Numerical and Experimental Studies on Subscale Behaviors of Ultrasonic Surface Peening

Displacement-controlled and force-controlled surface-striking conditions were applied for finite element method simulations of the single path ultrasonic nanocrystal surface modification (UNSM) surface peening process. The single path UNSM treatment was done on a 4 mm-thick 6061 T6 Al sheet, and the surface topology and through-thickness residual stresses were measured and compared to the simulation results. Both displacement- and force-controlled simulations predicted maximum compressive residual stresses near the surface and subsurface for the stresses along the strike progression direction and transverse direction, respectively. The experimentally measured through-thickness residual stress profile also showed the maximum compressive residual stress near the subsurface. Both simulation conditions showed a topological pile-up on the surface in front of the leading strike. However, only the force-controlled simulation showed the pile-up breakdown at a critical number of strikes followed by the redistribution of subsurface strain fields, and lead to topological undulations along the edge and valley of the UNSM path, which were confirmed to be consistent with the surface topology measured from the experiment. It was concluded that the actual UNSM strike is close to the force-controlled surface-striking condition.