Study on Micro-mechanisms of Fatigue Crack Initiation and Propagation

通过直接观察和断口学研究，探讨了3%晶粒取向硅铁疲劳裂纹萌生和扩展的微观机制。在选择垂直于材料滚动方向的拉伸加载轴进行疲劳裂纹扩展试验时，发现II型裂纹沿滑移方向的连续扩展被限制在2 mm左右的极短长度内，并且似乎存在单滑移系统运行阻碍裂纹扩展的机制。在平行于轧制方向拉伸加载的试样中，虽然存在面外滑移分量，且断裂延展性大，屈服强度低，但几乎没有观察到III型裂纹的推进，裂纹以脆性条纹形成机制扩展。在场发射型扫描电镜直接观察疲劳裂纹生长过程中，二次电子信号通过高速DMA型a /D转换器直接输入微机。借助于图像处理技术，可以测量裂纹尖端张开位移（CTOD）、裂纹尖端附近变形和裂纹扩展速率，并讨论了与宏观参数相关的微观机制。结果表明，在变幅载荷作用下，CTOD仍是裂纹扩展的控制参数。而在瞬态变荷载作用下，由于裂纹的扩展方向和模式发生了变化，使得CTOD等细观参数与宏观参数的相关性不是很好。另一方面，在稳态变荷载作用下，CTOD与有效应力强度因子等宏观参数相关。研究了空气和真空中裂纹扩展速率的差异，并讨论了晶粒取向和晶界对裂纹扩展行为的影响。

Microscopic mechanisms of fatigue crack initiation and propagation were investigated through direct observation and fractographical studies on a grain oriented 3 % silicon iron. In the case of fatigue crack growth tests where a tensile loading axis was chosen perpendicular to rolling direction of material, continuous crack advance of Mode II along the slip direction was found to be limited in the very short length of about 2 mm, and there seemed exist a mechanism to obstruct crack increment by operation of single slipping system. In the specimen subjected to a tensile loading parallel to rolling direction, although there exist slip components of out-of-plane, and large fracture ductility and low yield strength, Mode III crack advance was hardly observed and crack was found to grow in a brittle striation formation mechanism.During direct observations of growing fatigue cracks by field emission type scanning electron microscope, secondary electron signals were taken directly into the micro-computer by using a high speed DMA type A/D converter. With the aid of image processing technique crack tip opening displacement (CTOD), deformation near crack tip and crack growth rate could be measured and microscopoc mechanisms were discussed related with macroscopic parameters. It was found that the CTOD is the governing parameter of fatigue crack growth even under variable amplitude loadings. Under the transient varying loadings, however, since crack changed growth direction and mode, correlation of microscopic parameter such as CTOD to macroscopic parameter was not found good. On the other hand, under the stationary varying loadings CTOD was found to correlate to the macroscopic parameter such as the effective stress intensity factor. Moreover, in this study, the difference of crack growth rates between in air and in vacuum was investigated, and the effects of grain orientation and grain boundary on crack growth behaviors were discussed.