Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. In order for the dimple to be created, the surface of the material must be yielded in tension. Below the surface, the material tries to restore its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression.
Nearly all fatigue and stress corrosion failures originate at the surface of a part. Further, it has been well established that cracks will not initiate or propagate in a compressively stressed zone. Since the overlapping dimples from shot peening create a uniform layer of compressive stress at metal surfaces, the process provides considerable increases in part life. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. The maximum compressive residual stress produced just below the surface of a part by shot peening is at least as great as one half the yield strength of the material being peened.
Shot peening is also used to induce the aerodynamic curvature in metallic wing skins used in advanced aircraft designs. Additional applications include work hardening through cold work to improve wear characteristics, closing of porosity, improving resistance to intergranular corrosion, straightening of distorted parts, surface texturing, and testing the bond strength of coatings.
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