable matrix and different microcrack morphologies, both qualitatively as well as quantitatively. It is also shown, that in computational homogenization procedures, the effective permeability around the percolation threshold is strongly dependent on the discretization of the numerical REV. © 2017 Elsevier Ltd
    view abstract10.1016/j.mechmat.2017.09.001
  • Cascade continuum micromechanics model for the effective permeability of solids with distributed microcracks: Self-similar mean-field homogenization and image analysis
    Timothy, J.J. and Meschke, G.
    Mechanics of Materials 104 (2017)
    The transport and fluid flow in heterogeneous materials such as rocks, ceramics and concrete with a distributed random microcrack network is strongly influenced by the density and the topology (distribution and connectivity) of microcracks. The overall fluid flow characteristics of such microcracked solids can be quantified in terms of an effective permeability. In the paper, a semi-analytical formulation for the effective permeability is proposed within the framework of the mean-field homogenization method using the cascade continuum micromechanics model considering long range and short range interactions. We compare model predictions of the percolation threshold i.e. critical volume fraction of microcracks beyond which a solid with distributed microcracks becomes permeable, using results from numerical simulations. The model reveals a new perspective into the self-similar characteristics of the microcrack morphology near the threshold volume fraction of microcracks at which the microcrack structure changes from multiple disconnected microcracks to a connected self-similar microcracked structure. © 2016 Elsevier Ltd
    view abstract10.1016/j.mechmat.2016.10.005
  • Degradation in concrete structures due to cyclic loading and its effect on transport processes—Experiments and modeling
    Przondziono, R. and Timothy, J.J. and Weise, F. and Krütt, E. and Breitenbücher, R. and Meschke, G. and Hofmann, M.
    Structural Concrete 18 (2017)
    According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali-silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement. © 2017 fib. International Federation for Structural Concrete
    view abstract10.1002/suco.201600180
  • Recurrent neural networks and proper orthogonal decomposition with interval data for real-time predictions of mechanised tunnelling processes
    Freitag, S. and Cao, B.T. and Ninić, J. and Meschke, G.
    Computers and Structures (2017)
    A surrogate modelling strategy for predictions of interval settlement fields in real time during machine driven construction of tunnels, accounting for uncertain geotechnical parameters in terms of intervals, is presented in the paper. Artificial Neural Network and Proper Orthogonal Decomposition approaches are combined to approximate and predict tunnelling induced time variant surface settlement fields computed by a process-oriented finite element simulation model. The surrogate models are generated, trained and tested in the design (offline) stage of a tunnel project based on finite element analyses to compute the surface settlements for selected scenarios of the tunnelling process steering parameters taking uncertain geotechnical parameters by means of possible ranges (intervals) into account. The resulting mappings of time constant geotechnical interval parameters and time variant deterministic steering parameters onto the time variant interval settlement field are solved offline by optimisation and online by interval analyses approaches using the midpoint-radius representation of interval data. During the tunnel construction, the surrogate model is designed to be used in real-time to predict interval fields of the surface settlements in each stage of the advancement of the tunnel boring machine for selected realisations of the steering parameters to support the steering decisions of the machine driver. © 2017 Elsevier Ltd.
    view abstract10.1016/j.compstruc.2017.03.020
  • Simulation based evaluation of time-variant loadings acting on tunnel linings during mechanized tunnel construction
    Ninić, J. and Meschke, G.
    Engineering Structures 135 (2017)
    In the design of machine driven tunnels, the loadings acting on the segmental lining are often adopted according to simplified assumptions, which improperly reflect the actual loading on the linings developing during the construction of a bored tunnel. A coupled 3D Finite Element model of the tunnel advancement process including the ring-wise installation of the lining and the hardening process of the grouting material serves as the basis for the analysis of the actual spatio-temporal evolution of the loading on the lining during tunnel construction. The distribution of the loadings in the different construction phases is calculated using a modified surface-to-surface contact condition imposed between the solidifying grouting material in the tail gap and the lining elements. An extensive parametric study investigates the influence of the initial grouting pressure, the pressure gradient, the temporal stiffness evolution, the soil permeability as well as the interface conditions between the grouting material and the tunnel shell on the temporal evolution of the loading on linings. © 2016 Elsevier Ltd
    view abstract10.1016/j.engstruct.2016.12.043
  • The intrinsic permeability of microcracks in porous solids: Analytical models and Lattice Boltzmann simulations
    Timothy, J.J. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 41 (2017)
    The paper presents a synthesis of analytical modeling and computational simulations of the intrinsic permeability of microcracks, embedded in porous materials taking into account the interaction of the fluid flow in the microcrack with the surrounding porous material. In the first part of the paper, using the DARCY, STOKES, BRINKMAN, and the BEAVERS–JOSEPH approximations, we derive the intrinsic permeability of a plain non-rough microcrack in terms of the microcrack geometry and the permeability of the porous material surrounding the microcrack. In the second part of the paper, the intrinsic permeability of a microcrack is determined by means of computational simulations using the framework of the lattice Boltzmann method with partial bounceback conditions. The comparison of predictions from the analytical model and the numerical simulations show an excellent agreement. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd.
    view abstract10.1002/nag.2673
  • Wave dispersion and propagation in state-based peridynamics
    Butt, S.N. and Timothy, J.J. and Meschke, G.
    Computational Mechanics 60 (2017)
    Peridynamics is a nonlocal continuum model which offers benefits over classical continuum models in cases, where discontinuities, such as cracks, are present in the deformation field. However, the nonlocal characteristics of peridynamics leads to a dispersive dynamic response of the medium. In this study we focus on the dispersion properties of a state-based linear peridynamic solid model and specifically investigate the role of the peridynamic horizon. We derive the dispersion relation for one, two and three dimensional cases and investigate the effect of horizon size, mesh size (lattice spacing) and the influence function on the dispersion properties. We show how the influence function can be used to minimize wave dispersion at a fixed lattice spacing and demonstrate it qualitatively by wave propagation analysis in one- and two-dimensional models of elastic solids. As a main contribution of this paper, we propose to associate peridynamic non-locality expressed by the horizon with a characteristic length scale related to the material microstructure. To this end, the dispersion curves obtained from peridynamics are compared with experimental data for two kinds of sandstone. © 2017, Springer-Verlag GmbH Germany.
    view abstract10.1007/s00466-017-1439-7
  • A cascade continuum micromechanics model for the effective elastic properties of porous materials
    Timothy, J.J. and Meschke, G.
    International Journal of Solids and Structures 83 (2016)
    The elastic properties of porous materials with a disordered pore structure are estimated using the mean-field Eshelby homogenization scheme together with the principle of recurrence to generate a cascade of effective microstructures as a function of the porosity and the cascade level n. Starting with the Hashin-Shtrikman upper bound for porous materials, the proposed cascade micromechanics model generates a hierarchy of micro-structures which evolve from an initial configuration of a porous material with spherical pores embedded within an elastic solid phase consistent with the Mori-Tanaka matrix inclusion morphology to a porous material characterized by a hierarchic distribution of spherical elastic grains. The model is explicit and allows for an easy computational implementation. It predicts physically consistent threshold porosities, characteristic for the specific morphology of the porous material under consideration, beyond which the material loses its stiffness. The validity of the cascade micromechanics model is evaluated against experimental data for various materials ranging from foam to ceramics with different pore structures. © 2015 Elsevier Ltd.
    view abstract10.1016/j.ijsolstr.2015.12.010
  • A finite element model for propagating delamination in laminated composite plates based on the Virtual Crack Closure method
    Marjanović, M. and Meschke, G. and Vuksanović, D.
    Composite Structures 150 (2016)
    In the paper, a simple and efficient algorithm to track a moving delamination front of arbitrary shape, using a laminated finite plate element model in conjunction with the Virtual Crack Closure Technique (VCCT), is proposed. The solution requires the calculation of the virtually closed area in front of the delamination, which is approximated by means of a 6-node-polygon, the delamination opening behind this front and the reaction forces in the nodes at the delamination front. These quantities are calculated in a local coordinate system (LCS) defined in the nodes along the delamination front. Using the proposed algorithm, arbitrary meshes composed of 4- and 9-node quadrilateral finite elements can be considered. The proposed model is developed in the context of a layered finite element plate model. To prevent interlaminar penetration of adjacent layers in the delaminated region, an algorithm recently proposed by Marjanović et al. (2015) is adopted. The model performance is demonstrated by re-analyses of the Double-Cantilever-Beam problem, for which analytical solutions exist, and by transient analyses of laminated composite plates with propagating delamination fronts. © 2016 Elsevier Ltd.
    view abstract10.1016/j.compstruct.2016.04.044
  • A higher-order stress-based gradient-enhanced damage model based on isogeometric analysis
    Thai, T. Q. and Rabczuk, T. and Bazilevs, Y. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 304 (2016)
    The micro-damage associated with diffuse fracture processes in quasi-brittle materials can be described by continuum damage mechanics. In order to overcome the mesh dependence of local damage formulations, non-local and gradient-enhanced approaches are often employed. In this manuscript, a higher-order stress-based gradient-enhanced formulation is proposed, which exploits the higher-order continuity of B-spline functions in isogeometric analysis (IGA). The proposed formulation does not require the decomposition of the fourth-order model into two second-order models. Two numerical examples are presented to demonstrate the performance of the formulation and to compare the obtained solutions with results from conventional gradient-enhanced damage formulation. (C) 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cma.2016.02.031
  • A micromechanics model for molecular diffusion in materials with complex pore structure
    Timothy, J.J. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 40 (2016)
    Molecular diffusion in fully saturated porous materials is strongly influenced by the pore space, which, in general, is characterized by a complex topological structure. Hence, information on macroscopic diffusion properties requires up-scaling of transport processes within nano-pores and micro-pores over several spatial scales. A new model in the framework of continuum micromechanics is proposed for predicting the effective molecular diffusivity in porous materials. Considering a representative volume element, characterizing a porous material without any information about the pore space microstructure complexity, the uniform flux is perturbed by recursively embedding shape information hierarchically in the form of the ESHELBY matrix-inclusion morphology to obtain the effective diffusivity as a function of the recurrence level and the porosity. The model predicts a threshold value for the porosity, below which no molecular diffusion can occur because of the presence of isolated pore clusters that are not connected and unavailable for transport. The maximum porosity, below which no molecular transport is possible, is predicted as one-third for spherical inclusions. The model allows for extensions to more complex morphologies of the inclusions. We also identify, that the effects of the micro-structure on molecular transport are characterized by porosity dependent long-range and short-range interactions. The developed framework is extended to incorporate realistic pore size distributions across several spatial scales by means of a distribution function within the hierarchical homogenization scheme. Available experimental results assert the model predictions. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstract10.1002/nag.2423
  • A new mixed finite-element approach for the elastoplastic analysis of Mindlin plates
    Kutlu, A. and Meschke, G. and Omurtag, M.H.
    Journal of Engineering Mathematics 99 (2016)
    The objective of this paper is to develop an accurate and efficient solution procedure for elastoplastic problems in structural mechanics in the framework of a two-field mixed variational principle. A novel solution algorithm is proposed and applied to the elastoplastic analysis of Mindlin plates. The Hellinger–Reissner principle is adopted to obtain the global finite-element equations of the problem. Instead of a static condensation, the stress-type field variables are preserved during the solution. According to the proposed approach, the strain increments within a nonlinear solution step are obtained directly at the nodal points from matrix operations instead of gradients of a displacement field. In the present implementation, the von Mises yield criterion with linear hardening is adopted. For the integration of the elastoplastic constitutive rate equations at the nodal points, a 3D fully implicit algorithm is employed. A layered approach is followed to enable the resolution of the plastic strains through the plate thickness. The mixed formulation of the Mindlin plate theory is shear-locking free by construction. The proposed solution strategy is verified by solving several benchmark problems that demonstrate the high accuracy and convergence rate of the presented layered mixed formulation for elastoplastic analyses. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10665-015-9825-7
  • Advanced finite element modeling of excavation and advancement processes in mechanized tunneling
    Alsahly, A. and Stascheit, J. and Meschke, G.
    Advances in Engineering Software 100 (2016)
    Mechanized tunneling is characterized by complex interactions between the shield machine and the surrounding ground during the TBM advancement process. In this paper, a new computational framework is developed to enable an efficient and realistic three-dimensional modeling of the tunneling process for arbitrary alignments using the finite element method. A new steering algorithm for the advancement of the Tunnel Boring Machine (TBM) for arbitrary alignments during shield tunneling is incorporated in the proposed model. This algorithm simulates the shield behavior and accordingly provides the numerical model with the required information to keep the TBM on track during the simulation. However, the utilization of this algorithm is only possible using a finite element discretization which adapts to the actual motion path of the shield machine. For this purpose, a re-meshing technique is proposed in order to automate the process of mesh generation in the vicinity of the tunnel face, denoted as the region of interest, within the advancing process. The combination of a computational steering algorithm and a 3D automatic adaptive mesh generation procedure form a novel framework for process oriented finite element simulations of the mechanized tunnel construction process. The applicability of the proposed modeling technique for predicting the shield behavior and the soil-tunnel interactions during tunneling along curved alignments is demonstrated by means of selected examples. © 2016 Elsevier Ltd
    view abstract10.1016/j.advengsoft.2016.07.011
  • Cascade lattice micromechanics model for the effective permeability of materials with microcracks
    Timothy, J.J. and Meschke, G.
    Journal of Nanomechanics and Micromechanics 6 (2016)
    Within the framework of mean-field homogenization methods, a lattice version of the cascade micromechanics model for the estimation of the effective permeability of microcracked materials with a physically consistent percolation threshold is proposed. Also, a link is investigated between the cascade scheme, self-similarity, and critical phenomena. The cascade lattice micromechanics model is compared with the dilute, the Mori-Tanaka and the self-consistent lattice schemes and validated by means of numerical experiments using a computational lattice microcrack-pore network model. Different scenarios of the microcrack probability and the permeability contrast between the pore channels and the microcracks are investigated. © 2016 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)NM.2153-5477.0000113
  • Glaucoma and structure-based mechanics of the lamina cribrosa at multiple scales
    Grytz, R. and Meschke, G. and Jonas, J.B. and Downs, J.C.
    Structure-Based Mechanics of Tissues and Organs (2016)
    Glaucoma is among the leading causes of blindness worldwide. The disease involves damage to the retinal ganglion cell axons that transmit visual information from the eye to the brain. Experimental evidence indicates that biomechanical mechanisms at different length scales are involved in pathophysiology of glaucoma, where chronic intraocular pressure (IOP) elevation at the organ level initiates axonal insult at the level of the lamina cribrosa. The lamina cribrosa consists of a porous collagen structure through which the axons of retinal ganglion cells (RGCs) pass on their path from the retina to the brain. The extent to which the structural mechanics of the lamina cribrosa contribute to the axonal insult remains unclear. In this book chapter, we give a short review of the present understanding of the structural mechanics of the lamina cribrosa and its role in glaucoma. The main aim is to present a first computationally coupled two-scale analysis of the lamina cribrosa that translates the IOP load at the macroscale to the mechanical insult of the axons within the mesostructure of the lamina cribrosa. The numerical results of two-scale analysis suggest that the collagen structures of the lamina cribrosa and its surrounding peripapillary sclera effectively provide mechanical support to the axons by protecting them from high tensile stresses even at elevated IOP levels. However, in-plane shear stresses in the axonal tissue may increase with increasing IOP at the posterior lamina insertion region and contribute to a mechanical insult of the RGC axons in glaucoma. © Springer Science+Business Media, LLC 2016.
    view abstract10.1007/978-1-4899-7630-7_6
  • Multilevel computational model for failure analysis of steel-fiber-reinforced concrete structures
    Zhan, Y. and Meschke, G.
    Journal of Engineering Mechanics 142 (2016)
    A multilevel modeling framework for the failure analyses of structures made of steel-fiber-reinforced concrete (SFRC), which allows researchers to follow the effects of design parameters such as fiber type, distribution, and orientation from the scale of fiber-matrix interaction to the structural behavior, is proposed. The basic ingredient at the level of single fibers is an analytical model for the prediction of the pullout response of straight or hooked-end fibers. For an opening crack in a specific SFRC composite, the fiber bridging effect is computed via the integration of the pullout response of all fibers intercepting the crack, taking anisotropic fiber orientations into consideration. For the finite-element analysis of the failure behavior of SFRC structures, interface solid elements are used to represent cracks. The softening behavior of opening cracks is governed by cohesive tractions and the fiber bridging effect. The use of an implicit/explicit integration scheme enhances the computational robustness considerably. Numerical analyses of selected benchmark problems demonstrate that the model is able to predict the structural response for different fiber cocktails in good agreement with experimental results. © 2016 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)EM.1943-7889.0001154
  • Optimization of artificial ground freezing in tunneling in the presence of seepage flow
    Marwan, A. and Zhou, M.-M. and Zaki Abdelrehim, M. and Meschke, G.
    Computers and Geotechnics 75 (2016)
    Artificial ground freezing is an environmentally friendly technique to provide temporary excavation support and groundwater control during tunnel construction under difficult geological and hydrological ground conditions. Evidently, groundwater flow has a considerable influence on the freezing process. Large seepage flow may lead to large freezing times or even may prevent the formation of a closed frozen soil body. For safe and economic design of freezing operations, this paper presents a coupled thermo-hydraulic finite element model for freezing soils integrated within an optimization algorithm using the Ant Colony Optimization (ACO) technique to optimize ground freezing in tunneling by finding the optimal positions of the freeze pipe, considering seepage flow. The simulation model considers solid particles, liquid water and crystal ice as separate phases, and the mixture temperature and liquid pressure as primary field variables. Through two fundamental physical laws and corresponding state equations, the model captures the most relevant couplings between the phase transition associated with latent heat effect, and the liquid transport within the pores. The numerical model is validated by means of laboratory results considering different scenarios for seepage flow. As demonstrated in numerical simulations of ground freezing in tunneling in the presence of seepage flow connected with the ACO optimization algorithm, the optimized arrangement of the freeze pipes may lead to a substantial reduction of the freezing time and of energy costs. © 2016.
    view abstract10.1016/j.compgeo.2016.01.004
  • Strong discontinuity approaches: An algorithm for robust performance and comparative assessment of accuracy
    Cazes, F. and Meschke, G. and Zhou, M.-M.
    International Journal of Solids and Structures 96 (2016)
    The Strong Discontinuity Approach (SDA) is a popular method to incorporate cracks as displacement discontinuities into finite elements. In the first part of the paper, different SDA formulations (denoted as SOS, KOS and SKON) are assessed numerically based upon different error norms including a norm to evaluate the variational consistency of SDA elements. Results are compared with analytical solutions as well as with results from interface elements and the element erosion technique for cohesionless cracks. In the second part of the paper, a new method to improve the computational robustness of SDA analyses is proposed. In addition to using arc-length control to solve the nonlinear equation system, a sequential un- and reloading scheme is proposed with the crack state frozen during unloading to avoid, that more than one new crack segment is activated within an increment. The performance of the algorithm is demonstrated by numerical analyses of a tension test on a dog-bone shaped specimen by means of different variants of the SDA and, for comparison, also using interface elements. © 2016
    view abstract10.1016/j.ijsolstr.2016.05.016
  • A Generalized Finite Element Method for hydro-mechanically coupled analysis of hydraulic fracturing problems using space-time variant enrichment functions
    Meschke, G. and Leonhart, D.
    Computer Methods in Applied Mechanics and Engineering 290 (2015)
    In computational simulations of hydraulic fracturing problems, consideration of interactions between the propagating fracture zone and the fluid flow through the porous material requires an appropriate up-scaling procedure from the spatial scale of the local crack, which usually is much smaller compared to the scale of typical finite elements in poromechanics problems. This scale transition refers to both the displacement field (discontinuity across cracks) as well as to the fluid flow (accelerated flow within cracks and the interaction with the flow in the bulk material). To resolve the small and the large scale portion of the solution, the Generalized Finite Element Method (GFEM) exploiting the partition of unity property of shape functions is used. Accordingly, the displacements u and the liquid pressure p<inf>l</inf> are locally enriched to better resolve their distribution in the vicinity of cracks by means of an additive decomposition into a large and a small scale part. As far as the representation of cracks is concerned, the Extended Finite Element Method (XFEM) is used by enriching the displacement field by means of a jump function as well as crack tip functions. In the framework of the GFEM physically motivated enrichment functions for the local enrichment of the C1 discontinuity of the liquid pressure field across cracks are proposed in the paper. The space and time variant analytical solutions obtained from the 1D transient response of saturated porous materials subjected to the liquid pressure within the crack are applied as enrichment functions to locally improve the approximation of the liquid pressure field at discontinuities. Applying these space and time variant functions, which are the exact solutions of the pressure field in the vicinity of cracks, as local enrichment functions lead to a significant improvement in the local approximation of the pressure field at discontinuities. The new GFEM model is formulated in a poromechanics framework for fully saturated porous materials. Representative analyses demonstrate the improvement of the solution quality compared to existing FEM and XFEM models. © 2015 Elsevier B.V.
    view abstract10.1016/j.cma.2015.03.005
  • Computational Modeling of Concrete Degradation Due to Alkali Silica Reaction
    Timothy, J.J. and Nguyen, M.N. and Meschke, G.
    CONCREEP 2015: Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures - Proceedings of the 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures (2015)
    This paper presents a computational multi-level model for the description of alkali and moisture transport in concrete structures coupled to a macroscopic ASR induced phase-field damage model. Concrete is modeled as a heterogeneous material consisting of a partially saturated pore space with diffusively distributed microcracks and the solid skeleton (cement paste and potentially reactive and inert aggregates). The influence of the topology of the pore space and the presence of oriented microcracks on ion diffusion and moisture transport is taken into account through a novel continuum micromechanics homogenization model. The transport model is connected to a phenomenological reaction kinetics model to account for the ASR induced volume expansion of the affected aggregates. At the macroscopic scale, crack propagation and effects of induced topological changes on the fluid and ion transport are taken into account using a phase-field model. © ASCE.
    view abstract10.1061/9780784479346.023
  • Degradations in concrete due to cyclic loading and its effects on transport processes with regard to ASR damage
    Przondziono, R. and Timothy, J.J. and Nguyen, M. and Weise, F. and Breitenbücher, R. and Meschke, G. and Meng, B.
    Beton- und Stahlbetonbau 110 (2015)
    According to the goals of the research group 1498, this paper deals with the effects of cyclic flexural loading in a four-point bending test on the fluid transport processes within a concrete structure. Therefore, the degradation of the microstructure is characterized through ultrasonic wave measurements as well as microscopic crack analysis. In order to numerically model these processes, experiments on the penetration behavior of water into the concrete were carried out. The penetration behavior over time as well as the influence of degradation on the water transport were investigated. To predict the influence of concrete degradation on alkali diffusivity, a multi-scale continuum micromechanics model is incorporated into the numerical model, which accounts for the topology and the three-dimensional distribution of microcracks. As expected, the numerical simulation predicts larger alkali-penetration in pre-damaged concrete. Regarding the micro-crack distribution, an anisotropic distribution of micro-cracks tangential to the direction of the alkali and water flux increases their penetration depth. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
    view abstract10.1002/best.201400095
  • Geometrically nonlinear transient analysis of delaminated composite and sandwich plates using a layerwise displacement model with contact conditions
    Marjanović, M. and Vuksanović, D. and Meschke, G.
    Composite Structures 122 (2015)
    In the paper, a computational model for the transient response of laminated composite and sandwich plates with existing zones of partial delamination, subjected to dynamic pulse loading is proposed. Laminated composite and sandwich plates are modeled using the extended version of the Generalized Laminated Plate Theory. For the numerical solution, the finite element method with layered finite elements is used. Delamination between individual layers is considered as discontinuities in the displacement field using Heaviside step functions. Since the status of delaminated layers may change in dynamic loading conditions, leading to the so-called "breathing" phenomenon, contact conditions allowing for the opening/closing of delaminated layers are proposed. Nonlinear kinematics in the sense of small strains and moderately large rotations is accounted for according to the von Kármán assumptions. The material of the individual layers is assumed as orthotropic and linearly elastic. The governing spatial-temporal partial differential equations are integrated in time by means of the implicit Newmark's method. After verification of the proposed model for intact plates, the effect of the size and the position of embedded delamination zones on the transient response of geometrical nonlinearity of composite and sandwich plates is investigated numerically by means of a number of numerical applications. © 2014 Elsevier Ltd.
    view abstract10.1016/j.compstruct.2014.11.028
  • Hybrid surrogate modelling for mechanised tunnelling simulations with uncertain data
    Freitag, S. and Cao, B.T. and Ninić, J. and Meschke, G.
    International Journal of Reliability and Safety 9 (2015)
    For numerical reliability analyses of engineering structures, highly efficient computational models are required, in particular if numerical simulations of construction processes are to be performed in real time during the construction. Adopting process-oriented finite element simulations in mechanised tunnelling as a specific field of application, surrogate models are proposed to substitute inevitable expensive complex finite element simulations with uncertain parameters. The focus in this paper is laid on the uncertainty of geotechnical and tunnelling process parameters described by stochastic numbers, intervals and interval stochastic numbers. A new hybrid surrogate modelling concept is presented, which is based on combining artificial neural networks and the proper orthogonal decomposition method. Thereby, uncertain time variant surface settlements of several monitoring points are predicted by recurrent neural networks. Based on these predictions, the complete surface settlement field of each time step is computed using the gappy proper orthogonal decomposition approach. The hybrid surrogate model is applied for reliability analyses of a mechanised tunnelling process, where limit states at multiple surface positions are evaluated. Copyright © 2015 Inderscience Enterprises Ltd.
    view abstract10.1504/IJRS.2015.072717
  • Model update and real-time steering of tunnel boring machines using simulation-based meta models
    Ninić, J. and Meschke, G.
    Tunnelling and Underground Space Technology 45 (2015)
    A method for the simulation supported steering of the mechanized tunneling process in real time during construction is proposed. To enable real-time predictions of tunneling induced surface settlements, meta models trained a priori from a comprehensive process-oriented computational simulation model for mechanized tunneling for a certain project section of interest are introduced. For the generation of the meta models, Artificial Neural Networks (ANN) are employed in conjunction with Particle Swarm Optimization (PSO) for the model update according to monitoring data obtained during construction and for the optimization of machine parameters to keep surface settlements below a given tolerance. To provide a rich data base for the training of the meta model, the finite element simulation model for tunneling is integrated within an automatic data generator for setting up, running and postprocessing the numerical simulations for a prescribed range of parameters. Using the PSO-ANN for the inverse analysis, i.e. identification of model parameters according to monitoring results obtained during tunnel advance, allows the update of the model to the actual geological conditions in real time. The same ANN in conjunction with the PSO is also used for the determination of optimal steering parameters based on target values for settlements in the forthcoming excavation steps. The paper shows the performance of the proposed simulation-based model update and computational steering procedure by means of a prototype application to a straight tunnel advance in a non-homogeneous soil with two soil layers separated by an inclined boundary. © 2014 Elsevier Ltd.
    view abstract10.1016/j.tust.2014.09.013
  • Adaptive computational simulation of TBM-soil interactions during machine-driven tunnel construction in saturated soft soils
    Alsahly, A. and Stascheit, J. and Meschke, G.
    Geotechnical Special Publication (2014)
    In soft, partially or fully saturated ground conditions, machine-driven tunnel construction causes short- and long-term ground deformations resulting from the disturbance of the virgin stress state of the soil and changes in the pore water conditions. These variations are, in turn, influenced by the heading face support, shield skin friction and by the gap grouting. Realistic large-scale 3D simulations are, therefore, increasingly required to investigate the interaction between machine-driven tunnel construction and the surrounding soil in order to provide reliable estimates of the expected settlements and associated risks of damage for existing structures, respectively, in particular in urban tunneling projects. If performed properly, these simulations involve complex interactions between individual components of the numerical model. The presented paper is concerned with recent advances in the process-oriented adaptive computational simulation of the excavation and steering processes in mechanized tunneling in soft soils using the finite element method. A novel automated adaptive mesh-refinement procedure is proposed to allow a refined resolution of the region of interest in the vicinity of the tunnel face during the TBM advancement. This procedure allows for an accurate assessment of the tunnel face stability and for the investigation of the immediate soil deformation and pore pressure changes around the tunnel. Furthermore, selected aspects of the numerical treatment - such as the stabilization of low order, two-phase, finite elements and the sub-stepping schemes inherent in the numerical integration of elasto-plastic models -,are also addressed in the presentation. © 2014 American Society of Civil Engineers.
    view abstract10.1061/9780784413449.075
  • An ALE-PFEM method for the numerical simulation of two-phase mixture flow
    Dang, T.S. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 278 (2014)
    A new finite element method is proposed to simulate two-phase mixture problems with free surfaces and/or large moving boundaries. The two phases are treated as interpenetrating continua within a two-fluid model. In contrast to the conventional two-fluid model formulations, in which the governing equations are written in the Eulerian-Eulerian description, the modified two-fluid model has the governing equations of one fluid phase written in the Lagrangian description while the respective equations of the other phase is formulated in the Arbitrary Lagrangian-Eulerian (ALE) description with relative velocities defined with respect to the moving mesh associated with the first fluid phase. The Finite Element Method (FEM) together with stabilization techniques-the Characteristic-Based Split (CBS) and the Algebraic-Flux Correction (AFC) method-is employed to solve the equations numerically in space and time. The nodes of the computational mesh are updated together with the flow of the phase described in the Lagrangian description. The computational mesh is updated by applying techniques used in the so-called Particle Finite Element Method (PFEM), characterized by regenerating the mesh in each computational step according to the updated nodal positions while the current boundary of the domain is identified using α-shape method. The performance of the method is demonstrated by means of three 2D and 3D examples, including a mixing process of two fluids in a stirred tank, a free surface flow, a 3D two-phase channel flow, a phase separation problem and the flow of two immiscible fluids. © 2014 Elsevier B.V.
    view abstract10.1016/j.cma.2014.06.011
  • Analytical model for the pullout behavior of straight and hooked-end steel fibers
    Zhan, Y. and Meschke, G.
    Journal of Engineering Mechanics 140 (2014)
    In the context of multiscale-oriented computational analyses of fiber-RC (FRC) structures, the modeling of single fiber pullout behavior represents the basic constituent to provide traction-displacement relations to be used for the modeling of FRC on a macroscopic scale. This essential ingredient needs to be formulated such that it only requires minimal computational effort. To this end, an analytical model for the pullout behavior of single fibers embedded in a concrete matrix for various configurations of fiber type, matrix strength, and embedment condition is proposed. An interface law is developed for the frictional behavior between the fiber and matrix. In the case of inclined fibers, the plastic deformation of the fiber and the local damage of concrete are also considered. For hooked-end fibers, the anchorage effect due to the deformed topology of the fiber ends is taken into account in the formulation. By combining these submodels, the pullout response of single fibers embedded in a concrete matrix is predicted. In addition, numerical simulations of pullout tests are performed to obtain insight into the local fiber-concrete interactions and provide supporting information for analytical modeling. The model is successfully validated by means of representative experimental results. © 2014 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)EM.1943-7889.0000800
  • Beam-solid contact formulation for finite element analysis of pile-soil interaction with arbitrary discretization
    Ninić, J. and Stascheit, J. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 38 (2014)
    This paper presents an embedded beam formulation for discretization independent finite element (FE) analyses of interactions between pile foundations or rock anchors and the surrounding soil in geotechnical and tunneling engineering. Piles are represented by means of finite beam elements embedded within FEs for the soil represented by 3D solid elements. The proposed formulation allows consideration of piles and pile groups with arbitrary orientation independently from the FE discretization of the surrounding soil. The interface behavior between piles and the surrounding soil is represented numerically by means of a contact formulation considering skin friction as well as pile tip resistance. The pile-soil interaction along the pile skin is considered by means of a 3D frictional point-to-point contact formulation using the integration points of the beam elements and reference points arbitrarily located within the solid elements as control points. The ability of the proposed embedded pile model to represent groups of piles objected to combined axial and shear loading and their interactions with the surrounding soil is demonstrated by selected benchmark examples. The pile model is applied to the numerical simulation of shield driven tunnel construction in the vicinity of an existing building resting upon pile foundation to demonstrate the performance of the proposed model in complex simulation environments. © 2014 John Wiley & Sons, Ltd.
    view abstract10.1002/nag.2262
  • Coupled computational simulation of excavation and soil transport in earth-pressure balance shield tunneling machines using a viscous two-phase fluid model for soil-foam mixtures
    Dang, T.S. and Meschke, G. and Wessels, N. and Hackl, K.
    Geotechnical Special Publication (2014)
    The excavation process ofearth-pressure balance (EPB) shield machines involves the cutting of the ground at the tunnel face and the transport of the soil paste in the excavation chamber. For the numerical simulation of these two processes, a computational strategy, characterized by the coupling of two partial models using the discrete element method (DEM) and the finite element method (FEM) has been developed (Wessels et al., 2013). Excavation is simulated using DEM, with the fracture process being represented by the release of interaction forces between the particles. The transport of the excavated soil mixed with the soil conditioning foam, yielding a pasty soil-foam mixture within the pressure chamber, is simulated by means of a two-phase fluid model in Eulerian description. The two momentum and mixture mass equations are discretized in time by the characteristic-based split method (CBS) (Zienkiewicz et al., 2005). The phase volume fraction equations are solved using upwind weighting functions according to the improved Mizukami-Hughes method (Knobloch, 2006). The proposed model is applied to a coupled analysis of the excavation and transport-mixing flow inside a simplified pressure chamber with foam injections. The mixing process, due to the rotation in the chamber, is preliminarily investigated by the mixing flow in a 2D cavity test case. © 2014 American Society of Civil Engineers.
    view abstract10.1061/9780784413449.076
  • Experimental, analytical and numerical analysis of the pullout behaviour of steel fibres considering different fibre types, inclinations and concrete strengths
    Breitenbücher, R. and Meschke, G. and Song, F. and Zhan, Y.
    Structural Concrete 15 (2014)
    The pullout behaviour of single steel fibres embedded in a concrete matrix is investigated for various configurations of fibre types and embedment lengths and angles by means of laboratory tests and analytical models. Laboratory tests for fibre pullout are performed to investigate the fibre-matrix bond mechanisms. Parameters influencing the fibre pullout response, such as fibre shape, fibre tensile strength, concrete strength and fibre inclination angle are systematically studied. The effects of these parameters on the pullout force versus displacement relationship, fibre efficiency and fibre/matrix failure response are analysed based on the experimental results. For the analytical modelling of the fibre pullout behaviour of straight fibres, an interface law is proposed for the frictional behaviour between fibre and matrix. In the case of inclined fibres, the plastic deformation of the fibre and the local damage to the concrete are also considered. For hooked-end fibres, the anchorage effect due to the hook is analysed. Combining these sub-models allows the pullout response of single fibres embedded in a concrete matrix to be predicted. In addition, numerical simulations of pullout tests are performed to obtain insights into the local fibre-concrete interactions and to provide supporting information for the analytical modelling. The models are successfully validated with the experimental results. Copyright © 2014 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
    view abstract10.1002/suco.201300058
  • Numerical modeling of artificial ground freezing: Multiphase modeling and strength upscaling
    Zhou, M.-M. and Meschke, G.
    Geotechnical Special Publication (2014)
    In geotechnical applications of artificial ground freezing, safe design and execution require a correct prediction of the coupled thermo-hydro-mechanical behavior of soils subjected to freezing. In the context of thermo-poro- plasticity (Coussy, 2005), a three-phase finite element model of freezing soils is presented: (1) considering solid particles, liquid water and crystal ice as separate phases; and (2) mixture temperature, liquid pressure, and solid displacement as primary field variables. Through three fundamental physical laws (overall entropy balance, mass balance of liquid water and crystal ice, and overall momentum balance) and corresponding state relations, the model captures the most relevant couplings between the phase transition, the liquid transport within the pores, and the accompanying mechanical deformation. Particularly for the description of the poro-plastic mechanical behavior of the soil model, the enhanced Barcelona Basic Model (Nishimura et al., 2009) is adopted within a unified effective-stress-based framework. The macroscopic strength criterion of the freezing soil composite is improved through multi-scale strength homogenization based upon the linear comparison composite method (Ortega et al. 2011). The performance of the proposed model is demonstrated by re-analysis of a soil freezing test and AGF processes during tunneling. © ASCE 2014.
    view abstract10.1061/9780784413401.021
  • Numerical Predictions of Surface Settlements in Mechanized Tunneling: Hybrid POD and ANN Surrogate Modeling for Reliability Analyses
    Freitag, S. and Cao, B.T. and Meschke, G.
    Vulnerability, Uncertainty, and Risk: Quantification, Mitigation, and Management - Proceedings of the 2nd International Conference on Vulnerability and Risk Analysis and Management, ICVRAM 2014 and the 6th International Symposium on Uncertainty Modeling and Analysis, ISUMA 2014 (2014)
    Computational reliability analyses of engineering structures are often time-consuming, in particular, if time-dependent structural behavior is considered. If (almost) real time prognoses are required, surrogate models may be used to approximate the structural behavior described by advanced numerical models. In this paper, a hybrid surrogate modeling strategy based on a combination of Proper Orthogonal Decomposition (POD) and Artificial Neural Networks (ANN) is introduced. The hybrid approach is developed for the approximation of time variant surface settlements in mechanized tunneling due to uncertain geological and process parameters. The approximation capabilities are demonstrated by means of an example. The new hybrid surrogate model can be applied for numerical, efficient, real-time reliability analyses in mechanized tunneling. © 2014 American Society of Civil Engineers.
    view abstract10.1061/9780784413609.057
  • Strength homogenization of matrix-inclusion composites using the linear comparison composite approach
    Zhou, M.-M. and Meschke, G.
    International Journal of Solids and Structures 51 (2014)
    A homogenization procedure to estimate the macroscopic strength of nonlinear matrix-inclusion composites with different strength characteristics of the matrix and inclusions, respectively, is presented in this paper. The strength up-scaling is formulated within the framework of the yield design theory and the linear comparison composite (LCC) approach, introduced by Ponte Castaneda (2002) and extended to frictional models by Ortega et al. (2011), which estimates the macroscopic strength of composite materials in terms of an optimally chosen linear thermo-elastic comparison composite with a similar underlying microstructure. In the paper various combinations for the underlying material behavior for the individual phases of the composite are considered: The matrix phase can be a quasi frictional material characterized either by a Drucker-Prager-type (hyperbolic) or an elliptical strength criterion, which predicts a strength limit also in hydrostatic compression, while the inclusion phase either may represent empty pores, pore voids filled with a pore fluid, rigid inclusions, or solid inclusions, whose strength characteristics also maybe described by a Drucker-Prager-type or an elliptical strength criterion. For generating the homogenized strength criterion efficiently in such general cases of matrix-inclusion composites, a novel algorithm is proposed in the paper. The validation of the proposed strength homogenization procedure for selected combinations of strength characteristics of the matrix material and the inclusions is conducted by comparisons with experimental results and alternative existing strength homogenization models. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijsolstr.2013.10.002
  • A three-phase thermo-hydro-mechanical finite element model for freezing soils
    Zhou, M.M. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 37 (2013)
    Artificial ground freezing (AGF) is a commonly used technique in geotechnical engineering for ground improvement such as ground water control and temporary excavation support during tunnel construction in soft soils. The main potential problem connected with this technique is that it may produce heave and settlement at the ground surface, which may cause damage to the surface infrastructure. Additionally, the freezing process and the energy needed to obtain a stable frozen ground may be significantly influenced by seepage flow. Evidently, safe design and execution of AGF require a reliable prediction of the coupled thermo-hydro-mechanical behavior of freezing soils. With the theory of poromechanics, a three-phase finite element soil model is proposed, considering solid particles, liquid water, and crystal ice as separate phases and mixture temperature, liquid pressure, and solid displacement as the primary field variables. In addition to the volume expansion of water transforming into ice, the contribution of the micro-cryo-suction mechanism to the frost heave phenomenon is described in the model using the theory of premelting dynamics. Through fundamental physical laws and corresponding state relations, the model captures various couplings among the phase transition, the liquid transport within the pore space, and the accompanying mechanical deformation. The verification and validation of the model are accomplished by means of selected analyses. An application example is related to AGF during tunnel excavation, investigating the influence of seepage flow on the freezing process and the time required to establish a closed supporting frozen arch. © 2013 John Wiley & Sons, Ltd.
    view abstract10.1002/nag.2184
  • An edge-based smoothed finite element method for 3D analysis ofsolid mechanics problems
    Cazes, F. and Meschke, G.
    International Journal for Numerical Methods in Engineering 94 (2013)
    The edge-based smoothed finite element method (ES-FEM) was proposed recently in Liu, Nguyen-Thoi, and Lam to improve the accuracy of the FEM for 2D problems. This method belongs to the wider family of the smoothed FEM for which smoothing cells are defined to perform the numerical integration over the domain. Later, the face-based smoothed FEM (FS-FEM) was proposed to generalize the ES-FEM to 3D problems. According to this method, the smoothing cells are centered along the faces of the tetrahedrons of the mesh. In the present paper, an alternative method for the extension of the ES-FEM to 3D is investigated. This method is based on an underlying mesh composed of tetrahedrons, and the approximation of the field variables is associated with the tetrahedral elements; however, in contrast to the FS-FEM, the smoothing cells of the proposed ES-FEM are centered along the edges of the tetrahedrons of the mesh. From selected numerical benchmark problems, it is observed that the ES-FEM is characterized by a higher accuracy and improved computational efficiency as compared with linear tetrahedral elements and to the FS-FEM for a given number of degrees of freedom. © 2013 John Wiley & Sons, Ltd.
    view abstract10.1002/nme.4472
  • An imbricate finite element method (i-fem) using full, reduced, and smoothed integration
    Cazes, F. and Meschke, G.
    Computational Mechanics 52 (2013)
    Abstract A method to design finite elements that imbricate with each other while being assembled, denoted as imbricate finite element method, is proposed to improve the smoothness and the accuracy of the approximation based upon low order elements. Although these imbricate elements rely on triangular meshes, the approximation stems from the shape functions of bilinear quadrilateral elements. These elements satisfy the standard requirements of the finite element method: continuity, delta function property, and partition of unity. The convergence of the proposed approximation is investigated by means of two numerical benchmark problems comparing three different schemes for the numerical integration including a cell-based smoothed FEM based on a quadratic shape of the elements edges. The method is compared to related existing methods. © Springer-Verlag Berlin Heidelberg 2013.
    view abstract10.1007/s00466-013-0860-9
  • Diffusion in fracturing porous materials: Characterizing topological effects using cascade micromechanics and phase-field models
    Timothy, J.J. and Meschke, G.
    Poromechanics V - Proceedings of the 5th Biot Conference on Poromechanics (2013)
    The diffusion properties of fracturing porous materials, such as concrete or geological materials, are strongly influenced by the complex and random topological structure of the pore space, the state of distributed micro-cracks inevitably caused by processes such as autogenous and drying shrinkage of concrete, and finally by propagating cracks caused by various loading conditions. Information on macroscopic diffusion properties of the porous material requires up-scaling of transport processes within nano- and micro-pores over several spatial scales. The macroscopic transport coefficients are computed using a cascade continuum micromechanics model recently proposed by the authors. The cascade continuum micromechanics model recursively embeds shape information in the form of the ESHELBY matrix-inclusion problem to obtain the homogenized effective diffusion coefficient. The model is able to predict mathematically and physically consistent percolation thresholds. To consider the effects of oriented, diffusely distributed micro-cracks on the diffusion properties, the homogenization scheme for in intact concrete is enhanced by representing micro-cracks as additional ellipsoidal inclusions within the aforementioned homogenized porous matrix. Finally, the effect of propagating macro-cracks on the diffusion process is taken into consideration by weakly coupling the diffusion model and a fracture energy based staggered phase-field model to simulate brittle fracture. © 2013 American Society of Civil Engineers.
    view abstract10.1061/9780784412992.264
  • Parallelized computational modeling of pile-soil interactions in mechanized tunneling
    Meschke, G. and Ninić, J. and Stascheit, J. and Alsahly, A.
    Engineering Structures 47 (2013)
    The construction of tunnels in soft ground causes short and long term ground deformations resulting from the disturbance of the virgin stress state of the soil and the changing pore water conditions. In particular in urban tunneling, in each stage of the construction process, interactions between the construction process, the soil and existing building infrastructure need to be evaluated to limit the risk of damage on existing buildings and to decide on appropriate mitigation measures. Besides conventional tunneling, mechanized tunneling is a well established and flexible technology in particular in urban areas, which allows for tunnel advances in a wide range of soils and difficult conditions. The paper presents a finite element model for the simulation of interactions between mechanized tunnel construction, the surrounding soil and existing buildings resting on pile foundations in the framework of a process-oriented simulation model for mechanized tunneling. The performance of the model is demonstrated by means of selected prototype analyses. As a consequence of the high computational demand connected with this type of spatio-temporal simulations, problem specific parallelization techniques are investigated to increase the numerical efficiency of the numerical analyses. © 2012 Elsevier Ltd.
    view abstract10.1016/j.engstruct.2012.07.001
  • Strength homogenization for partially frozen soil using linear comparison composite approach
    Zhou, M.-M. and Meschke, G.
    Poromechanics V - Proceedings of the 5th Biot Conference on Poromechanics (2013)
    Adopting the linear comparison composite (LCC) method within the framework of yield design theory [1], this paper presents a novel multi-scale strength homogenization procedure for a three-phase, partially-frozen soil composite where the solid particle phase (S) and the crystal ice phase (C) are assumed to be characterized by two different Drucker-Prager strength criteria and the liquid water phase (L) has zero shear strength capacity. Based upon a multi-scale thought model shown in Fig. 1, the macroscopic strength criterion for partially frozen soil can be upscaled through a two-step homogenization process. For each step, the LCC methodology is implemented by estimating the strength criterion of a two-phase nonlinear matrix-inclusion composite in terms of an optimally chosen linear thermo-elastic comparison composite with a similar underlying microstructure. In addition, the pressure melting of ice is considered in this model by incorporating a temperature and pressure dependent phase transition between ice and water. The predicted macroscopic strength criterion for partially frozen soil shows qualitatively a good agreement with observed phenomena, such as strengthening of soil during freezing and weakening of soil during pressure melting. © 2013 American Society of Civil Engineers.
    view abstract10.1061/9780784412992.067
  • An edge-based imbricate finite element method (EI-FEM) with full and reduced integration
    Cazes, F. and Meschke, G.
    Computers and Structures 106-107 (2012)
    An Edge-based Imbricate Finite Element Method (EI-FEM), aiming at combining the ease of mesh generation using triangular elements and the improved accuracy of quadrilateral elements, is proposed. Two finite elements for 2D analyses that imbricate each other while being assembled are designed based upon the triangulation of the domain. By means of two numerical benchmark analyses, we show that similar improvements with respect to numerical accuracy as obtained by the recently proposed Edge-based Smoothed Finite Element Method (ES-FEM) can be obtained, without sacrificing the standard concepts of the Finite Element Method. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.compstruc.2012.04.011
  • Computational simulation of mechanized tunneling as part of an integrated decision support platform
    Meschke, G. and Nagel, F. and Stascheit, J.
    International Journal of Geomechanics 11 (2012)
    Mechanized tunneling is characterized by a staged procedure of excavation and lining erection and continuous support of the soil by means of supporting fluids (or compressed air) at the tunnel face and pressurized grouting of the tail gap. The interactions between the tunnel boring machine (TBM), the support measures, and the soil, including the groundwater, determine the efficiency, safety, and effects on the existing infrastructure. In this paper, a process-oriented numerical simulation model for mechanized tunneling and its integration in the context of an integrated optimization platform for tunneling (IOPT) is addressed. The simulation model is based upon the finite-element method and considers the transient excavation process and all relevant components, support measures, and processes, along with their interactions during tunnel advance. In particular, the model allows the investigation of the effects of drilling and stand-still periods upon the generation of a filter cake at the tunnel face. This is demonstrated by the numerical analysis of a straight tunnel advance by means of a hydroshield machine in water-saturated soft soil. © 2011 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)GM.1943-5622.0000044
  • Lamina cribrosa thickening in early glaucoma predicted by a microstructure motivated growth and remodeling approach
    Grytz, R. and Sigal, I.A. and Ruberti, J.W. and Meschke, G. and Crawford Downs, J.
    Mechanics of Materials 44 (2012)
    Glaucoma is among the leading causes of blindness worldwide. The ocular disease is characterized by irreversible damage of the retinal ganglion cell axons at the level of the lamina cribrosa (LC). The LC is a porous, connective tissue structure whose function is believed to provide mechanical support to the axons as they exit the eye on their path from the retina to the brain. Early experimental glaucoma studies have shown that the LC remodels into a thicker, more posterior structure which incorporates more connective tissue after intraocular pressure (IOP) elevation. The process by which this occurs is unknown. Here we present a microstructure motivated growth and remodeling (G&R) formulation to explore a potential mechanism of these structural changes. We hypothesize that the mechanical strain experienced by the collagen fibrils in the LC stimulates the G&R response at the micro-scale. The proposed G&R algorithm controls collagen fibril synthesis/degradation and adapts the residual strains between collagen fibrils and the surrounding tissue to achieve biomechanical homeostasis. The G&R algorithm was applied to a generic finite element model of the human eye subjected to normal and elevated IOP. The G&R simulation underscores the biomechanical need for a LC at normal IOP. The numerical results suggest that IOP elevation leads to LC thickening due to an increase in collagen fibril mass, which is in good agreement with experimental observations in early glaucoma monkey eyes. This is the first study to demonstrate that a biomechanically-driven G&R mechanism can lead to the LC thickening observed in early experimental glaucoma. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.mechmat.2011.07.004
  • Numerical simulation of interactions between the shield-supported tunnel construction process and the response of soft water-saturated soils
    Nagel, F. and Stascheit, J. and Meschke, G.
    International Journal of Geomechanics 12 (2012)
    During the design and construction of shield-driven tunnels, a reliable analysis of the construction process is required for the prognosis of the process-induced surface settlements, changes in soil stresses, and changes in groundwater conditions, as well as for the determination of the loads acting on the tunnel tube and on the tunnel-boring machine. In this context, numerical simulation methods like the finite-element method allow for a realistic description of the construction process and its impact on the surrounding underground. The investigated problem is governed by the interactions between the tunneling process and the surrounding underground and its constituents-soil grains, groundwater, and pore air. The tunnel-construction process interacts with the surrounding underground via the heading face support, by frictional contact between shield skin and soil, and because of grouting of the annular gap. Considering these interactions, a holistic simulation model is presented for the process-oriented simulation of shield-supported tunnel advance and its interactions with fully saturated, partially saturated, or nonsaturated soft soil. Its applicability is demonstrated by selected simulations of real-scale examples. Parametric studies are performed to investigate the influence of soil conditions and of process parameters on the time-variant settlements and groundwater conditions, showing its capabilities with respect to the simulation of the soil-process interactions in front, above, and behind the tunnel-boring machine. © 2012 American Society of Civil Engineers.
    view abstract10.1061/(ASCE)GM.1943-5622.0000174
  • Grout and bentonite flow around a TBM: Computational modeling and simulation-based assessment of influence on surface settlements
    Nagel, F. and Meschke, G.
    Tunnelling and Underground Space Technology 26 (2011)
    Adequate consideration of the various interactions between the Tunnel Boring Machine (TBM) and the surrounding underground is a pre-requisite for reliable prognoses in shield supported tunneling based upon numerical analysis. In addition to face support and the grouting of the annular gap the contact conditions along the shield skin between the moving TBM and the surrounding, deforming soil constitute the most relevant component of TBM-soil interactions in mechanized tunneling. This paper is concerned with the analysis of the interface conditions between the shield skin and the soil and its adequate numerical representation in the context of a process-oriented numerical simulation model for mechanized tunneling. The situation around the shield skin is influenced by the design of the Tunnel Boring Machine, the deformational behavior of the surrounding underground and by a possible inflow of process liquids into the steering gap. A novel simulation method is proposed which allows to model the viscous flow of the process liquids into the steering gap and its interactions with the face support, the tail void grouting, the deforming soil and the moving TBM. The proposed numerical model for the TBM-soil interaction is part of a recently developed three-dimensional, process-oriented finite element model for shield tunneling (Nagel et al., 2010). It allows to investigate the effects of the inflow of process liquids into the steering gap during TBM advance considering realistic machine-related and geological conditions. It is, in particular, capable to compute the pressure distribution within the developing liquid film in association with the face support and grouting conditions and to predict its influence on the surface settlements and the overall TBM-soil interaction affecting, e.g. the hydraulic jack forces or shield deformations. © 2010 Elsevier Ltd.
    view abstract10.1016/j.tust.2010.12.001
  • The collagen fibril architecture in the lamina cribrosa and peripapillary sclera predicted by a computational remodeling approach
    Grytz, R. and Meschke, G. and Jonas, J.B.
    Biomechanics and Modeling in Mechanobiology 10 (2011)
    The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.
    view abstract10.1007/s10237-010-0240-8
  • 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 (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 abstract10.1007/s10237-009-0173-2
  • A hybrid modeling concept for ultra low cycle fatigue of metallic structures based on micropore damage and unit cell models
    Hommel, J.-H. and Meschke, G.
    International Journal of Fatigue 32 (2010)
    The paper presents a concept for life-time predictions of metallic structures subjected to ultra low cycle fatigue. The proposed hybrid strategy is characterized by a combination of unit cell analyses on a microstructural level and a micropore damage model used for structural analyses on the macroscopic level. To account for the large plastic deformations evolving during cyclic loading, an advanced elasto-plastic model using a Bari-Hassan-type kinematic hardening rule based on a superposition of several kinematic hardening laws according to Armstrong-Frederick is employed. Micromechanically oriented unit cell analyses are used for a calibration of the model parameters of a macroscopic Gurson-type model. Numerical results include the validation of the macroscopic Gurson model based on laboratory test results on steel specimens as well as a prototype application to a life-time prediction of a metallic spherical pressure vessel subjected to earthquake loading. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijfatigue.2010.06.006
  • An elasto-plastic three phase model for partially saturated soil for the finite element simulation of compressed air support in tunnelling
    Nagel, F. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 34 (2010)
    This paper presents a fully coupled finite element formulation for partially saturated soil as a triphasic porous material, which has been developed for the simulation of shield tunnelling with heading face support using compressed air. While for many numerical simulations in geotechnics use of a two-phase soil model is sufficient, the simulation of compressed air support demands the use of a three-phase model with the consideration of air as a separate phase. A multiphase model for soft soils is developed, in which the individual constituents of the soil-the soil skeleton, the fluid and the gaseous phase-and their interactions are considered. The triphasic model is formulated within the framework of the theory of porous media, based upon balance equations and constitutive relations for the soil constituents and their mixture. An elasto-plastic, cam-clay type model is extended to partially saturated soil conditions by incorporating capillary pressure according to the Barcelona basic model. The hydraulic properties of the soil are described via DARCY's law and the soil-water characteristic curve after VAN GENUCHTEN. Water is modelled as an incompressible and air as a compressible phase. The model is validated by means of selected benchmark problems. The applicability of the model to geotechnical problems is demonstrated by results from the simulation of a compressed air intervention in shield tunnelling. © 2009 John Wiley & Sons, Ltd.
    view abstract10.1002/nag.828
  • continuum mechanics

  • fracture

  • mechanized tunneling

  • micromechanics

  • numerical methods

  • porous materials

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