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)

为了对表面裂纹构件的完整性进行工程评估，了解在宽裂纹长度范围和各种加载条件下一致控制裂纹扩展速率的严重程度的措施是有用的。在本研究中，进行了疲劳裂纹扩展试验并测量了裂纹尖端附近的裂纹张开位移范围。在相同的裂纹扩展速率下，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DELTACOD分布可以被认为是控制裂纹扩展速率的严重程度的量度。另一方面，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分布。因此，裂纹张开的分布也是衡量裂纹严重程度的一个指标。利用扫描激光显微镜的现场观察台对裂纹张开行为进行研究是非常有用的。在空气中进行疲劳试验时，可获得高达3500倍的实时高倍率图像。

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.