Fretting Fatigue Initial Damage State to Cracking State: Observations and Analysis

Interrupted fretting fatigue experiments were performed to demonstrate the capabilities of a confocal microscope related to characterizing fretting damage and to correlate that damage with cycles to failure. Fretting damage was established at stepped down levels of 100% (baseline), 80%, 60%, 40%, 20%, and 10%. The damage levels were calculated as a percentage of the total cycles to fracture for the baseline fretting fatigue specimen. The baseline or 100% specimen was subjected to axial fatigue forces and normal forces to induce a fretting situation for 100% of its life or total cycles to fracture. The baseline specimen was cycled to fracture without interruption and experienced fretting due to normal forces throughout the experiment. Other specimens were subjected to cyclic forces for their respective percentage cycles under fretting fatigue conditions (i.e. with the applied normal force). After the prescribed number of cycles was attained each specimen was removed and inspected for fretting damage and characterized utilizing a confocal microscope and a scanning electron microscope. Each specimen was then cycled to fracture or run-out without the applied normal force. A damage threshold was demonstrated for fretting damage at 60% or less for these experimental conditions. Fretting was characterized as depth of damage (wear and/or pitting) and surface cracking. A key factor was the location of the fretting damage, not necessarily the magnitude of that damage. Fretting damage located near the edge of the fretting pad contact area was more detrimental than damage near the center of the fretted area.