Shot peening is a method of cold working in which compressive stresses are induced in the exposed surface layers of metallic parts by a stream of shot, directed at the metal surface at high velocity under controlled conditions. It differs from blast cleaning in primary purpose and in the extent to which it is controlled to yield accurate and reproducible results. Although shot peening cleans the surface being peened, the major purpose of shot peening is to increase fatigue strength.
Media used for peening can be iron, steel, or glass shot, or cut steel or stainless steel wire. Metallic shot is designated by numbers according to size. Shot numbers, as standardized by MIL-S-13165, range from S70 to S930. The shot number is approximately the same as the nominal diameter of the individual pellets in ten thousandths of an inch. The effectiveness of the shotpeening operation is measured via the almen strip. This is a thin flat piece of steel that is clamped to a solid block and exposed to the blast of shot, which produces a curvature. The extent of this curvature on a standard sample serves as a means of measurement of the intensity of the peening.