Pressure-shear impact investigation of strain rate history effects in oxygen-free high-conductivity copper

Abstract

A combined experimental and computational investigation of the behavior of oxygen-free high-conductivity copper under very high shear rates is presented. Pressure-shear plate impact is used for conducting constant strain rate tests and strain rate change texts in which the specimen is strained at shear rates up to 10 6s −1 for 1μs and then strained at substatially lower shear rates for another microsecond. The specimen is sandwiched between two hard elastic plates to impose conditions of simple shear at very high strain rates and constant hydrostatic pressure. Marked increases in flow stresses are observed at strain rates of 10 5s −1 and higher. Flow stresses decrease gradually after a sharp drop in strain rate in all strain rate change tests. Homogeneous equiaxed dislocation cells are found as the predominant substructure in the deformed specimens. Theoretical analyses of the nonlinear wave propagation within the specimen are carried out using a general internal variable formulation in which the hardening rate depends on the rate of deformation. The governing system of hyperbolic partial differential equations is solved using a finite-difference scheme; computational results are compared with the experimental results. Both small- and finite-deformation formulations are considered. Only the internal variable model which incorporates a strong rate sensitivity of strain hardening is successful in describing the observed response to the change in strain rate. The enhanced rate sensitivity at high strain rates is concluded to be related primarily to the rate sensitivity of strain hardening, not the rate sensitivity of the flow stress at constant dislocation structure. The generation and evolution of dislocation cells appears to be the dominant micromechanical process during the high- rate deformation of pure metals.