Graphite/epoxy honeycomb core sandwich permeability under mechanical loads

Abstract

The air permeability of two honeycomb core sandwich materials as a function of applied shear stress was characterized. The honeycomb core sandwich specimens were provided to Analytical Services and Materials, Inc. by The Boeing Company for evaluation. The core material for the test specimens was either Hexcel HRP-3/16-8.0 or DuPont Korex-1/8-4.5 and was nominally 1.27 cm (0.5 in.) thick. The facesheets where made of Hercules' AS4/8552 graphite/epoxy (Gr/Ep) composites and were nominally 0.15 cm (0.059 in.) thick. The permeability of the sandwich specimens during both static (tension) and dynamic (reversed and non-reversed) shear loads were measured. The permeability was measured as the rate of air flow through the core from a 2.54-diameter circular area of the core exposed to an air pressure of 68.9 kPa (10.0 psig). In both the static and dynamic testing, the Korex core experienced sudden increases in core permeability corresponding to a core catastrophic failure, while the HRP core generally experienced a gradual increase in the permeability prior to a bond line failure. The Korex core specimens failed at lower loads than the HRP core specimens both in the transverse and ribbon directions.