Estimate of Fatigue Life Distribution on ADI Using .RAD.area Distribution of Fatigue Crack Origins.

We carried out internal pressure tests on a large number of thin-wall cylinders and CT tests of austempered ductile iron (ADI). As a result of internal pressure tests, we observed that the fatigue life distribution at the low-pressure level was divided into two groups : a short-life group below l x 106 cycles and a long-life group fracturing at near l x 107 cycles. At the high-pressure level and in the short-life group of the low-pressure level, the fatigue crack origins were the casting defects, and the √area distribution of these was fitted with the extreme value distribution. On the other hand, it was observed that the crack growth rate obeyed the Paris law in CT tests. Thus we explained the form of the fatigue life distribution using the √area distribution of the fatigue crack origins and the crack growth rate based on the Paris law.