Katsuhisa Jinbo

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.

Influence of Casting Defects on High Cycle Fatigue Behavior of Austempered Ductile Iron Cylinders under Internal Pressure

We carried out internal pressure tests on a large number of thin wall cylinders of austempered ductile iron (ADI), and investigated the influence of the casting defects on fatigue life distributions. As a result, it was observed that the fatigue life distributions at low-pressure levels were divided into two groups : a short-life group below 1×106 cycles and a long-life group fractured at near 1×107 cycles. After the SEM observation of the fracture surface around the casting defects, the short-life group showed significant roughness, and the long-life group tended to be flat ; it was supposed that the fracture mechanisms of the two were different. Specifically, in the short-life group, a proportional relationship was observed between the initial stress intensity factor of the casting defect and the fatigue life on the double log scale. It is apparent that crack propagation of the short-life group was caused under the Stage 2 process in which the Paris law was effective.

The correlations among weibull parameters of fatigue life distribution and one-parameter expression.

Using the three-parameter Weibull distribution function, we analyzed two kinds of fatigue life data. Three parameters estimated by the Maximum Likelihood Method are dependent on the stress, and have strong relationships between each other. Using these relationships, we proposed a new formula of Weibull distribution, which has only one parameter, shape parameter m. and three material constants. We clarified the meaning of each material constants included in the formula of the distribution function by numerical simulation. We applied this new distribution to the actual data, namely, the rotating bending test data of S 45 C and repeated internal pressure test data of A 7075-T 73 cylinders, which proved the validity of the new equation.