Fatigue limits and crack growth thresholds in cyclic tension and bending of a stainless steel sheet

Many parts made of thin steel sheet are subjected to two types of loads: tension and bending. In this study, we define and suggest novel fatigue failure criteria based on the outcomes of testing specimens with various notch lengths under these two loading modes at constant and different stress ratios. Large cyclic plastic strains occurred near the notch, which were taken into account in the evaluation of the threshold stress intensity range OKth. The fatigue limit under tension load was lower than that under plane bending load, and it had an inflection point on the relationship line with notch length. In mode I fracture, OKth became a constant value at the notch length corresponding to the boundary between short and long cracks, but no such inflection point was observed in mode IIII fracture. A calculation model for the critical stress intensity range OKthc, which is a constant value regardless of the length of the notch or crack, was presented. Using the strength factor to quantify the effect of stress ratio, the change in OKth was evaluated. A prediction model for the effective stress intensity range that takes into account the plasticity-induced crack closure effect was also presented.