Analysis of the formation of plastic deformation layer on the surface of polycrystalline metals subjected to a micro-size high-rate shot impact

• The grain-order work-hardening during a single shot impact was simulated.
• A small shot introduces intense work hardening beneath the surface.
• Metal surface is significantly work-hardened after high-rate shot impact.

The effect of current peening techniques that modify the surface layer of metallic materials on the fatigue strength of those materials has been enhanced by employing a micro-size, high-rate shot impact. The present paper evaluates the effect of impact velocity and impact size on work hardening on the surface of polycrystalline metals subjected to peening. A single-shot impact was modeled based on a polycrystal plasticity finite-element analysis in order to address the effect of grain-order work hardening. Using the finite-element analysis, the effect of the relative size of the shot and the individual grains on the surface work hardening of polycrystalline metal was investigated. Simulated results reveal that the deformation progresses preferentially along the grain boundary rather than inside the grain after a large shot impact, while peening with a small shot can introduce intense work hardening inside the grain just beneath the surface. Moreover, we compared a shot impact and a static indentation whose dimples were almost the same size and confirmed that a high-rate shot impact can generate a significantly work-hardened layer beneath the impacted surface.