| Summary: | Recently, morphing structures have found increased interest due to their potential for combining conflicting requirements of strength, flexibility and low mass. Many concepts depend on continuously powered actuators to deform the structure. Others possess multiple stable states, i.e. multi-stability and power is only needed to change the shape, not to hold it. This dissertation explores the design space of a helical morphing, composite, twisting structure capable of large deformations along its axis of twist with non-linear stiffness properties. Multi-stability is achieved by a combination of pre-stress, geometry of the structure and material properties. Multi-stability is fully exploited by demonstrating the capability of the helix to be bi-stable or, depending on the design parameters, to hold any twisted configuration; hence presenting the remarkable property of zero torsional stiffness. Three proof-of-concept case studies are detailed: simple helices, a wind turbine twisting tip and a bi-stable I-beam. In the first instance, the helical structure is investigated and analytically modelled. Prototypes are developed to verify the non-linear stiffness properties of the structure and experimental results are correlated against analytical and finite element model data. Three different stability characteristics are explored and detailed. Multi-stability is analysed using a simple analytical model, predicting the positions of stable and unstable states for different design parameters and material properties. Actuation using piezoelectric material is also explored in a separate analytical study. Then, the negative stiffness property of the pre-stressed structure is incorporated into a half-scale twisting wind turbine blade. Overall, the manufactured blade achieves zero stiffness in torsion for angles of twist between _5° to +5° due to the added helical structure. Finally, a common structural component, the I-beam, is redesigned to show bi-stability.
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