A statistically-formulated fracture mechanics model for crack growth under sustained load is used to analyze crack growth data from 23 compact tension specimens of IN100, a turbojet engine disk material. The procedures characterize crack growth rates assuming that the growth rate is a lognormal random variable. The mean and standard deviation of the growth rate are determined from test data using the method of maximum likelihood. From these estimates, a lognormal creep crack growth rate model is developed from which is derived a statistical distribution of the crack size at any time. The distribution of time to reach some critical crack size is also presented. These distributions allow for the determination of the effect of hold time in the loading cycle on the life prediction of gas turbine engine disks.

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