Knockdown Factor Estimation of Stiffened Cylinders under Combined Loads - A Numerical Study 2024-26-0417

Airframe section of rockets, missiles and launch vehicles are typically cylindrical in shape. The cylindrical shell is subjected to high axial load and an external pressure during its operation. The design of cylinders subjected to such loads is generally found to be critical in buckling. To minimize the weight of cylinders, it is typically stiffened with rings and stringers on the inner diameter to increase the buckling load factor. Conventionally the buckling load estimated by analytical or numerical means is multiplied by an empirical factor generally called Knockdown factor (kdf) to get the critical buckling load. This factor is considered to account for the variation between theory and experiment and is specified by handbooks or codes. In aerospace industry, NASA SP 8007 is commonly followed and it specifies the kdf as a lower bound fit curve for experimental data . Recent literature have shown promising results for kdf estimation of cylinders under axial compression by numerical methods. In this paper the novel problem of kdf estimation of cylinders under external pressure using numerical methods is studied. Methods for kdf calculation including Single perturbation load analysis (SPLA) and Energy barrier criteria (EBC) are studied and found that they are giving consistent results. Also the kdf predicted is higher than given by handbook which will lead to a lighter mass of the structure. Then study on the kdf for a cylinder under combined axial compression and external pressure case is studied and the effect of different stiffener parameters on kdf is estimated .