The scatter factor in the reliability assessment of aircraft structures

The lack of reliability of the analytical methods of prediction of fatigue lives of structural parts and complete structures requires the use of factors to be applied to the results of full-scale fatigue tests of aircraft components and of structures under loading representative of conditions of service. These factors, so far, have been selected on the basis of subjective judgment based on engineering experience and therefore cannot be related to any specific level of reliability. An attempt is made to define the scatter factor in probabilistic terms as the ratio between the best estimator of the fatigue life obtained in a small number of tests and the time to the first failure in a fleet of aircraft of specified size, at a specified reliability level. It is shown that with this definition the scatter factor can be related to the result of a single full-scale test of a complete structure and associated with a specific reliability level. The effect of the minimum fatigue life on the scatter factor is studied and its significance demonstrated in a set of tables computed on the basis of the derived equations.