In the shot peening process small particles bombard a metal

In the shot peening process, small particles bombard a metal surface to increase the yield strength. What strengthening mechanism is operative here? Justify your answer.

Solution

-Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals.

-It entails impacting a surface with shot (round metallic, glass or ceramic particles) with force sufficient to create plastic deformation. It is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion: each particle functions as a ballpeen hammer. In practice, this means that less material is removed by the process, and less dust created.

-Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface.

-Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage, shot peening can increase fatigue life from 0–1000%.

-Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress in the interior. The tensile stresses deep in the part are not as problematic as tensile stresses on the surface because cracks are less likely to start in the interior.

-In shot peening, small spherical media called shot bombard the surface of a part.

-During 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.

-Nearly all fatigue and stress corrosion failures originate at the surface of a part, but cracks will not initiate or propagate in a compressively stressed zone. Because the overlapping dimples from shot peening create a uniform layer of compressive stress at metal surfaces, shot peening provides considerable increases in part life.

-The basic shot peening process principle is used by many research scholars to improve the surface characteristics. They have modified the process to enhance its capability to be used for different material surfaces.