Residual stresses are introduced during processing and can remain in materials after production. They can be deliberate or accidental and can cause failures if uncontrolled. Residual stress can also be desirable for certain applications, but measuring it is difficult.
Residual stresses are stresses in the material that are introduced during the processing stage but persist after the product is finished, rather than resolving once the source of the stress is removed. One might think of residual stresses as stresses that are left in manufactured components. Sometimes these stresses are deliberately introduced to keep parts in tension for the purpose of prolonging wear life and providing other characteristics. In other cases, residual stresses are introduced accidentally and are undesirable.
Machining materials such as 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. Similarly, stresses can be created with plastics that are heated and injection molded. Other processes such as lamination can create similar stresses in the materials used to shape or otherwise work.
When residual stress is unwanted and carefully controlled, it can become a problem. Components may fail due to uncontrolled residual stresses; a bolt could explode without any pressure, for example. Residual stresses are sometimes blamed for accidents such as bridge failures when other causes cannot be determined. These stresses are difficult to measure, especially after a component failure has damaged a part, making it difficult to study and difficult to distinguish the cause of cracks and other problems.
In other cases, the residual stresses are actually wanted. A classic example is the stress associated with hammering in a nail. When a nail is driven into wood, the residual stresses created during the process are what hold the nail in place instead of allowing it to come out. Likewise, blades and other metal components are often manufactured with residual stresses to limit cracking and fatigue. In these cases, careful calibrations are performed to determine the desired stress level and control it during production so that components with the specified characteristics are produced.
Equipment is available that can be used to make residual stress measurements. Measuring such stress is tricky because most equipment is designed to measure ongoing stress, such as the strain created by using a tool. Stress that is essentially stuck in one component is harder to measure. Companies that manufacture equipment that can be used for residual stress measurement may also offer materials testing services that allow companies to send components for testing rather than conducting their own tests.
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