Composite structures have been proposed for noise attenuation or vibration reduction for example around machinery or in buildings. Other uses of composites are in construction of boats and component parts of aircraft. Conventional structural composites are acoustically and vibrationally live, as a consequence of which noise reduction palliatives are applied retrospectively at considerable cost and with a subsequent increase in weight which may make them unsuited to their original purpose.

In the case of boats and aircraft, the main purpose of composites has been to obtain a structurally sound material, to replace existing metal components with a lighter material.

In submarines there is a particular problem in that the rudder, fins, hydroplanes and other free flooded structures made from a steel framework with steel coating are very prone to corrosion, giving rise to regular and expensive remedial action. An additional requirement of a submarine is that it wishes to remain undetected by sonar systems. This is normally achieved by attaching tiles which possess anechoic and transmission-loss properties to the surface of the submarine, with filler in the gaps.

Tiles are labour intensive to make and apply to the submarine, so they are an expensive solution. The cost of tile application is typically four times the cost of the tile itself and the necessity to apply tiles after building the steel structure adds to the overall time taken to build the submarine. The tiles also tend to become detached due to vibration or impact in some areas, particularly around the rudder and propeller. In addition they add weight and may lead to instability in the vessel.

In accordance with the present invention, a composite structure comprises first and second fibre reinforced composite layers; a plurality of through direction support members connected between the first and second composite layers; and an absorbent material positioned between the layers; wherein the support members are curved, such that the structure is compliant in the through direction of the structure and has a high bending stiffness perpendicular to the through direction.

The present invention allows existing metal or composite components to be replaced with a light material incorporating intrinsic absorption measures by being

compliant in one direction and so able to absorb energy, yet being stiff and resistant to bending in the other two of its three dimensions and so structurally acceptable. This enables noise and vibration reduction to be achieved in a more cost effective way than conventional methods, reduces the overall fabrication for the structures by incorporating the absorption measures in the construction process, and minimises through life costs of metal structures which are prone to corrosion by using material which requires minimal maintenance.

Preferably, each support member and composite layer comprises a fibre mesh.

The fibres ofthe mesh may be wound at any angle between 30° and 600 , but preferably they are wound at + / - 450 to its axis.

A fibre mesh wound at this angle has been found to provide a high shear stiffness link or support between the outer reinforcing composite layers necessary for high bending stiffness of the composite structure.

Preferably, the support members comprise a stretched "D" shaped fibre mesh.

A stretched "D" shaped mesh can be filled with absorbent material and multiple filled meshes can be positioned adjacent one another between the composite layers.

Preferably, the relative volume fraction of the support member to the absorbent material does not exceed 2.5%.

This gives the required through direction compliance.

In order to allow the acoustic energy to couple into the absorbent filler it is necessary for the outer reinforcing composite layer to be a transparent structure and therefore preferably, the mesh comprises no more than 50% of the composite layer.

Typically, the fibre mesh comprises carbon, glass, polyester or kevlar fibres.

Preferably, the absorbent material comprises at least one of an acoustic, vibration or electromagnetic absorbing material.

The choice of material is dependent upon the use to which the structure will be put. With acoustic or vibration absorbing material, this construction can be used as a mounting for machinery to reduce vibration and noise transmission. Acoustic absorbing material can be used to reduce a submarine's likelihood of detection by sonar or other means such as magnetic sensors. Electromagnetic stealth features can be added by substituting or at least part substituting the outer reinforcing composite layer with electromagnetic absorbing material.

An example of a composite structure according to the present invention will now be described with reference to the accompanying drawings in which:- Figure 1 illustrates a first example of a composite structure according to the invention; Figure 2 illustrates a second example of a composite structure according to the invention; Figure 3 illustrates in more detail a support member for the structure of Fig. 1 or Fig. 2; Figure 4 illustrates one application of a composite structure according to Fig. 1 or Fig. 2; and, Figure 5 illustrates a second application of a composite structure according to Fig. 1 or Fig. 2.

Fig. 1 shows a first example of a composite structure 1. The structure 1 has fibre reinforced composite layers 2,3 which are joined together by curved supports 4.

Contained in the area defined by the layers 2,3 are the supports 4 and an absorbent material 5. The absorbent material is chosen to have properties according to the use of the structure. For a machine raft, the material will be a vibration absorber (damping material) such as polyurethane rubber; for submarine secondary structure, the material will be acoustically absorbing material such as a foamed polyurethane rubber that is acoustically tuned via its thickness and compliance to the operational frequencies of interest. The curved supports 4 couple the composite layers 2,3 in shear to give a structure which is very stiff and strong in bending. In one example, this is equivalent to the bending stiffness of a 12mum thick steel plate.

In Fig. 2 an alternative arrangement is shown. In this case, the reinforced composite layers are joined together by supports 6 which are of a stretched "D" shape.

These could also be oval. These supports 6 contain appropriate absorbent material 5 in the same way as the example of Fig. 1, but can be manufactured by fabricating the "D" or sections with fibre braid, filling the centre of the braid with absorbing material 5 and positioning these between the layers 2,3 as desired before the whole of the composite is bonded together. As in the example of Fig. 1, the supports have a curved shape

between the two composite layers 2,3 which is compliant under pressure, but enhances the bending stiffness of the structure.

Fig. 3 illustrates an example of a support member in more detail. Each of the composite layers and the supports are made up of not more than 50% fibres 8,9 joined by matrix material 10. The fibres for the supports or webs of the composite layers are wound preferentially at +/- 45" (although any angle within the range of 30 to 60 degrees would be acceptable) to the longitudinal axis of the support or layer, which has been shown to give the best bending stiffness. The matrix material 10 is an epoxy/vinyl ester resin which is reinforced by the fibres 8,9 and of substantially constant thickness throughout. The material can be polyester, polyurethane or an epoxy resin.

Applications of the composite of the invention include construction of submarine secondary structures, particularly the free-flooded parts which must be strong, but compliant to absorb acoustic energy to avoid detection. Construction of certain parts of the submarine directly in this way overcomes the disadvantages of applying tiles to avoid detection and has the additional benefit that by using composites, corrosion in free-flooded structures is reduced. The cost of initial construction is comparable to the costs of a conventional steel structure, but the through life costs are significantly reduced. Furthermore, the acoustic performance is significantly better than that achieved using tiles. Another application is in construction of machine rafts and isolation mounts for mounting machinery to reduce vibration and noise transmission. A mount with damping incorporated into it is easier and cheaper to manufacture than a mount to which damping is added. The requirement for compliance in the through direction and high bending stiffness perpendicular to this direction is the same in both applications.

In a waterborne environment it is essential for an acoustically absorbing material to have a high compliance compared to water (i.e. a low stiffness/bulk modulus). Typically, an acoustic absorbing material is l/lOth the effective bulk modulus of water. This is essential as it helps to ensure that the applied cladding is thin and has low added weight and material costs. Such a material however, has only minimal structural qualities and could not act as a substitute for a submarine casing fin or rudder skin. The present invention is particularly suited to waterborne applications, providing a highly anisotropic structure which is compliant in one direction and so able

to absorb energy, but is structurally strong in the direction required for structural purposes.

Fig. 4 shows an application of the structure of the present invention, which is in construction of a submarine rudder 11. The rudder 11 has an internal framework 12 of fibre reinforced epoxy/vinyl ester resin on which a skin 13 made from the composite structure of the invention is mounted. The skin 13 completely surrounds the rudder 11 but has vent holes (not shown) at the top and bottom to allow the structure to free flood. The outermost surface ofthe skin 13 should be acoustically transparent, so a carbon fibre reinforced plastic composite with no more than 50% carbon fibres is preferred to prevent reflections directly off the skin. This gives only moderate in-plane extensional stiffness and strength. An electrically insulating layer of glass reinforced epoxy/vinyl ester layer may be provided between the framework and the skin if required.

The curved supports of the present invention are well suited to this application because in combination with the absorbing material chosen, they allow the stiffness normal to the skin to be optimised for acoustic matching to water. The supports 4, absorbing material 5 and composite layers 2,3 are bonded together with resin. A layer 14 of low density, low-modulus, transmission-loss rubber, is bonded to the inner composite layer 3 ofthe skin 13 ofthe rudder. This is non-structural, but is acoustically important and causes a z-phase-change on reflection and allows quarter wavelength tuning of the absorption peak with minimal overall panel thickness. This material reflects sound generated inside the submarine freeflood structure to prevent sound energy reradiating from an internal structure back into the surrounding water.

The rudder is structurally and hydrodynamically similar to a conventional rudder, but with better acoustic performance and reduced through life costs.

Fig. 5 shows another application of the invention which is in constructing machine rafts 15 for mounting machinery 16. The raft 15 comprises a composite structure as described above which supports the vibrating machine 16. The intrinsic loss characteristics of the composite ensure that vibrational energy from the machine 16 is effectively absorbed within the composite and does not transmit to the raft edges 17 where it is attached to a ship's hull 18. By aligning the supports 4 as shown the most tortuous route for the vibrations is provided because the loss characteristics are highest in a direction perpendicular to the curved supports.

These applications of the structure are not exhaustive and other uses envisaged include panelling in noisy buildings, machinery spaces, aircraft, ships and trains or other situations where there is transmitted noise or vibration.