2007 Volume 1 Issue 5 Pages 667-678
Threshold stress intensity factor ranges, ΔKths, of spheroidal cast iron, which are composed of ferritic, ferrite-pearlitic, or pearlitic structures, have been estimated by ΔK-increasing tests for small cracks and by ΔK-increasing and ΔK-decreasing tests for long cracks at several stress ratios. With respect to the effects of the hardness and stress ratio, ΔKth estimated by the ΔK-decreasing tests for long cracks is evidently different from that for small cracks, while ΔKth estimated by the ΔK-increasing tests for long cracks is similar to that for small cracks. Moreover, in the case of long cracks, the values of ΔKth estimated by the ΔK-decreasing tests were larger than those estimated by the ΔK-increasing tests, irrespective of the type of matrix structures. These tendencies are attributed to the fact that the crack closure stress of the long cracks in the ΔK-decreasing tests is relatively higher than that of the long cracks in the ΔK-increasing tests. On the fracture surface of the long cracks in the ΔK-decreasing tests, oxide layers produced by fretting are observed at sites around the crack tip, although such layers are not observed on the fracture surface in the ΔK-increasing tests.
