Compression facing wrinkling of composite sandwich structures

A thorough investigation was conducted of face wrinkling failures of sandwich columns under compression and beams in three- and four-point bending and cantilever beams under end loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and closed-cell PVC foam cores. The constituent materials were fully characterized. Face wrinkling failures were observed in sandwich columns and beams with foam cores, but not in those with honeycomb cores. Several wrinkling failure loads were measured and compared favorably with an early expression of Hoff and Mautner for the case of columns and long beams where the core is in the linear elastic range. However, for short span beams, core failure precedes face wrinkling. Core yielding and stiffness loss reduce core support of the facing and precipitate facing wrinkling at a lower stress. For this case a modified Hoff and Mautner expression, where the values of the reduced core Young's and shear moduli in the through-the-thickness direction are introduced, appears more appropriate. It was concluded that failure by wrinkling is prevalent in the case of low through-the-thickness stiffness and long beam spans. In other cases, other failure modes including shear core failure, compressive facing failure, face sheet debonding or indentation failure may occur.