In-situ observation rig using a scanning laser microscope and its application to the COD measurement of fatigue crack (Unifying the measure of severity controlling the crack growth rate in small and large cracks)

对于工程评估的完整性与表面裂纹的组成部分，它是有用的，知道的严重程度的措施，始终控制裂纹扩展速率的情况下，广泛的裂纹长度和各种加载conditions.In这项研究中，疲劳裂纹扩展试验进行和裂纹张开位移范围（DELTACOD的）沿着裂纹附近的裂纹尖端进行了测量。相同裂纹扩展速率下的DELTACOD分布几乎相同，与裂纹长度、外加应力水平和加载方式无关。因此，裂纹尖端附近的Δ COD分布可以作为控制裂纹扩展速率的严重程度的量度，而弹塑性有限元方法表明，裂纹尖端的最大塑性应变可以作为严重程度的量度。裂纹尖端相同的最大塑性应变保证了相同的应变和裂纹张开度分布。因此，裂纹张开度分布也是衡量裂纹严重程度的一个尺度。使用扫描激光显微镜的原位观察装置对于研究裂纹张开行为是有用的。在空气中进行疲劳测试时，可以获得实时高倍率图像（高达 *3500）。

For engineering assessments of the integrity for component with a surface crack, it is useful to know the measure of severity controlling consistently the crack growth rate in the case of wide range of crack lengths and various loading conditions.In this research, fatigue crack growth tests were carried out and crack opening desplacement ranges (DELTACOD's) along a crack were measured in the neighborhood of the crack tip. The DELTACOD distributions under the same crack growth rate are almost the same, independent of crack length, applied stress level and manner of loading. Therefore, the DELTACOD distribution near a crack tip can be considered as a measure of severity controlling the crack growth rate.On the other hand, a elastic-plastic finite element approach shows that the maximum plastic strain at a crack tip is considered as a measure of severity. The same maximum plastic strain at a crack tip guarantees the same strain and crack opening distribution. Therefore the crack opening distribution is also a measure of severity.In-situ observation rig using a scanning laser microscope is useful for investigating crack opening behavior. Realtime high magnification image (up to*3500) could be obtained during fatigue tests in air.