Fatigue Crack Growth Behavior Of CT Specimen Under High-low Sequence Loading

Many mechanical components are subjected to the variable amplitude loading sequence. The fatigue fracture is one of the most common failure forms. Therefore, it is of great significance to investigate the delayed retardation of fatigue crack growth under variable amplitude loading sequence.The fatigue crack growth behavior under variable amplitude loading sequence (with the same stress ratios or with the identical maximum load) was investigated numerically by modeling the compact tension specimens of16MnR steel. The Armstrong-Frederick type cyclic plasticity model proposed by Jiang and Sehitoglu, which was implemented into Abaqus by the user subroutine UMAT, was used to capture the cyclic plasticity characterization of16MnR steel. The contact elements were used to mimic the potential contact of the crack surfaces, and the dynamic crack propagation model was used to consider the effect of loading history. Based on the numerical results of elastic-plastic stress-strain and normal displacement of crack surface, the detailed evolution processes of the stabilized stress-strain hysteresis loops, fatigue crack closure, cyclic plasticity zone and accumulated fatigue damage were obtained, which were then used to comparatively analyze the interactive influence of loading history, crack closure and cyclic plasticity zone on the fatigue crack growth behavior. Under the loading case with the same stress ratios, the discontinuous "gap closure" status was observed during the delayed retardation period of fatigue crack growth under the lower loading sequence. Finally, based on the unified model of fatigue initiation and crack growth, the retardation behavior of fatigue crack growth under variable amplitude loading sequence was predicted. The numerical results agree well with the experimental data.