This work intends to demonstrate the importance of geometrically nonlinear crosssectional analysis of certain composite beambased fourbar mechanisms in predicting system dynamic characteristics. All component bars of the mechanism are made of fiber reinforced laminates and have thin rectangular crosssections. They could, in general, be pretwisted and/or possess initial curvature, either by design or by defect. They are linked to each other by means of revolute joints. We restrict ourselves to linear materials with small strains within each elastic body (beam). Each component of the mechanism is modeled as a beam based on geometrically nonlinear 3D elasticity theory. The component problems are thus split into 2D analyses of reference beam crosssections and nonlinear 1D analyses along the four beam reference curves. For thin rectangular crosssections considered here, the 2D crosssectional nonlinearity is overwhelming. This can be perceived from the fact that such sections constitute a limiting case between thinwalled open and closed sections, thus inviting the nonlinear phenomena observed in both. The strong elastic couplings of anisotropic composite laminates complicate the model further. However, a powerful mathematical tool called the Variational Asymptotic Method (VAM) not only enables such a dimensional reduction, but also provides asymptotically correct analytical solutions to the nonlinear crosssectional analysis. Such closedform solutions are used here in conjunction with numerical techniques for the rest of the problem to predict multibody dynamic responses, more quickly and accurately than would otherwise be possible. The analysis methodology can be viewed as a threestep procedure: First, the crosssectional properties of each bar of the mechanism is determined analytically based on an asymptotic procedure, starting from Classical Laminated Shell Theory (CLST) and taking advantage of its thin strip geometry. Second, the dynamic response of the nonlinear, flexible fourbar mechanism is simulated by treating each bar as a 1D beam, discretized using finite elements, and employing energypreserving and decaying time integration schemes for unconditional stability. Finally, local 3D deformations and stresses in the entire system are recovered, based on the 1D responses predicted in the previous step. With the model, tools and procedure in place, we shall attempt to identify and investigate a few problems where the crosssectional nonlinearities are significant. This will be carried out by varying stacking sequences and material properties, and speculating on the dominating diagonal and coupling terms in the closedform nonlinear beam stiffness matrix. Numerical examples will be presented and results from this analysis will be compared with those available in the literature, for linear crosssectional analysis and isotropic materials as special cases.
