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.
Highlights
A thin cylinder is the most important structural element for space launch vehicles
A detailed model considering modeling of numerous stiffeners and an equivalent model without modeling of stiffeners were used for the finite element modeling of the isogrid-stiffened cylinders
The geometrically initial imperfection of the cylinder was modeled using the single perturbation load approach (SPLA), and Newton-Raphson method with the displacement control method was applied for the nonlinear static postbuckling analyses
Summary
A thin cylinder is the most important structural element for space launch vehicles. It is extremely vulnerable to axial compressive loads. In general, numerous stiffeners are used in both the axial and circumferential directions of the thin cylinder to improve its stiffness and strength. The stiffeners are usually arranged in a grid form, such as the orthogrid- and isogrid-stiffened systems (Figure 1) or in a latticed structure [1,2,3]. The isogrid-stiffened system includes a lattice of intersecting ribs, which form an array of equilateral triangles, as illustrated, and it provides the isotropic behaviors of the stiffened structure while improving the buckling load of the cylindrical structure. The isogrid-stiffened cylinder is frequently used in the propellant tanks and intertanks of space launch vehicles
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