A comparison of crack growth behaviour in several full-scale airframe fatigue tests

Metal fatigue plays a major role in the design of optimised flight vehicles. Economics dictate that modern air vehicles be optimised for weight to maximise performance, therefore an ability to accurately design against the possibility of fatigue failures is paramount. An essential element of the airframe design and certification process is the full-scale fatigue test, where an airframe is subjected to realistic variable amplitude loads which are representative of those predicted to occur over the life-of-type. Generally these fatigue tests are cycled until the equivalent of several lifetimes is achieved or an unrepairable failure occurs. The results of the full-scale fatigue testing of aircraft structures are generally considered to be the foundation for confirming and managing continuing structural airworthiness. This paper reviews fatigue crack growth data from a significant number of fatigue tests on several different military aircraft types. The data analysed generally include the primary crack(s) leading to failure of the test article. The aircraft considered include the F/A-18, F-16, F-111, A4,A7, Mirage III-0, P3C, T37B and the PC-9. It was instructive to compare the crack growth histories from these tests as in general these airframes were designed for different requirements, using different metallic materials, manufacturing processes and analysis tools, and generally were the result of different design regulations or requirements. As a result of this review of the cracking found in these tests it was concluded that a relatively simple crack growth model adequately represents typical crack growth and that this model can be used to aid in the optimisation of fatigue design such that an airframe will just survive the certification fatigue test requirements. The results of this investigation should assist in the development of future optimised structures and allow a better understanding of the limitations caused by fatigue crack growth in airframes. (c) 2006 Elsevier Ltd. All rights reserved.