Influences of surface grain size and gradient variation along depth on fatigue life of metallic materials

The Navarro-Rios (N-R) model was used to investigate the effects of surface grain size and the grain size gradient along depth (characterized as parameter g) induced by surface nanocrystallization (SNC) on the fatigue damage of the metallic materials in this paper. Two equations were deduced and the results showed that the crack arrest of metallic materials decreased when the surface grain became nanocrystalline. Nevertheless, the crack arrest of the SNCed materials increased as the top layer grain size decreased and it rose with the increase of the value of g when the surface grain size was smaller than 100. nm. The transition stress diminished with the increase of the surface grain size and the decrease of the value of g. The short crack growth rate descended with the decrease of surface grain size and grain size gradient. It explained why the SNC could improve the fatigue life of the metallic materials effectively.