High Strain Rate Behavior of Metals

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Experimental results on the high strain rate response of polycrystalline metals are reviewed, with emphasis on the behavior of pure metals. A strong increase in flow stress with increasing strain rate is reported for strain rates of approximately 105s−1 and higher. This increase is observed in pressure-shear plate impact experiments at nominally constant strain rates from 105s−1 to 106s−1. To improve understanding of the increased rate sensitivity at high strain rates, pressure-shear, strain-rate-change experiments have been conducted on OFHC copper specimens. These experiments have been analyzed using a conventional viscoplasticity formulation and an internal variable formulation in which the hardening rate depends on the rate of deformation. Only the latter formulation is successful in describing the observed response to the change in strain rate. This observation is discussed in terms of its implications for interpreting other dynamic plasticity experiments and for improved understanding of the underlying dislocation mechanisms. 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 structure.