A Novel Fibre Steering Technology that Allows for Nature Inspired Composite Aircraft Designs: CTS

Hello everyone, my name is Evangelos Zympeloudis
and the topic of my presentation is on CTS, or Continuous Tow Shearing, which is a novel
manufacturing process which expands the overall design space for composites. As you all know composites are only strong
in the fibre direction and we compensate for this in order to give sufficient strength
in all the critical directions by stacking layers of straight fibres together. But if we look at nature, we hardly see any
straight fibres. What you see here is articular cartilage,
a high tech composite found within our bodies, where as you can see not only the density,
but also the orientation of the collagen fibres constantly changes, in order to adapt to the
complex loading condition. Could we then design structures like these, structures
which mimic nature and would allow us to improve the efficiency of our components. Well, the first step in doing that would be
the ability to control the path of the fibres, something known as fibre steering. Fibre steered designs could yield significant
performance benefits. For example what you see here is a fibre steered
wing skin, where the optimally steered paths could distribute the load directly onto the
wing stiffeners and thus achieve a weight reduction of approximately 10%. Furthermore as fibre steering allows us to
tailor the stiffness of a structure at any given point, it makes it possible to aeroelastically
tailor aircraft structures. More importantly however, since this is a
new field in structural engineering, we haven’t yet fully explored the expanded design space
that it opens. This is all great, apart from the fact, that
modern placement machines cannot produce such optimised designs and to understand this we
would have to look at the current state of the art in composites manufacturing; the automated
placement machines, which basically place bundles of fibres on the surface of a mould
using a material deposition head. Although these machines are excellent at laying
in straight lines, they have very limited steering capabilities and the reason is that
they try to bend the tapes in order to steer their path and this creates buckling; and
bucking leads to all sorts of defects. To overcome this manufacturing complications
the concept of Continuous Tow Shearing was developed here at the University of Bristol
by Eric Kim. CTS exploits the material shear deformation
in order to steer the fibre path and results in no buckling and no defects whatsoever. So since conventional machines rely on bending,
there is a strong coupling between the material width and the minimum steering radius; this
is the radius that can be achieved, without the introduction of significant defects. Basically what this means is that for AFP
and ATL the narrower the tape, the better the steering capabilities. On the other hand since CTS relies on shear
deformation there is no coupling between the material width and the steering radius and
we can exploit this by laying up much wider tapes in order to boost the productivity of
the process. So let’s take an example: 100 mm is a standard
tape width for ATL and this would result in a minimum steering radius of 11000 mm. CTS on the other hand can do 50 mm. This is several orders of magnitude lower
and essentially CTS is the first process that allows for wide tape layup with steering capabilities. This is a video showing layup with CTS using
dry fabric. When we use dry fabric we have two options:
either we can attach the tape on the surface of the mould using binder, or we can attach
a resin tape within the head to impregnate the preform. As you can see with the second option, the
head layout is much more complicated but it reduces the manufacturing steps. While shearing dry fabric the quality is primarily
affected by material parameters, such as the weft yarn pattern and although with these
materials we cannot achieve the best quality possible, it is an overall low cost and very
robust process with highly flexible material selection. Very recently we were able to expand CTS for
use with prepreg and what was key here was the ability to control the tension of the
fibres while shearing. With prepreg we can achieve much higher quality
and due to the high mechanical properties of the material we can produce components
of high structural integrity as well. So what we have seen is that nature designs in a completely
different way to man and to mimic nature and improve the efficiency of our structure, we
would have to escape the straight fibre paradigm. The first step in doing that would be to enable
fibre steering capabilities in modern placement machines and to do that we have developed
the concept of CTS, which operates in a niche application field, as it allows for wide tape
layup with steering capabilities, something which was not possible before. More importantly however, since this technology
expands the overall design space, this technology could shape the next generation of ultra efficient
aircraft. Thank you very much for your attention.

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