2000 年 49 巻 4 号 p. 448-452
The effect of the specimen thickness on the fatigue crack propagation in polycarbonate plates was investigated. Fatigue cracks were propagated under axial load conditions with constant amplitude loading, a single peak overload and two-step loading. The crack propagation rates both on the surface and in the interior of the specimen were measured. The crack closure behavior and the appearance of fracture surface were also examined. The fatigue crack propagation rate, da/dN, under constant amplitude loading increased with increasing specimen thickness at low ΔK levels. The crack closure behavior was not appreciably affected by the specimen thickness. The measurements of shear lip width on the fracture surface indicate that the thickness dependence of da/dN is closely related with the formation of shear lips. The crack extension during a single peak overload was larger in the thick specimen. The fatigue crack retardation after the peak overload was more marked in the thin specimen, which was similar to the case of stress decrease under two-step loading. These results can also be understood in terms of the contribution of shear lips to fatigue crack propagation.