COMPOSITE MATERIALS FOR MILITARY VEHICLES
By Dr Mark French, Principal Engineer, Qinetiq

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 we