Effect of Lubricant Degradation on Contact Fatigue

Abstract

It is well known that contact fatigue is affected by contact pressure, frictional stress, residual stress, initial distribution of material flaws, and so on. The behavior of contact pressure and, primarily, the frictional stress is determined by the viscous properties of the lubricant used. It is also recognized that lubricants degrade while passing through lubricated contacts. Degradation of lubricants causes viscosity loss that, in turn, reduces the frictional stress and raises contact fatigue life. The objective of this study was to find out the extent to which lubricant degradation may change contact fatigue life of elastic surfaces completely separated by lubricant. The analysis was performed numerically based on the models of contact fatigue and lubricant degradation recently developed by the author. The results showed that contact fatigue life of solids completely separated by lubricants with the same ambient viscosity may vary significantly due to the specific way lubricants are formulated. In particular, contact fatigue life is strongly affected by the initial molecular weight distribution of the polymeric additive (viscosity improver) in the lubricant and contact operating conditions, which in some cases promote fast lubricant degradation caused by high lubricant shearing stresses.