Experiments on heat generated during plastic deformation and stored energy for TRIP steels

The goal of this study is to analyze the effect of heat generation during plastic deformation in new sheet steel with high strength on the mechanical behavior during relaxation test σ(t)∣εpσ(t)∣εp and during tension at constant strain rate. In the first part an estimation of the fraction β of plastic work converted into heat has been analyzed with and without phase transformation [Rosakis P, Rosakis AJ, Ravichandran G, Hodowany J. On the partition of plastic work into heat and stored energy in metals: Part II. Theory, Internal SM Report No. 98-8. California Institute of Technology, June 1998]. It is shown that β   increases with plastic deformation. The latent heat due to phase transformation plays an important role. In order to analyze those effects, a fast hydraulic machine has been used combined with an infrared technique allowing measurement of temperature increase with plastic deformation. This experiment has been performed in adiabatic conditions, ε˙⩾10s-1. The value found for β vary within the limits: 0.82 ⩽ β ⩽ 1. In the second part the temperature effect due to plastic deformation has been analyzed via relaxation test. Due to high stress level and high ductility, relatively high temperature increase ΔT was found in fast and quasi-static loading. The temperature effect is relatively important for the creep observed during testing. Such behavior is completely different in comparison to mild steel where the temperature increase is much lower [Rusinek A, Zaera R, Klepaczko JR, Cheriguene R. Analysis of inertia and scale effects on dynamic neck formation during tension of sheet steel. Acta Mater 2005;53:5387–400]. In the third part, experimental results are reported concerning plastic instability, notably Lüder’s band propagation. An estimation of temperature increase in the band has been performed.