Multiaxial Low-cycle Fatigue Behavior and Life Prediction of 7075-T651 Aluminum Alloy Thin-walled Tubular Specimens

Tension-torsion fatigue tests with different loading conditions were carried out on 7075-T651 aluminum alloy thin-walled tubular specimens aiming at studying multiaxial fatigue failure of aviation aluminum alloy. With the decrease of equivalent stress amplitude, the multiaxial fatigue life increases. Under the same equivalent stress, the multiaxial fatigue life is prolonged as the stress amplitude ratio augments while the tension-torsion phase has little effect on fatigue life. Under high stress amplitude, the materials are mainly softened in the axial and torsional directions while the hardening and softening alternately occur under the low stress amplitude. The platform area of macro fracture decreases with the increase of stress amplitude ratio. The microscopic fracture exhibits the multiple crack sources outside the wall of the tube. Fatigue striations and secondary cracks can be observed in the crack propagation region. Mixed mode dimples appear in the instantaneous fracture region. Based on Basquin's criteria, a modified model was proposed, and fine life prediction results were obtained, with the life prediction values in 2X scatter bands.