Analysis and Test of Graphite/Epoxy Sandwich Panels Subjected to Internal Pressures Resulting from Absorbed Moisture

A simple analysis procedure is developed to predict graphite/epoxy sandwich panel internal pressures at elevated temperatures, based on moisture characteristics of the panel constituents, prior environmental exposure, and test temperature time histories. Pressures so predicted are shown to correlate well with measured panel pressure test results. It is further shown that, due to environmental absorbed moisture, internal panel pressures of 65 psig (4.5 kg/cm²) or greater can be produced at 350°F (177°C) in graphite/epoxy sandwich panels with Nomex core. Panel internal pressure test failures at 350°F (177°C) for various initial environmental exposures are also shown to be dependent on the moisture level in the METLBOND 329-7 adhesive at time of failure. Panel failure pressures in excess of 150 psig (10.5 kg/cm²) at 350°F (177°C) are reported for low adhesive moisture levels (room temperature equilibrium relative humidity of 20 percent or less), but as low as 60 psig (4.5 kg/cm²) for high adhesive moisture levels (70 percent relative humidity equilibrium at room temperature). Lap shear and flatwise tension test results of the METLBOND 329-7 adhesive at room temperature and 350°F (177°C) with absorbed moisture are also presented; these tests exhibit similar moisture strength degradation and failure modes as that demonstrated by the panel pressure test specimens.