Stress Analysis of Tapered Sandwich Panels with Isotropic or Laminated Composite Facings

Structural sandwich construction is used in many air and space vehicles, cargo containers, boats and ships. Connection of the sandwich construction component to a framework or substructure is a critical issue in the detail design for sandwich construction. The tapered connection where the facings are drawn together at the support is one of the most efficient types of connections in composite sandwich construction. We present a tapered sandwich theory that is simple to use, yet accurately predicts the stresses and deflection of both symmetric and non-symmetric tapered sections. In this investigation we assume that the facings are relatively thin and therefore in a state of plane stress. The core is assumed to be inextensible in the thickness direction and carry only transverse shear stress. Since the facings are tapered, the plane stress conditions for each facing are established in a local coordinate system with axes oriented parallel and normal to the facing to obtain the reduced stiffness for the tapered laminae. The force and moment resultants are obtained by integrating the stresses in the facings and core. The resulting resultants are related to the reference surface strains and curvatures through the familiar [A], [B] and [Dl matrices. The deflections are computed using an energy method approach. The shear and peeling stresses at the interface between the core and the facings, which may cause delamination at the interface, are computed by integrating the three-dimensional equilibrium equations along a straight path that is perpendicular to the facings. We also have systematically derived a total of 12 elastic stiffnesses that couple the force and moment resultants to the transverse shear deformation. Six of the twelve elastic couplings are due to the tapered sandwich construction itself, irrespective of whether the facings are isotropic or anisotropic, whereas the remaining six elastic couplings are present only for anisotropic laminated facings. Their influence on the behavior of tapered sandwich beams of the stiffnesses is investigated. Analytical models have shown that the behavior of the tapered section is counterintuitive and that, for a tapered cantilever sandwich beam with fixed dimensions at the clamped edge, there is an optimum taper angle where the tip deflection is a minimum. This decrease in deformation with increasing taper angle is due to the participation of the facings in resisting transverse shear loads. Results from the tapered sandwich theory show very good comparison with finite element models for several case studies. The theory enhances our understanding of tapered sandwich beams and clarifies the causes of premature failure encountered at the interfaces between the core and facings