SEM study of overload effects during fatigue crack growth using an image analysing technique and potential drop measures

To study the mechanisms affecting the crack propagation rate for fatigue cracks exposed to an overload, an in situ scanning electron microscope technique was used, together with potential drop measurements. High-resolution images were analysed with an image analysis program to measure the displacements along the crack, and the potential drop technique was employed to measure the electrical contact between the fatigue crack surfaces. The crack closure level could, by image analysis, be determined as close as 1 mu m from the crack tip. The indications from the image analysis pointed towards a somewhat lower closure load as compared to the potential drop technique. The effect of an overload on the crack propagation rate was found to depend on the magnitude of the overload in combination with the steady-state conditions. Both overload induced crack retardation and crack acceleration was noticed to occur.