Using a split Hopkinson bar to specify the parameters of thermomechanical models of flow under high strain rate deformation

Abstract

By way of numerical simulation, a method is developed to determine the parameters of the thermomechanical Bodner-Partom model of flow under high strain rate deformation using a split Hopkinson bar. The classical method is generalized in two directions. To evaluate the kinematic hardening parameters, the wave reflected from the free end of the bar is used. The thermomechanical parameters that are responsible for the stored energy of cold work are calculated from measurements of temperature changes in the specimen Keywords: split Hopkinson bar, Bodner-Partom model, Baushinger effect, temperature change, finite-element method, stored energy of cold workIntroduction. The coupling of mechanical and thermal fields is of theoretical and applied importance. While in service, many structural elements of modern engineering are subjected to cyclic or impulsive loads, which may induce inelastic residual strains and temperature changes, either reversible (thermoelastic) or irreversible (dissipative) [12,13,30]. Monotonic quasistatic, high strain rate, and very high strain rate loading may give rise to local regions of large inelastic strains and high temperature (thermal shear bands) [22,23,25,28], which may lead to failure.On the other hand, thermal effects of deformation represent a number of prominent aspects of material behavior, which are either difficult or impossible to analyze within the framework of purely mechanical models. Among them are stored energy of cold work, dissipation of mechanical energy as heat, etc. A conceptual factor is that a characteristic variable of an adiabatic process is temperature, which can be measured experimentally. Thermal effects also carry information useful for diagnostics and study of damage kinetics, structural transformations, etc. [18,26].The reliability of data obtained in studying nonstationary thermomechanically coupled dynamic processes in high strain rate (10 2 -10 3 sec -1 ) and very high strain rate (10 3 -10 6 sec -1 ) deformation is strongly dependent on the choice and individualization of constitutive equations. A criterion for choosing a model should be its capability of describing viscosity, dependence of the yield stress on the strain rate, hardening, thermal effects, damage, etc.A split Hopkinson bar is widely used to test dynamic properties [15,17,19,20,24]. The equipment developed in [21] to measure temperature in a sample through which a wave propagates allows us to study the effects of thermomechanical coupling.The thermodynamically consistent Bodner-Partom model [9][10][11] was used in [27] to determine, for AMg-6 aluminum alloy, the portion of the inelastic work that does not dissipate as heat and accumulates by hardening ("cold work"). The results obtained there were used for numerical simulation of thermal (temperature) effects in a disk under impulsive radial loading. An analysis revealed that a chief task in studying such effects is to individualize the parameters of a thermodynamic model of flow under high strain rate loading.In the present paper, we carry o...