Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. Residual stress may be desirable or undesirable. For example, it is used in toughened glass such as Gorilla Glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. It is also used in an interference fit, in which friction and residual stress are used to fasten parts together. Residual stresses occur through a variety of mechanisms including inelastic (plastic) deformations, temperature gradients (during thermal cycle) or structural changes (phase transformation). Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Another example occurs during semiconductor fabrication and microsystem fabrication when thin film materials with different thermal and crystalline properties are deposited sequentially under different process conditions. The stress variation through a stack of thin film materials can be very complex and can vary between compressive and tensile stresses from layer to layer.
Premature failure 
Castings may also have large residual stresses due to uneven cooling. Residual stress is often a cause of premature failure of critical components, and was probably a factor in the collapse of the Silver Bridge in West Virginia, United States in December 1967. The eyebar links were castings which showed high levels of residual stress, which in one eyebar, encouraged crack growth. When the crack reached a critical size, it grew catastrophically, and from that moment, the whole structure started to fail in a chain reaction. Because the structure failed in less than a minute, 46 drivers and passengers in cars on the bridge at the time were killed as the suspended roadway fell into the river below.
Controlled residual stress 
While uncontrolled residual stresses are undesirable, some designs rely on them. In particular, brittle materials can be toughened by including compressive residual stress, as in the case for toughened glass and pre-stressed concrete. The predominate mechanism for brittle failure is for cracks to form and then for the cracks to be opened up; the crack tips concentrate stress, resulting in local stress much higher than the average stress on the bulk material. Compressive residual stress prevents cracks from opening up because even when the material is stretched from "rest" its local stress goes from compressed toward zero stress rather than from zero stress into tension.
A demonstration of the effect is shown by Prince Rupert's Drop, a material-science novelty in which a molten glass globule is quenched in water: Because the outer surface cools and solidifies first, when the volume cools and solidifies, it "wants" to take up a smaller volume than the outer "skin" has already defined; this puts much of the volume in tension, pulling the "skin" in, putting the "skin" in compression. 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.
Bolted joints use residual stress to avoid subjecting bolts to fatigue. A gradient in martensite formation leaves residual stress in some swords with particularly hard edges (notably the katana), which can prevent the opening of edge cracks.
In certain types of gun barrels made with two tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the gun is fired. Parts are often heated or dropped into liquid nitrogen to aid assembly...
Compressive residual stress 
Measurement techniques 
There are several techniques that are used to measure the residual stress. They can be classified as destructive and non-destructive methods. Mechanical methods or dissection uses the release of stress and its associated strain after doing a cut, hole or crack. Nonlinear elastic methods as ultrasonic or magnetic techniques requires a reference sample. X-ray diffraction is a non-destructive method which allows the measurement of residual stress in isolated spots spaced distances as small as 100 micrometres. Neutron diffraction is an alternative non-destructive method which allows measurement of residual stress in isolated spots. The choice between these two techniques depends on the design of the mechanical part to be tested.
See also 
- Khan, Z. et al. (2005). "Ceramic rolling elements with ring crack defects—A residual stress approach". Materials Science and Engineering: A 404: 221. doi:10.1016/j.msea.2005.05.087.
- Khan, Z. et al. (2006). "Residual stress variations during rolling contact fatigue of refrigerant lubricated silicon nitride bearing elements". Ceramics International 32: 751. doi:10.1016/j.ceramint.2005.05.012.
- Khan, Z. et al. (2007). "Manufacturing induced residual stress influence on the rolling contact fatigue life performance of lubricated silicon nitride bearing materials". Materials & Design 28: 2688. doi:10.1016/j.matdes.2006.10.003.
Further reading 
- Hosford, William F. 2005. “Residual Stresses.” In Mechanical Behavior of Materials, 308–321. Cambridge University Press. ISBN 9780521846707
- Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3.
- - Energy-saving technology and computerized equipment for vibration stabilization of residual stress (the only CIS 35-year-old Scientific School, who presented in 1988, the world's first installation of computer diagnostics) are intended to stabilize the residual stresses in welds, castings and other non-rigid dynamic metal
- Residual stress at TWI (The Welding Institute)
- Comprehensive resources on Residual stresses at Cambridge University