Derivation of knockdown factors for grid-stiffened cylinders considering various shell thickness ratios

Abstract

PurposeThe purpose of this paper is to derive knockdown factor functions in terms of a shell thickness ratio (i.e. the ratio of radius to thickness) for conventional orthogrid and hybrid-grid stiffened cylinders for the lightweight design of space launch vehicles. Design/methodology/approachThe shell knockdown factors of grid-stiffened cylinders under axial compressive loads are derived numerically considering various shell thickness ratios. Two grid systems using stiffeners – conventional orthogrid and hybrid-grid systems – are used for the grid-stiffened cylinders. The hybrid-grid stiffened cylinder uses major and minor stiffeners having two different cross-sectional areas. For modeling grid-stiffened cylinders with various thickness ratios, the effective thickness (teff) of the cylinders is kept constant, and the radius of the cylinder is varied. Thickness ratios of 100, 192 and 300 are considered for the orthogrid stiffened cylinder, and 100, 160, 200 and 300 for the hybrid-grid stiffened cylinder. Postbuckling analyses of grid-stiffened cylinders are conducted using a commercial nonlinear finite element analysis code, ABAQUS, to derive the shell knockdown factor. The single perturbation load approach is applied to represent the geometrical initial imperfection of a cylinder. Knockdown factors are derived for both the conventional orthogrid and hybrid-grid stiffened cylinders for different shell thickness ratios. Knockdown factor functions in terms of shell thickness ratio are obtained by curve fitting with the derived shell knockdown factors for the two grid-stiffened cylinders. FindingsFor the two grid-stiffened cylinders, the derived shell knockdown factors are all higher than the previous NASA’s shell knockdown factors for various shell thickness ratios, ranging from 100 to 400. Therefore, the shell knockdown factors derived in this study may facilitate in the development of lightweight structures of space launch vehicles from the aspect of buckling design. For different shell thickness ratios of up to 500, the knockdown factor of the hybrid-grid stiffened cylinder is higher than that of the conventional orthogrid stiffened cylinder. Therefore, it is concluded that the hybrid-grid stiffened cylinder is more efficient than the conventional orthogrid-stiffened cylinder from the perspective of buckling design. Practical implicationsThe obtained knockdown factor functions may provide the design criteria for lightweight cylindrical structures of space launch vehicles. Originality/valueDerivation of shell knockdown factors of hybrid-grid stiffened cylinders considering various shell thickness ratios is attempted for the first time in this study.

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.…”

Impact resistance and energy absorption ability of bio-mimetic thin-walled structures with hollow columns under impact loading

“…10 Isogrid-stiffened thin-walled structures with the smeared stiffener can obviously improve structural stability and have been widely used in aerospace structural components. 11,12 For the thin-walled structures with similar geometry but different materials, the influence factor of the buckling capacity of the shells needs to be compared by the different theories and codes. 13,14 For the stiffened plate with defects, the study results obtained by the post-buckling and imperfection sensitivity analysis show that the secondary stiffened structure can effectively improve the buckling load and ultimate strength of the thin-walled plate.…”