Plastic strain distribution underneath a Vickers indenter: role of yield strength and work hardening exponent

Abstract

The effects of yield strength, σ_y, and strain hardening exponent, n, on the plastic strain distribution underneath a Vickers indenter were explicitly examined by carrying out macro- and micro-indentation experiments on Al-Zn-Mg alloy that was aged for different times so as to obtain materials with different σ_y but with similar n, or the same σ_y but different n. Large Vickers indents were made (using a load of 700 N) that were subsequently sectioned along the median plane and the plastic strain distribution was determined by recourse to microindentation mapping. Comparison of the iso-strain contours shows that for similar values of n, higher σ_y leads to a smaller deformation field in both the indentation (z) and lateral (x) directions. Higher n, at the same σ_y, leads to a shallower deformation field. In all cases, it was found that strain fields are elliptical, with the long axis of the ellipse coinciding with the z-direction. The strain field ellipticity is sensitive to the work hardening behavior of the material, with higher n leading to lower ellipticity. While the lateral strain distribution is in agreement with the expanding cavity model, strain distribution directly ahead of the indenter tip appears to follow the Hutchison, Rice, and Rosengren fields ahead of a crack tip in an elastoplastic solid.