The process of machining, welding, fabrication, forming and additive manufacturing all introduce surface discontinuities which may adversely affect the performance of the material.
When designing a component the stress analysis is usually performed on a perfect model of the component, whereas in reality this component will have many surface imperfections introduced by the manufacturing process. These imperfections can act as crack initiators and significantly reduce the fatigue life of the component. Once the crack has initiated crack growth may also increase due to the nature of surface imperfection. It is often not feasible to remove these imperfections by polishing so shot peening remains a significant way of restoring the fatigue life by producing a homogeneous surface finish often with the added benefit of a surface compressive layer.