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

为了能够记录疲劳裂纹在服役载荷下的扫描电子显微镜图像，利用高速直接存储器存取(DMA)型A/D转换器将扫描电子显微镜的二次电子信号直接送入微型计算机。一次可存储0.1-1 Hz循环载荷下连续5帧扫描电子显微镜图像，并开发了图像处理技术。对一种含3%硅铁的颗粒进行了两级高强度加载和重复两级变幅加载下的疲劳裂纹扩展试验，研究了疲劳裂纹的扩展方向、裂纹尖端变形、裂纹扩展速率、裂纹尖端张开位移(CTOD)和裂纹闭合等微观和宏观行为，以阐明疲劳裂纹扩展机理。结果表明，在两种加载条件下，CTOD是疲劳裂纹扩展的控制参数。而在原载荷作用下，疲劳裂纹在载荷变化后以之字形扩展，即使用有效应力强度因子keff也不能解释裂纹扩展的滞后行为。另一方面，在重复的变幅载荷下，CTOD与Kef的关系与恒幅载荷的关系相似。因此，可以用有效应力强度范围来估算静态变化载荷条件下的疲劳裂纹扩展速率。此外，通过断口分析，探讨了变幅载荷作用下弹塑性疲劳裂纹在屈服区的扩展机理和扩展速率的估算方法。

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.