Numerical Investigation of Tapered Sandwich Closeouts with Isotropic Functionally Graded Cores

Abstract

Tapered closeouts that transition a sandwich composite to a solid laminate enable sandwich composites to be attached to other structures using mechanical fasteners. Eliminating the core provides increased normal compression and bearing strengths required at fastening locations. The non-uniform geometry at closeout sites leads to localized stress concentrations. Tapered closeouts in sandwich composites are prone to fatigue failure by delamination. The goals of this study were to elucidate the deformation and interface stresses in sandwich tapered closeouts, and to investigate the use of functionally graded material (FGM) for the cores at the closeout locations to mitigate the large deformation and stress gradients in this region. 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, if there is a concomitant increase in the strength of FGM materials with increase in stiffness. Decreasing the core modulus at the closeout apex regions was found to be most effective in reducing the stresses, while leading large localized deformations and reduction in structural stiffness. The analyses indicate that a FGM material with decreasing inplane modulus, but increasing out of plane and shear modulus, and increased interface strength properties will be an optimum choice for the core material at the closeout location.