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Improved survivability and lighter weight driving the use of composite materials
An armoured vehicle (AV) must balance a range of competing requirements, including survivability, mobility and load carrying capacity. The need to improve the survivability addressing all layers of the “survivability onion,” is providing the impetus for the development of lighter-weight vehicle structures using composite materials. Traditionally, armour materials applied to the metallic hull of a vehicle such as ultra high hardness steels, lightweight polymers and ceramics have been parasitic in nature, providing increased occupant protection at the expense of payload and mobility, and requiring stiffer base structures to support their added weight. For this reason, fibre reinforced composite materials are of significant interest in the design of ballistic and blast-protected vehicles, providing a unique combination of structural performance and occupant protection, as well as the potential for reducing visibility to radar by the integration of absorbing materials. Furthermore, the use of composite in place of a conventional metallic hull reduces or even eliminates the need for a dedicated spall liner, since the debris produced during an overmatch event is much less damaging than the relatively large secondary fragments produced when a metallic target is overmatched.
Spall liners
Despite the improvements that can be made in occupant survivability, it is important to remember that protection can never be guaranteed, and that all vehicles are potentially vulnerable to threats which exceed those considered during their design. This is particularly relevant in the case of light armoured vehicles, where the ability to carry the mass of applied armour is limited by the need to retain a high level of mobility and, in some cases, the strength and stiffness of the base vehicle chassis. Due to the risk of overmatching threats, therefore, a spall liner is generally required inside the metal hull of vehicles. The purpose of the spall liner is to minimise the spread of primary and secondary projectiles within the vehicle in the event of an armour overmatch, and its effectiveness is measured by identifying the angle of spread of the fragments, which may be represented by a cone. The smaller the cone angle of fragments, the higher the probability that a particular occupant will survive the penetration of the hull, and the better the spall liner’s performance. Traditionally, the spall liner material is non-structural, non-metallic, and it is attached to the metallic hull purely to mitigate threat damage and protect the crew and internal components.
The use of composite materials in vehicles
The use of composite materials for military vehicles has been proposed for a number of years, but has been typically limited to spall liners and a limited number of components such as ammunition bins. One exception is the CAV 100 vehicle commonly know as Snatch from NP Aerospace, which uses a composite pod on a metal chassis. In this case, the composite pod fulfils multiple roles, forming the structure of the rear of the vehicle as well as providing protection against ballistic, blast and fragmentation threats.
One of the reasons for the apparent reluctance to use composite materials is that current AFV manufacturers are not well-versed in the design and production methods needed to produce a composite hull. The incompatibilities of composite materials with existing metals processes includes different design requirements (e.g. isotropy and homogeneity), manufacture (processability, dimensional control, cycle times, temperature tolerance, and assembly methods), and performance (coefficients of thermal expansion, electrical conductivity and ballistic efficiency). While substituting individual metal components with composite replacements can lead to both weight and cost reductions alongside improved durability, the dimensions of the composite components must conform to those of existing component designs in order to allow interchangeability between units. This design approach may lead to a less than optimum design for the composite component, and the potential weight savings and performance improvements for the composite component may not be fully realised. In order to achieve the maximum advantage from composite materials, the complete vehicle design strategy must be re-evaluated to effectively realise the advantages offered by composite materials. Moving from incremental, part-by-part substitutions to whole platforms, shifts the emphasis from making composites compatible with metals to exploiting composites’ unique benefits for solving system issues such as hull weight, hull stiffness and the financial cost per hull.
The development of a composite AFV
Following in the success of the CAV 100 vehicle, two composite demonstrator vehicles were produced in the UK, these being The Advanced Composite Armoured Vehicle Platform (ACAVP), and the High Survivability Concept (HSC) vehicle
The ACAVP programme demonstrated that glass fibre composite materials could be used to form the hull of an armoured fighting vehicle (AFV) [1]. In this application, the composite acts both as a structural material, capable of carrying the loads imposed from the suspension / running gear and associated equipment, and as a ballistic material to provide the required level of ballistic and spall protection. ACAVP was the first all composite hulled AFV to be produced that relied entirely on the composite hull material to carry directly the loads reacted at the hull suspension joint, without the need for supporting metallic structures. The ACAVP hull was manufactured using vacuum infusion moulding (VIM) to produce a glass fibre reinforced plastic (GFRP) hull. The vehicle, illustrated in Figure 2, was evaluated by the British Army between March 2000 and March 2001.
As well as forming an integrated structural spall liner, composite hulls can also improve performance against mine blast loading by eliminating the presence of weld joints, which are used in metallic hulls to join the belly to hull side plates. Due to their location, these welds can be susceptible to cracking under the pressure of the mine, allowing the blast to enter the hull.
A composite hull bottom can be produced in one piece, including shaping, while the hull can also be designed such that the joints are above the region directly affected by the blast. This approach was used on ACAVP, Figure 4 shows the top composite moulding being placed on the lower composite hull moulding.
While the ACAVP was being conducted in the UK, the Composite armoured vehicle (CAV) was being undertaken in the USA. The CAV programme aim was the same as that for ACAVP, to demonstrate a composite hulled vehicle using a mass production manufacturing process. Both programmes were looking to feed into the US Future Scout and UK TRACER cooperative programmes to identify a lightweight reconnaissance vehicle. Unfortunately TRACER programme did not lead to an in-service vehicle. The findings from CAV were fed into the USA Crusader programme for a lightweight mobile artillery vehicle. This programme was intending to produce the top of the vehicle using an integrated ceramic and composite structure. Again this programme was cancelled.
Composite Patrol Vehicle
The UK HSC demonstrator was looking specifically at the potential use of composites as structural/ballistic material for future protected patrol vehicles for Northern Ireland operations with the key requirements being:
–Design and manufacture a composite vehicle with limited metallic components, e.g. drive line and suspension to minimise metallic spall generation
The HSC was seen as taking the NP CAV 100 idea further by eliminating the metallic chassis. While the HSC vehicle demonstrated it was possible to design armour solution that was 30% lighter than on the existing in service vehicle, with a 25% increase in protected volume for the vehicle, MOD and industry did not take up the approach as hostilities in NI were reducing. Had the development of a HSC type vehicle continued, its introduction could probably have contributed to reducing casualties in recent conflicts. As such it can still be seen as a precursor the current UK LPPV programme, since both the Supacat and Force Protection Europe (FPE) vehicles use all composite crew pods.
The Ocelot axles, wishbones and suspension units are mounted on the metal armoured spine called the “skateboard” that has an armoured belly plate which houses the fuel tank, differentials, and transfer box. The power-pack which includes the bonnet is then bolted onto the front of the skateboard and the composite crew pod mounted onto the skateboard which matches the ‘V’ shape of the pod with the ‘V’ shape of the skateboard giving a continuous blast protection. The approach of integrating a composite structure with external steel blast protection has been investigated via the European CAFV programme and is being addressed in the current UK LIMBS programme managed by BAES. The LIMBS programme is assessing the optimum material combination to provide lightweight structure with blast and spall performance. To enhance the protection offered by the Ocelot composite pod, LAST® Armor has been integrated into the internal and external surfaces of the pod. LAST® is a unique attachment system that installs without any cutting, welding or drilling on the base vehicle. It is designed to be easily integrated into vehicles either at the initial vehicle design stage reducing manufacturing time at least fourfold, or as a retrofit to an existing vehicle platform. LAST® Armor uses the QinetiQ patented Hook and Loop Velcro system for attachment, which eliminates the presence of metallic fixings which can contribute to the spall threat. By attaching the liner to the hull via the large surface area of hook and loop this helps reduce the loading on the liner during an overmatch situation, which can lead to the liner being forced off conventional fixings.
In the area of patrol vehicles, in the US an “all composite” vehicle, built on the exiting HMVEE design, has been produced. However, a composite vehicle for operational use has yet to be announced.
Maximising survivability while maintaining the lightweight
To complement the composite structure of a vehicle, QinetiQ has a number of products and developments which address the requirement for increased survivability while minimising both weight and the presence of metallic spall. These cover RPG attack, increased energy storage, improved transparent armour and composite suspension components.
QinetiQ North America has developed a lightweight RPG (rocket-propelled grenade) defeat capability that uses nets. The RPGNet™ system is an adaptable solution capable of providing RPG protection on a wide range of tactical and lightly armoured vehicles. The system is ultra-lightweight, (anywhere from 1/3 to 1/20 the weight of bar armour technologies), low-cost and easily adaptable to a variety of platforms. The RPGNet system uses a net design developed jointly by QinetiQ North America, DARPA and ONR. It has been subjected to extensive live fire and laboratory tests with results indicating performance levels matching or exceeding competitor bar armour solutions. Due to the design of the net system, tests also indicate that vehicles are provided with multi-hit RPG defeat protection from all angles (360 degrees) including overhead protection. This reduction in weight allows for RPG protection on vehicles that cannot support the excessive weight of other technologies. This is particularly appropriate for light patrol vehicles where excessive weight can cause a roll-over risk, wear and tear on vital vehicle operating systems, and/or excessive fuel consumption. Due to the use of QinetiQ’s patented hook and loop attachment technique for both the net and frame, the system supports installation and repair by the soldier in the field and the collapsible frame allows for easy storage.
To reduce the weight of transparent armour, QinetiQ has been exploring the development of hybrid glass and glass fibre plastic (GRP) composite solution. Obviously to achieve this it requires the GRP to be transparent. This is achieved by matching the refractive index (RI) of the glass fibres to the plastic resin. Figure 8 illustrates the dispersion curve for a fibre and resin measured against wave length. To achieve optimum visual performance for the system, the resin curve should sit within three decimal places of the fibre curve.
In addition, the GRP must have zero defects in the terms of voids and delaminations. These two requirements are extremely difficult to achieve but significant progress has been made to identify a cost effective solution.
The suspension of a vehicle represents a considerable mass. Lighter weight composite leaf springs have been available since the 70s and Formula 1 and rally cars use composite suspension components. The potential use of composites for military vehicle suspensions has been demonstrated by a proof of concept programme looking to use composites for a suspension arm. QinetiQ produced both glass fibre and carbon fibre suspension arms reducing the weight of the arm from 64 kg in steel to 22 kg in carbon fibre. The composite arms (Figure 9) demonstrated the same structural performance matched to the performance and space envelope of the original steel arm.
One of the aspirations for a designer is to achieve multi-functionality from a component. This is being explored by investigating the ability to integrate battery storage into structural composite components. GRP spall liner panels have been produced with battery elements integrated through the structure, Figure 10. These cells can provide additional electrical energy storage, for scenarios such as silent watch and can be used in combination with integrated super capacitors cells to replace a vehicle’s standard lead acid batteries, thereby removing both weight and releasing space within the vehicle.
Testing has shown that the presence of cells has minimal reduction in ballistic and cone angle performance of the liners. To improve performance a method to introduce “Z” directional out of plane reinforcement called tufting has been demonstrated by QinetiQ. The presence of tufting significantly reduces the delamination associated with ballistic impact.
Conclusion
This article has provided a brief overview of the use of composite materials for military vehicles. The requirement to produce lighter vehicles necessitates that materials used in the design of AFVs must provide a multifunctional response in terms of being able to contribute to the vehicle’s structural and ballistic requirements including the need to minimise the overmatch of the armour and provide spall protection. This is a key discriminator when choosing between metallic and composite materials for the construction of a vehicle hull. The ACAVP, HSC and CAFV programmes have demonstrated the use of composites for the construction of future military vehicles by providing evidence that alternatives to existing metallic hulls, in terms of both performance (defined by ballistic, blast and structural requirements) and cost are viable. The key advantage offered by composites is the ability to make a lighter base structure onto which modular survivability products can be integrated. Using the composite structure therefore increases the options for maintaining long term performance.
AFV manufacturers may remain reluctant to use composites for their vehicle designs until fully working composite vehicles have been successfully demonstrated over a period of time so that current unknown issues, such as long term durability and associated whole life costs, have been determined. Consequently to date composites structures have only been applied to lightweight wheeled vehicles such as Snatch. The proposed UK programme to re-hull a number of tracked CVRT vehicles would provide an excellent opportunity to use composites in a tracked vehicle.
REFERENCES
* 1 French M (2000), “ACAVP”- Demonstrating the Potential use of Composite Materials for Future AFVs, ECCM9, June 2000
* 2 Wright, A, and French, M (2008), The response of carbon fibre composites to blast loading via the Europa CAFV programme, J Mater Sci (2008) 43:6619-6629
Dr Mark French, Principal Engineer, QinetiQ, Room 2012, Building A5, Cody Technology Park, Farnborough, Hampshire, GU14 6EN, UK +44 (0) 1252 394377/MAFRENCH@QINETIQ.COM
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