Full-field strain rate measurement by white-light speckle image correlation

This paper explains a numerical procedure to process sequences of digital images and to return a full-field evaluation of the strain rate. The processing procedure is based on a nonlinear least squares fitting performed globally, on the whole image, and simultaneously on several images. The use of a highly optimised code allows the analysis of long sequences in a few minutes. The results of calculations are presented as movies built by blending the colour maps of the measured strain field with the specimen pictures used in the correlation procedure. Our application is focused on studying the plastic behaviour of metals and, in particular, on highlighting any transient phenomena that might occur during yielding and strain-hardening phases on thin sheets used in the manufacture of sheet metals. A typical example for such phenomena is the Portevin-Le Châtelier effect, a repetitive yielding of alloys during plastic deformation. © 2008 The Authors. Journal compilation © 2008 Blackwell Publishing Ltd