The comparison of complex load sequences tested at several stress levels by fractographic examination

Abstract

The Australian Defence Science and Technology Organisation (DSTO) is currently assessing the fatigue life of the F/A-18 aircraft amongst other activities. Projects include full-scale testing as well as coupon test programs. One of the coupon test programs recently completed at DSTO was undertaken to evaluate fatigue performance under a variety of aircraft operational flight profiles. The program consisted of six test phases representing four different reference stress levels with the fifth phase designed to evaluate the effect of overload spikes on fatigue life and the sixth phase designed to evaluate material batch effects. While six loading sequences or spectra were evaluated in the test program only two were tested in all five phases. The coupons were manufactured from the aluminium alloy 7050-T7451 and were representative of the material and geometry of a typical fatigue critical part of the F/A-18 aircraft structure. A minimum of five specimens were tested for at least the two spectra in each of the first four phases. A minimum of three specimens was used in phases five and six. Consequently, more than 120 specimens in total were tested and 50 examined fractographically in this test program. This paper presents a summary of both the total life results as well as the crack growth rates for these specimens. Other aspects considered include the influence of the surface finish of the coupons and the identification of the nature of the crack initiation. This test program allowed two quite different spectra to be compared at several reference stress levels to determine the relative severity of each. Also sufficient numbers of specimens were tested to allow trends and scatter in the data to be fully evaluated.