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

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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.