Evaluation of Composite Honeycomb Sandwich Panels Under Compressive Loads at Elevated Temperatures

Fourteen composite honeycomb sandwich panels were tested to failure under compressive loading. The test specimens included panels with both eight and twenty-four ply graphite-bismaleimide composite facesheets and both titanium and graphite-polyimide core materials. The panels were designed to have the load introduced through fasteners attached to pairs of steel angles on the ends of the panels to simulate double shear splice joints. The unloaded edges were unconstrained. Test temperatures included room temperature, 250°F, and 300°F. For the room and 250°F temperature tests, the 24-ply specimen failure strains were close to the unnotched allowable strain values and failure loads were well above the design loads. However, failure strains much lower than the unnotched allowable strain values, and failure loads below the design loads were observed with several of the 8-ply specimens. For each individual test temperature, large variations in the failure strains and loads were observed for the 8-ply specimens. Dramatic decreases in the failure strains and loads were observed for the 24-ply specimens as the test temperature was increased from 250°F to 300°F. Due to initial geometric imperfections, all specimens exhibited varying degrees of bending prior to failure. All 8-ply specimens appeared to have failed in a facesheet strength failure mode for all test temperatures. The 24ply specimens displayed appreciably greater amounts of bending prior to failure than the 8-ply specimens, and panel buckling occurred prior to facesheet strength failure for the 24-ply room and 250°F temperature tests. For the two 24-ply specimens tested at 300°F, one panel failed due to facesheet strength and the other due to a disbond between the facesheet and core materials.