Assessing relative spectra severity using single fatigue test coupons

Abstract

A common requirement of airframe fatigue analysis is to know the relative fatigue severity of two variable amplitude load spectra when they are applied to the same components or structures. For example, when the predicted service spectrum used in a full-scale airframe fatigue test is later found to be unrepresentative of the average fleet usage, it is necessary to determine the extent to which the service spectrum is more or less severe than the test spectrum. In such cases, methods are required to allow the spectra to be compared so that the results of the test may be used to represent in-service usage. Whilst conventional fatigue prediction tools/models are available for this task, these methods usually contain uncertainties which are difficult to assess without resorting to extensive coupon test programs. In this paper, a very small coupon test program is used to asses the relative severity of two spectra for crack growth from small natural cracks to failure. The two spectra were applied sequentially to fatigue coupons and the crack growth data was assessed using quantitative fractography. This allowed direct comparison of the crack growth rate from the two spectra at different times in the crack growth life.