Fatigue: SEM based studies of crack initiation and early growth

This paper reports on the use of an SEM based diffraction method in the study of some fatigue problems. The electron channelling contrast imaging (ECCI) method has allowed us to image dislocation substructures in bulk samples in the SEM. Some advantages afforded by ECCI are (i) large areas can be observed, (ii) repeated intermittent observations of the same area can be made, and (iii) sample containing cracks are readily studied. We have used the method to follow the evolution of matrix vein and persistent slip band ladder structures during cyclic deformation of copper single crystal samples. Examples of direct imaging of dislocation configurations at the tips of stage I and stage II cracks will be given