Post buckling of curved skin-stiffened metal panels under shear loading

Abstract

The established theory of diagonal tension (i.e., post-buckling of skin-stiffened structures under shear load) for curved skin is based on experiments conducted on stiffened cylindrical shells at NACA in the 1940s-50s. However, the experiments were unable to capture all aspects of the phenomenon, leaving knowledge gaps in the understanding of the local distribution of the compressive loads generated on the stiffener, which were filled using the theory of diagonal tension for flat skins. Furthermore, due to the limitations of the time, the secondary bending effects on the stringers were not captured well.

This MSc thesis attempted to fill this gap by performing virtual tests using non-linear finite element modelling, which is enabled by the significant improvements in computational hardware of today. A virtual testing framework is developed based on the stiffened cylinders tested at NACA using the ABAQUS finite element modelling software. The skin and the stringers are modelled using continuum shell elements and the rivets are modelled using point-based fasteners. Furthermore, the generation of the models were parameterised using python scripting with provisions to model imperfections on the cylinders in the shape of the buckling eigenmodes. To validate the modelling framework, FE models of 8 cylinders were created and the results were compared with experimental data. Satisfactory correlation was found between the virtual testing results and the experimental results given the under-reported experimental data and the unknown imperfections in the original test

A custom post-processing method was developed that was able to capture the axial and bending loads that were generated in the stringers, which were used to develop a more detailed understanding of the load-transfer mechanism under diagonal tension. The results were also compared with the semi-empirical design method and the theory of diagonal tension, and significant discrepancies were found pertaining to the local variation of the axial loads and the secondary bending of the stiffeners.