Accurate stress computation in plane strain tensile tests for sheet metal using experimental data

Abstract

The advances achieved in phenomenological constitutive laws and their implementation in finite element codes for predicting material behavior during forming processes have motivated the research on material identification parameters in order to ensure prediction accuracy. New models require experimental points describing a bi-axial stress state for proper calibration, and the features of the plane strain tensile test have made it one of the most used. The test's principal inconvenience is the influence of the free edges on strain field homogeneity and stress computation. Experimental measurements of the strain field over the gauge zone on a plane strain tensile test specimen during deformation reveals the evolution of the size of the specimen area that represents a plane strain state. This article proposes a methodology, based on a numerical analysis of a plane strain tensile test for different materials and specimen geometry, to experimentally identify the evolution of the homogeneous strain field zone during deformation. This research defines an expression for computation of the actual stress in the specimen's plane strain state zone along the loading direction using experimental data and including the edge effect evolution in terms of plastic strain. The influence of the specimen's geometry and material anisotropy over the stress computation error is discussed and quantified. The stress computation expression proposed here can be adapted to other specimen geometries.