A Sublaminate Generalized Unified Formulation for buckling and wrinkling of sandwich plates

This contribution deals with thermal and elastic instabilities of sandwich panels, from long wavelength buckling to short wavelength wrinkling. A recently proposed Sublaminate Generalized Unified Formulation (S-GUF) is employed to derive the two-dimensional plate stability equations through the introduction of axiomatic assumptions within the classical displacement based approach. The S-GUF is a variable kinematics modeling approach, which permits to subdivide the composite stack into an arbitrary number of sublaminates, into each of which the displacement field is expanded across the thickness by means of combinations of Legendre polynomials up to an arbitrary order. As a result, variable kinematics Equivalent Single Layer (ESL) and Layer-Wise (LW) models can be combined within the composite cross-section. This approach is particularly meaningful for assessing computational models for sandwich panels for the thin and stiff skins demand different approximations with respect to the relatively thick and soft core. The plate stability equations are obtained in the framework of Euler’s linearized adjacent equilibrium method. The corresponding eigenvalue problem for the plate is constructed by referring to a Ritz solution method. The complete and admissible functional space can be spanned by Chebyshev polynomials or trigonometric functions, depending on the application and the involved boundary conditions. The developed software is very flexible as it can manage a large number of configurations and provide a consistent assessment of different plate models. Buckling and wrinkling results for sandwich panels under various boundary conditions and initial stress states are presented and compared against reference solutions. Initial stresses are included that may be of thermal nature. Moreover, variable stiffness composite faceplates can be handled, in which the local fiber orientations of the plies can be tailored for optimizing the structural response. It is demonstrated that high-order LW models are capable of providing highly accurate results. The result of the plate models assessment can be used to identify guidelines for constructing a suitable reduced model that can cope with the specific response characterizing the sandwich instability.