Composite parts for subs

Future submarines could be less costly to maintain, more manoeuvrable and possibly stealthier with the use of composites. So think engineers at Oxford University who are leading a project to investigate the use of such materials to replace hull components currently made of metal.

Composites — materials made up of two or more structurally complimentary substances — are engineered to be stronger and stiffer than metal. For the same strength, composites are lighter than steel and aluminium and also less susceptible to corrosion. Composites are attractive in naval platforms because they can maintain the stability margins and buoyancy needs for the complex combat systems used in next-generation submarines.

Navies around the world are increasingly incorporating the material into warships. There is also a requirement to reduce procurement and the cost of ownership while maintaining capability.

Dr Vito Tagarielli, one of the project investigators and a composite materials specialist at Oxford, previously worked on testing composites for warships and predicts similar successful results with their integration in submarines.

’With warships, composites have shown to perform better than metals in terms of resistance to explosions,’ he said.

The Oxford team believes, in the future, composites could replace nickel aluminium bronze — a metal alloy used for critical components such as submarine hull valves.

Nickel aluminium bronze was first used in submarine applications in the 1960s after demonstrating it had better strength, toughness and shock loading resistance compared to gunmetal, which was previously the standard material. However, components cast in nickel aluminium bronze were found to be susceptible to corrosion.

’When you have a metallic structure that is sitting in the water you have to maintain it a lot and check it for corrosion,’ said Tagarielli. ’There is a potential cost saving using composites because essentially this material might have a better relation with the water.’

Composites may also mitigate cost and supply problems associated with complex high-density nickel aluminium bronze castings.

The Oxford team hopes to develop a new composite structure with fibre and hybrid architectures that can withstand years submerged under water and potential blasts from naval ordnance.

Their research will begin by gaining a clearer understanding of how composites respond to high strains while submerged in water. The Oxford researchers have developed a testing method that simulates the exact type of loading an explosion would cause on materials.

The team will take a small sample of a composite and place it at the end of a tube filled with water. A sliding piston will enclose the opposite end.

With the high velocity of a torpedo, a heavy metal projectile will be shot at the piston and create a shockwave in the water that hits the sample. Using high-speed cameras the Oxford team will observe the sample to monitor the deformation and determine its effect.

The tests will be performed on samples that have spent various amounts of time submerged in water. This will determine what detrimental effect — if any — water has on the properties of composites.

With the additional use of computer modelling, Tagarielli and his team expects to have discovered the optimum composite structure within five years.

There are many different architectural possibilities, he said, including 3D reinforced structures and composite sandwiches.

A sandwich solution, Tagarielli said, would consist of two plates separated by a soft material such as foam. ’There is a body of literature that shows for a given weight a sandwich structure is much stronger than a solid structure,’ he said.

The basic goal will be to find a material that is stronger and lighter weight than metal. ’We hope to reduce the weight of the submarine so there is less inertia and it can have higher acceleration and easier manoeuvrability,’ added Tagarielli.

The use of composites could also have strategic advantages. ’If a submarine is made of a composite it makes it invisible to modern sea mines that detonate when they recognise a specific magnetic or acoustic signature,’ he said. ’These are all advantages [the UK] MoD is attracted by.’

The five-year Oxford University project, which begins this April with the aid of a £690,000 EPSRC grant, is receiving support from the Ministry of Defence and the Defence Science and Technology Laboratory. Industrial partners include BAE Systems Advanced Technology Centre, Rolls-Royce Naval Marine, Materials Modelling NPL and Weidlinger Associates.

Siobhan Wagner