What are residual stresses and how do they affect component performance?

Residual stresses are self-equilibrating stresses present without external loads, caused by manufacturing (welding, machining, heat treatment, forming). Tensile residual stresses are detrimental to fatigue life as they add to service stresses. Compressive residual stresses are beneficial, which is why processes like shot peening, laser peening, and autofrettage are used to improve fatigue performance.