Bi-Stable Structures with Embedded Actuation

Introduction

This research, started on 1 April 2000, aims to develop structures that can change their shape in a controlled fashion. Application will include deployable structures and adaptive surfaces to air flow or lighting in buildings.

Research team: Dr. E. Kebadze, D. Galletly , K. Iqbal, Prof. S. Pellegrino, Dr. S.D. Guest
Collaboration: Rolatube Technology Ltd, Arup and Partners

Background

Rolatube Technology Ltd have developed a way of making laminated composite tubes that exploit the natural bi-stability of cylindrical shells. By-stable tubes have two stable configurations, straight (Fig.1a) and rolled up (Fig.1b). Tubes with different lay-up form coils of different radii. We have conducted an in-depth study of these laminated composites, leading to a computer model that predicts those natural radii of coiling.

[Rolatube in extended configuration] [Rolatube in rolled-up configuration]
Figure 1: Bi-stable tube in (a) extended and (b) rolled up configurations.

Current work

Bi-stable metallic shells

We have extended this work to metallic shells (Fig.2) and are able to achieve bi-stability by inducing residual stresses through a forming process. We have developed a computer model which simulates the forming process and predicts the natural radii of both configurations.

[Metallic bi-stable shell in extended configuration] [Metallic bi-stable shell in rolled-up configuration] [Metallic bi-stable shell between the stable configurations]
Figure 2: Bi-stable metallic shell in (a) extended, (b) rolled up configurations, and (c) between those configurations where it is not stable.

Finite element simulation

We can also simulate the snap-through behaviour of these shells with finite element method using ABAQUS package. The residual/initial stresses which are applied at integration points through the shell thickness are taken from the model which simulates the forming process (Fig.3a).

If the initial/residual stress signs are reversed, which is equivalent of applying the forming process in opposite direction, then shell is not stable anymore and losses its stability through twist (Fig.3b).

Figure 3: Finite element simulation of (a) the snap-through behaviour of bi-stable shells and (b) losing the stability through the twist.

However, we have discovered that shells with some particular residual stress distributions have an infinite number of neutrally stable configurations with different twist angles (Fig.4).

Figure 4: Neutrally stable shell.

Shell with embedded actuations

We are also working on triggering bi-stable configuration changes in metallic shells using shape memory alloy wires. Having understood the interaction between the wires and an isotropic shell the work will be extended to the more complex composite shells.

[ Cambridge University | CUED | Structures | Deployable Structures ]

Last updated on the 4th of April, 2001

E. Kebadze - ek235@eng.cam.ac.uk