A state-based fatigue crack growth model considering the interaction effects under variable amplitude loading

Many testing observations have revealed that the plasticity plays a significant role in fatigue crack propagation. In this study, the plasticity variation ahead of the crack tip is measure by our previously proposed in-situ microscopy experiment. With these testing results, a crack closure model based on the near-tip plastic state is modified. Furthermore, a state-based fatigue crack growth formulation in integral form including the above modified crack closure model is developed. In this model, the time-based fatigue crack growth is considered as a continuous process, which is determined by the state left by the previous load sequence. This approach can evaluate the multi-scale fatigue crack growth behavior. Since the crack propagation during the loading part of a cycle can be calculated, the crack growth under arbitrary tension-tension variable amplitude loading can be precisely estimated. Extensive experimental data of different metallic materials under the various loading spectra are used to validate the proposed model. Good agreements are observed. And some special variable amplitude loading cases are discussed.