A new constitutive equation is developed to model the flow stress on a metal surface undergone high speed impacts that result in strain hardening. The new equation is based on the Johnson-Cook model and has considered the effects of strain, strain rate, grain refinement, twin formation and twin spacing. Two mechanisms for the strain hardening are proposed: Grain refinement or twin formation, depending on the strain rate. At low strain rate, the Hall-Petch relation is obeyed, while at high strain rate, the flow stress is controlled by the formation of deformation twins. The theoretical estimation of flow stress agrees well with experimental data for stainless steel 304. According to the new model, the flow stress can be as high as 1.46 GPa at a strain rate of 105 /s.
Keywords: SMAT, flow stress, grain refinement, twin spacing, metal plasticity.
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