How Surface finish vs fatigue life of components?

The fatigue life of metals and some plastics too, can be affected by their surface finishes. Fatigue nearly always starts as a surface crack which spreads across the cross-section of the material and weakens it until it is unable to carry the load and it fails either by direct tension of shear stress on the uncracked remnant of the original cross-section.

The surface finish of the material contributes adversly to the initiatian period before a crack starts, and when the surface is rough a stress concentration is present there. When there is no roughness present the crystaline nature of the material itself may be the cause of the crack starting. But when the surface is rough this process is faster due to having greater local stresses. So with increasingly rougher surfaces the fatigue life becomes progressively shorter.

Depending on the spectrum of applied loads, this very local stress is responsible for the initiation of the crack which spreads with time. In sheet metal the spread is slow but in heavy solid pieces of metal, the speed of the crack is much faster so the start and finishing times are not very different.