Hot/Wet Environmental Degradation of Honeycomb Sandwich Structure Representative of F/A-18: Discolouration of Cytec FM-300 Adhesive

摘要

Bonded honeycomb sandwich panels, consisting of graphite-epoxy face- sheets adhesively bonded to a honeycomb core, are used on military aircraft such as the F/A-18 due to their high stiffness and low weight Cytec FM-300 is the adhesive extensively used in bonding these composite structures. Water entering these structures can seriously degrade the adhesive bonds between the skin and core and within the core itself. Such degradation poses an issue for structural integrity as shown by a recent study conducted at AMRL. It is often difficult to know whether the internal structure has had prior exposure to water as the structure may dry yet the bonds may be degraded. It has been observed that in some failed components the adhesive changes colour from its original post cure green colour to a light/dark brown. A laboratory experiment was conducted on Cytec FM-300 adhesive to assess the colour change after exposure to a range of hot/wet environments typical of aircraft service. It was found that the adhesive undergoes differing colour changes depending on exposure conditions. The light brown colour seen in some failed components was produced when the relative humidity was > 95% and the temperature exceeded 90 deg C for an exposure period in excess of 20 weeks. The study concludes that Cytec FM-300 adhesive that has changed colour to a light or dark brown colour has been directly exposed to a hot/wet environment.