Residual stress

For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones.

Residual stresses can result from a variety of mechanisms including inelastic (plastic) deformations, temperature gradients (during thermal cycle) or structural changes (phase transformation).

This causes the initial crack to enlarge quickly (propagate) as the surrounding material is overwhelmed by the stress concentration, leading to fracture.

The manufacture of some swords utilises a gradient in martensite formation to produce particularly hard edges (notably the katana).

As a result, the solid globule is extremely tough, able to be hit with a hammer, but if its long tail is broken, the balance of forces is upset, causing the entire piece to shatter violently.

Common methods to induce compressive residual stress are shot peening for surfaces and High frequency impact treatment for weld toes.

For example, the four point bend allows inserting residual stress by applying a load on a beam using two cylinders.

Some of these work by measuring the diffraction of high frequency electromagnetic radiation through the atomic lattice spacing (which has been deformed due to the stress) relative to a stress-free sample.

The Ultrasonic and Magnetic techniques exploit the acoustic and ferromagnetic properties of materials to perform relative measurements of residual stress.

The thermal method involves changing the temperature of the entire part uniformly, either through heating or cooling.

Stress relief bake should not be confused with annealing or tempering, which are heat treatments to increase ductility of a metal.