Study on the material properties for improving the life estimation of fatigue crack

根据力学环境和材料极限值的比较，对各种断裂现象进行了评价。然而，疲劳强度是使用S-N关系评估的，疲劳裂纹的扩展是使用da/dN-ΔK关系评估的，两者都是仅使用机械环境评估的。众所周知，疲劳裂纹扩展过程中，不仅应力幅和裂纹长度，而且载荷历史和载荷大小等，也有很大的影响。众所周知，当峰值载荷作用于等幅载荷时，即使在等幅载荷恢复之后，也会出现疲劳裂纹的减速扩展。还已知的是，这种现象不能使用目前使用的da/dN-ΔK关系进行评估。近年来，已经揭示了疲劳裂纹在扩展过程中在裂纹尖端反复打开/闭合的现象。我们已经开发了一个系统，测量的滞后回线的裂纹尖端，由于疲劳裂纹尖端的打开/关闭现象而产生的。使用这个系统，我们已经发现了很好的匹配的形状的磁滞回线和疲劳裂纹的传播。在本研究中，我们提出了一种方法，它也可以评估减速的疲劳裂纹的传播速度峰值加载后，通过比较的力学环境和材料的特性。此外，在疲劳裂纹扩展极限中使用的材料的特性中，我们利用了压缩塑性变形之后的伸长。

Various fracture phenomena are evaluated according to a comparison of mechanical environment and the limit values of materials. However, fatigue strength is evaluated using the S-N relation and propagation of fatigue cracks is evaluated using the da/dN-ΔK relation, both being evaluated using only the mechanical environment. It is known that in propagation of fatigue cracks, not only stress range and crack length, but also loading history and load size etc., also significantly influence. It is also well known that when an spike loading acts on constant amplitude loading, even after constant amplitude loading returns, for a while the deceleration propagation of a fatigue crack occurs. It is also known that this phenomenon cannot be evaluated using the da/dN-ΔK relation that is currently used. In recent years, the phenomenon of a fatigue crack opening/closing repeatedly at the crack tip during propagation has been revealed. We have developed a system that measures the hysteresis loop of the crack tip that arises due to the opening/closing phenomenon of the fatigue crack tip. Using this system we have revealed good matching of the shapes of hysteresis loops and the propagation of fatigue cracks.In this research, we have proposed a method whereby it is possible to also evaluate the deceleration in the propagation rate of a fatigue crack after spike loading, through a comparison of the mechanical environment and characteristics of a material. Also, in the characteristics of a material used in the limit of the propagation of a fatigue crack, we utilized the elongation that follows compressive plastic deformation.