Efficient Stiffened Composite Plate Analysis

The high specific strength, high specific stiffness and tailorability amongst many other advantages have made composites an excellent choice in comparison with other material for aircraft. Latest advances in composite fabrication technology like co-curing/co-bonding have resulted in more efficient integral structures. These stiffened co-cured shells or panels constitute a major part of the primary aircraft structures such as wing, fuselage etc. To ensure the safety of the structures, the designers need to understand the behavior of these panels as they are subjected to three dimensional stresses. This is because many unexpected failures of composite structures in the recent past were later ascertained to be directly or indirectly due to out-of-plane loads, ignored during the design. Such failures have demonstrated the acute need to overcome the difficulties encountered in predicting out-of-plane loads and the effects of such loads on strength of laminated, co-cured or co- bonded structures. Hence, many researchers are constantly attempting to develop and/or improvise upon efficient methods for the accurate prediction of deformations, stresses and failures of composite structures. The present work comprises the structural analysis of a stiffened skin construction under out-of-plane loading. The Variational Asymptotic Method (VAM) is used to systematically reduce the dimensionality of the stiffened structure by taking advantage of the smallness of the thickness of the stiffener flange and skin compared to the in-plane dimensions of the structure. The original 3-D elasticity problem is systematically reduced into a linear 1-D through-the-thickness analysis (which provides the 2-D constitutive law and 3-D recovery relations) and a geometrically nonlinear 2-D plate analysis. This problem is solved through a 1-D finite element method using the generalized Reissner-Mindlin model in a VAM- based program developed by Yu et al, namely Variational Asymptotic Plate And Shell Analysis (VAPAS).10 The program leads to separate 2-D constitutive laws for the stiffened plate, at the location of the stiffener and the rest of the plate; and corresponding sets of recovery relations that are used for expressing the 3-D field variables in terms of the 2-D plate variables to be calculated in a separate geometrically nonlinear plate Finite Element Analysis (FEA). Solutions to the 3-D field variables obtained thus are compared with those from 3-D FEA using ABAQUS, a standard commercial finite element softwar