Study on Micro-mechanisms of Fatigue Crack Initiation and Propagation

点击翻译按钮获取中文摘要

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.