Residual stresses can be sufficient to cause a metal part to suddenly split into two or more pieces after it has been resting on a table or floor without external load being applied. While this is not a common occurrence, experienced people in the metal working industry have witnessed this phenomenon. While there may be additional factors causing this to occur residual stresses help explain these occurrences.
Residual stresses are stresses that are inside or locked into a component or assembly of parts. The internal state of stress is caused by thermal and/or mechanical processing of the parts. Common examples of these are bending, rolling or forging a part. Another example are the thermal stresses induced when welding.
Residual stresses can play a significant role in explaining or preventing failure of a component at times. One example of residual stresses preventing failure is the shot peening of component to induce surface compressive stresses that improve the fatigue life of the component. Unfortunately, there are also processes or processing errors that can induce excessive tensile residual stresses in locations that might promote failure of a component.
It must be kept in mind that the internal stresses are balanced in a component. Tensile residual stresses are counter balanced by compressive residual stresses. To better visualize residual stresses it is sometimes helpful to picture tension and compression springs to represent tensile and compressive residual stresses. While this is an aid to understanding, it must be kept in mind that residual stresses are three-dimensional.
Residual stresses can result in visible distortion of a component. The distortion can be useful in estimating the magnitude or direction of the residual stresses.
Thermal residual stresses are primarily due to differential expansion when a metal is heated or cooled. The two factors that control this are thermal treatment (heating or cooling) and restraint. Both the thermal treatment and restraint of the component must be present to generate residual stresses.
A good common example of mechanically applied residual stresses is a bicycle wheel. A bicycle wheel is a very light and strong because of the way in which the components are stressed. The wire spokes are aligned radialy and tightening the spokes creates tensile radial stresses. The spokes pull the rim inward, creating circumferential compression stresses in the rim. Conversely, the spokes pull the tubular hub outward. If the thin spokes were not under a proper tensile preload load the thin wire spokes could not adequately support the load of the rider.