MODELING THE INITIAL STATE OF A MATERIAL AND ITS EFFECT ON FURTHER DEFORMATION

摘要

We consider a strain rate and temperature dependent, large deformation plasticity model in which two internal variables have been introduced to describe the state of the material. A scalar variable is introduced to describe the isotropic hardening or change in radius of the yield surface, while a tensor variable is introduced to describe any anisotropic hardening. The evolution equations for both variables is cast in a hardening minus recovery format, and an analytic solution can be determined for each variable for the case of isothermal constant true strain rate. We utilize these analytic solutions in conjunction with reverse loading data to determine the parameters for the model. We then investigate whether these material properties for an annealed material can be used to model the behavior of a rolled plate. Many materials exhibit a Bauschinger effect, that is an apparent softening upon load reversal. The Bauschinger effect has been shown to strongly depend upon the definition of yield stress. For a small strain offset yield, the material appears to exhibit a large Bauschinger effect, and as the strain offset definition of yield is increased the material appears to harden more isotropically. In this paper we utilize reverse loading data to determine the independent growth and saturation of the two tensor variables. Reverse loading tests at various strain levels were conducted on OFHC copper. From these tests, values of both state variables could be determined at the various strain levels. The parameters of the model are then determined utilizing the analytic solution for each variable and a nonlinear regression analysis. Having determined the parameters for an annealed material, we then model the rolling of a tapered plate to determine the non uniform state of the plate. These initial values are then utilized to investigate the effect of the non uniformity of state on reloading of compression specimens which were machined out of the plate in the rolling direction.