Postbuckling analyses and derivations of Knockdown factors for hybrid-grid stiffened cylinders

In this work, postbuckling behaviors of grid-stiffened cylinders under compressive loads are examined in an attempt to establish an analytical method to derive the shell Knockdown factor for space launch vehicle structures. Two grid systems using stiffeners – traditional orthogrid and hybrid-grid systems – are considered for the stiffened cylinders. The hybrid-grid system consists of major and minor stiffeners, which have two different cross-sectional sizes. Additionally, the baseline and minimum weight models are considered for each grid-stiffened cylinder model. The single perturbation load approach is used to represent the geometrical imperfection of cylinders. A commercial finite element analysis code, ABAQUS, is used for the postbuckling analyses. The present numerical simulations investigate the global and local instabilities for both stiffened cylinders using orthogrid and hybrid-grid systems. The global buckling load for the hybrid-grid stiffened cylinder is higher than that for the traditional orthogrid stiffened cylinder for both the baseline and minimum weight models. However, the postbuckling behaviors for the hybrid-grid stiffened cylinders are more complex, as compared to those for the orthogrid stiffened cylinders. Additionally, the shell Knockdown factors are derived using obtained numerical analysis results for the grid-stiffened cylinders. Since the shell Knockdown factor of the hybrid-grid stiffened cylinder is higher than that of the traditional orthogrid stiffened cylinders for both the baseline and minimum weight models, the hybrid-grid stiffened cylinder is more efficient in terms of the buckling design when compared to a traditional orthogrid stiffened cylinder. Furthermore, the derived Knockdown factors in the present work are higher than the values by NASA's buckling design criteria.