Composite material suitable for a morphing skin

Abstract

A morphing skin for an air vehicle structure, the skin being formed by a composite material which includes: a multiplicity of fibers, a matrix incorporating the fibers, and a thermo-sensitive material, such as a thermo-plastic. The thermo-sensitive material can be reversibly transitionable in response to a change in temperature, between (i) an ambient temperature mode in which the thermo-sensitive material is capable of transferring loads in the composite material, and (ii) a heated mode in which the ability of the thermo-sensitive material to transfer the loads is reduced. This can allow the stiffness of the skin to be temporarily reduced to enable the skin to be morphed into a different shape.

Claims

1. A morphing skin for an air vehicle structure, the skin comprised of a composite material comprising: a multiplicity of fibers coated with a thermo-sensitive material which is reversibly transitionable in response to a change in temperature, between: (i) an ambient temperature mode in which the thermo-sensitive material will transfer loads in the composite material, and (ii) a heated mode in which an ability of the thermo-sensitive material to transfer loads is reduced, and a matrix material that incorporates said fibers coated with a thermo-sensitive material, the matrix material remaining stiff when the thermo-sensitive material is heated; wherein: the thermo-sensitive material reduces skin stiffness in the heated mode, allowing a change in shape of the skin in the heated mode and maintaining a changed shape of the skin in the ambient mode.

2. A morphing skin according to claim 1, wherein the thermo-sensitive material is a thermoplastic.

3. A morphing skin according to claim 1, wherein when in the heated mode, the thermo-sensitive material is at a temperature above a transition temperature, such that the thermo-sensitive material has changed state.

4. A morphing skin according to claim 1, wherein the composite material comprises: a first layer comprising said multiplicity of fibers coated with the thermo-sensitive material and incorporated into said matrix material; and a layer of thermo-sensitive material, adjacent to the first layer.

5. A morphing skin according to claim 4, wherein the composite material comprises: a plurality of layers, each of which comprises said multiplicity of fibers coated with the thermo-sensitive material and incorporated into said matrix material, the plurality of layers interleaved with layers of thermo-sensitive material.

6. A morphing skin according to claim 1 wherein the fibers in said multiplicity of fibers are uni-directional.

7. A morphing skin according to claim 1, wherein the skin comprises, or is configured for connection to, a heating apparatus for heating the thermo-sensitive material from the ambient temperature mode to the heated mode.

8. A morphing skin according to claim 7, wherein the fibers in said multiplicity of fibers are electrically conductive and the heating apparatus is configured to pass a current through the multiplicity of fibers to heat the thermo-sensitive material.

9. A morphing skin according to claim 1, wherein the fibers in said multiplicity of fibers are carbon fibers.

10. A morphing skin according to claim 1, comprising: an actuator for changing and reverting a shape of the skin when the thermo-sensitive material is in the heated mode.

11. A method of morphing a skin on an air vehicle, comprising: (i) heating the skin, the skin being configured to reversibly reduce in stiffness in reaction to an increase in temperature; (ii) deforming the skin; and (iii) cooling the skin in the deformed state, the skin being configured to return to higher stiffness in reaction to a decrease in temperature; wherein the skin comprises a composite material comprised of a multiplicity of fibers coated with a thermo-sensitive material, and a matrix material that incorporates said fibers coated in a thermo-sensitive material; wherein the thermo-sensitive material is reversibly transitionable in response to a change in temperature, between: (i) an ambient temperature mode in which the thermo-sensitive material will transfer loads in the composite material, and (ii) a heated mode in which an ability of the thermo-sensitive material to transfer loads is reduced, thereby reducing skin stiffness; the matrix material remaining stiff when the thermo-sensitive material is heated, and the thermo-sensitive material thereby reducing a stiffness of the composite material in the heated mode, allowing a change in shape of the skin in the heated mode, and maintaining a changed shape of the skin in the ambient mode.

12. A method according to claim 11 wherein heating of the skin comprises passing an electrical current through the multiplicity of fibers.

13. An article comprising a composite material which comprises: a multiplicity of fibers coated with a thermo-sensitive material, and a matrix material that incorporates said fibers coated in a thermo-sensitive material; wherein the thermo-sensitive material is reversibly transitionable in response to a change in temperature, between: (i) an ambient temperature mode in which the thermo-sensitive material will transfer loads in the composite material; and (ii) a heated mode in which an ability of the material to transfer the loads is reduced, thereby reducing a stiffness of the composite material; the matrix material remaining stiff when the thermo-sensitive material is heated, the thermo-sensitive material thereby reducing a stiffness of the composite material in the heated mode, allowing a change in shape of the article in the heated mode, and maintaining a changed shape of the article in the ambient mode.

14. The morphing skin according to claim 1, wherein said thermo-sensitive material is a thermoplastic polymer and said matrix is an epoxy matrix.

15. The morphing skin according to claim 14, wherein said thermoplastic polymer is selected from poly(methyl methacrylate-co-acrylamide), polystyrene, and liquid crystal polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Example embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

(2) FIG. 1 is a schematic perspective view of a morphing skin according to a first embodiment of the invention;

(3) FIG. 2 is a schematic exploded perspective view of a morphing skin according to a third embodiment of the invention;

(4) FIGS. 3a and 3b are sectional images of the morphing skin of the third embodiment of the invention in unmorphed and morphed states;

(5) FIG. 4 shows in principle a simple actuating mechanism for use with a morphing skin of an embodiment of the invention; and

(6) FIG. 5 shows a cross section of a wing having active trailing and leading edges incorporating a morphing skin according to an embodiment of the invention.

DETAILED DESCRIPTION

(7) FIG. 1 is a schematic view of a composite material 1 according to a first embodiment of the invention. The composite material 1 is for use on a morphing skin forming the outer surface of a wing of an air vehicle (not shown).

(8) The composite material 1 comprises a plurality of unidirectional carbon fibres 3 incorporated into an epoxy matrix 5, and a thermo-sensitive material 7. As shown in FIG. 1, the surface of the fibres 3 is coated in the thermo-sensitive material 7 such that the thermo-sensitive material 7 forms an interface between the carbon fibres 3 and the matrix 5.

(9) In the first embodiment of the invention, the thermo-sensitive material 7 is a poly(methyl methacrylate-co-acrylamide) copolymer (p(MMA-co-AAm)). MMA is the monomer for PMMA, an amorphous polymer with glass transition temperature (Tg) around 105-120 C. AAm was chosen as co-monomer as it was found to provide effective adhesion between carbon fibres and an epoxy matrix.

(10) The composite material 1 was manufactured as follows:

(11) (i) The p(MMA-co-AAm) copolymer was coated on carbon fibres in-situ. MMA and AAm were dissolved in dimethylformamide (DMF), then, lithium perchlorate (electrolyte salt) was added to the solution.

(12) (ii) Carbon fibres (AS4 unsized 12 k, Hexcel) were passed continuously through a stainless-steel tube which was immersed in the electrolyte bath at constant processing speed (1.9 mm/s), fibre tension (150 g) and temperature (65 C.). Constant electric current (1.2 A) was applied to the setup during the electrocoating process. Carbon fibres were used as anode while the stainless-steel tube was the cathode.

(13) (iii) The coated carbon fibres were washed thoroughly with acetone and distilled water to remove any trapped and unreacted monomers and extract any salt residue, then, they were dried overnight in a drying oven (55 C.).

(14) (iv) Filament winding with constant tension and speed was used to wind the p(MMA-co-AAm) electrocoated carbon fibres onto a stainless steel plate which was covered with a release film. Mixed epoxy resin (Epoxy resin (LY556, Huntsman) and hardener (XB3473, Huntsman) was applied in between each layer of coated fibre laminates by brushing.

(15) (v) Next, the laminates were placed into a resin infusion under flexible tooling (RIFT) setup in order to infuse more epoxy resin through the laminates under vacuum and also remove any air bubbles from the laminates. The coated fibre laminates were cured at 120 C. for 2 hours and 180 C. for 4 h under vacuum (105 Pa).

(16) The thermo-sensitive material 7 in the composite material 1 of the first embodiment is reversibly transitionable between a glass phase in an ambient temperature mode when it is below 114 C. (the glass transition temperature) and a rubber phase in a heated mode (when it is above the glass transition temperature). In the ambient temperature mode the thermo-sensitive material 7 is capable of transferring loads in the composite material, whereas in the heated mode the ability of the material 7 to transfer the loads is reduced.

(17) This behaviour can be demonstrated in a flexural test (three point bending) using a piece (10 mm width40 mm length1.1 mm thickness): At room temperature, the thermo-sensitive material 7 (p(MMA-co-AAm)) is in ambient temperature mode and can transfer loads into the composite material 1. Accordingly, the flexural modulus of the p(MMA-co-AAm) electrocoated carbon fibre-epoxy composite is 857 GPa. When the composite 1 is heated to 130 C., which is above glass transition temperature (114 C.) of p(MMA-co-AAm), the thermo-sensitive material 7 transitions into a heated mode in which its ability to transfer loads to the surrounding composite material 1, and in particular the carbon fibres 3, was reduced. Accordingly, the flexural modulus of the composite material 1 at 130 C. reduces to 398 GP. This corresponds to a 54% reduction in stiffness.

(18) Furthermore, the p(MMA-co-AAm) is reversibly transitionable between these heated and ambient temperature modes. Accordingly, when cooled back to ambient temperature the thermo-sensitive material 7 is once again capable of transferring loads in the composite material (the composite material 1 was found to have a flexural modulus of 822 GPa (i.e. 96% recovery)).

(19) The above-described composite material 1 is especially beneficial for use in a morphing skin (for example on an aircraft wing) because it can be repeatedly altered between a stiff state in which it acts as a suitable aerodynamic surface, a heated state in which the stiffness of the skin is reduced to enable morphing of the wing skin into a different shape, and then cooled to stiffen the skin in that morphed shape.

(20) An aircraft (not shown) using the composite material of the first embodiment comprises a wing skin formed from the composite material 1, a heater for heating the composite material 1 and an actuator for morphing the wing. The heater comprises an electrical power source electrically coupled to the carbon fibres 3 such that when the power source is switched on, current runs through the carbon fibres 3 and heats them up. This heat is then transmitted through to the surrounding thermo-sensitive material 7, which, once hotter than its transition temperature, softens, thereby reducing its ability to transfer loads in the composite material 1 and thus reducing the stiffness of the composite material. The actuator is arranged to exert a deforming force of the wing skin (for example via hydraulics, and/or using cams) once the stiffness has been reduced. Once the wing skin is then in the required shape, the power source is switched off and the thermo-sensitive material 7 naturally cools to below its transition temperature thereby re-stiffening the wing in its morphed shape.

(21) In a second embodiment of the invention (not shown), the thermo-sensitive material is a liquid crystalline polymer (LCP). LCPs combine the useful properties of polymers with those of liquid crystals forming mesophases upon melting before turning into an isotropic melt. LCPs provide more phase transitions than the thermo-sensitive materials of the first embodiment because they can be changed between crystalline, liquid crystalline and isotropic melt states. The properties of the thermo-sensitive material can therefore be switched reversibly between more stages than those in the first and third embodiments.

(22) There are various possible methods to coat the carbon fibres with LCP (for example dipped coating, polymerisation coating and plasma coating). The LCP-coated carbon fibres can then be manufactured into the composite material following the procedures described in the first embodiment.

(23) FIG. 2 is a schematic exploded view of a morphing skin according to third embodiment of the invention. The skin is located on the engine air-intake of an air-vehicle (not shown). The skin comprises a composite material 101 having interleaved layers of 125 m Carbon-Fibre reinforced thermoset (CFRP) 104 and 130 m polystyrene 107. The CFRP 104 is made up of unidirectional carbon fibres 103 and an epoxy matrix 105 (not separately visible in FIG. 2).

(24) The composite material 101 is made up of seventeen alternating layers of CFRP 104 and polystyrene 107 (only some of which are shown in FIG. 2). To form the composite material, the layers were placed into a hot press and cured at 175 C. and 100 Psi for 1 hour.

(25) FIGS. 3a and 3b are sectional images of the composite material in a flat state (FIG. 3a) and a morphed (bent) state (FIG. 3b). The morphed state is achieved by heating the polystyrene layers 107 to 120 C. such that the elastic modulus of the polystyrene 107 reduces, thereby reducing the ability of the polystyrene to transfer loads in the composite material, and thus reducing the stiffness of the wing skin. The skin is then actuated into the deformed state, and allowed to cool (by removing the heat source) such that it stiffens in the morphed shape.

(26) Only an end portion of the composite material providing the morphing skin 1 is shown in FIGS. 3a and 3b, showing the relative positions of the ends of the CFRP layers 103, 105 in the flat and morphed states of the polystyrene layers 107. A greater length of the polystyrene layers 107 is shown in sketches to the right in FIGS. 3a and 3b, these showing the curve of the body of the polystyrene layer 107 in the morphed state and a change in shape of the endface 300 thereof.

(27) As shown in FIG. 3b, when the thermo-sensitive material 107 is in the heated state, the layers of CFRP 104 are able to move relative to one another, thereby reducing the stiffness of the wing skin. In this third embodiment of the invention, the flexural modulus of a test piece was measured as 65 GPa at room temperature (when the thermo-sensitive material was in ambient temperature mode) and 1 GPa at 120 C. (when the thermo-sensitive material was in heated mode). This equates to a 98% stiffness loss. Upon returning to the ambient temperature mode the flexural modulus of the composite material was measured as 65 GPa (indicating a substantially 100% stiffness recovery).

(28) Actuators for producing changes in shape of a structure such as an aircraft wing are known. FIG. 4 shows a very simple mechanical actuator for flexing a morphing skin 1 having a generally curved cross section. The actuator is based on a three slot structure in which two slots 425 are aligned and a third slot 430 is at right angles and directed towards the gap between the aligned slots 425. Shafts 420 attach to points towards the edges of the skin 1, their free ends being hingedly attached to a mounting in the third slot 430. If the mounting moves in the third slot 430, as indicated by the vertical arrow 400, it causes the attachment points of the shafts 420 to the skin 1 to move closer or further apart, as indicated by the horizontal arrows 410, thereby flexing the skin 1 to and fro, changing and reverting it between different shapes, in this case of greater and lesser curvature. Movement of the mounting of the shafts 420 in the third slot 400 might be produced for example hydraulically.

(29) FIG. 5 shows a slightly different arrangement of actuator shafts 420 controlled from a rigid wingbox structure 510 in a wing and extending through forward and aft active regions 500, 510 of the wing encased in a morphing skin 1. This mechanical actuation arrangement also changes and reverts the aerodynamic wing profile while the morphing skin 1 is in heated mode.

(30) Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For example, the heating apparatus may comprise an exhaust pipe located in proximity to the thermo-sensitive material such that some of the heat emitted from the exhaust is transferred to the thermo-sensitive material. By way of another example, the composite material need not necessarily be used in an air vehicle and could, for example, be used when manufacturing complex component parts, to move non-aerospace components such as a valve in a duct. In a further example, the skin is configurable between two heated modes; in a first heated mode only selected locations of the wing skin are heated to partially reduce the stiffness of the skin, whereas in a second heated mode additional locations of the skin are also heated to yet further reduce the stiffness and configure the skin.

(31) Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.