Crack Propagation in Clad 7079-T6 Aluminum Alloy Sheet Under Constant and Random Amplitude Fatigue Loading

This paper presents the results of a comprehensive experimental study, mainly using square panels, of a recently introduced high-strength aluminum alloy sheet material and its resistance to the propagation of fatigue cracks. An axial-load fatigue machine, applying both constant and Rayleigh random loads, was developed for the test program, and fatigue cracks in the panels were continuously monitored by a new servo-controlled eddy-current crack follower. The resulting traces of crack length permitted the study of several crack growth laws, by manipulation of the loads to maintain a theoretically constant crack growth rate. From the tests where the load levels were not altered, an optimum thickness of sheet (yielding a minimum average crack growth rate) was indicated. Load-shedding, a phenomenon inherent in the failure of elements in redundant (fail-safe) structures, was simulated in many of the tests. It was found that appreciably different load-shedding histories still yielded the same instantaneous crack growth rate for a given instantaneous level of stress intensity factor. It was found that fracture mechanics provided the most effective approach for the study of fatigue crack propagation, either by constant amplitude or by Rayleigh random amplitude loading.