Effect of Hardness and Stress Ratio on Threshold Stress Intensity Factor Ranges for Small Cracks and Long Cracks in Spheroidal Cast Irons

Threshold stress intensity factor ranges ZIKeh on spheroidal cast irons, which are composed of ferritic, ferrite-pearlitic and pearlitic structures, have been measured by 4K-increasing tests for small cracks, and by 4K-increasing and UK-decreasing tests for long cracks at several stress ratios. With respect to the effect of hardness and stress ratio, 2Ifth for the long cracks under the UK-decreasing tests and 41(th for the small cracks are evidently different, while JIGhs for the long cracks under the 4K-increasing tests and for the small cracks are similar. Moreover, regardless of the type of matrix structures, the values of JICth for the long cracks under the 4K-decreasing tests were larger than those for long cracks under the 4K-increasing tests. These tendencies are due to the fact that the long cracks under the 4K-decreasing tests have a relatively higher crack closure stress than those under the 4K-increasing tests. On the fracture surface of the long cracks under the 4K-decreasing tests, oxide layers caused by fretting were observed around the crack tip on the surface, although oxide layers were not observed under the 4K-increasing tests.