Non-linear stability of isotropic stiffened composite channels under compression

In this study, compressed laminated channels with isotropic stiffeners on their free longitudinal edges are investigated. The use of edge stiffeners in the form of clamps or angles made of an isotropic material significantly affects the behaviour of the structures under study. Two cases of laminate layup are examined, assuming that the columns are freely supported. It is assumed that there are strong interactions between the eigenmodes. In addition to that, in the two analysed cases, the lowest eigenmode is local and does not cause structural failure. Thus, the expected ultimate load-carrying capacity values will be greater than the lowest eigenmode. The perturbation method and the finite element method are employed to solve an eigenproblem and a problem of nonlinear stability loss. The adequacy of the models and boundary conditions used in both methods is validated by comparing local, global, symmetric and antisymmetric as well as distortional eigenmodes. Postbuckling equilibrium paths and ultimate load-carrying capacity values are determined for all analysed cases. In the perturbation analysis, results obtained with the 4- and 5-mode approaches are compared for low pre-deflection amplitudes. Results obtained by all methods showed a very high qualitative agreement. The geometric imperfections used in numerical simulations were assumed to be identical to the eigenmodes with low amplitudes. The FEM results show that this has no effect on the postbuckling equilibrium paths and ultimate load-carrying capacity of the columns. A low amplitude value is enough to disturb the lowest eigenmode.