Fatigue and crack-growth analyses on 7075-T651 aluminum alloy coupons under constant- and variable-amplitude loading

The wings on two Navy aircraft, EA-6B and P-3, are made of 7075-T651 aluminum alloy. These aircraft are both involved in extensive life-extension programs being conducted by the US Navy. The objective of this paper is to present some of the efforts to conduct fatigue and crack-growth analyses with the FASTRAN life-prediction code using Small-Crack Theory. FASTRAN is an advanced non-linear life-prediction code based on cyclic plasticity and the crack-closure concept. For aluminum alloys, such as those used in the older naval aircraft, the fatigue process has been shown to be primarily crack growth from microstructural features, such as inclusion-particle clusters, voids or pits. This paper presents the results of small- and large-crack-growth-rate tests conducted on the 7075-T651 alloy over a wide range of constant-amplitude loading to establish the baseline crack-growth-rate curves for the fatigue analyses. Crack-growth behavior was found to be different in the thickness and in-plane directions. These test data are compared with results from a NASA/CAE Cooperative Program on small- and large-crack growth in a 7075-T6 thin-sheet alloy. Using inclusion-particle-cluster sizes and shapes, like those that are observed in tests, the fatigue and crack-growth lives are predicted on various specimens used to simulate details in a wing panel under either constant-amplitude or simulated wing spectrum loading. The predicted results compared well with the test data.