Complementary energy theorem for thin composite plates in postbuckling

The thin composite von Kármán plates in postbuckling are considered. Using the first Piola stress tensor and the displacement gradient tensor, the complementary energy variational theorem is proven. The Kirchhoff assumptions are adopted. The plate lay-up is symmetric and pointwise. According to the theorem, at the actual stress state of the plate the complementary energy (as a functional of the internal forces and of the moments) reaches its stationary value. The stationary feature of the actual state is valid as compared to other feasible states satisfying the static equilibrium and the static boundary conditions. The theorem is a consent of the static variational principle. The principle leads to the linear relations between forces/moments, created by the corresponding first Piola stress tensor components, and the 2D-strains/curvatures. An illustrative plate example is given.