Defining the future of defence

user experience. In the defence sector in particular, an overhaul in armour technology is long overdue, but with products and material development now falling into place, Simon Lott looks at how the sector can be expected to develop.

The central problem faced by the military is probably the most universally common one where composites are concerned. Current generation armoured vehicles are becoming too encumbered by their sheer size and weight, leading to crucial reductions in speed and mobility. In turn, the increasing weight causes a demand for more powerful, thirstier engines which are not only wasteful financially and environmentally, but provide increased logistical challenges for supplying such vehicles.

As an example of this need, at the 2009 Defence Vehicle Dynamics (DVD) event, the MoD outlined its desire for a Future Protected Vehicle (FPV), describing its ideal of ‘an electric 30 tonne armoured fighting vehicle with the ‘punch’ of a current main battle tank (MBT)’. Included in these requirements is the ‘effectiveness and survivability’ currently associated with current MBTs, but with high tactical mobility, reduced logistic footprint and strategic mobility of a rapidly deployable, air-portable system. This would employ a modular, open architecture approach to underpin a future generation of mission configurable platforms and offer a ‘Troop Carrier’ variant, with initial test bed demonstrators by 2013.

Starting from scratch

And with that announcement, it was up to the industry to respond. Interestingly, the myriad of technologies and design options that are now surfacing thanks to progress in the composites arena means that companies further down the supply chain are seeing greater opportunities to get involved in product development and the role played by subcontractors can be just as important as the research institutes. One such company is the Delta Group, which has been a dedicated producer of complex composite components since its inception in 2002. Initially the company comprised of F1 and motorsport specialist Delta Composites, however two years ago, the Group set-up Alpha Composites to deal with the large number of alternative applications it had started to get involved in. Since then it has designed and produced specific products for military applications such as the short gap crossing – a lightweight, modular carbon fibre bridge, ladder and stretcher system - and is now working with several key defence contractors on how a future protective vehicle will take shape.

The main area of opportunity as far as director Ian O’Dell is concerned, is to produce a tough composite monocoque structure, which would likely sit inside a much thinner metallic shell. Based on manufacturing technologies familiar from its F1 work, the way in which composite structures are manufactured allows for further opportunities to optimise designs beyond the weight aspect. For example, vehicles currently in operation require a lot of cabling that is often left hanging around the cabin, whereas this could be built into a tailored composite structure and integrated into the monocoque. Critically for survival in an attack, materials can also be designed with specific failure modes so that they are designed to break up on impact, much like surfaces outside the survival cells of modern racing cars. The ability to tailor materials to allow particular levels of flexibility will also ensure, with careful design, that structures are more tolerant of damage and there is also a comfort factor due to the fact the carbon and Kevlar absorb sound substantially more than their metallic counterparts.

A further potential benefit using a concept developed by Alpha Composites, is to embed circuitry into a composite structure. While the company cannot discuss the manufacturing process, the concept is already in production for high security applications. This involves a briefcase with a circuit embedded throughout the shell which if broken, will automatically shred any documents inside. Such a concept (applied to both circuitry and cabling) could potentially be modular, so that components can be easily replaced and retain their functionality. With a little bit of lateral thinking, there are also many other opportunities for metal replacement, which may be as simple as replacing the heavy toolboxes and their contents carried by support vehicles.

Novel materials

While the MoD will typically place the onus on the manufacturers to come up with the innovations required, much of the technology trickles down from institutions such as the Composite Systems Innovation Centre (CSIC) at Sheffield University, established by Professors Frank Jones and Costas Soutis. The centre joins around 30 researchers from its Mechanical Engineering and Engineering Material departments. Its main areas of development are composites and hybrids for improved ballistic and blast resistance, self-healing materials, biopolymer composites, recycled polymers in composites for structural applications, and advanced technologies for high-temperature composites. Naturally, as part of Sheffield University, CSIC also has a strong partnership with the Advanced Manufacturing Research Centre with Boeing.

As a member of Team MAST, an MoD research consortium administered by QinetiQ, some of the university’s projects have been directly funded through these channels and works closely with the major defence contractors and manufacturers. As far as technologies for future protective vehicles are concerned, there are currently two key areas of research being carried out – composite/metal hybrids and self-repairing structures.
CSIC manager Dr Alma Hodzic explains her department’s research into hybrid materials: “The first challenge is to identify two suitable materials in a way that neither would be compromised in a hybrid structure. Carbon fibre composites, which have no parallel among lightweight structures in terms of stiffness and strength, possess lower ballistic resistance compared to their metallic counterparts, so hybridisation is required to improve that aspect, without seriously compromising lightweight performance and other structural properties. “A new system that we are designing involves composite scientists and metallurgists from The University of Sheffield and Imperial College. The initial trials have shown that a small addition of a particular light metallic alloy, developed at Imperial 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 efficiency available to the operator negates 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, so that 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.

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