Fatigue crack closure for inclined and kinked cracks

Abstract

This paper presents recent work on finite element modelling of plasticity-induced fatigue crack closure occurring in cracks which are inclined to the loading axis, and which may then develop a kink. Such crack deviations occur naturally during crack growth, and improved life prediction models require understanding of the effects of such inclined sections on closure, and of the parameters that influence this response. A full description of the closure response for such cracks, requires insight into both plasticity-induced and roughness-induced closure. The work reported here develops a finite element (FE) model for plasticity-induced closure model, which implicitly considers macroscopic roughness effects through crack inclination and kink development. Some examination of micro-roughness effects is planned for future work. Results from the model were tested against experimental data obtained from 2 mm thick SENT specimens, intended to simulate a state of plane stress, and reasonable agreement found between predicted and observed closure values. The FE model was used to simulate the effects of some fundamental factors affecting fatigue crack closure behaviour for inclined cracks, including the effects of initial angle to the transverse specimen axis, crack path kink angle, friction between crack surfaces, stress or load ratio (R =S _min/S _max), and stress state. Two parameter crack growth modelling (ΔK _eff and K _max) was performed as a function of stress ratio, using nonlinear regression to fit model-based ΔK _eff values to experimentally observed growth rates for a kinked crack. Good agreement was observed, indicating the utility of the FE model.