Damage Resistance and Damage Tolerance of Thin Composite Facesheet Honeycomb Panels

An experimental and analytical investigation was made into the behavior of sandwich panels with two- and three-ply woven fabric graphite/epoxy facesheets subjected to low-velocity impacts. The facesheets were made from AW193PW/3501-6, a plain wave fabric made from Hercules AS4 fibers and 3501-6 epoxy. The core material was 3.0 pcf Nomex. Damage was observed to begin at an energy level of 1.0 ft lbs. Damage consisted of core crushing, matrix cracking, fiber breakage, and delaminations. Panels were also subjected to static indentation which produced damage indistinguishable from impact damage. Damaged panels were then tested in compression to failure. Panels with (±45) facesheets were found to be notch-insensitive and failed by local facesheet buckling (also called "facesheet wrinkling") at a constant value of net-section stress. Panels with (0,90) facesheets were notch-sensitive and failed in the same manner at a lower value of net-section stress. Analytical models of the impact event and and compression test are able to predict the amount of damage caused and the residual strength of the damaged panel. These models comprise a fast, preliminary analysis tool for use in design of sandwich panels