Three-dimensional boundary element and experimental analysis of lubricant ceramic surface ring cracks in rolling contact.

Abstract

Surface defects such as ring cracks are located on silicon nitride ceramic rolling elements.The random nature of ring crack position creates difficulties in predicting rolling contact fatigue (RCF) life. The influence of the lubricant, contact pressure and crack geometry on the RCF mechanism is difficult to determine due to the variable nature of RCF life when the crack location is not defined. In this study, rolling contact fatigue was tested using a novel experimental technique. The test ceramic ball specimen with identified surface ring cracks was precisely located in the contact path. The rolling contact fatigue tests were conducted using a modified four-ball machine in a hybrid ceramic/steel combination with five different types of lubricating mediums at various contact pressures. Detailed analysis of damage progression was carried out to study the mechanisms of fatigue failure. A boundary element analysis was used to determine the stress intensity factors around the crack front and stress distributions on the surface in the presence of the surface crack. The RCF tests of silicon nitride balls with surface ring cracks showed that fatigue life was strongly influenced by load/contact stress, crack size,lubricant and material type. Ring crack spalling failure is not only influenced by sub-surface fatigue crack propagation but also strongly influenced by secondary surface cracks. The formation of the secondary surface cracks is a key factor which dominates spalling of rolling contact fatigue. Modelling work showed that the crack length was an important parameter.Increasing the crack length for given crack radius resulted in higher tensile surface stresses ahead of the original ring crack and also gave higher absolute K11 values. In addition, the crack gap and crack face friction had a significant effect on the formation of secondary surface cracks and fatigue crack propagation behaviour. Increasing the crack face friction decreased both the surface tensile stress and absolute magnitude of K11. Numerical calculation results were consistent with the experimental observations.