Moisture Effects and Peel Testing of Polymethacrylimide and Honeycomb Core in Sandwich/Skin Structures.

摘要

One of the greatest concerns of the aerospace materials engineer is how a manufactured final part will change structurally, dimensionally, and weight-wise during its lifetime on an aircraft. Polymethacrylimide (PMI) foam has been chosen as a core material for use on the Army's Advanced Composite Airframe Program (ACAP), and is a candidate for use in other current and future Army helicopters (JVX, LXH). The main objective of this paper is to answer a few of the questions concerning structural integrity, dimensional stability, and weight gain of this closed cell thermoplastic structural foam by processing typical sandwich/skin structures, and determining the effects of moisture absorption on peel strength. Results of the PMI foam's exposure to the combined effects of elevated temperature and high humidity conditions were compared directly to identical sandwich/skin constructions using aramid honeycomb core in both 4 and 6 lb per square foot densities, used widely by the aerospace industry. Polymethacrylimide foam cores and honeycomb cores were used both in the "as-is" condition received by the manufacturers, and in an annealed or heat treated condition prior to layup of the sandwich constructions to determine their respective dimensional stability during a standard autoclave curing cycle. Two 350 degree F. cure structural film adhesive prepregs were used for autoclave bonding +/- 45 degree S-2 glass/epoxy precured facesheets onto both the foam and honeycomb constructions to determine their respective surface bonding ability and relative moisture barrier properties. The cured sandwich panels were then finally machines to ASTM D 1781 climbing drum peel test specimens and subjected to 95% relative humidity at 160 degrees F. for exposures up to 720 hours and subsequent redry both with and without RTV-sealed edges to determine comparative moisture weight gains and their effect on peeling torque.