Evaluation of Strain-Induced Surface Changes by Optical Correlation

Abstract

Previous experiments in our laboratory [1,2] have demonstrated that changes in optical correlation intensity can be used to monitor small strain applied, for example, to aluminum alloy specimens in simple tension. By analyzing data separately for specimen translation, elastic strain, and plastic strain, respectively, we inferred that irreversible changes in correlation intensity associated with plastic strains of the order of 10^-3 are caused by accompanying changes in surface topography [2]. In this paper, we present results which link metallographic observations of surface slip and related topographic changes directly to corresponding changes in optical correlation intensity, during uniaxial tensile straining of a stainless steel heat treated to two different initial conditions. The results show that in a given material, correlation intensity is not a unique function of plastic strain, but that it depends on the detailed topographic changes which occur when strain is applied.