Residual stresses are stress fields that prevail in non-loaded structures. For instance, a normal bolt resting in the palm of your hand may experience large measures of stress. This is not because of your demeanor or the way you are holding it, instead it is due to the way the bolt has been manufactured. A quality thread is rolled rather than cut. Thus the material has been subjected to significant plastic deformation, which creates compressive stresses in the creases of the thread. The treatment also generates tensile stresses, as required by the law of force equilibrium.
Compressive residual stresses in the thread are favorable to the fatigue life of the bolt, and are thus desirable. The corresponding tensile stress's, on the other hand, may have a negative impact on the maximum load carrying capacity. However, this weakening effect is small thanks to the material's ability to yield and redistribute the stress fields in the event of an extreme load.
Residual stresses may have positive, negative or no practical effects on a structure's mechanical integrity. Whatever the case may be, knowing about them is important. This chapter explains common causes and possible effects of residual stresses, and thoroughly prepares the reader for related topics in subsequent chapters.