Effect of specimen thickness on fatigue crack opening in A7075-T6 alloy

Fatigue crack growth simulations were performed on the A7075-T6 alloy using a 3D finite element method. The simulations were conducted under several maximum stress intensity factors ( K ) with specimen thicknesses ( B ) ranging from 1 to 11 mm and a zero stress ratio. The simulation results show that the crack opening stress intensity factor ( K op ) increases with increasing fatigue crack growth before reaching a constant value. The crack opening level ( K op / K max ) at the specimen surface is approximately 0.5, and that at the center of the specimen thickness is approximately 0.2, where K max is the maximum stress intensity factor. For thin specimens, the average K op close to the surface is consistent with the experimental results (unloading elastic compliance method). However, the average K op for the full specimen thickness is approximately 0.24, which is substantially different from the experimental results. For thick specimens, the influence of the K op on the surface layer diminishes, and its influence inside the specimen becomes dominant. Furthermore, the thickness dependence present in the K op –Δ K relationship (where Δ K = K max − K min is the stress intensity factor width) disappears in the K op / B –Δ K / B relationship. This result also holds true for other Al alloys, and the underlying mechanism was investigated. In addition, the K op generation mechanism was studied through a detailed analysis of the in-plane and out-of-plane stresses and displacements obtained from the simulations.