A scaling methodology for axial buckling of sandwich composite cylindrical shells

Abstract

This thesis examines the buckling behavior of sandwich composite cylindrical shells, which are integral to the primary structure of launch vehicles. The central aim is to develop a scaling methodology based on nondimensional parameters, enabling large-scale composite structures to be scaled down to laboratory-size models while maintaining their buckling response.

The study begins with an overview of existing analytical, numerical, and experimental techniques for shell buckling analysis, alongside a review of current scaling methods in structural mechanics. Building on insights from the literature, a scaling framework is introduced by reformulating classical buckling equations in a nondimensional form. This approach allows the scaling laws to be directly derived from the equations' components but requires a comprehensive and adaptable nondimensional formulation of the structural behavior.

To establish this formulation, the work extends the nondimensional framework by first incorporating the effects of transverse shear deformations, relevant when the shells are reduced in size. Then the framework is extended to included the theoretical impact of imperfection sensitivity, tackled by incorporating a trigonometric imperfection model into the nondimensional framework.

With these extensions in place, a systematic scaling methodology is proposed. Two distinct strategies are developed: one that directly scales sandwich composite shells while preserving their structural characteristics, and another that substitutes them with equivalent composite laminate shells. While the first strategy offers greater theoretical accuracy, it is constrained by practical limitations in manufacturing scaled thicknesses. The second strategy, although more feasible for experimental implementation, introduces new complexities in ensuring equivalence between different structural configurations.

The proposed methodologies are validated through comparisons between analytical predictions, numerical simulations, and experimental results. Scaled laboratory models, produced using the more practical laminate-based strategy, show an 8% discrepancy in nondimensional buckling loads in theory compared to full-scale counterparts. However, experimental observations reveal a larger deviation of approximately 22%, underscoring the limitations of current imperfection modeling and the need for refinement in the scaling approach.

The thesis concludes by emphasizing the contributions made to nondimensional scaling methods for composite structures and highlights the importance of further experimental validation. In particular, improved modeling of imperfections and expanded laboratory testing are recommended to bridge the gap between theoretical predictions and real-world behavior. This work lays a solid foundation for the development of scaled testing protocols, advancing the design and verification of large-scale sandwich composite shell structures in aerospace applications.