Shape changing or morphing structures enable optimisation of structural configuration to suit current operating conditions. Conventional techniques for achieving shape change often result in weight and complexity penalties, which may outweigh the potential benefits of greater shape changing ability. The research presented in this thesis focuses on the use of shape memory materials to achieve a reversible change in shape of an innovative compliant composite structure, which may enable shape change without the drawbacks of conventional shape changing techniques. An initial concept was evaluated using a glass fibre reinforced shape memory polymer which was heated and deformed locally to fix the actuated shape. It was found that a large change in shape can be achieved. However, due to the high shear strain between the fibres during large deformations, fibre/matrix debonding occurred and propagated with repeated cycling. An alternative topology, consisting of a shape memory polymer reinforced with comparatively large diameter precured CFRP composite rods, was proposed and successfully demonstrated to reduce matrix shear strains, thereby reducing damage during deformation. The increased reinforcement size also improved load carrying ability when the shape memory polymer was in its low stiffness state. Initial testing of a rod reinforced composite beam with a low stiffness elastomer matrix indicated that shape memory alloy actuators wound helically around the composite beam could be effectively used to provide Significant rotational actuation. A constitutive model of the shape memory alloy thermo-mechanical behaviour was compared to the experimental findings for different configurations of shape memory alloy winding around the composite beam. A composite beam with a shape memory polymer matrix was found capable of 'locking' and 'releasing' mechanically introduced rotational shape change. The composite beam topology used initially consisted of circular rods and relied upon an adhesive for torsional rod restraint within the end alignment fittings, often resulting in failure of the adhesive and large non- returnable rotations at the temperatures required for softening of the shape memory polymer matrix. Rectangular cross section rods were used to enable the end fittings to mechanically restrain the rods in torsion. The rectangular rod topology also gave a large increase in bending stiffness compared to circular rods, with both topologies having similar torsional rigidities. A key aspect of the actuator performance was found to be the rate at which the shape memory materials could be activated by heating or cooling. This was found to be a particular problem for the shape memory polymer. To increase the rate of heating for the composite beam with a shape memory polymer matrix, efforts were made to incorporate carbon nanotubes to improve the thermal response of the material. However, only a modest change in thermal properties was achieved, combined with some undesirable detrimental effects on mechanical and therrno- mechanical properties. Further work is needed to optimise this combination. A final composite beam demonstrator combining both helically wound shape memory alloy wires for actuation and a shape memory polymer matrix for shape fixing was constructed. The shape memory polymer matrix was heated using an embedded heating element. By activating the shape memory alloy actuators when the shape memory polymer matrix was in its soft state, a rotation was achieved. Cooling the shape memory polymer matrix before the shape memory alloy actuators fixed the rotated, which was returned upon reheating the shape memory polymer matrix.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:556738 |
| Date | January 2011 |
| Creators | Walls-Bruck, Marcus |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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