Study on Fatigue Crack Growth Mechanism under Service Loadings with the Aid of Direct Observations by Scanning Electron Microscope

为了能够记录使用载荷作用下疲劳裂纹的扫描电镜图像，利用高速直接存储器存取（DMA）型a /D转换器将扫描电镜的二次电子信号直接输入到微机中。在0.1-1 Hz的循环载荷下，连续5帧的扫描电镜图像可以在一次操作中存储，并开发了图像处理技术。采用Hi-Lo两步加载和重复两步变幅加载对3%硅铁进行了疲劳裂纹扩展试验，研究了裂纹扩展方向、裂纹尖端变形、裂纹扩展速率、裂纹尖端张开位移（CTOD）和裂纹闭合等细观和宏观行为，阐明了疲劳裂纹扩展机制。结果表明，CTOD是两种载荷下疲劳裂纹扩展的控制参数。而在原加载情况下，载荷变化后疲劳裂纹呈“之”字形扩展，即使用有效应力强度因子keff也不能解释裂纹扩展的迟滞行为。另一方面，在反复变幅加载下，CTOD与Keff之间的关系与恒幅加载相似。因此，可以用有效应力强度范围估计稳态变载荷条件下的疲劳裂纹扩展速率。此外，通过断口学研究，探讨了变幅载荷作用下后屈服区弹塑性疲劳裂纹扩展机理和扩展速率的估计方法。

In order to make it possible to record the scanning electron microscope(SEM) images of fatigue crack under service loadings, secondary electron signals of the SEM were taken directly into the micro-computer by using a high speed direct memory access(DMA) type A/D converter. Sequential five frames of SEM images under cyclic loading of 0.1-1 Hz could be stored in a single operation and image processing techniques have been developed.Fatigue crack growth tests under Hi-Lo two step loadings and repeated two step variable amplitude loadings were performed on a grain opiented 3 % silicon iron, and microscopic and macroscopic behaviors of fatigue crack such as crack growth direction, deformation near crack tip, crack growth rate, crack tip opening displacement(CTOD) and crack closure were investigated to elucidate the fatigue crack growth mechanisms. It was found that the CTOD is the governing parameter of fatigue crack growth under both loading conditions. In the case of former loading, however, fatigue crack was found to grow in a zig-zag manner after load changing, and retardation behavior of crack growth could not be explained even by the effective stress intensity facter, keff. On the other hand, under the repeated variable amplitude loadings, the relationship between CTOD and Keff was found to be similar with that of constant amplitude loading. Therefore, it was conformed that the fatigue crack growth rate under stationary varying loading conditions could be estimated in terms of the effective stress intensity range. In addition, growth mechanisms and estimation method of growth rate of elastic-plastic fatigue crack under variable amplitude loadings in the post-yield region were discussed through the fractographical investigations.