Many parts made of thin steel sheet are subjected to two types of loads: tension and bending. In this study, we define and suggestnovel 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 ΔKth. 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, ΔKth 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 ΔKthc, 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 ΔKth was evaluated. A prediction model for the effective stress intensity range that takes into account the plasticity-induced crack closure effect was also presented.

中文翻译：

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不锈钢板循环拉伸和弯曲中的疲劳极限和裂纹扩展阈值


许多由薄钢板制成的零件承受两种类型的载荷：拉伸和弯曲。在本研究中，我们根据在恒定和不同应力比下这两种加载模式下具有不同缺口长度的试样的测试结果，定义并提出了新的疲劳失效准则。缺口附近出现大的循环塑性应变，在评估阈值应力强度范围 ΔKth 时考虑到了这一点。拉伸载荷下的疲劳极限低于平面弯曲载荷下的疲劳极限，且在与缺口长度的关系线上有一个拐点。在 I 型断裂中，ΔKth 在对应于短裂纹和长裂纹边界的缺口长度处变为常数值，但在 IIII 型断裂中未观察到此类拐点。提出了临界应力强度范围 ΔKthc 的计算模型，无论缺口或裂纹的长度如何，ΔKthc 都是一个常数值。使用强度因子量化应力比的影响，评估 ΔKth 的变化。还提出了一个考虑塑性诱导的裂纹闭合效应的有效应力强度范围的预测模型。