Eva Kirkby
This thesis describes the development of a smart carbon fibre polymer composite that is capable of sensing its own damage and self-healing. Structural composites are used in the aerospace and marine industries, for example. During their lifetime impact events are inevitable, which can result in extensive internal damage. Once damaged, the only options to date are manual repair or replacement. If self-sensing and self-healing properties could be imparted to the next generation of structural composites, this would bring greater safety and reduced maintenance costs for aircraft, as well as an extended operational lifetime for equipment such as satellites. The concept of the new material is to embed three additional components into a fibre-reinforced composite material: (i) microcapsules containing a liquid healing agent, together with a solid catalyst in the matrix, (ii) optical fibre Bragg grating (FBG) strain sensors and (iii) woven shape memory alloy (SMA) wire actuators. An impact event causes a crack to propagate at the damage site, rupturing the microcapsules in its path. This releases the liquid healing agent into the crack, where it comes into contact with the catalyst and begins to polymerise. The strain shock pulse radiating out from the impact site is detected by a sparse array of FBG sensors, which locate the impact position by time-of-flight. With this information, the SMA wires in the impact region are thermally activated using resistive heating, causing them to contract and exert a compressive force, closing the crack. The SMA wires then remain activated during the polymerisation period of the healing agent. The new material is developed in several stages. In the first stage, SMA wires and microcapsules are combined in an epoxy matrix. The addition of SMA wires results in a significant improvement in healing efficiency; the healed fracture toughness approaches that of the virgin material. The improvement results both from crack-closure and also from the heating effect of the wires, which increases the degree of polymerisation of the healing agent. In the second stage, a liquid composite moulding cure schedule that allows straightforward integration of the SMA wires during composite fabrication is developed. With this process, the SMA wires do not need to be maintained in place with an external frame, even though the peak post-cure temperature exceeds the activation temperature of the wires. The wires remain bonded to the matrix both during processing and subsequent activation. In the third stage, impact sites are localised on composite plates to a precision of a few centimetres using three FBG sensors spaced several tens of centimetres apart. In the final stage, a prototype self-healing carbon-fibre composite with embedded microcapsules, Grubbs' catalyst and woven SMA wires is fabricated and tested. While the results presented in this thesis represent only a first step towards a fully-functional Active Sensing and Repair Composite, they successfully demonstrate the advantages of combining the three components in a single material, as well as validate the scientific concept of the system.
EPFL2009
