In shot peening, the surface of the finished part is bombarded with round steel shot in custom built machines under fully-controlled conditions. Every piece of shot acts as a minute peening hammer. When the surface has been peened by the abundance of impacts, the resulting compressed surface layer, is highly resistant to the formation of stress cracks.
It is well known that a crack will not expand into a compressed layer. Nearly all fatigue and stress corrosion failures originate at the surface of a part, the layer of compressive stress induced by shot-peening produces a significant increase in the useful life of the part. The maximum compressive residual stress produced at or near the surface is at least as great as half the ultimate tensile strength of the material.
Shot peening is used to eliminate failures of present designs, or to allow the use of higher stress levels, which, in turn, permit weight reduction for new designs.