Conclusions

This research has presented various techniques to enhance the sustainability of healing capacity. To appreciate previous development in self-healing composites and reveal key barriers in practice, the thesis provides an overview of the various self-healing concepts proposed over the past decades, and a comparative analysis of healing mechanisms and fabrication techniques for building capsules and vascular networks. Based on the analysis, factors that influence healing performance are presented to highlight the importance of developing sustainable self-healing capability to allow materials to recover regardless of ambient temperatures and material types.

The first part of the research was to experimentally investigate the effects of vessel configurations on the healing performance and flexural properties of fibre-reinforced composites. It also analysed the crack behaviour caused by bending and healing processes in laminates. Experimental results indicate that vessels should be placed in the core of the laminates instead of under the surface. The vessels should penetrate multiple layers and have wide in-plane coverage so that large-scale delamination could be cured. When the damage is minor, mainly consisting of debonding and delamination, the composites are able to achieve a high healing efficiency, even a full recovery. However, the fractured structural fibres becoming the primary crack site should be avoided. When the composites are designed using a correct vessel configuration, the vessels can have a minor effect on the flexural properties, and a healing efficiency higher than 100% is achievable. Otherwise, the healing efficiency can be very poor. The results also indicate the importance of developing new healing agents that can repair not only the matrix but

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also the structural fibres. In this part of the study, Objective 1 was accomplished and Hypothesis 1 was verified.

The aim of the second part of the research was to enable sustainable self-healing at very low ambient temperatures. The thesis has presented a structural composite able to maintain its temperature to provide a sustainable self-healing capability – similar to that in the natural world where some animals keep a constant body temperature to allow enzymes to stay active. The composite incorporates three-dimensional hollow vessels with the purpose of delivering and releasing healing agents, and a porous conductive element to provide heat internally to defrost and promote healing reactions. Healing in fibre- reinforced composites at a temperature around -60 C was achieved with an average recovery of 108% in fracture energy and 96% in peak load. The effects of the sheets on the interlaminar and tensile properties have been experimentally investigated. It was found that the introduction of a carbon nanotube sheet increased the tensile strength of polymer composites, but had negative effects on interlaminar properties. In this work, Objective 2 was accomplished and Hypothesis 2 was verified.

The third part of the research focussed on an approach to recover carbon fibres, a common material adopted in structural fibre-reinforced composites. The thesis reports the development of a carbon fibre composite that can repair its structural fibres and restore its mechanical properties after it has been subjected to damage, by using an embedded vascular self-healing system. Damage is healed through the application of an epoxy-based resin containing short carbon fibres that can reconnect the fractured carbon fibres upon electric alignment and curing. Different variables and parameters were investigated to observe their effects on the healing performance until the optimum healing agent composition and conditions were found. A model based on the Rule of Mixture can be

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used to analyse and predict healing performances. The optimum conditions demonstrate that the healing efficiency and performance were increased by 3788% and 153% respectively compared to an unprocessed epoxy resin used in conventional self-healing materials, and as high as 45% of the original strength could be restored. In this study, Objective 3 was accomplished and Hypothesis 3 was verified.

The final part of the research dealt with another type of materials, conductive porous elements, which can be automatically repaired by using a modified mechanism based on the third part. Conductive elements are popular in the development of multifunctional composite materials. The composite incorporated hollow vessels which could deliver a healing agent containing short carbon fibres to the damaged areas to repair broken carbon nanotube sheets. The recovery process took place in an electric field to aid the alignment of the short carbon fibres. The healing agent composition was changed as well as parameters such as the electric field intensity and short carbon fibre content to determine their effects on the recovery of the carbon nanotube sheet. The results show that the intensity of the electric field, the addition of silver paste to the healing agent, and the addition of dispersant to the healing agent lead to optimum recovery. Experimental results demonstrate that an average recovery of 54% is achievable, and recovery as high as 100% has been observed. Thus, Objective 4 was accomplished and Hypothesis 4 was verified.

The work presented in the thesis has demonstrated new techniques and a novel design enabling structural composites to recover regardless of ambient conditions and material types. Therefore, the aim of the research has been achieved.

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