Investigation of functionally graded material cores for tapered closeouts in sandwich composites

Includes bibliographical references (p. 105-109)In tapered closeouts designs, the sandwich laminate is transitioned to a solid laminate, with high compressive strength, by eliminating the core. Tapered closeouts are found to be prone to fatigue failure by delamination. The delaminations initiate at the region where the upper and lower facesheet meet to form a solid laminate, due to the presence of voids or resin rich areas. These cracks then grow along the facesheet-core interfaces and lead to fatigue failure by delamination. Improving the fatigue life of sandwich closeouts requires a better understanding of the stresses and the damage growth mechanisms. The goal of this study is to elucidate the deformation mechanisms, load transfer, and interface stresses in the tapered close out section of a sandwich. The investigation of sandwich closeouts were performed using finite element analyses (FEA). The effects of geometry and facesheet to core stiffness on the deformation and stresses in the closeout regions are presented. The symmetry of the taper geometry and the interaction of the facesheet and core local deformation at the closeout regions are shown to provide anisotropic shear-bending coupling and shear-extension coupling. Large facesheet-core interfacial shear and normal stresses arise at the closeout region due to the localized bending at this region. The preliminary investigations also showed that the gradient in the geometry of the core leads to gradients in deformation and stress in the region. To compensate for stiffness reduction in the core geometry, use of functionally graded material (FGM) for the cores at the closeout locations are investigated. FGM cores with various gradations in the elastic modulus of the core along the closeout region were analyzed. The results indicate that increasing the core modulus at the tapered closeout apex region is effective in reducing the local deformations. However, these increases lead to small increases in stress. The increase in stress in the FGM core is not a significant concern, because in FGMs increasing stiffness can also lead to increase in strength. Decreasing the core modulus at the closeout apex regions was found to be most effective in reducing the stresses. However, such a design has large localized deformations and reduced structural stiffness. The analyses indicate that a FGM material with decreasing inplane modulus, but increasing out of plane and shear modulus will be an optimum choice for the core material at the closeout location