Fatigue life and crack growth direction in 7075-T6 aluminium alloy specimens with a circular hole under biaxial loading

This work shows the results of a set of tests in 7075-T6 aluminium alloy specimens. The tests were under load control at R = -1. From the tests, the corresponding S-N curves were built, and the endurance limits were calculated for 1 million cycles. The ratio between the pure torsion and tension endurance limits was 0.58, i.e., the behaviour of this material in fatigue is of the von Mises type. Specimens with circular holes of various diameters under tension, torsion and in-phase biaxial loading were tested. The directions of the cracks that grew from the holes were studied with an optical microscope, a scanning electron microscope (SEM) and a non-contact 3D optical profiler. In particular, the points where the cracks began and the direction in which the cracks propagated along the first 150 mu m were analysed. In general, the initiation point was close to the maximum principal stress point, and the crack direction was close to the maximum principal stress direction. The endurance limit predictions were close to the experimental results.