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

扫描电镜直接观察研究工作载荷下疲劳裂纹扩展机制

基本信息

批准号：
62550073
负责人：
JONO Masahiro
金额：
$ 1.28万
依托单位：
Osaka University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1987
资助国家：
日本
起止时间：
1987 至 1988
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-62550073/
关键词：
Fatigue Crack Service Loading Scanning Electron Microscope Direct Observation Fatigue Crack Growth Mechanism Crack Closure Crack Tip Opening Displacement 画像処理き裂開閉口挙動動変動荷重

项目摘要

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.

为了记录疲劳裂纹在工作载荷下的扫描电镜图像，采用高速直接存储器（DMA）式A/D转换器，将扫描电镜的二次电子信号直接送入微机。利用扫描电子显微镜（SEM）对0.1 ~ 1Hz循环载荷下的5帧连续图像进行了一次存储，并开发了图像处理技术，对含3%硅铁的试样进行了Hi-Lo两级加载和两级变幅重复加载的疲劳裂纹扩展试验，研究了疲劳裂纹扩展方向、裂纹尖端变形、通过对裂纹扩展速率、裂纹尖端张开位移（CTOD）和裂纹闭合的研究，阐明了疲劳裂纹扩展机理。结果表明，CTOD是两种加载条件下疲劳裂纹扩展的控制参数。而在前加载情况下，载荷变化后，疲劳裂纹呈锯齿形扩展，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，在反复变幅载荷下，CTOD和Keff之间的关系被发现是类似的恒幅载荷。因此，可以用有效应力强度范围来估计定常变载荷条件下的疲劳裂纹扩展速率。通过断口分析，探讨了变幅载荷下屈服后区弹塑性疲劳裂纹的扩展机制和扩展速率的估算方法。