FAILURE MECHANISMS OF COMPOSITE SANDWICH STRUCTURES

Sandwich construction is of particular interest and widely used, because the concept is very suitable and amenable to the development of lightweight structures with high in-plane and flexural stiffness. Sandwich panels consist typically of two thin face sheets (or facings, or skins) and a lightweight thicker core. They display various failure modes under general bending, shear and in-plane loading. The failure modes can be predicted by conducting a thorough stress analysis and applying appropriate failure criteria in the critical regions of the beams. The analysis is difficult because of the nonlinear and inelastic behavior of the constituent materials and the complex interactions of failure modes. A thorough investigation of the failure mechanisms of composite sandwich beams under four- and three-point bending and cantilever beams was undertaken. The beams were made of unidirectional carbon/epoxy (AS4/3501-6) facings and a PVC closed-cell foam (Divinycell) core. Two types of core material H100 and H250 with densities 100 and 250 kg/m3, respectively, were used. The thickness of the facings and core were 1 mm and 25.4 mm, respectively. The uniaxial tensile, compressive and shear stress-strain curves of the facings and core materials were obtained. It was found that unidirectional carbon/epoxy after an initial linear part exhibits a characteristic stiffening nonlinearity in tension and a softening nonlinearity in compression. The longitudinal strength in tension is about fift