Effects of heat generation in dynamic material testing

This paper investigates the significance of thermal softening effects when using Johnson-Cook (JC) constitutive equation to model high-speed impact testing. The results of using the Johnson-Cook model for characterizing medium carbon steel (steel 1045) and the results of computational simulation using the derived material parameters are presented. Parameters for the material model are obtained via dynamic materials testing using a Split Hopkinson Pressure Bar (SHPB) set-up. By assuming that cylindrical test specimens undergo adiabatic deformation, changes in their temperatures can be. approximated and included in the temperature term of the JC model to incorporate thermal softening. The values of the parameters are also determined for the JC model without the temperature term to investigate the significance of the effects of temperature. Both material models - with and without temperature dependency - are then used in the finite element simulation of the Taylor rod impact problem. In general, there are significant deviations in the model parameters of the two material models. This means that heat effects or the consideration of the temperature term is important and should not be ignored. Results from computational simulation and actual Taylor rod tests show that both models are able to predict the impact-deformed geometries of the steel rods despite large differences in the model parameters. However, stresses and equivalent plastic strains developed within the rods were very different.