Postbuckling Analysis of Composite Shell Structures: Toward Fast and Accurate Tools With Implicit Fem Methods

Ideas and tools for improving the response time of composite shell postbuckling calculations by means of the finite element (FE) method are presented. While traditional FE tools are able to predict elastic response relatively accurately, they are slow in analysis response time, a situation which becomes far worse if damage phenomena are to be included in the simulations. This paper describes the B2000++ FE-based tool for postbuckling analysis of composite panlels, one of the objectives of the COCOMAT EC-funded research project. The developments comprise a static nonlinear FE continuation solver with artificial damping, a line search technique for improving the convergence of the Newton algorithm, and an implicit dynamic nonlinear solver with linear multistep time integration and variable time step size controled by a local time integration error estimation. In addition, an efficient and robust family of elements — specifically, shell elements with assumed natural strain and enhanced assumed strain to reduce locking, automatic selection between five or six degrees of freedom per node and continuity of the shell normal between elements for proper handling of shell intersections and for correct transfer of twisting moments in curved shell models — is described. If the material is linear, the elements feature preintegration through the thickness of linear materials to reduce the cost of evaluating the elements' first and second variations. The solvers and elements are implemented in the B2000++ FE simulation environment, which provides fast sparse linear solvers and FE assemblers running in threaded mode on multicore processors, thus resulting in short analysis response times, particularly for postbuckling analysis of composite panels. Validation of the developments is illustrated.