Shot peening is a 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 ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created. It was common practice for blacksmiths to hammer peen the concave side of leaf springs; this process enhanced the life of the springs, although the exact mechanism was unknown. The max tensile stresses are located on the surface of the concave portion of leaf springs; the peening effectively offsets the tensile stress, when the compressive stresses were induced by hammer peening (with a ball peen hammer).
Aircraft Metal Finishing
Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Shot peening is often called for in aircraft metal finishing when carrying out repairs to relieve tensile stresses built up in the grinding process and replacing 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. Surface compressive stresses confer resistance to metal fatigue and to some forms of stress corrosion. 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.
Shot peening may be used for cosmetic effect. The surface roughness resulting from the overlapping dimples causes light to scatter upon reflection. Because peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced.
A study done through the SAE Fatigue Design and Evaluation Committee showed what shot peening can do for welds compared to welds that didn't have this operation done. The study claimed that the regular welds would fail after 250,000 cycles when welds that had been shot peened would fail after 2.5 million cycles, and outside the weld area. This is part of the reason that shot peening is a popular operation for aircraft metal finishing.
Popular methods for propelling shot media include air blast systems and centrifugal blast wheels. In the air blast systems, media is introduced (by various methods) into the path of high pressure air and accelerated through a nozzle; which is directed at the part to be peened. The centrifugal blast wheel consists of a high speed paddle wheel and shot media is introduced in the center of the spinning wheel; the media is propelled by the centrifugal force by the spinning paddles and the media is directed towards the part by adjusting the media entrance location.
After some development of the process, an analog was needed to measure the effects of shot peening. John Almen noticed that shot peening made the side of the sheet metal that was exposed begin to bend and stretch. He also created the Almen strip to measure the compressive stresses in the strip created by the shot peening operation. One can obtain what is referred to as the "Intensity of the Blast Stream" by measuring the deformation on the Almen strip that is in the shot peening operation. As the strip reaches a 10% deformation, the Almen strip is then hit with the same intensity for twice the amount of time. If the strip deforms another 10%, then one obtains the "intensity of the blast stream". Read more about the history of shot peening.