The anisotropy of fatigue crack nucleation in an AA7075 T651 Al alloy plate

Four-point bend fatigue tests were conducted on the L–T (Rolling–Transverse), L–S (Rolling–Short transverse) and T–S planes of an AA7075 T651 alloy plate, respectively, at room temperature, 20 Hz, R=0.1, in air. The populations of crack initiation sites (i.e., fatigue weak-links), measured on these surfaces, were found to be a Weibull-type function of the applied maximum cyclic stress, from which fatigue weak-link density and strength distribution could be determined. The alloy showed a profound anisotropy of fatigue weak-links with the weak-link density being 11 mm−2, 15 mm−2 and 4 mm−2 on the L–T, L–S and T–S planes, respectively. The fact that cracks were predominantly initiated at Fe-containing particles on the L–T and L–S planes, but only at Si-bearing particles on the T–S plane, profoundly demonstrated that the pre-fractured Fe-containing particles were responsible for crack initiation on the L–T and L–S planes, since the pre-fracture of these particles due to extensive deformation in the L direction during the prior rolling operation could only promote crack initiation when the sample was cyclic stressed in the L direction on both the L–T and L–S planes. The fatigue strengths of the L–T, L–S and T–S planes were measured to be 243.6, 273.0 and 280.6 MPa, respectively. The difference in grain structure and particle between these planes was responsible for the anisotropy of fatigue strength on these planes.