An analytical investigation of deformation-induced heating in tensile testing

Abstract

A numerical method for analyzing non-isothermal viscoplastic deformation problems has been developed. The physical problem is cumbersome because the thermal and deformation effects are coupled both ways, i.e. plastic deformation generates heat and the temperature rise affects material flow behavior. As an application of this method, sheet tensile tests conducted in air have been analyzed using a two-dimensional finite-element formulation. A modified Bishop's method is used to solve the thermo-plasticity problem in decoupled form at each time step. The analysis consists of two main parts: a rigid-plastic finite-element method to analyze the deformation and a transient heat transfer finite-element method, and each part is assumed to occur at sufficiently small, consecutive time steps. Using the present method, the various factors affecting the non-isothermal ductility of material and flow characteristics can be investigated completely. The accuracy of the analysis is confirmed by comparison with experimental tensile test data for interstitial free (I.F.) steel. The uniform elongation is found to drop by 0.1 to 2.7% at moderate rates (10 −4−10 −1 s −1), while total elongation decreases by 0.3 to 4.0% during tensile testing in air compared to the isothermal case. The predicted drop of uniform and total elongation between isothermal and adiabatic cases is about 2.7 and 4.8%, respectively, at those rates. The effect of deformation heating becomes more pronounced as necking develops and at higher testing speeds. The development of a temperature gradient has a detrimental effect on ductility opposite to the stabilizing effect of rate sensitivity. Consequently, better formability can be achieved by controlling heat transfer during forming.