Study on Elastic-Plastic Fatigue Crack Growth under Service Loading Conditions

工程结构在使用过程中经常会受到超过材料屈服条件的大振幅载荷作用。因此，为了保证结构在损伤容限设计中的安全性，了解一种在变载荷条件下的弹塑性疲劳裂纹扩展速率的估算方法变得非常重要。从这一点来看，在这项研究中，载荷控制疲劳裂纹扩展试验进行了几种结构材料下的恒定和变幅载荷在很宽的范围内，从线弹性区域的屈服后，和断裂力学参数的疲劳裂纹扩展速率进行了调查。用卸载弹性柔度法监测了裂纹扩展增量和裂纹闭合行为，发现有效应力强度范围<DELTA>Keff不仅在线弹性区，而且在极限条件下的大范围屈服区也是控制裂纹扩展速率的参数。而在屈服后区域，循环塑性变形和疲劳诱导单向变形的发生与材料、试样形状和应力比有关，并影响裂纹扩展行为。结果表明，这些影响可以用循环J积分<DELTA>J和最大J积分Jmax来解释，而弹塑性疲劳裂纹扩展速率可以用<DELTA>J/E（1-Jmax/C）的幂律来表示，与材料、试样形状和试验条件无关。由模拟多种使用载荷的两步重复加载试验结果发现，利用上述断裂力学参数，基于裂纹扩展的线性累积规律，可以很好地估算不同载荷条件下的弹塑性疲劳裂纹扩展速率。

Engineering structures are often subjected to large amplitude loads in service which exceed the yield conditions of materials. Therefore, it becomes important to know an estimation method of elastic-plastic fatigue crack growth rates under varying loading conditions, in order to secure the safety of structures in damage tolerant design. From this point of view, in this study, load controlled fatigue crack growth tests were carried out on several kinds of structural materials under constant and variable amplitude loads over a wide range from the linear elastic region to the post-yield one, and fracture mechanics parameters which governed the fatigue crack growth rates were investigated. Crack growth increment and crack closure behavior were monitored through fatigue tests by the minicomputer-aided unloading elastic compliance method.The effective stress intensity range, <DELTA> Keff, was found to be a governing parameter of fatigue crack growth rate even in the large scale yielding region within the limit conditions as well as in the linear elastic region. In the post-yield region, however, cyclic plastic deformation and fatigue induced one-directional deformation took place remarkably depending on materials, specimen configurations and stress ratios, and affected crack growth behaviors. It was found that these effects could be explained by considering the cyclic J-integral, <DELTA> J, and the maximum J-integral, Jmax, and the elastic-plastic fatigue crack growth rate could be expressted by the power law of <DELTA> J/E(1-Jmax/C) irrespective of materials, specimen configurations and test conditions. From the test results with repeated two-step loadings simulating several kinds of service loadings, it was found that the elastic-plastic fatigue crack growth rate under varying loading conditions could be well estimated based on the linear accumulation law of crack growth using the above mentioned fracture mechanics parameter.