The elastic field around the crack tip measured by scanning electromicroscopy

Abstract

A whole field method was developed for the evaluation of the relative displacements and strains around the tip of a cracked plate loaded elastically, but using measurements with a scanning electron microscope (SEM). The radial displacements were measured at data points defined by the positions of fine grains of a powder layer sprayed on the top of an aluminium coating. By using the possibilities of the SEM for deep focussing, it was possible to develop a versatile method, whose sensitivity is increased with large magnifications of the instrument. This fact allowed the use of the method with the same accuracy and sensitivity not only very close to the crack tip, where strong strain gradients exist but also far away from it, where moderate strain gradients develop. The method is versatile and highly accurate everywhere in the stress field around the crack, since it defines, in advance, the number of terms in the Williams series development of the expressions for displacements and strains. The method is convenient not only for elastic, but also for plastic strain fields, provided the appropriate relationships are introduced in the evaluation of strains.