Degree Name

Doctor of Philosophy


School of Mechanical, Materials, Mechatronic and Biomedical Engineering


Large-scale torsional actuation occurs in twisted fibres and yarns as a result of volume change induced electrochemically, thermally, photonically, and other means. When formed into spring-like coils, the torsional actuation within the fibre or yarn generates powerful tensile actuation per muscle weight. For further development of these coil actuators and for the practical application of torsional actuators, it is important to standardise methods for characterising both the torsional stroke (rotation) and torque generated. This thesis introduces such a method for use in the free rotation of a one-endtethered fibre, when operating against an externally applied torque (isotonic) and during actuation against a return spring fibre (variable torque). The torsion mechanics approach has been verified and allows the prediction of torsional stroke under any external loading condition based on the fundamental characteristics of the actuator: free stroke and stiffness. The second thesis aim was to develop a better understanding of the link between fibre / yarn volume change and the induced torsional actuation. The developed theoretical analysis was based on experimental investigation of the effects of fibre diameter and inserted twist on the torsional stroke and torque measured when heating and cooling nylon 6 fibres over a certain temperature range. The results show that the torsional stroke depends only on the amount of twist inserted into the fibre and is independent of fibre diameter.