Optical measurement of the plastic zone size in a notched metal specimen subjected to low-cycle fatigue

This paper describes an optical method used for the determination of the plastic zone size generated on the surface of a notched metal specimen under conditions of low-cycle fatigue. The proposed optical method uses white-light scattering generated by plastically induced surface roughness on the specimen polished surface. By digital processing two images of the specimen surface, one recorded before and the other after the generation of the fatigue deformation, it is possible to evaluate the accumulated plastic damage. The plastic zone measured in the crack initiation stage is compared with the results obtained by a recrystallization technique.