Surface roughness characterization for fatigue life predictions using finite element analysis

Fatigue cracks usually initiate at the surface in homogenous materials, thus the surface state has a high influence on fatigue life. The most important surface parameters are surface roughness, residual stress, and microstructure. Methods for determining residual stresses and microstructure are well established, whereas surface roughness cannot readily be characterized in terms of fatigue life. Common methods employing reduction factors and roughness parameters are notoriously inaccurate, leading to large safety factors and overly conservative designs. A new method is proposed, in which microscopic surface measurements are used to create finite element models of surface topography. The influence of surface roughness on fatigue life can then be based on stress solutions instead of empirically derived reduction factors.