For many common types of loading, such as bending and torsion, the maximum stress occurs at the surface. Furthermore, the surface is exposed to harsh environments such as corrosion, or unexpected loads such as scratch and impact. In addition, experiments show that even the failure of axial loading usually begins at the surface. With all the evidences pointing to the surface, it is necessary to understand the effect of surface roughness and the treatments improving surface properties.
Experiments done on SAE 3130 steel specimens under completely reversed stress at 655 MPa show that a uniform and smooth surface does have positive influence on the fatigue life.
|Median Fatigue Life
|Ground and polished
A more systematic approach to understand the "surface factor" is to plot it against the fatigue limit of the material. Each line represent a different surface treatment.