Evaluation of Crack Propagation Criterion and Sensitivity Analysis in Middle-Crack Tension Specimens under Extremely Low Cycle Fatigue

Fatigue life assessment under ultra-low cycle fatigue conditions remains under investigation due to the effects of cyclic large plastic deformation and constraint conditions, which limit the applicability of conventional fracture parameters such as ΔK, ΔJ, Weibull stress and CTOD. To address this issue, previous studies proposed a crack propagation criterion using stress triaxiality and equivalent plastic strain increment and validated it using compact tension (CT) specimens made of SUS316 stainless steel. However, it is challenging to obtain stress triaxiality and equivalent plastic strain increment directly from experiments. In this study, the applicability of the proposed criterion was evaluated for middle-crack tension (MT) specimens made of A2024-T3 aluminum alloy. Crack growth experiments were conducted on MT specimens with a stress ratio of R = −0.5, and generation phase analysis was performed to reproduce the experimental behavior and extract stress triaxiality and equivalent plastic strain increment. Furthermore, a sensitivity analysis was conducted to assess the influence of crack length and hardening model on the analysis results, with the aim of improving the accuracy and reliability of the simulation. The results indicate that the proposed criterion accurately captures crack propagation behavior at the specimen’s center of thickness. We would like to discuss the applicability of the crack propagation conditional equation.