Abstract
Fatigue crack propagation properties of about 500 nm-thick freestanding copper (Cu) films have been investigated. Fatigue crack propagation rate da/dN-stress intensity factor range ΔK relationship clearly depended on stress ratio R in entire ΔK region. On the other hand, da/dN-maximum stress intensity factor K_<max> relationship was independent of R in high K_<max> (K_<max> ≥ 4.5 MPa・m^<1/2>) region and was dependent on R in low K_<max> (K_<max> ≤ 4.5 MPa・m^<1/2>) region. In order to clarify the effects of stress ratio on fatigue crack propagation mechanisms, fracture surfaces and fatigue damage around crack tip were clearly examined by a field emission scanning electron microscope (FE-SEM). In high K_<max> region, microstructure-insensitive or chisel-point fracture was observed in the fracture surface, and slip deformation and necking were observed at the crack tip of both R=0.1 and 0.8. At R=0.1 in low K_<max> region, microstructure-sensitive fracture was observed in the fracture surface, and intrusion/extrusion-like damage was observed ahead of the crack tip. At R=0.8 in low K_<max> region, chisel-point fracture was partly observed in the fracture surface and the fatigue crack propagated in a similar mechanism to the fatigue crack propagation in high K_<max> region.
