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 fourand threepoint 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/m, 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 fifty percent higher than that in compression. The core material showed quite different stress-strain behavior in tension and compression. In compression the stress-strain curve shows a linear elastic – perfectly plastic behavior. At some point the load increases rapidly as the cells fully collapse and densification takes place. For the H100 core tensile strength along the longitudinal direction is eighty percent higher than the compressive strength, while for the H250 material the tensile strength is fifty percent higher. Both materials are anisotropic with the through-thickness tensile strength higher than the longitudinal tensile strength. The failure behavior of the core materials was modeled by the Tsai-Wu failure criterion. Sandwich beams were loaded under bending moment and shear and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, core failure and facing wrinkling. The various modes have been studied separately and both initiation and ultimate failure have been determined. Initiation of a particular failure mode and triggering and interaction with other failure modes was also investigated. A detailed stress and failure analysis was performed taking into account that a biaxial state of stress is developed in both the core and the facings. The stress state was combined with the Tsai-Wu failure criterion for both the core and the facings. The initiation of the various failure modes depends on the material properties of the constituents (facings, adhesive, core), geometric dimensions and type of loading. The appropriate failure criteria should account for the complete state of stress at a point, including twoand three-dimensional effects. Failure modes were discussed according to the type of loading applied. In sandwich columns under compression, or beams in pure bending, compressive failure of the skins takes place if the core is sufficiently stiff in the through-the-thickness direction. Otherwise, facing wrinkling takes place. In the case of beams subjected to bending and shear the type of failure initiation depends on the relative magnitude of the shear component. When the shear component is low (long beams), facing wrinkling occurs first while the core is still in the linear elastic range. The critical stress at wrinkling can be predicted satisfactorily by an expression by Hoff and Mautner and depends only on the facing and core moduli. When the shear component is relatively high (e.g., short beams), core shear failure takes place first and is followed by compression facing wrinkling. Wrinkling failure follows but at a lower than predicted critical stress. The predictive expression must be adjusted to account for the reduced core moduli.