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Residual stresses develop during deformation. A small portion of the applied stress — perhaps about 10% — is stored within the structure as a tangled network of dislocations. The residual stresses increase the total energy of the structure.
Residual stresses are not uniform throughout a deformed metal. For example, high compressive residual stresses may be present at the surface of a rolled plate and high tensile stresses may be stored in the center. If a small amount of metal is machined from one surface of a cold-worked part, the metal that contains only the compressive residual stresses is removed. To restore that balance, the plate must distort.
Residual stresses also affect the ability of the part to carry a load. If a tensile stress is applied to a material that already containes tensile residual stresses, the total stress acting on the part is the sum of the applied and residual stresses. If, however, compressive stresses are stored at the surface of a metal part, an applied tensile stress must first balance the compressive residual stresses. Now the part may be capable of withstanding a larger than normal load. The compressive residual stresses can be harmful or beneficial. Assume a bending force applies a tensile stress along the top of a cantilever beam. Since there are already tensile residual stresses at the top, the load-carrying characteristics are poor. If the top surface contains compressive residual stresses then the load-carrying characteristics are very good.
Sometimes components that are subject to fatigue failure can be strengthened by shot peening.
Consider a rod that is stretched beyond its yield point. As the load is removed, the rod does not regain its original length; it is permanently deformed. However, after the load is removed, all stresses disappear. But, this will not always be the case. Indeed, when only some of the parts of an indeterminate structure undergo plastic deformations, or when different parts of the structure undergo different plastic deformations, the stresses in the various parts of the structure will not, in general, return to zero after the load has been removed. Stresses, called residual stresses, will remain in the various parts of the structure.
The computation of the residual stresses in an actual structure can be quite involved.
Plastic deformations caused by temperature changes can also result in residual stresses.
Residual stresses also occur as a result of the cooling of metals which have been cast or hot rolled. In these cases, the outer layers cool more rapidly than the inner core. This causes the outer layers to reacquire their stiffness (E returns to its normal value) faster than the inner core. When the entire specimen has returned to room temperature, the inner core will have contracted more than the outer layers. The result is residual longitudinal tensile stresses in the inner core and residual compressive stresses in the outer layers.
Residual stresses due to welding, casting, and hot rolling can be quite large (or the order of magnitude of the yield strength). These stresses can be removed, when necessary, by reheating the entire specimen to about 600°C, and then allowing it to cool slowly over a period of 12 to 24 hours.
Residual stresses may either improve the endurance limit or affect it adversely. Generally, if the residual stress in the surface of the part is compression, the endurance limit is improved.
High residual stress means low toughness and danger of cracking.
Stress relieving is done to relieve residual stresses and prevent parts from warping when getting machined.
Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting all can induce residual stress into metal.
welding, in particular, because of the rapid thermal expansion and contraction created along a very localized area, is a prime source of residual stress.
All welds will have some residual stress, and it will never be totally reduced to zero strain. But the level of stress can be very high depending on certain conditions. Heat input, base metal thickness, cooling rate, restraint of the weldment, and welding process play roles in the level of residual stress induced into a weldment.
Residual stress is an internal stress that is not a result of externally applied loads. If stress buildup in the weldment is excessive, the fatigue life of the metal is reduced.
Annealing at a low temperature may be used to eliminate the residual stresses produced during cold working without affecting the mechanical properties of the finished part. High temperatures can cause annealing, which may remove beneficial residual compressive stresses.
Due to the processing problems inherent in large components, there is a greater chance of having residual stresses, which may adversely affect fatigue strength.
Material properties may be altered and residual stresses introduced via heating or deformation that may occur during manufacture, storage, transportation, or construction.