An analysis of fretting fatigue
This thesis describes a series of fretting fatigue experiments carried out under closely controlled conditions of partial slip. These experiments confirm the existence of a size effect whereby the fretting fatigue life of an aluminium alloy is shown to vary with contact size. The configuration chosen, of cylindrical fretting pads contacting a plane specimen is amenable to classical stress analysis and the surface tractions between the contacting bodies are derived. The effects of tension in the specimen, finite specimen thickness, differing elastic constants, and surface roughness are all investigated and incorporated into the analysis where appropriate. A technique is then developed to calculate stress intensity factors for plane cracks growing under the contact load at an arbitrary angle to the free surface. The analysis is then applied to the experimental results and three possible explanations for the size effect are proposed, based on statistical effects, crack arrest, and crack initiation. These are examined in the light of the experimental evidence and it is proposed that the variation of fatigue life with contact size is due to an increase in the amount of fretting damage above a threshold level for crack initiation. A composite parameter is chosen to characterise the severity of fretting conditions and this is shown to describe the experimental results accurately. Finally, the use of this parameter in design calculations is discussed.