FEMMAS Contribution Buckling of composite panels

Abstract

This document is a follow-up study of the validation of BEOS (Buckling loads of Eccentrically Stiffened Orthotropic Shells) as described in FEMMAS-RP-DLR-01. In that document, the validation of BEOS for stringer stiffened panels was performed using ABAQUS models with high degree of detail. A high correlation of the resulting buckling loads was found for panels without stringers or panels with only “blade” type stiffeners. For T-stiffeners the buckling load computed by BEOS was up to 30 % lower than that calculated for the ABAQUS model. Buckling loads of panels stiffened by omega stiffeners showed even higher deviations. The main reason for these findings was attributed to the neglect of the skin stiffening effect of the stringer feet on the adjacent skin. Since BEOS uses bar elements to model stiffeners, this effect is not accounted for. Two different approaches are presented here to evade this shortcoming: firstly, so-called “dummy stringers” are used. In this case the stringer stiffness formerly represented through one bar element is now divided into several bar elements to model foot and blade stiffness separately. Since these results seemed not satisfactory, BEOS was extended to allow for different skin stiffnesses for the skin elements. In this way, stringer feet are modelled by adding their stiffness to the stiffness of the adjacent skin.