FATIGUE LIFE DISTRIBUTION AND ITS SIMULATION IN SPHEROIDAL GRAPHITE CAST IRONS

Abstract

Statistical fatigue tests have been carried out using smooth specimens of a spheroidal graphite cast iron (SGI) with ferritic microstructure and the fatigue life distribution was examined. It was found that fatigue cracks initiated from casting defects and thus fatigue life could be regarded as crack growth life. The scatter of fatigue life was primarily attributed to the scatter of the size of casting defects. The prediction of fatigue life was performed using the measured growth characteristics of small fatigue cracks. The results obtained agreed well with the experimental data. From the practical viewpoint, similar prediction was conducted using the growth characteristic of large cracks. When the crack opening stress was assumed to be zero, the results obtained gave longer lives than the experimetal data at high stress, but were coincident with the experimental data at low stress. Furthermore, a Monte Carlo simulation was performed using the measured distribution function of casting defect size and the similar results were obtained. In order to discuss the universality of the proposed prediction and simulation, additional fatigue tests were conducted on two SCI's with pearlitic and bull's eye microstructures, and it was found that the fatigue strength was independent of microstructure. The distribution of casting defect size in pearlitic microstructure was examined, which agreed closely with that in ferritic microstructure. This indicated that if the distribution of the size of casting defects from which cracks initiated was the same in different SCI's, then fatigue strength would be independent of microstructure, and the validity of the prediction and simulation of fatigue life distribution was confirmed.