Stiffnesses and Bucklings

In principle, sandwich materials can be analyzed as RP structures. Structures and sub-structures composed of a number of different components and/or materials, including traditional materials, obey the same principles of design analysis. Stresses, strains, and displacements within individual components must be related through the characteristics (anisotropy, viscoelasticity, etc.) relevant to the particular material also loads and displacements must be compatible at component interfaces. Thus, each individual component or sub component must be treated using the relevant methods. 

Load and support conditions for individual components depend on the complete structure (or system) analysis. For example, if a panel is mounted into a much more rigid structure, then its support conditions can be specified with acceptable accuracy. However, if the surrounding structure has comparable flexibility to the panel, then the interface conditions will depend on the flexural analysis of the complete structure. In a more localized context, structural stiffness may be achieved by ribbing, and relevant analyses may be carried out using available design formulae (usually for elastic behavior) or finite element analysis. But necessary anisotropy or viscoelasticity complicate the analysis, often beyond the ability of the design analyst. 

The primary structural role of the face/core interface in sandwich construction is to transfer transverse shear stresses between faces and core. This condition stabilizes the faces against rupture or buckling away from the core. It also carries loads normally applied to the panel surface. They resist transverse shear and normal compressive and tensile stress resultants. For the most part, the faces and core that contain all plastics can be connected during a wet lay-up molding or, thereafter, by adhesive bonding. In some special cases, such as in a truss-core pipe. 

With most typical sandwich constructions, the faces provide primary stiffness under in-plane shear stress resultants direct stress 

The stiffness of the face and core elements of a sandwich RP must be sufficient to preclude local buckling of the faces. Local crippling occurs when the two faces buckle in the same mode (anti-symmetric). Local wrinkling occurs when either or both faces buckle locally and independently of each other. Local buckling can occur under either axial compression or bending compression. Resistance to local buckling is 

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Amos, R. J. (1999). On stiffness and buckling of adhesively bonded rocket motor tubes at high temperature. Extended Abstracts to Adhesion 99, lOM Communications, pp. 129-134. 

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