ON RATE SENSITIVITY OF POLYCRYSTALLINE ALUMINUM AT HIGH-STRAIN RATES

An analysis is presented on plastic behavior of polycrystalline alumin um within a wide range of strain rates. A consistent approach to const itutive modeling is based on evolution of physical state variables whi ch characterize a microstructure. A one-variable approach has been app lied to describe evolution of a microstructure at different strain rat es and temperatures. It has been assumed that the mean dislocation den sity controls the evolution, including the mobile dislocation density. The thermal activation strain rate analysis is employed for both the kinetics of glide and the kinetics of structural evolution. Special em phasis has been put on analysis of the high strain rate region, 10(3) s-1 less-than-or-equal-to GAMMA less-than-or-equal-to 10(5) s-1, where GAMMA is strain rate in shear, that is in the region where the mechan ical threshold is expected due to a transition from the thermally acti vated plasticity to the drag controlled plastic deformation. Numerical analyses have shown that the mechanical threshold is rate and structu re dependent. Another numerical simulations have been performed with a n instantaneous reduction in strain rate from GAMMA(i) almost-equal-to 10(4) s-1 to GAMMA(r) = 0.4 GAMMA(i), where GAMMA(i) is the initial s train rate and GAMMA(r) is the strain rate after reduction. Numerical results were compared with experimental data for polycrystalline 5 N a luminum reported in Ref. 15) and Ref. 26). A general discussion is mad e about the effect of structural evolution on the mechanical threshold transition.