User:SHLS1993/sandbox

Shot Peening/Blasting Shot peening is a cold working process in which innumerable small round steel shots continuously bombard the surface of a part. During the shot peening process, each piece of shot that strikes the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. To create the dimple, the surface of the material must yield 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. Overlapping dimples develop a uniform layer of residual comprehensive stress. It is well know that cracks can hardly initiate or propagate in a compressively stressed zone. Since all fatigue and stress corrosion failures originate at or near the surface of a part, comprehensive stress induced by shot peening provides significant increases in part’s life. The magnitude of comprehensive stress created by shot peening must come up to as half as the tensile strength of the part material being peened In most modes of long-term failure, the common denominator is tensile stress. These stresses can result from external applied loads or the residual stress produced by manufacturing processes such as welding, grinding or machining. Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation. Comprehensive stress squeezes the surface grain boundaries together and will greatly delay the development of fatigue cracking. Because crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance