Reliability-Based Scatter Factors. Volume 1. Theoretical and Empirical Results
Final rept. Mar 1977-Mar 1978
MODERN ANALYSIS INC RIDGEWOOD NJ
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A definition of scatter factor is introduced that is rational and at the same time can directly be related to the reality of aircraft design and certification as well as of the full-scale and also coupon fatigue test of structural elements or components. Specifically, the scatter factor is defined as the ratio of the MLE maximum likelihood estimate of the scale parameter of the two-parameter Weibull distribution assumedly describing the life distribution of structural elements or components, to the time to first failure among a fleet of nominally identical elements of components subjected also to nominally identical operating conditions. Freudenthal has used the same definition of the scatter factor, however, under much simplified conditions He assumes that the shape parameter of the Weibull distribution is known. Unfortunately, however, such an assumption is inconsistent with the reality where the Weibull shape parameter easily ranges from 2.0 to 10.0 reflecting the fact that structural elements or components suffer from a variety of sources of randomness in fatigue strength not only from the probabilistic variation of the material property but also from the statistical variation in workmanship associated with, for example, drilling rivet holes in the process of airframe fabrications.