Many structural elements external to the fuselage of transport aircraft are produced using polymer sandwich honeycomb composites; composite rudders or ailerons, for example. These components experience the environmental extremes typical of Class A airspace. In particular, temperatures at altitude often fall to -50ºC or below. Meanwhile, it has been shown that the humidity within the core region of a polymer honeycomb panel will slowly increase with time due to exposure to typical terrestrial humidity levels. Although immediately after production the internal core humidity level of sandwich composites is normally low (perhaps approaching zero humidity), over time water molecules diffuse through otherwise undamaged gelcoat and composite facesheets, slowly increasing internal humidity levels. If internal humidity becomes high enough then internal water vapor may condense and freeze within the core while the aircraft is at altitude. This implies that a condense-freeze-thaw-evaporate cycle may occur within a sandwich structure during the normal duty cycle of a commercial transport aircraft. The objective of this study is to determine if this condense-freeze-thaw-evaporate cycle leads to internal damage that may be detrimental to the mechanical performance of honeycomb sandwich structures. Specifically, the bending stiffness and mode I strain energy release rate (GIc) of honeycomb composites will be measured, under four conditions:
- as-produced (dry),
- following thermal cycling from room temperatures to -50 ºC under dry conditions,
- following exposure to 70%RH and 50%C for several months, and
- following exposure to 70%RH and 50%C for several months as well as thermal cycling from room temperatures to -50 ºC.