Peening

When a metal undergoes strain hardening its yield strength increases but its ductility decreases.

When peening is used to induce residual stress or work-harden an object, care needs to be taken with thin parts not to stretch the work-piece.

Scythe and sickle blades have traditionally been sharpened by occasional peening followed by frequent honing in the field during use.

Later applications to improve metal strength include the hammering of artillery gun barrels in the 18th century.

[7] Likewise, blacksmiths typically used a ball peen hammer to shape and improve the life of carriage springs.

[8] First scientific investigations of the properties of metals were carried out in the 19th century, notably in the context of fatigue failures in railway development and the industrial revolution.

Wöhler, e.g. carried out extensive work on the fatigue strength of metals subjected to cycles of stress.

[11] It was only in the early 20th century that surface treatments of metals began to develop into technical processing methods, with shot peening — effectively a myriad of small hammer blows[8] — coming into focus as an alternative to rolling for increasing fatigue strength.

Independently in 1930, a few engineers at Buick noticed that shot blasting (as it was originally termed) made springs resistant to fatigue.

[12] A significant innovation in hammer peening technology in the early 1970s was Efim Statnikov’s development of ultrasonic impact treatment (UIT),[13] which uses guided rod-shaped indenters to transmit high-frequency impulses to the treated surface.

[14] In the early 1970s, peening experienced a further major innovation when researchers such as Allan Clauer at Battelle labs in Columbus, Ohio applied high-intensity laser beams onto metal components to achieve deep compressive residual stresses, which they patented as Laser Shock Peening, and became known as laser peening in the late 1990s, when it was first applied to gas-fired turbine engine fan blades for the U.S. Air Force.