Mechanics of Fatigue Crack Propagation in Submicron-Thick Freestanding Copper Films

Abstract

The dominant mechanics and mechanisms of fatigue crack propagation in approximately 500-nm-thick freestanding copper films were evaluated at three stress ratios, R = 0.1, 0.5, and 0.8. The fatigue crack propagation rate (da/dN) versus stress intensity factor range (ΔK) relation was dependent on the stress ratio (R): da/dN increased with increasing R. Plots of da/dN versus the maximum stress intensity factor (Kmax) exhibited coincident features in the high-Kmax region (Kmax 4.5 MPam1/2) irrespective of R, indicating that Kmax was the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode, or chisel-point fracture, irrespective of the R value. In contrast, in the low-Kmax region (Kmax < 4.5 MPam1/2), da/dN increased with decreasing R. In this region, the fracture mechanism depended on R. At the higher R value (R = 0.8), the fatigue crack propagated in the tensile fracture mode similar to that in the high-Kmax region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appeared: within several grains, intrusions/extrusions formed ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagated preferentially through the intrusions/extrusions.