Fatigue life distribution and its simulation in spheroidal graphite cast irons

Statistical fatigue tests have been carried out using smooth specimens of a spheroidal graphite cast iron (SGI) with ferritic microstructure and the fatigue Life distributions were examined. It was found that fatigue cracks were initiated from casting defects at the early stage of fatigue life and thus fatigue life could be regarded as crack growth life. The scatter in fatigue lives was primarily attributed to the scatter in casting defect size. The fatigue life prediction was performed using experimentally determined crack growth characteristics. The obtained results agreed well with the experimental data. From practical viewpoint, similar prediction was conducted using the growth characteristics for large cracks (CT specimen). When the crack opening stress was assumed to be zero, the prediction showed longer fatigue lives than the experimental data at high stress level, while a good agreement at low stress level. Furthermore, a Monte Carlo simulation was performed using measured distribution function of casting defect size, and the similar results were obtained. Additional fatigue tests were conducted on both pearlitic and bull's eye materials prepared with heat treatment of different SGI from ferritic SGI. It was found that the fatigue strength of both materials was almost the same and also similar to that of ferritic material. The distribution of casting defect size in pearlitic material agreed closely with that in ferritic material. This indicated that if the distribution of the sizes of casting defects from which crack was initiated were the same in different SGI's, then the fatigue strength would be independent of microstructure.