Full-field evaluation of the onset of microplasticity in a steel specimen

Abstract

This work deals with the study of the local onset of microplasticity in a steel specimen under tensile test through the use of a full-field strain measurement method at the micrometric scale. The kinematic measurements were performed by the so called grid method. In order to produce the gratings, direct interferential photolithography was used. The gratings were digitized through the use of a white-light interferometric microscope. An adapted image stitching algorithm, applied to the phase maps, has been developed and implemented in order to extend the size of the region of interest. With this method, and by paying a particular attention to the conduct of the mechanical tests, the strain resolution obtained ranges from 1 to 2*10^-3 for a spatial resolution of about 20 µm. In this paper, the method was applied to the study of the onset of local elasto-plastic strains in a ferritic steel coupon. The results show that the method is well suited to study small localized strains, for which few alternatives exist. To the best knowledge of the authors, it is the first time that white light interferometric microscopy is used to measure full-field strains. The technique is reasonably easy to set up although a number of limitations exist. The paper is concluded by some considerations about further work required to extend the range of applicability of the method.