An Investigation of Scatter and Distributions for Crack Growth Lives Under Constant Amplitude Stress Intensity Factor Cycling
AIR FORCE FLIGHT DYNAMICS LAB WRIGHT-PATTERSON AFB OH
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It has been recognized that scatter in the crack growth phase as well as in the initiation phase is important in assessing the life of aerospace structures using reliability methods. First estimates of the potential effects of this scatter have recently been evaluated. Selection of potential distributions for the initiation phase evolved over many years both on theoretical bases as a weakest link theory and as an asymptotic distribution and from fits of actual data. Two distributions, the Weibull and the lognormal, have been widely used to fit the distribution of times to crack initiation. In this report, it will be shown that 1 the summation process of incremental crack growth can be used to restrict the class of distribution functions appropriate for describing cycles for a given increment of crack growth 2 nonparametric tests for independence of crack growth increments can be developed and 3 the summation process of crack growth leads to a decreasing standard deviation of cycles as a percentage of the mean number of cycles to grow a crack a given increment the coefficient of variation with increasing length of increments of crack growth.