1) Shot peening introduces compressive residual stresses near the surface layers. The surface plastically deformed by repeated impingement of hard steel balls can improve fatigue life since the compressive residual stresses first need to be compensated before crack propagation can take place.

2) Carburizing and nitriding introduce compressive residual stresses at the surface while increasing also the strength/hardness. Decarburization lowers the fatigue strength.

3) A fine grain size improves fatigue life.

4) The crack initiation almost always happens from a site of stress concentration. In design, fillets of adequate radius of curvature should be provided at places where a sudden change of cross section exists. Shafts are usually highly polished to reduce the chance of surface irregularities.

5.1.8