Official Answer
Answer
Shot peening is a surface enhancement process where the ring is bombarded with spherical media (shot). This induces a residual compressive stress layer on the surface, typically extending 0.1 to 0.2 mm deep. Since fatigue cracks almost always initiate at the surface under tensile stress, the residual compressive stress must be overcome by the applied axial load before the surface actually experiences tension. For a spiral ring in a pulsating load application, shot peening can increase fatigue life by a factor of 1.5 to 3. The intensity of peening is measured using Almen strips, and for thin spiral rings, care must be taken to prevent distortion or 'warping' of the turns due to the unbalanced surface stresses.
