The fatigue critical structures of military aircraft are generally subjected to variable amplitude flight spectrum loading. Maintaining aircraft structural integrity to ensure safe operation of the fleet is critically dependent on accurate analysis and reliable prediction of fatigue crack growth in those structures under service operating conditions. To achieve this goal, laboratory experimental methods that can accurately measure and monitor fatigue crack growth under variable amplitude loading are required. This can be challenging as no test standard exists to guide the process of fatigue crack growth measurement under variable amplitude loading conditions to ensure the accuracy of the test results. This challenge was addressed by developing a modified compliance method as described in this paper. The results presented employed a modified compliance method complemented with a travelling microscope technique and marker band loads. The modified compliance method developed is able to measure in-situ, fatigue crack growth of standard compact-tension specimens under a fighter flight spectrum loading. The marker band loads and microscope readings were used to assist the post-test validation using quantitative fractography. The results from this study have demonstrated that the modified compliance method can produce consistent and accurate fatigue crack growth data under variable amplitude loading conditions.
The Effect on Fatigue Crack Growth Under Spectrum Loading of an Imposed Placard “G” Limit
