Approximate Internal Load Prediction in Composite Structures with Locally Buckled Panels

Most aerospace structural designs are complex assemblies of a large number of panel components. In designing such a structure, it is customary to divide the structure into many regions and design them independently/semi-independently. A coarse finite element model of the entire structure is first used to determine loads in each component, and subsequently the components are designed using local constraints and performance requirements for these fixed loads. This two-level approach works well if a linear static behavior is considered. For cases in which local components display postbuckling behavior, the internal load distribution in the structure is altered requiring nonlinear global analysis, which is computationally expensive. This paper focuses on developing a computationally efficient approximate methodology for predicting internal load distribution in a structure considering some or all panels to be buckled. In this paper, a semi-analytical approach based on the Rayleigh-Ritz method and the perturbation approach is developed to compute the postbuckled stiffness of buckled panels. An iterative procedure is suggested to predict the internal load distribution with successive buckling during loading. Three different examples are presented. A simply supported flat composite panel subjected to different edge compression is used to verify the perturbation approach used for the calculation of the postbuckled panel stiffness. A two-panel arrangement with different thicknesses subjected to end compression and a 48-panel composite wing structure subjected to bending load are used as cases for structures with buckled panel components. In all cases, the present approach agrees with full nonlinear analysis in predicting the internal load distribution.