Residual stresses are internal forces contained within a part after the original source of those stresses has been removed. Typical sources of stress include loads applied in deforming operations such as bending, forging or extruding, and temperature gradients such as those encountered in casting and welding. When a metal part is permanently deformed, as in bending or forging, these residual stresses are deposited into the part. Similarly, expansion and contraction from temperatures encountered in casting and welding also deposit residual stresses. If no further change is made to the part, these residual stresses may simply remain contained within the part with little or no affect. However, if a section of the part is machined away or if the part is heated, these stresses can be re-distributed in a manner which will cause the part to distort. This distortion can result in misaligned bores, threaded holes, and bearing surfaces.
Residual stresses are cumulative. In other words, if a beam has the capacity to carry a load of 1000 pounds and contains residual stresses in the same orientation as that load carrying capacity of 100 pounds, then any load applied to the beam that exceeds 900 pounds will overload (100 pounds residual + 900 pound load = 1000 pounds) the beam’s carrying capacity and cause it to fail. This is an obvious factor to consider in the design and manufacture of load carrying parts.