Fatigue crack propagation under various kind of loading

通过建立Dugdale模型，建立了疲劳裂纹开闭现象的仿真模型。结果表明，模拟的再拉伸塑性区生成载荷与实验值吻合较好。通过再拉伸塑性区出现时间对应的有效应力强度因子范围DELTAKAPPA_<RP>，可以定量评价最大载荷减小后对疲劳裂纹扩展的延迟延迟效应，疲劳裂纹扩展的停止条件为DELTAKAPPA_<RP>*0。模型还证实了塑性区尺寸omega受疲劳损伤与DELTAKAPPA_<RP>的2次幂成正比。利用ω可以定量评价残余应力分布场和平均载荷变化的“风暴模型”载荷下的疲劳裂纹扩展曲线。然而，当接收到很大的尖峰荷载时，会产生一个问题，被评估为不安全侧。在疲劳萌生初期，表面小裂纹相继萌生并重复合并。因此，各表面裂纹的纵横比具有较大的散点。而在融合完成之前，最深处K值与附近表面裂纹干涉作用导致的K值增加相同，且最深处表面裂纹的深度相同的具有代表性的表面裂纹，其长径比与深度呈线性关系。此外，还介绍了用串联弹簧模型将表面裂纹建模为二维问题的方法。

A Simulation model of fatigue crack opening and closing phenomena is introduced by developing Dugdale model.It is confirmed that the simulated RPG load (Re-tensile plastic zone Generated load) is in good agreement with experimental one.It becomes clear that a delayd retardation effect on fatigue crack propagation just after decreased maximum load can be also quantitatively assessed by effective stress intensity factor range DELTAKAPPA_<RP> corresponding to the period in which the re-tensile plastic zone appears, and stopping condition of a fatigue crack propagation is that DELTAKAPPA_<RP>*0.It is also confirmed from the model that plastic zone size omega received fatigue damage is proportional to a two power of DELTAKAPPA_<RP>. It is reported that a fatigue crack propagation curve in residual stress distribution field and under "storm model" loading with changing mean load can be quantitatively assessed by using omega. However a problem occures in case of receiving very large spike load, which is assessed to unsafe side.In the early stage of fatigue initiation, small surface cracks emanate and repeat coalescence one after another. Therefore aspect ratio of each surface crack has a large scatter. However the aspect ratio of a flctitious representative surface crack of which K value at the deepest point is the same as an increased K value due to interference effect of vicinity surface cracks and the depth is the same of the same of the deepest surface crack among surface cracks changes to be linear to the depth before the completion of coalesence. Moreover the method of a surface crack to be modeled into two dimensional problem by series springs model is also introduced.