Sandwich structures composed of two thin sheets separated by a core which can support only transverse shear are both statically and kinematically determinate “in-plane”, allowing in-plane elastic-plastic behavior of such structures to be found in terms of sheet material properties. Unlike homogeneous shells, the shear load-deformation relations separate from in-plane behavior, and all the usual difficulties associated therewith disappear. The method for deriving in-plane constitutive relations for thick (t/R terms not negligible compared to unity), doubly curved, nonsymmetric shells with arbitrary-known sheet elastic-plastic behavior is presented. The form of the equations for yield and limit behavior is discussed, including yield surfaces and flow laws. Comparison of thin versus thick shell theory shows that nonconservative strength predictions can result from neglecting t/R terms. Use of the theory is illustrated by an example which shows application to fibrous composite materials and the effect of shell thickness.

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