Coupling between mesoplasticity and damage in high-cycle fatigue

The multiaxial fatigue loading in the high-cycle regime leads to localized mesoscopic plastic strain that occurs in some preferential directions of individual grains for most metallic materials. Crack initiation modeling is difficult in this fatigue regime because the scale where the mechanisms operate is not the engineering scale (macroscopic scale), and local plasticity and damage act simultaneously. This article describes a damage model based on the interaction between mesoplasticity and local damage for the infinite and the finite fatigue life regimes. Several salient effects are accounted for via a simple localization rule, which connects the macroscopic scale with the mesoscopic one, and by the model presented here, which describes the coupled effects of mesoplasticity and damage growth. Irreversible thermodynamics concepts with internal state variables are used to maintain a balance between extensive descriptions of plastic flow and damage events. Cyclic hardening behavior is described by a combined isotropic and kinematic hardening rule while the damage evolution is governed notably by the accumulated plastic mesoscopic strain. In this study, predictions are compared to fatigue tests performed on a mild steel (C36) under different loading modes. All the experiments are carried out under in-phase loading conditions: reversed tension, torsion, and combined tension-torsion. The mean stress effect is also studied through tests conducted under tension. The predicted Wohler curves under any loading mode can be readily obtained with this model, but the main feature of this approach is to ensure a clear link between the mesoscopic parameters like the hardening behavior of individual grains and the subsequent local damage.