Numerical Derivation of Buckling Knockdown Factors for Isogrid-Stiffened Cylinders with Various Shell Thickness Ratios

Abstract

This study is aimed at providing a numerical derivation of the shell knockdown factors of isogrid-stiffened cylinders under axial compressive loads. The present work uses two different analysis models such as the detailed model with modeling of numerous stiffeners and the equivalent model without modeling of stiffeners for isogrid-stiffened cylinders. The single perturbation load approach is used to represent the geometrically initial imperfection of the cylinder. Postbuckling analyses using the displacement control method are conducted to calculate the global buckling loads of a cylinder. The shell knockdown factor is numerically derived using the obtained global buckling loads without and with the initial imperfection of the isogrid-stiffened cylinder. The equivalent model is more efficient than the detailed model in terms of modeling time and computation time. The present knockdown factor function in terms of the shell thickness ratio (radius to thickness) for the isogrid-stiffened cylinder is significantly higher than NASA’s knockdown factor function; therefore, it is believed that the present knockdown factor function can facilitate in developing lightweight launch vehicle structures using isogrid-stiffened cylinders.

Buckling Knockdown Factors of Composite Cylinders under Both Compression and Internal Pressure

“…The control nodes are connected rigidly to the edge of the thin-walled composite cylinder (Figure 3); therefore, the applied displacement is transmitted to the edge of the cylinder with a uniform magnitude [26]. Furthermore, artificial damping of 5% is applied for numerical stabilization to dissipate the released strain energy using an artificial velocity for each node in the nonlinear static analysis when local instabilities occur [26][27][28][29][30]. The internal pressure on the curved side surfaces of a cylinder is represented by distributed pressure loads.…”

“…In addition, since the geometric initial imperfection of a thin-walled shell structure is independent of loading conditions, the SPLA is enough to study the trend of the amount of increment in the buckling knockdown factors due to internal pressure when the geometric initial imperfection only is considered. The procedures of nonlinear post-buckling analysis using the SPLA [26][27][28][29][30] used in this study are as follows.…”