Scanning electron microscope measurements of crack-opening stress onfatigue cracks exposed to overloads

摘要

An in-situ scanning electron microscope technique was used to investigate the effects of a single overload on fatigue cracks inInconel 718.The displacements at the crack tip during the load cycle were measured from scanning electron microscopeimages, using an image analysing program.The direct current potential drop technique was simultaneously used to produceinformation about the crack opening via the electrical contact between the crack surfaces.During the overload cycle, the compliance curves show large influence from the enlarged plastic zone and the changes instress-strain levels at the crack tip.This affects the opening/closing stresses and the remaining displacements.Also a variationin the deformation field perpendicular to the load direction was detected.The potential drop signal showed a higher value forthe crack tip opening/closing stresses, indicating that the overload increases the electrical contact between the crack surfacesduring reloading.This signals a misfit between the crack surfaces caused by the overload.The misfit effect decreasescontinuously for each cycle following the overload cycle.