Abstract: Systems and methods for generating beneficial residual stresses in a material by impacting the surface of the material with particles that are softer than the material to be peened (“target material”). Shock waves emanate through the target material from the soft particle impacts to generate residual stresses without significantly deforming the surface of the target material. A high pressure liquid is accelerated through a peening nozzle to generate a high-speed liquid jet that is used to accelerate the soft particles that impact the surface of the target material.