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Scatter in fatigue test results is one of the more important factors to be considered in predicting a safe service life for aircraft. A scatter factor is developed using statistical techniques to cover the variation in fatigue life among nominally identical specimens subjected to identical loading histories. A procedure is included to handle those cases where the life demonstrated in the laboratory is a least-of-two test result. The statistical analysis is supplemented by a review of numerous constant- and variable-amplitude test results which reveals that the log normal distribution is applicable and the standard deviation of log life is constant within a certain range of average cycles to failure. Fatigue test results of 120 composite aluminum structures are utilized to compute an unbiased sample standard deviation. A significant development of the derived statistical approach is the removal of the necessity to determine confidence intervals for the true mean and standard deviation. This is accomplished by the introduction of a new random variable from which these unknown population parameters vanish.
fatigue (materials), aircraft, aluminum alloys, statistical distribution, probability, standard deviation, load spectrum
Impellizzeri, L. F.
Senior strength engineer, McDonnell Aircraft Corp., St. Louis, Mo.