Investigation using quantitative fractography of an unexpected failure in an F/A-18 centre fuselage bulkhead in the FINAL teardown program

As part of the structural life assessment of the F/A-18 aircraft in service in the Royal Australian Air Force (RAAF), the Defence Science and Technology Organisation (DSTO) have completed the teardown of several ex-service F/A-18 wing attachment centre barrels in the Flaw IdentificatioN through the Application of Loads (FINAL) program. Prior to teardown, each centre barrel was fatigue cycled under a wing root spectrum in a test rig until each of the three fracture-critical bulkheads (denoted Y453, Y470.5 and Y488) had failed. The Y488 bulkhead of the third centre barrel cycled in the FINAL program failed after approximately 24 blocks of applied loading from fatigue cracking in one of the left-hand side main landing gear uplock attachment holes. Even with the addition of the bulkhead’s service life, the time to failure was well short of the demonstrated fatigue test life on which the RAAF fleet is managed. Thus, the cause of this failure needed to be investigated.A failure investigation using quantitative fractography and finite element analyses of these fatigue cracks, as well as an examination of fatigue cracks at similar locations on other FINAL centre barrels was conducted. Crack depth versus time during FINAL cycling data were generated and these data were used to show that the rate of crack growth for the cracks that grew from the failed hole was much greater than the rate of crack growth at the other identically loaded main landing gear uplock holes. Examination of the crack growth led to the conclusion that this hole had not been cold expanded correctly in production, whereas the other holes examined had. In addition, the examination of the other cracks at this location showed that cracking is likely to be found in service at these holes regardless of the cold expansion process, and that these cracks are very unlikely to cause a failure within the planned life of the centre barrel when the cold expansion process has been implemented correctly.