DESIGN & DEVELOPMENT


Blast modelling comparison between aerospace grade aluminium (left) and GLARE hybrid aluminium/fibreglass material (right). The perforated aluminium sheet was subjected to an 85g C4 charge whereas the GLARE withstood 150g


with MAST funding, significantly improves the ballistic performance of the composite and with an appropriate design is capable of arresting a bullet. The advantage of this alloy is that although it possesses high modulus and high elongation to break, which aids its ballistic performance, it can also be rolled into thin layers comparable to the thickness of composite laminates. A structure comprised of CFRP and metallic thin layers provides a high number of interfaces that help distribute the ballistic impact into shear rather than compression only, and thus the whole structure dissipates the impact energy, not only a small area of the material.” Another advantage of these structures is that the production


process is not much different from a standard composite lay-up, as the metallic sheets are rolled prior to hybrid manufacturing, and in any case the higher payloads and fuel efficiencies available to the operator negate the additional manufacturing costs. At the moment, the project is still in its conceptual design stage and full results have not yet been presented to the industry. However, the technology is gathering pace with partners from the MAST consortium taking a keen interest.


Staying strong


At a similar stage on the technology maturity curve, self- repairing material structures is another area being investigated by the University of Sheffield, led by Dr Simon Hayes, a lecturer in aerospace engineering. While it is not the first to publicise its research, the approach taken by the university differs substantially from other methods currently employed for thermosetting resins. Rather than relying on liquid resin delivery, as other systems do, it employs a solid-state system, in which a conventional epoxy resin is modified with a completely soluble thermoplastic healing agent. So how does this actually function in practice? Hayes


explains: “Upon impact, there is generally substantial matrix damage in the form of matrix cracks and delamination. In this event, by and large the cracks are closed, rather than open. In our system, application of heat to the panel will enable the soluble thermoplastic to mobilise and diffuse through the


thermosetting network. As the crack faces are closed, some of the thermoplastic chains will diffuse across the crack face, and thus upon cooling the crack will be bridged and mechanical performance is recovered. To date we can recover between 40% and 70% of the pre-damaged strength.” Given the unusual nature of these materials, certification


will be an interesting task and certain steps have been taken to smooth the process. Hayes continues: “In order that self- repair can be employed in safety critical sectors such as defence and aerospace, it is essential that the effectiveness of repair can be easily assessed. We have always intended that our self-repair system will incorporate sensing to identify the damage, and also to instigate a targeted repair process. “As well as the solid-state self-healing technology, we have


a self-sensing system which uses changes in resistance to monitor the location and extent of damage. This can follow the changes in the panel arising during healing and detect subsequent impacts. In addition, because we have electrical contacts, we can apply a power source to them and cause the panel to heat locally in the region of damage. We are working with Airbus, with the intention of combining these in order to produce a system that identifies and responds to its own damage state, and has the capability to warn users that it has done so and appropriate additional checks can be made.” The next stage will be to come up with a manufacturable


product, and this is where further research will be required. The main problem lies in that the resin tends to have high viscosity due to the presence of the healing agent. Reducing the viscosity of the resin will mean that techniques such as resin transfer moulding can be employed. Current commercial systems use the more mature liquid resin delivery method, but early adopters such as the defence sector are beginning to take an interest in alternative concepts in order to more firmly establish the way forward for armoured vehicles.


Ó www.alphacomposites.co.uk Ó www.shef.ac.uk


Autumn 2010 | Composites in Manufacturing | 31


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