Investigation of Fatigue Cracking on LAU-7/A Launcher Housing,
DEFENCE SCIENCE AND TECHNOLOGY ORGANIZATION CANBERRA (AUSTRALIA)
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A number of LAU-7A launcher housings on Australian FA-18 aircraft have been found to be fatigue cracked at the location of the forward missile hanging bracket. This was initially ascribed to poor aft snubbing of the missile in the launcher guide rail. An investigation of the response of the LAU-7A launcher housing configuration to static and fatigue loading was undertaken to determine the failure mechanism. A test fixture was designed to apply static loads to a launcher housing via a dummy AIM-9 missile. The results showed that the strains at the points of engagement of the missile hanging brackets with the launcher guide rail were not significantly influenced by the effectiveness of the aft snubbing of the missile. The launcher housing was then fatigue tested using spectrum loading. The loads applied to the test articles were derived from a number of Nz load spectra of Australian FA-18 aircraft. The load levels were factored up to account for the dynamic effects of the flexible wings of the aircraft. To ascertain whether the loads achieved in the experimental study were appropriate for the fatigue testing of the component, the fracture surfaces derived from the fatigue test were compared with several surfaces removed from launcher housings which had failed under operational loads. The results showed that the use of factored Nz loads was only an approximate simulation of the wing tip environment, but in the absence of a wing tip spectrum these loads gave approximately similar fatigue fracture surfaces to those of the components in service. The results showed that the cracking was largely induced by the inertial loads experienced by the missile, which are transferred to the guide rails of the LAU-7A housing.