Multi-functional structures integrate multiple capabilities within a single system, enhancing performance and adaptability. Among these, variable stiffness structures and morphing structures are of significant interest for aerospace and soft robotics due to their ability to alter mechanical properties or shape in response to external stimuli. However, achieving both high stiffness and large deformability within a single structure poses a fundamental challenge, as these properties are often mutually exclusive. This study proposes a potential solution to this trade-off by developing a lightweight multilayer composite that combines active morphing using Nickel-Titanium shape memory alloys and controllable stiffness via thermoplastic softening.

The research begins with a literature review to identify current limitations and gaps, followed by three main investigative parts. The first part (Chapter 3) explores composites incorporating thermoplastic interlayers, examining various interlayer configurations activated by chamber heating. The effects of interlayer thickness and quantity on stiffness reduction at elevated temperatures are analysed in detail. Composite with multiple PET interlayers showed a maximum of 56% stiffness reduction at elevated temperature relative to at room temperature.

In the second part (Chapter 4), building upon the study of interlayer configurations, an optimal configuration was identified and integrated with an active heating system. This system enabled localised heating of the thermoplastic interlayer to induce softening, replacing the previously employed chamber heating approach. The corresponding electro-thermo-mechanical behaviour of the composite was subsequently investigated. For the composite incorporating multiple PET interlayers with active heating, a maximum stiffness reduction of 56% was achieved within 60 seconds of heating, relative to its initial stiffness at room temperature.

The third part (Chapter 5) introduces a shape memory alloy hybrid composite, combining active stiffness modulation with shape-morphing capability. This design retains high stiffness when unactuated and achieves significant deformability upon activation. End-tip deflection tests under both chamber heating and Joule heating confirmed enhanced deformability when actuated, while maintaining rigidity in the unactuated state. An improvement of 36\% in normalised deflection was observed under chamber heating, and a distinct transition in deflection was captured during Joule heating at excitation currents above 5.5A.

Overall, this thesis provides new insights into variable stiffness composites, evaluating interlayer configurations and heating strategies for de-stiffening, and successfully demonstrates an integrated system with both active variable stiffness and morphing capabilities. Conclusions and future research directions are presented in Chapter 6.