Residual stresses are stresses in material which are introduced during the processing phase but endure after the product is finished, instead of resolving once the source of the stress is removed. One could think of residual stresses as stresses which are left behind in manufactured components. Sometimes these stresses are deliberately introduced to hold parts in tension for the purpose of prolonging wear life and providing other features. In other instances, residual stresses are introduced by accident and they are not desirable.
Processing of materials like metals and plastics often involves heat, which can contribute to the development of residual stresses. Welding residual stress, for example, is a problem with some metal components. Likewise, stresses can be created with plastics which are heated and injection molded. Other processes like rolling can create similar stresses in the materials they are used to shape or otherwise work.
When a residual stress is not desired and carefully controlled, it can become a problem. Components might fail as a result of residual stress which is not controlled; a bolt might blow apart without any pressure, for instance. Residual stresses are sometimes blamed for incidents like bridge failures when no other causes can be determined. These stresses are hard to measure, especially after a component failure has damaged a part, making it difficult to study and hard to distinguish the cause of cracking and other problems.
In other instances, residual stresses are actually wanted. A classic example is the stress associated with hammering in a nail. When a nail is hammered into wood, the residual stresses created through the process are what holds the nail in instead of allowing it to pop back out. Likewise, blades and other metal components are often made with residual stresses to limit cracking and fatigue. In these cases, careful calibrations are performed to determine the desired level of stress and control for it during production so that components will be produced with the specified characteristics.
Equipment which can be used to take residual stress measurements is available. Measuring such stress is tricky because most equipment is designed to measure ongoing stress, such as tension created by the use of a tool. Stress which is essentially locked into a component is more difficult to measure. Companies which manufacture equipment which can be used for residual stress measurement may also offer materials testing services which allow companies to send out components for testing rather than conducting their own tests.