Torsional Fretting Fatigue Damages of 7075 Aluminum Alloy

The torsional fretting test of 7075 aluminum alloy was carried out on the independently designed fretting fatigue test rig based on the multi-axis fatigue testing machine. The effects of different torsional shear stress on fretting fatigue life under the same normal load were discussed and S-N curve for the torsional fretting fatigue of 7075 aluminum alloy was obtained. The damage regions of torsional fretting fatigue were analyzed based on scanning electron microscopy, energy dispersive spectroscopy and electron probe micro-analyzer. The damage physical model of the surface damages, the crack initiation and propagation in mixed fretting regime was set up to reveal the damage mechanism of torsional fretting fatigue. The experimental results show that the fatigue life was greatly reduced for the effect of the fretting; the S-N curve of the torsional fretting fatigue presented the characteristics of " ε” curve. The oxidation of the damage regions near the loading end was the most serious and the oxidation degree of the contact area increased with the number of cycles increasing. The torsional fretting fatigue cracks initiated in the subsurface and the main cracks on both sides of the contact central area propagated, crossed and then continued propagating perpendicularly to the contact surface until the test sample was fractured.