Flaw IdeNtification Through The Application Of Loading (FINAL)

The teardown and inspection of aircraft, which have completed a significant period of service, is a central requirement of many Aircraft Structural Integrity Management Plans (ASIMP). The reasons for this include a need to inspect for the potential onset of wide spread fatigue damage and to assess the impact of corrosion and in-service mechanical damage. Furthermore, service life data from fleet aircraft are required to confirm laboratory fatigue test results and substantiate the assumptions made during safe-life calculations or probabilistic risk and reliability studies. A teardown and inspection of the fracture critical F/A-18 wing attachment bulkheads (or centre barrel - CB) has been initiated to achieve these goals for the RAAF's F/A-18 fleet. Use is being made of ex-service CBs supplied from the Canadian Forces and U.S. Navy (USN) CB replacement programs. Investigations suggest that the largest 'likely' cracks in the critical bulkheads will be less than 1 mm deep at the time a CB is replaced. Since the detectable crack depth threshold for current NDI (using high frequency eddy current (HFEC) detection) is 1.0 mm or greater, these cracks may not be found. To significantly improve the probability of detecting cracks that are below the lower threshold of NDI, an increase in their size by accelerated fatigue testing of the CBs has been implemented. Cyclic loads (using the mini-FALSTAFF spectrum) are applied to the wing attachment lugs of ex-service CBs in a test rig to simulate in-flight wing loads. The loading is of sufficient magnitude and duration to ensure that any existing cracks will be grown to a size that ensures their detection under laboratory conditions. Quantitative fractography has been performed on observed cracking to obtain crack growth data and to determine the size, nature and cause of discontinuities that initiate fatigue cracking. This paper will provide a summary of the teardown philosophy, methodology and preliminary results