Analysis of strain localization during tensile tests by digital image correlation

This paper presents an imaging technique developed to study the strain loca lization phenomena that occur during the tension of thin, flat steel sample s. The data are processed using digital speckle image correlation to derive the two in-plane components of the displacement vectors. The authors obser ve that the calculation of the intercorrelation function reveals a systemat ic error and propose a numerical method to limit its influence. Plastic inc ompressibility and thin-sheet assumptions are used to derive the third disp lacement component and, hence, the various strain and strain rate component s. Numerous checks are presented at each step in processing the data to det ermine the final accuracy of the strain measurements. It is estimated that this accuracy is quite sufficient to track the inception and the developmen t of localization. Examples of possible application are presented for mild steels whose strain localization mechanisms appear to be precocious and gra dual.