Numerical Estimation of Knockdown Factors for Externally Pressurized Cylinders Using Energy Barrier Approach

Abstract Thin-walled cylinders exhibit a significant difference between the theoretically calculated critical load and experimental buckling loads. The inconsistency arises primarily due to the presence of geometric imperfections in the shell, which cannot be accurately determined during the design process. To mitigate this, designers apply a buckling knockdown factor (KDF) to theoretical estimates when sizing such critical structures. Industry guidelines like NASA SP 8007 specify KDF, which is a lower-bound experimental data fit and is overly conservative. Recent studies have shown significant progress in the numerical estimation of KDFs for cylinders subjected to axial compression. However, other critical loading conditions, such as external pressure, remain underexplored. This paper addresses this gap by employing numerical methods to estimate KDFs for cylinders under external pressure, enabling the design of lighter structures. The applicability of energy barrier analysis (EBA) is investigated for KDF estimation, and its efficacy is validated by comparing it with experimental results. The effect of geometry on KDF estimation is studied through numerical experiments, and new KDF curves are proposed as a function of the Batdorf parameter and the L/R ratio. A comparison with a detailed dataset of various pressurized cylinder buckling experiments demonstrates the accuracy of the suggested KDF curves. The studies indicate that the KDF curves enhance the load-carrying capacity by up to 20% compared to the conservative standards. These findings contribute to the development of lighter, optimized airframe structures and enhance the understanding of buckling behavior under external pressure.