With advanced deformation models, typical material phenomena such as strain hardening, creep, relaxation and strain rate effects can be described in such a way that microstructural changes and their effects on the mechanics can also be taken into consideration. As these changes in the material can occur on different time scales, their integration into the models takes place in a differentiated way. The formation of dense networks of secondary carbides and the associated cyclical strain hardening is already evident in austenitic cast steel 1.4849, for example, at moderate temperatures in the range of 600°C after a few minutes to a few hours in low-cycle strain tests. This microstructural change can usually be modeled very well over the cyclically cumulative plasticity.
For application related temperatures, the microstructural change process for the aluminum alloy EN-AW2618A plays out over a significantly longer time scale. The maturation of the S-phase which provides strength is essentially driven by temperature and time, so that model parameters for any points in the lifetime can be determined on the basis of a precalculated microstructure and the model prediction scaled accordingly.
Materials for which the macroscopic stress response depends significantly on the load direction include cast-iron materials. Gray cast-iron materials, in particular, display what is referred to as tension compression asymmetry even for loads in the low cycle fatigue range because of their plate shaped graphite structure. This mechanical material effect can be modeled very well through phenomenological extensions of the model equations.
Using mechanism-based damage models, crack propagation under fatigue and creep fatigue loading can be described. The models take into account the material characteristics that change with temperature and thus allow lifetime assessment, including under thermomechanical load.
The starting point for the damage models is the description derived from fatigue mechanics for ductile crack propagation. If the mechanism changes or the complexity of the sequence of loads increases, the basic equations can be extended in relation to material and load. For example, AlSi alloys include brittle phases which can break because of the high matrix strengths at low temperatures under low cyclical fatigue loads; the brittle fracture mechanism overlaps with ductile crack propagation. High strength materials such as nickel-based alloys and austenitic steel show a dependence on the ambient medium. Overlapping damage due to oxidation can be taken into account in accordance with oxygen partial pressure, temperature and load. Complex load scenarios can be dealt with in a similar way. If, for example, in the case of a thermomechanical fatigue load, overlapping thermal fluctuations or mechanical (high frequency) loads occur, these can be integrated into the model by means of corresponding extensions. In this context, not only the current load state, but also the load history can be taken into account for prediction of the current crack propagation.
Fraunhofer Institute for Mechanics of Materials IWM