Influence of a single axisymmetric asperity on surface stresses during dry rolling contact

The effect from contact loading of some single axisymmetric asperities as a potential mechanism for surface initiated rolling contact fatigue was investigated numerically using FEM. Computational results were compared to properties of some rolling contact fatigue craters, or spalls, in the teeth surfaces of four driving gear wheels. The gears were geometrically identical but had experienced slightly varying load conditions. The residual surface stresses of a used teeth with spalls were measured using the hole drilling technique. The combined cylinder asperity contact was first modelled with a stationary model in which an asperity was introduced at the contact rim. By varying asperity height, width, position and contact load dangerous asperity configurations were sought for. The gear contact close to the rolling circle was modelled as two rolling cylinders. A single asperity was introduced into the contact surface of one of them. Due to the presence of the asperity a three-dimensional contact model was required. The simulation included residual stresses from heat treatment and plastic deformation due to the first roll cycle. Thus, the stress results were computed from the second roll cycle. The important overall conclusion was that a single asperity may serve as a stress raiser in the contact surfaces. Furthermore, the computed values of the increased surface stresses were comparable to those that are reported in the literature to give cracks. Example of dangerous asperity dimensions were noted and changes in residual stresses from moderate plastic deformation during rolling were estimated. The asperity deformed plastically during over-roll but remained sufficiently high. The trajectory of the largest principal stress was computed, starting from the position in front of a loaded asperity with maximum tensile stress. The trajectory was compared to the spalling entry angle of a representative spall. For some asperity-cylinder configurations a convex region with large stress was found in the surface. The presence of such a convex stress region was compared to the convex shaped of the spalling tip that sometimes could be found.