An Effect of Load Variation on High-cycle Fatigue Behavior and Fatigue Life

近年来的一些疲劳研究表明，在超高周（nbbb10 ^7）或千兆周疲劳状态下，即使在低于常规疲劳极限的应力幅下也会发生疲劳破坏，并且S-N曲线根据破坏机制显示出多阶段的形状，即由试样表面或内部缺陷引起的破坏。通过旋转弯曲试验获得的高碳铬钢S-N曲线表明，内部缺陷诱发断裂（鱼眼型断裂）的疲劳数据与表面滑移诱发断裂的疲劳数据不同。结果表明，S-N曲线由两条直线组成，一条直线对应的是由于表面断裂导致的较短寿命，即疲劳极限；另一条直线对应的是由于内部夹杂物断裂导致的超长寿命。在高水平应力高于常规疲劳极限、低水平应力低于常规疲劳极限的条件下，进行了两步重复加载试验。基于Miner规则的累积疲劳损伤值D，在低水平应力不导致疲劳损伤的情况下，鱼眼型断裂的累积疲劳损伤值大于1，说明常规S-N曲线对疲劳损伤的估计过高，而表面断裂的累积疲劳损伤值较小。结果一致表明，应力低于疲劳极限时可能产生疲劳损伤。本文讨论了基于修正Miner规则的疲劳损伤，通过外推低于表面断裂疲劳极限的S-N曲线或使用鱼眼型断裂数据得到的D值，表面断裂的D值较为保守，而鱼眼型断裂的D值较小，说明高强度应力对内部夹杂物裂纹起裂的影响随着载荷的变化而变得更加不利。在循环频率为1kHz的条件下，对Cr-Mo钢进行推拉式疲劳试验，得到S-N曲线。在这种情况下，没有观察到鱼眼型断裂，S-N曲线显示了超过10^8次循环区域的疲劳极限。通过改变低应力水平，重复进行两步试验。低水平应力引起的疲劳损伤得到了清晰的识别，基于Miner规则的累积损伤值趋于不一致。恒幅试验发现修正S-N曲线位于S-N曲线下方，并扩展到远低于疲劳极限的应力水平。少

In recent years several fatigue studies have shown that fatigue failures can occur even at the stress amplitude below the conventional fatigue limit in the ultrahigh cycle ( N > 10^7) or gigacycle fatigue regime, and S-N curves indicate a multi stage shape depending on the failure mechanisms, i.e. failure originated from specimen surface or internal defect.S-N curves in a high carbon chromium steel obtained by rotating bending test revealed that the fatigue data for the internal defect induced fracture (fish eye type fracture) were different from those for surface slip induced fracture. It was found that the S-N curve consisted of two straight lines, one corresponding to shorter life due to fracture from surface, which shows the fatigue limit, and another to ultra-long life due to fracture from internal inclusions. Repeated two step loading tests were conducted on the condition that the high level stress is above the conventional fatigue limit and the low level stress below the fatigue … More limit obtained form the data for fracture from surface. Cumulative fatigue damage values based on the Miner rule, D, where the low level stress does not result in fatigue damage, represent the values larger than unity for fish-eye type fracture indicating the conventional S-N curve give an over estimation of fatigue damage, while that for surface fracture gives a smaller value .than unity indicating that the stress below fatigue limit may give a fatigue damage. Fatigue damages based on the modified Miner rule, D, which was obtained by extrapolating the S-N curve below the fatigue limit for surface fracture or by using the data for fish-eye type fracture are discussed, D for surface fracture shows a conservative value, but D for fish-eye type fracture shows fairly small values, suggesting that the effect of the high level stress on the crack initiation from internal inclusion becomes more detrimental by the load variation.S-N curves obtained by a push-pull type fatigue test in Cr-Mo steel, which were conducted with the cycle rate at 1kHz. In this case no fish-eye type fracture was observed and S-N curve showed the fatigue limit in the region over 10^8 cycles. Repeated two step tests were conducted by changing the low stress level. Fatigue damage due to the lower level stress was clearly recognized and cumulative damage values based on the Miner rule become less than unity. Modified S-N curve was found to lie below the S-N curve for constant amplitude test and to be extended to the stress level far below the fatigue limit. Less