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)

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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.