Buckling of Open Cross-Section Deployable Composite Thin Shells with Manufacturing Imperfections

Author: Carmi, Meital Oshrit

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Pellegrino, Sergio

Committee Members: Ravichandran, Guruswami; Pellegrino, Sergio; Lapusta, Nadia; Shaikeea, Angkur

Option: Space Engineering

Abstract

Thin composite shells are increasingly being used to support large area space systems due to their high strength to mass ratio and ability to withstand tight packaging for launch and then deploy once in space. This thesis specifically focuses on long, slender composite shells (longerons) with an open cross-section consisting of two circular arc flanges bonded along one edge. They are useful components of lightweight space structures, including the Caltech Space Solar Power Project spacecraft. The flanges of the longerons are less than 100 µm thick, and these extremely thin composite shells are prone to manufacturing imperfections and local buckling. Therefore, the primary objective of this thesis is to better understand the buckling behavior of open cross-section, ultra-thin composite shells that contain manufacturing imperfections, with the goal of informing the design of future space structures that are more resistant to buckling.

The first step towards understanding a structure’s imperfection sensitivity is to measure its imperfections. Thus, the thesis begins by characterizing the geometric imperfections present in experimental composite longerons. A method to measure and quantify the parameters of both local and global imperfections in thin shells is developed and applied. Results show that the longerons contain local imperfections as large as five to ten times the shell thickness, which can have serious implications for local buckling.

Once the imperfections were measured, both numerical and experimental studies are performed to study the effects of these imperfections on the buckling behavior and knockdown factor of longerons loaded in pure bending. In the numerical study, a finite element analysis is used to characterize the effect of a single imperfection, created using a simplified model based on the shape of the experimentally measured imperfections, with a wide range of geometric parameters. Then, experiments are performed to measure the buckling behavior of actual longerons, whose random manufacturing imperfections were characterized. The results of these studies show that imperfections, especially ones with large amplitudes, significantly reduce the longeron’s critical buckling load and bending stiffness. Good agreement between the experimental and numerical results was achieved, particularly for higher quality longerons with a single, dominant imperfection.

Motivated by the imperfections and their detrimental effects found in the earlier parts of the thesis, key parameters of the longeron's cross-section are varied with the goal of increasing its stability. The subtended angle of the longeron's flanges is varied in both experiments and numerical simulations of longerons loaded in bending. The results show good agreement between the experiments and simulations, with both showing a trend of increasing critical buckling load and bending stiffness with increasing flange subtended angle. Then, based on these promising results, the radius at the edge of the flange is decreased, which is shown to significant improve the longeron's stability and imperfection sensitivity without increasing its mass.

Finally, the effect of length on the buckling behavior of longerons loaded in bending is studied numerically with the goal of extending the current work to longer longerons. For lengths varying from 0.5 m to 5 m, both perfect and imperfect longerons with realistic geometric imperfections are studied. It is shown that for longer longerons, the critical buckling moment and imperfection sensitivity remain almost constant with increasing length, which is promising for future large space structures.