From the mechanistic modelling of fatigue crack formation to the complexity of component fatigue life assessment
Esteban Busso, Imperial College London, London, UK
The development and early propagation of fatigue cracks can be considered in terms of the basic damage mechanisms and the relative size of the crack with respect to applicable microstructural feature(s). In this work, a brief overview is first presented of mesoscopic / grain scale approaches used to study the fatigue crack formation and early growth in the micro-structurally small crack (MSC) growth regime , where fatigue crack growth rates are strongly dependent on microstructural features such as grain size, second phase particle size and spacing, etc. Particular attention will be given to approaches which rely on a correlation of fatigue indicator parameters (FIPs) with measured locations of fatigue crack formation within polycrystals, using representative crystal plasticity based finite element models of the microstructure. It will also be shown how various FIPs have increasingly been used as mesoscopic indicators to relate to processes of fatigue crack growth of MSCs.
In the second part of this presentation, alternative macroscopic fatigue formulations required when considering complex conditions of high temperatures, environmental effects and thermomechanical loadings seen by, e.g., discs and blades in jet engines, will be discussed. Such macroscopic formulations are based upon fundamental physical processes, and generally rely on a scalar measure of damage to account for complex loading conditions. A representative example will be given in the form of a recently proposed time-dependent (or incremental) fatigue damage formulation developed for single-crystal nickel-based superalloys . It accounts for the damage associated with microcracks lying on planes parallel to those of the crystallographic slip systems, and for the effect of oxidation on microcrack closure. A simplified approach is relied upon to enable an efficient numerical integration of the single crystal formulation within a cycle, and an extrapolating method to deal with periodic fatigue loading conditions. The predictive capability of the formulation under local multiaxial mechanical fields is illustrated on perforated specimens of the commercial superalloy AM1 subject to cyclic torsional loads at 950°C.
 Pineau, A., McDowell, D.L., Busso, E.P. and Antolovich, S., “Failure of Metals II:
Fatigue”, invited Overview Paper, Acta Materialia, V. 117 (2016), 484-507.
 Kaminski, M., Kanouté, P., Kruch, S. Busso, E.P. and Chaboche, J-L. “A high temperature fatigue damage model for single crystal superalloys”, Materials at High Temperatures, Vol. 33, Issue 4-5, (2016), 412-424.