Shot peening is a cold working process that imparts a small indentation on the surface of a part by impacting small spheres called shot onto the material surface. (Figure 3) This process creates the same effect that a peening hammer does by causing outer surface to yield in tension. The material directly beneath it is subjected to high compressive forces from the deformation and tries to restore the outer surface to its original shape. By overlapping the surface indentations, a uniform compressive layer is achieved at the surface of the material. The compressive layer squeezes the grain boundaries of the surface material together and significantly delays the initiation of fatigue cracking. As a result, the fatigue life of the part can be greatly increased