Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy

Fatigue crack growth experiments are performed using A7075-T6 compact tension (CT) specimens with various thicknesses t (1-21 mm). The stress intensity factor at the crack opening level K-op is measured, and the effects of t and the stress intensity factor range Delta K on K-op are investigated. In addition, the change in K-op value due to specimen surface removal is investigated. Furthermore, we clarify that the radius of curvature of the leading edge of the fatigue crack decreases as t becomes thinner. Using the three-dimensional elastoplastic finite element method, the amount of plastic lateral contraction (depression depth d) at the crack tip after fatigue loading is calculated quantitatively. The following main experimental results are obtained: In the region where Delta K is 5 MPam(1/2) or higher, the rate of fatigue crack growth da/dN at a constant Delta K value increases as t increases from 1 to 11 mm. The da/dN between t = 11 and 21 mm is the same. Meanwhile, in the region where Delta K is less than 5 MPam(1/2), the effect of t on da/dN is not observed. The effects of t and Delta K on the da/dN-Delta K relationship are considered physically and quantitatively based on d.