Abstract
The effects of stress ratio R on fatigue crack propagation rate and fatigue fracture surface appearance of polycarbonate at low stress intensity factor range ΔK level were investigated. Fatigue crack closure behavior of these specimens was also examined. From these experiments, it was found that the dependence of fatigue crack propagation rate on R, which was observed in the relation between crack propagation rate and ΔK, was considerably masked by using the effective stress intensity factor range ΔKeff. But, the effect of R on crack propagation mechanism was observed, and so the data of crack propagation rate at each R were not perfectly well expressed even in terms of ΔKeff.
Discontinuous growth bands (D. G. B.) were observed only when R was negative, and as R was decreased, D. G. B. seemed to be more easily formed. Fatigue crack closure also seemed to contribute to D. G. B. formation. These results suggested that the cyclic compressive stress at the crack tip played an important role for D. G. B. formation. This was considered to be due to the easy initiation of a single craze at the crack tip which was responsible for D. G. B. formation when cyclic tensile and compressive stress existed at the crack tip.
