GB2168009A - Materials - Google Patents

Abstract

A vehicle having a body at least part of which is fabricated from a resilient substrate including a microwave radiation absorbent or microwave radiation reflective material.

Description

SPECIFICATION Improvements in or relating to materials The present invention relates to materials and more particularly to radiation absorbing materials or radiation reflecting materials.

Radiation absorbing materials, such as microwave radiation absorbing materials, fall generally into two categories, broad band absorbers and tuned absorbers. Such materials are well known and are usually bonded to the surface of a vehicle to limit the possibility of detection of the vehicle by modern detection apparatus.

For certain vehicles the bonding of radiation absorbing materials to the body of the vehicle may give rise to difficulties. In the case of an inflatable craft, for example a dinghy, the main body of the craft must have resilient properties and these properties may not be compatible with the physical properties of a material having the desired radiation absorbing characteristics. Such difficulties may also arise in the case of radiation reflecting materials which are usually bonded to the body of a vehicle when it is desired to facilitate detection of that vehicle. Similar problems may also arise when there is a requirement to bond a material having a desired radiation absorbing or reflecting characteristic to resilient parts of vessels, for example to hovercraft skirts and fingers.

The present invention strives to overcome the above difficulties by providing a radiation, in particular microwave radiation used by radar, absorbent or reflective material incorporated into a resilient material for use in fabricating the body or at least part of the body of a vehicle.

According to the present invention there is provided a vehicle having a body at least part of which is fabricated from a resilient material, the resilient material being at least partly fabricated as a resilient substrate including a microwave radiation absorbent or microwave radiation reflective material.

In a preferred embodiment of the present invention the resilient substrate incorporates reinforcement material in addition to the microwave radiation absorbent or microwave radiation reflective material.

Conveniently the reinforcement may comprise fibrous, filamentary or fabric material, for example formed from Kevlar (Trade Mark) or Nylon.

In one embodiment of the present invention at least a part of the body of the vehicle has the microwave radiation absorbing material formed as at least one layer within the resilient substrate, a layer of conductive material also being provided in the substrate or on a surface of the substrate, so as to form with the microwave radiation absorbing material a tuned absorber.

In another embodiment of the present invention at least a part of the body of the vehicle has a graduated concentration of microwave radiation absorbing material embedded within at least a portion of the resilient substrate, thereby forming a broad band absorber by creating a gradual transition of the impedance in said at least portion of the resilient substrate.

In another embodiment of the present invention at least a part of the body of the vehicle is formed from a resilient substrate provided with a conductive material in a manner so that the prefabricated substrate behaves as a microwave radiation reflective material, the conductive material being flexible and preferably exhibiting elastomeric properties.

The resilient substrate may comprise natural or synthetic rubbers or rubber-like materials such as polyester elastomer materials or flexible plastic materials.

The resilient materials described above are prefabricated before they are subsequently fabricated into the shape of a vehicle body or part of a vehicle body, the resilient materials being inherently microwave radiation adsorbing or microwave radiation reflecting thereby eliminating the need for bonding onto their surface further microwave radiation absorbing or reflecting materials which may as explained above give rise to design difficulties.

In a preferred embodiment of the present invention the prefabricated microwave radiation absorbing or reflecting resilient materials form at least part of an inflatable body of a craft, for example a dinghy. In another preferred embodiment the vehicle is a craft, for example a hovercraft, having a skirt or fingers fabricated from the microwave radiation absorbing or reflecting materials.

The present invention will be described further, by way of examples, with reference to the accompanying drawings wherein the same reference numerals refer to the same features in each of the examples, and in which: Figure 1 is a cross-sectional view of a resilient material having microwave radiation absorbent properties for use in the body of a vehicle in accordance with an embodiment of the present invention; Figure 2 is a cross-sectional view of another form of resilient material having microwave radiation absorbent properties for use in the body of a vehicle in accordance with another embodiment of the present invention; Figure 3 is a cross-sectional view of another form of resilient material having microwave radiation absorbent properties for use in the body of a vehicle in accordane with another embodiment of the present invention:: Figure 4 is a cross-sectional view of a resilient material having microwave radiation reflecting properties for use in the body of a vehicle in accordance with another embodiment of the present invention; Figure 5 is a cross-sectional view of another form of resilient material having microwave radiation absorbent properties for use in the body of a vehicle in accordance with a further embodiment of the present invention; and Figure 6 is a graph illustrating the variation of impedence with depth through the resilient material of Figure 5.

The resilient material as shown in Figure 1 comprises a substrate 2 of elastomeric material such as natural rubber. Within the substrate 2 are embedded reinforcement members 4, such as Kevlar (Trade Mark) or Nylon, and a portion of the substrate 2 having a thickness "d" is impregnated with microwave radiation absorbing particles 6, such as iron powder. If desired the portion of the substrate 2 having the thickness "d" may also be provided with reinforcement members. The layer of microwave radiation absorbing particles 6 is formed adjacent a conductive layer of material 8, for example a brass wire gauze, which in Figure 1 is shown embeded within the substrate 2.

In Figure 1 the surface A indicates the surface of the substrate 2 which will form the outside surface of the vehicle body. The surface A may be provided with a thin surface layer (not shown) of elastomerfor its environmental protection. The concentration of the iron powder 6 and the thickness "d" is chosen so that the layer of iron powder 6 together with the conductive material 8 forms a tuned radiation absorber of the quarter-wave type. The radiation absorber is tuned to absorb substantially microwave radiation within a specific frequency band, that frequency band preferably lying somewhere within the overall frequency band of 2 to 100 GHz which covers the frequency of radiation commonly used with modern radar systems.

The resilient substrate 2 in Figure 2 is formed as a multiband radiation absorber. The portion of the substrate 2 lying between the layer of microwave radiation absorbing particles 6 and the surface A is either left free of or impregnated with microwave radiation absorbing particles 6 the concentration of which affects the frequency band or bands which are to be absorbed. The reinforcement members 4 may be embedded throughout the substrate 2 and in particular may be embedded within the portion of the substrate "d".

It will be appreciated that it is possible by alternative designs to provide structures which will absorb any frequency in the radar bands, or to design a two or more layer type structure which will absorb separate frequencies.

In the embodiment of Figure 3 the surface A of the substrate 2 is covered by at least one resistive layer 10, the layer 10 being formed from for example one or more carbon impregnated cloths. The resistive layer 10 may alternatively be located within the substrate 2 close to the front surface A. The embodiment in Figure 3 behaves as a quarter-wave absorber type the substrate 2 being impregnated with either a high or low concentration of microwave radiation absorbing particles 6 which concentration together with the thickness "d" determines the tuned frequency band which is to be absorbed. If desired reinforcement members 4 may be embedded within the substrate 2.

The resilient material as shown in Figure 4 comprises as in Figures 1 and 2 a resilient substrate 2 into which is embedded the reinforcement material 4. A conductive layer of material 8 is provided on the surface A of the substrate 2. Unlike the embodiment in Figures 1 to 3 radar absorbing particles 6 are incorporated and the resilient material because of the provision of the conductive layer 8 behaves as a microwave radiation reflecting material.

In an alternative embodiment of Figure 4 the conductive layer 8 is embedded within the substrate 2 a distance "d" away from the surface A of the substrate. The distance "d" is conveniently chosen to be substantially equal to nX 4v where n is an integer, X is the design wavelength to be reflected and v is the refractive index of the substrate material.

The resilient material in Figure 5 comprises the resilient substrate 2, reinforcement material 4, conductive layer 8 as a backing layer on the substrate 2 and microwave radiation absorbing particles 6. The microwave radiation absorbing particles 6, typically iron, or carbon loaded plastic, are impregnated within the resilient substrate 2 with graduated concentration so creating a graduated microwave absorbing index to provide a gradual transition in the impedence of the material. The concentration increases with depth below the surface A. The impedence z at a depth x in the material is defined by the equation

where ffi is the complex magnetic permeability and is the complex dielectric constant. The graph of Figure 6 illustrates the variation of impedence with depth below the surface A for the material of Figure 5. Such a material behaves as a broad band microwave radiation absorber.

The resilient materials described above with reference to Figures 1 to 6 can be used ideally for the construction of inflatable bodies, such as dinghies. Other craft having inflatable bodies include reconnaissance balloons or inflatable aircraft or parts of aircraft.

The resilient materials described above with reference to Figures 1 to 6 can also be used for the fabrication of hovercraft skirts or fingers, or side skirts for tanks.

Other designs of resilient materials in accordance with the present invention which are particularly suitable for use in fabricating the bodies of dinghies, or for use in hovercraft skirts or fingers, or for other inflatable craft or parts of vehicles are designs incorporating circuit analogue features. For example an array of resistive or metallic elements may be provided, by printing using the screen printing technique, on the surface A of the substrate 2. The array of resistive or metallic elements is arranged to resonate at microwave frequencies which it is desired to absorb.

Claims (12)

1. A vehicle having a body at least part of which is fabricated from a resilient material, the resilient material being at least partly fabricated as a resilient substrate including a microwave radiation absorbent or microwave radiation reflective material.

2. A vehicle as claimed in claim 1 wherein the resilient substrate incorporates reinforcement material in addition to the microwave radiation absorbent or microwave radiation reflective material.

3. A vehicle as claimed in claim 2 wherein the reinforcement material comprises fibrous, filamentary or fabric material.

4. A vehicle as claimed in any one of claims 1 to 3 wherein at least a part of the body of the vehicle has the microwave radiation absorbing material formed as at least one layer within the resilient substrate, a layer of conductive material also being provided in the substrate or on a surface of the substrate, so as to form with the microwave radiation absorbing material a tuned absorber.

5. A vehicle as claimed in any one of claims 1 to 3 wherein at least part of the body of the vehicle has a graduated concentration of microwave radiation absorbing material embedded within at least a portion of the resilient substrate, thereby forming a broad band absorber by creating a gradual transition of the impedence in said at least portion of the resilient substrate.

6. A vehicle as claimed in any one of claims 1 to 3 wherein at least part of the body of the vehicle is formed with the resilient substrate provided with a conductive material in a manner so that the prefabricated substrate behaves as a microwave radiation reflective material, the conductive material being flexible and exhibiting elastomeric properties.

7. A vehicle as claimed in any one of claims 1 to 6 wherein the vehicle is a craft having an inflatable body, the craft body or part of the craft body being formed from the prefabricated microwave radiation absorbing or reflecting resilient material.

8. A vehicle as claimed in any one of claims 1 to 6 wherein the vehicle is a hovercraft, at least a part of the skirt or fingers of which is fabricated from the microwave radiation absorbing or reflecting material.

9. A vehicle as claimed in any one of claims 1 to 8 wherein the resilient substrate is prefabricated from one or more of the group of materials including natural or synthetic rubbers, rubber-like materials such as polyester elastomer materials or flexible plastic materials.

10. A method of manufacturing the body of a vehicle as claimed in any one of claims 1 to 9, the method comprising the steps of fabricating the prefabricated resilient substrate into the shape of the vehicle body or part of the vehicle body, the prefabricated resilient substrate exhibiting microwave radiation absorbing or microwave radiation reflecting properties thereby eliminating the need for bonding onto the body of the vehicle further microwave radiation absorbing or reflecting materials.

11. A vehicle having a body at least part of which is fabricated from a resilient material, the vehicle being substantially as hereinbefore described with reference to, and as illustrated in any one of Figures 1 to 6 of the accompanying drawings.

12. A method of manufacturing the body of a vehicle, the method being substantially as hereinbefore described with reference to, and as illustrated in any one of Figures 1 to 6 of the accompanying drawings.