A demonstrator tank with a composite hull could save up to around 30 per cent in major component weight and enhance performance.

More significant, however, is probably its consequence for the commercial sector, where reducing the weight of commercial vehicles and machines has an immediate effect on operator profits.

The materials and manufacturing techniques used are not the expensive varieties associated with aerospace, but more akin to boat building, which puts the technology into the commercial arena.

The tank body under construction

The Advanced Composite Armoured Vehicle Platform (ACAVP) comes out of a desire to reduce the weight, acoustic noise and radar signature of armoured vehicles. The problem, as in much of modern mechanical design, is to achieve sufficient stiffness at minimum weight. Steel per unit cross section is stronger than high performance aluminium alloy which is stronger than steel. But stiffness of a sheet of material is proportional to Young’s Modulus times thickness cubed. This means that because composites are much less dense than aluminium alloys, which are less dense than steel, weight for weight, composites are considerably stiffer in bending than aluminium alloys, which are in turn stiffer than steels.

The demonstration task chosen was to make a complete hull for a scout reconnaissance vehicle of similar size and using the same components as the Warrior infantry-fighting vehicle. The Warrior carries troops but the new vehicle is a two-man ‘light’ reconnaissance tank weighing 18 to 25 tonnes. The hull supports the suspension, turrets, and running gear, as well as providing the inner layer of armour. The demonstrator vehicle uses a Warrior Perkins power pack, and Warrior running gear, but the metal Warrior hull is full of right angles and re-entrant corners, which would be difficult to mould in plastic. The three degree draft angle required to remove the base hull moulding from the mould also requires addition of interface plates to allow fitting of suspension components.

Despite the obviousness of the idea, the project has taken its time.

A feasibility study by GKN, Westland Aerospace and DRA (now Dera) began in 1991, followed by a moulding programme from 1991 to 1993 at the University of Plymouth and a manufacturing process study by PERA in 1993. Also in 1993, a composite door for a Warrior was made which was found to have the same ballistic performance as its steel equivalent but was 25 per cent lighter. Short Brothers was appointed as composite fabricator. The project to build a whole vehicle hull was approved by the Ministry of Defence in March 1993; detailed designs were completed by Vickers Defences Systems in October 1996, assisted by information supplied by their normal competitor, Alvis, who build the Warrior. In March 1997, Shorts withdrew, and the team turned to Vosper Thorneycroft to do the actual moulding fabrication.

Cost was an issue right from the beginning, so it was decided to go for E-glass non-crimp fabric reinforcement, at about £3/kg, as opposed to more exotic substances such as S2-glass at £11/kg, ‘Kevlar’ aramid fibre at £20/kg or carbon. The fabric has been supplied by Hexcel Composites. The matrix is Ciba’s ‘Araldite’ LY556 epoxy, which is chosen to be able to withstand engine bay temperatures of up to 130 deg C.

Much of the weight of an armoured fighting vehicle is its armour. For internal security work, the hull of the demonstrator vehicle is the armour, but if the threat is greater, extra armour plates are bolted onto the outside. The armour on the ACAVP is attached using studs, which screw into threaded inserts in threaded holes in the composite. The reason for this approach is to allow the base vehicle to be transported by air, and one of the requirements of the design programme was that the vehicle could fit into a C130 transport aircraft.

The composite hull comes out as around 60mm thick and weighs five and a half tonnes. The bottom half is made as a single three tonne piece, 6.5m long, with torsion bar suspension covers bonded and bolted on inside.

In the original concept, it would have been necessary to cure the moulding in a large autoclave, but one of the consequences of Shorts dropping out was going to Vosper Thorneycroft, which already knew how to make large, thick composite sections which only need curing in an oven.

The manufacturing process is a version of resin transfer moulding called Vacuum Infusion Moulding. In this, fabric reinforcement is laid into the mould enclosed in a plastic bag. A vacuum pump is attached to the bag, and resin is sucked in and distributed through a mesh on the top of the moulding. The challenge in this instances was to adapt a process usually used to make flat sheets, and work with a viscous resin suitable for surviving engine bay temperatures up to 130 deg C.

Computer aided design and finite element analysis were essential parts of all aspects of the design process. Mark Dean, chief engineer at Vickers Defence Systems, said that this was the first time his company has designed an armoured fighting vehicle using 3D CAD from beginning to end. Everyone concerned with the project stressed the importance of CAD, because they said there was only enough money in the budget to have one go, and so it all had to be right first time. The main CAD package used was EDS Unigraphics, although various other packages were used for specialist work, including the virtual reality mannequin, ‘Jack’ in order to ensure ease of operability by the two crew. The finite element analysis was undertaken using SDRC’s Master Series software.

CAD representation

Future development work under consideration by Dera includes making at least part of armoured fighting vehicle bodies out of fibre reinforced thermoplastics, possibly polypropylene, to further reduce costs, and possible use of lower cost £11/kg carbon fibre now coming onto the market.

The technology now being evaluated in trials with the ACAVP vehicles should be of more than passing interest to the commercial sector. The team is coy about revealing an exact figure for weight saving, but will say that some idea can be gleaned from the 25 per cent weight saving achieved in the experimental Warrior door, and a 30 per cent saving in hull weight "could reasonably be deduced". Since this represents an overall weight saving of only about two tonnes in an 18 to 25 tonne vehicle, this may not at first sight seem a lot. But it must be remembered that if a hull could usefully be made of composites, so could other components. And transferred to a commercial heavy goods vehicle, taking a tonne out of the overall weight means a tonne of extra cargo, and in these economically difficult times, the probable profit margin of the operator.

Nor are motor vehicles the only instances where cost effective weight saving gives commercial advantage. Top sides of oil platforms are often in danger of becoming over-weight, and drastic measures often have to be taken at a late stage of design to protect against a possible capsize. Commercial ships too, are constantly looking at ways of saving weight, hence the widespread use of aluminium alloy superstructures.

More information: Richard Kochanek rfkochanek@dera.gov.uk

http://www.dera.gov.uk

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