This thesis investigates the development of novel multifunctional composites which primarily
take advantage of the electrical conductivity of carbon fibres in high performance composites.
The multifunctionalities are targeted towards use in the aerospace industry, covering
manufacturing, and in-service, such as damage detection and de-icing. This work aims to
increase the matureness of these and related technologies, allowing the concepts to be built
on more easily, and reduce the barrier to industrial implementation.
The functionalities are Joule heating for direct electric cure (DEC), de-icing and heated tooling,
and electrical self-sensing for damage detection. These utilise the electrically conductive
nature of the carbon fibres in composites used in aerospace structural components. This
means that little modification to the materials and structure is required, crucial for aerospace
qualification and ease of use.
The low conductivity of composite matrices was identified as an initial problem to the scale
up and industrialisation of multifunctional composites. Carbon Nanotubes (CNTS) resin
composites were manufactured using 3 dispersion methods and tested for conductivity and
piezoresistivity. Despite showing these desired properties, quick agglomeration of the
particles and difficulty of manufacturing fibre reinforced composites meant that their use
cases were limited to the fibre-electrode interfaces of the multifunctional demonstrators.
DEC is developed as an alternative sustainable curing method, requiring 99% less input energy
when curing industrial scale components (2000 x 700 mm), using both pre-preg and vacuum
assisted resin transfer moulding (VARTM). The degree of cure for VARTM was only 0.82 %
lower than oven cured samples, however it was significantly lower in pre-preg samples. The
main challenge was to ensure even heating over the component, which was due to the plain
weave fibres used in these experiments. When the same heating technology was applied to
unidirectional (UD) fibres within a representative leading-edge section of a wing, the heating
performance for de-icing was more even, despite having power headroom issues due to
embedded CNTs causing localised overheating.
Evidence of even heating performance of UD led it to being embedded into composite tooling,
to enable the manufacture of low-cost heated tooling. The DEC style electrodes only had to
be applied to the composite tooling once, rather than every part manufactured, meaning that
the benefits of DEC can be achieved with an easier barrier to entry.
To further increase the benefits of these heating technologies, a cure kinetic and thermal
model was assessed to reduce the energy and length of cure cycles. It predicted the
exothermic energy output of composites during curing, and the heat dissipation to the
environment, to be able to run close to the exothermic limits of the cure system. It accurately
predicted the exothermic reaction of a commercial pre-preg system using open loop control,
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leading to a degree of cure 0.14 % lower than a post cured sample, whilst saving an estimated
95 % of input energy.
A novel damage detection for composites was developed, with the aim to enable and
automate damage detection without the requirement of external inspection equipment. The
system monitored electrical conductivity of fibres using embedded flexible printed circuit
boards (PCB) to enable implementation into a manufacturing process. Electrical self-sensing
hardware and software was developed, which were tested on 360 x 360 mm components,
with the aim to detect barely visible impact damage (BVID). These were then scaled up to a
modular system, which monitored a 560 x 400 mm section of a leading edge. The sensing
system was able to detect BVID damage in some configurations and higher impact energies
in most samples.
Novelty is in both the developments to increase the scale and robustness of the components
through improvements in manufacturing and testing such as with DEC and Self-sensing, but
as well as identifying opportunities of applications, such as with Joule heated tooling and
applied cure modelling.
The increase in maturity of these technologies has ensured that development has continued
in this area. Joule heated tooling continues to be developed through funding grants with
industrial partners such as Pentaxia, whereas self-sensing and Joule heating of components
has attracted private funding from Tier 2 aerospace suppliers.
