Slip regime explanation of observed size effects in fretting

Previous studies in fretting fatigue have observed size effects related to contact width such that a critical contact width exists where there is drastic change in the fretting fatigue life. Several fatigue or fracture mechanics based concepts have been invoked to explain this interesting observation by previous researchers. The experiments conducted in the present study were similar in nature to the previous studies, but analyses and conclusions are different. These tests were conducted to investigate the contact width effects as well as to characterize the contact condition as contact width changed. Two series of tests were performed in the present investigation which again showed the critical contact width phenomenon. Fretting hysteresis loops were recorded and the profiles of fretting scars were characterized. These showed that the critical contact width is a demarcation of contact condition between partial slip and gross slip on either side of this characteristic width in the present study. Either failure occurred in a relatively short fatigue life due to partial slip fretting condition or there was almost infinite fatigue life due to gross slip fretting condition. There was also a drastic difference in the magnitude of relative slip amplitude between pad and specimen in tests on either side of the critical contact width. Furthermore, the size of critical contact width was affected by the coefficient of friction.