Development of Jet Transport Airframe Fatigue Test Spectra

Aircraft development programs commonly employ large tests to substantiate new design features. Full-scale airframe static and fatigue tests are conducted to demonstrate that the structure has sufficient strength and will be economical to maintain in service. Full-scale fatigue testing is accelerated relative to actual usage so that areas that exhibit early fatigue problems can be located and corrected in a timely manner. Development of the airframe fatigue test spectra for two new jet transport models is described. These test spectra were based on the concept of applying loads as realistically as possible while conforming to program schedule and test equipment limitations. The frequency and magnitudes of loads were determined to meet observed statistical criteria without relying on damage model assumptions. Test loads were applied in blocks of 5000 flights using five different flight types. Each flight type employed up to 25 flight segments with five load levels for nine major gust and maneuver segments. Test time objectives were met by tailoring the spectra truncation levels. Ancillary panel tests were conducted to evaluate the impact of spectrum variables. The mixture of flights was constructed such that unique fracture face marking would result to simplify subsequent optical microscopy.