MASTering defence - UK work to improve defence

Materials World magazine
2 Mar 2010
FV432 vehicles with identical visual colour and differing coatings

Dr Eoin O’Keefe of QinetiQ Ltd, Farnborough, UK, and Head of the IOM3 Defence, Safety and Security Committee presents Team MAST case studies relating to the UK Ministry of Defence’s materials and structures research.

Team MAST has been delivering the UK Ministry of Defence’s (MOD’s) low to medium maturity materials and structures research since November 2007. It is a growing UK consortium of 27 companies, universities, research and technology organisations. The multimillion pound MOD Science Innovation and Technology research programme is led by three core UK-based partners, QinetiQ, BAE Systems and Imperial College London.

Three case studies are described as examples of the technical work delivered by the consortium –

• Cool coatings – coatings to lower solar heat absorption without compromising susceptibility to enemy detection. This is particularly relevant to summer operations in desert environments.

• Self healing of fibre-reinforced polymer composites (FRPCs) – to reduce design weight for safety critical components and structures.

• Integrated damage resistance and lightning strike protection in composite structures – enabling widespread use of FRPCs on military airframes while controlling fabrication costs.

Vehicle vestments

High external surface and internal temperatures are reached in vehicles in hot sunny environments. Vehicle skin temperatures frequently exceed 80°C and internal values can be over 50°C. Under these conditions equipment and personnel cannot perform optimally and require significant additional logistical support.

A three-month experimental study has developed and tested coating systems to reduce solar heating while maintaining key vehicle electromagnetic and electro-optic signature characteristics.

Coating systems have been formulated using selected pigments to provide identical visual colour (BSI 381c light stone) and gloss while providing higher reflectivity in other parts of the solar spectrum. In one case, a commercially available hollow microsphere thermal insulation material was added. Coatings were applied to identical test vehicles (FV432s pictured above), one of which remained in current specification paint. Water bourne highly elastic polymers have been used as the binder system to create tough weather resistant but easily removable peelable paint coatings.

The effect of solar loading on temperature for each of the paint systems has been assessed in a simulated high intensity solar and thermal environment test chamber over a simulated desert diurnal cycle. The solar heat reflective coatings show beneficial exterior and interior temperature reductions compared with current coatings, however, the insulation additive does not provide useful temperature reductions.

Conventional vehicle coatings primarily provide corrosion protection and camouflage. Military coatings increase survivability by minimising susceptibility to detection across the electromagnetic spectrum. Selection and testing of pigments, polymer binders and additives reduced deleterious signature effects. The vehicle signatures have been characterised using a range of battlefield sensors in laboratory and real-world environments – which necessitated the creation of a small ‘test desert’ in the UK. The outcome of this experiment has been used by MOD to inform decisions about coatings.

Helping with healing

Materials technologies are largely responsible for improving performance in military vehicle components and structures, and continue to drive reliability, performance and cost effectiveness. Fibre-reinforced polymer composite materials provide lightweight, high strength and high stiffness, and can improve the military effectiveness and cost of ownership of fighting vehicles in all theatres of operation. Due to their hierarchical internal architecture, engineered FRPCs are ideal vehicles for multifunctionality. However, following impact loading, a reduction in strength, stiffness and stability limits their use in safety critical components and structures.

Self-healing FRPCs have received significant interest as an alternative to applying conservative damage tolerant design that increase vehicle weight and consequently utility. Liquid-based self-healing of FRPCs is being developed at the University of Bristol, UK. Both compartmentalised (microcapsules or hollow fibres) and a novel vascular network approach to self-healing are being studied, together with customised healing agent systems that increase efficacy and durability.

A prototype vascular network has been integrated into the foam core of a composite sandwich structure. Rupture of the vascular channels by impact damage permitted the healing agent to infiltrate the damage and cure. The work has shown strength recovery in sandwich beams subject to flexure-after-impact and compression-after-impact testing.

Damage limitation

Research conducted at QinetiQ incorporates shape memory alloys (SMAs) into fibre-reinforced polymer composites to improve their resilience to impact loading. The SMAs are tough lightweight titanium- based alloys, with the composition chosen to absorb significant mechanical energy.

The use of SMAs to enhance the impact properties of carbon fibre composites (CFCs) found in the published literature requires SMA wires to be added at the surface or the laminate inter-layer. The technique being developed in the Team MAST project uses the advanced weaving skills of Team MAST member Sigmatex, Runcorn, to integrate SMA reinforcement. This has led to a commercially viable and scalable process.

Experimentation has covered variables such as different wire cross-sections and volume fractions introduced into the fabric warp and weft. For example, woven carbon fibre with an SMA wire on each edge of the warp and the weft on a 2D ‘over one, under two x under one, over four’ pattern subjected to impact tests show an increase in impact absorption from nine joules for unmodified CFC to 27J for CFC with 11% SMA.

In addition to better impact damage resilience, the SMA/CFC materials are sufficiently electrically conductive to provide lightning strike protection. The unique combination of mechanical, thermal and electrical properties of the SMAs makes them potentially useful as actuators and sensors.

Simulated lightning strike tests have been conducted on four ply laminates with nominal 5.9% and 11% SMA volume fraction. The tests have been repeated on conventional lightning strike protected CFC samples with no SMA reinforcement, but with a copper surfacing mesh, Astrostrike. Following simulated lighting strike, none of the samples showed damage or punctures on the back face. The SMA laminate front or ‘strike face’ had significantly less visible damage extending over a smaller area than the copper mesh protected sample.

For the 11% SMA sample, a small area of resin burn-off is seen close to the arc attachment point, exposing some carbon fibres. The five per cent SMA laminate has similar damage characteristics over a larger area. The copper mesh protected sample has suffered significantly more damage with erosion of the grid and tufting over an area of 100mm around the attachment point.

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