FATIGUE DESIGN AND TEST PROGRAM FOR THE AMERICAN SST

Summary The American supersonic transport will be a large, variable-sweep-wing airplane constructed primarily of titanium alloys. It will cruise at Mach 2·7 and have a service life in excess of 50,000 flight hours. In addition to the perplexing economic aspects of structural fatigue for today's airplanes, consideration must be given to the thermal environment and the various load distributions resulting from different wing positions. The temperature magnitudes and variations associated with supersonic flight affect the structural properties and produce thermal stresses. Long service life data for titanium airframe structure are lacking; however, the use of titanium permits higher cruise speeds and proportionately lower thermal stresses than are obtainable with aluminium alloys. The use of titanium also results in a lighter airframe than one constructed of steels or the high nickel super alloys. In addition, the movable wing has a significant effect on load distributions. The fatigue considerations for the SST necessitate a structural development and verification program more comprehensive and demanding than is required for today's airplanes. Fatigue structural integrity will be provided and demonstrated by an integrated program of analysis, development testing, and verification testing. A spectrum of operational flights is established, based on predicted SST usage. This spectrum, along with training and check flights, is used to establish the fatigue loads and temperatures for use in the fatigue analysis. In addition to parametric evaluations, fatigue analyses are used to develop fatigue test conditions, test life requirements, fatigue design stress levels, and fatigue life predictions. Full advantage is taken of a comparative type of approach. This method is based on integrating fatigue analysis, test, and service experience from the large fleets of contemporary Boeing airplanes and extrapolating this experience to the SST by the relationships between design stresses, operating stresses, structural fatigue quality, and fatigue environment. Fatigue tests are accomplished on numerous specimens ranging from small single-element coupons through full-scale components to, ultimately, the complete airframe. The smaller specimens are used primarily for screening materials, processes, methods of fastening, and temperature effects. Considerations of temperature magnitudes and time at temperature further complicate the already difficult problem of suitable simulation of service environment in laboratory testing. Considerable effort is devoted to the evaluation of techniques for condensing test time. As the number of variables is reduced, progressively larger and more representative specimens are tested to conditions more closely simulating those expected in actual service. Full-scale components of all major portions of the airframe will be tested. In addition, three identical full-scale specimens, representing a portion of the wing box, will be tested with different load and temperature programs. The fatigue integrity will be verified ultimately by the full-scale airplane fatigue test This, in essence, will be the second test for a major portion of the structure because of the extent of the full-scale component tests.