STRAIN SENSING IN COMPOSITE MATERIALS

Abstract

A method of sensing strain in a structural component, the method comprising the steps of providing, embedded within said structural component, at least one carbon fibre element (10) coated with an electrically conductive material (12), measuring the electrical resistance of said coated carbon fibre element and determining strain in respect of said carbon fibre element based on changes in said electrical resistance thereof. A method of manufacturing a carbon fibre element, a carbon fibre element so manufactured, and a carbon fibre reinforced structural component including such a carbon fibre element are also disclosed.

Claims

1. A method of sensing strain in a structural component, the method comprising: providing, embedded within said structural component, at least one carbon fibre element coated with an electrically conductive material; measuring an electrical resistance of said coated carbon fibre element; and determining strain in respect of said carbon fibre element based on changes in said electrical resistance thereof.

2. The method according to claim 1, wherein said electrically conductive material has a thermal coefficient of resistance which is of opposite sign and substantially equal magnitude to an electrical resistance of said carbon fibre.

3. The method according to claim 1, wherein a thickness of the coating of said electrically conductive material is such that a resistance per unit length of said electrically conductive material is substantially equal to a resistance per unit length of said carbon fibre.

4. The method according to claim 1, wherein said electrically conductive material exhibits negligible piezoresistance.

5. The method according to claim 1, wherein said electrically conductive material is a metal or a metal alloy.

6. The method according to claim 5, wherein said electrically conductive material comprises nickel.

7. The method according to claim 6, wherein said electrically conductive material comprises a nickel-copper alloy or phosphorous nickel plate.

8. A method of manufacturing a carbon fibre element for a carbon fibre reinforced structural component, the method comprising: providing at least one carbon fibre; selecting an electrically conductive material having a thermal coefficient of resistance which is of opposite sign and substantially equal magnitude to a thermal coefficient of said carbon fibre; and coating said carbon fibre with a coating of said electrically conductive material.

9. The method according to claim 8, wherein a thickness of said coating of electrically conductive material is selected such that a resistance per unit length of said electrically conductive coating is substantially equal to a resistance per unit length of said carbon fibre.

10. The method according to claim 8, wherein said electrically conductive material exhibits negligible piezoresistance.

11. The method according claim 8, wherein applying said coating comprises applying said electrically conductive material directly to said at least one carbon fibre.

12. The method according to claim 8, wherein said electrically conductive material comprises a nickel-copper alloy or phosphorous nickel plate.

13. The method according to claim 8, wherein applying said coating comprises electroless nickel plating.

14. A carbon fibre element for use in a carbon fibre reinforced structural component, said carbon fibre element comprising: a carbon fiber; and a coating of an electrically conductive material applied to said carbon fiber, said coating having a thermal coefficient of resistance that is of opposite sign and substantially equal magnitude to a thermal coefficient of resistance of the carbon fiber.

15. A carbon fibre reinforced structural component comprising: a structural component; a carbon fiber embedded in said structural component; and a coating of an electrically conductive material applied to said carbon fiber, said coating having a thermal coefficient of resistance that is of opposite sign and substantially equal magnitude to a thermal coefficient of resistance of the carbon fiber.

Description

[0028] Embodiments of the present invention will now be described, by way of examples only, and with reference to the accompanying drawings, in which:

[0029] FIG. 1 is a schematic diagram illustrating the structure of a fibre reinforced polymer material according to the prior art;

[0030] FIG. 2 is a schematic cross sectional view of a fibre reinforced material and strain sensing arrangement according to the prior art;

[0031] FIG. 3 is a schematic cross-sectional diagram illustrating a coated carbon fibre filament according to an exemplary embodiment of the invention;

[0032] FIG. 4 is a graphical representation illustrating the thermal coefficient of resistance for copper-nickel alloys;

[0033] FIG. 5 is a graphical representation illustrating the TCR response of plain carbon and mixed carbon/nickel plated carbon tows; and

[0034] FIG. 6 is a graphical representation illustrating the modelled strain error response of plain carbon fibre tows and thermally compensated tows, based on equal and opposite TCR and matched resistance per unit length.

[0035] Thus, embodiments of the present invention provide a fibre reinforced substrate in which at least one of the fibres is a carbon fibre coated with a conductive material which has a thermal coefficient of resistance (TCR) which is of opposite sign and at least closely matched magnitude to that of the underlying carbon fibre, and which has a thickness selected such that its resistance per unit length at least closely matches that of the underlying carbon fibre.

[0036] Numerous methods of plating or coating individual carbon fibres and carbon fibre tows have been proposed, and it is envisaged that many of these could be used to produce the metal-coated carbon fibres for use in embodiments of the present invention.

[0037] For example, electroplating can be used to coat carbon fibre filaments or a carbon fibre tow, by using electrical current to reduce dissolved metal cations so that they form a coherent metal coating on an electrode (i.e. in this case, the carbon fibre).

[0038] An alternative coating method is known as electroless nickel plating which is an autocatalytic chemical technique used to deposit a layer of nickel-phosphorous or nickel-boron alloy on a substrate (in this case, a carbon fibre filament or tow). The process relies on the presence of a reducing agent, for example, hydrated sodium hypophosphite (NaPO.sub.2H.sub.2.H.sub.2O), which reacts with the metal ions to deposit material. Thus, unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. Deposition of alloys with different percentages of phosphorus are possible, and the metallurgical properties of alloys tend to depend primarily on the percentage of phosphorous. In this regard, low, medium and high phosphorous electroless nickel are all general terms commonly used in the art, and properties of electroless nickel-phosphorous alloys are provided, for example, by Norio Miura, et al in “Electroless Nickel Resistors Formed in IMST Substrate”, IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-4, No. 4, December 1981.

[0039] It will be appreciated that other methods of coating a carbon fibre filament, or a tow containing one or more carbon fibre filaments, will be known to a person skilled in the art, and the present invention is not necessarily intended to be limited in this regard.

[0040] It will be further appreciated that the present invention is not necessarily intended to be limited in respect of the conductive material used for the coating. The coating material selected is dependent on the thermal coefficient of resistance (TCR) of the underlying carbon fibre, and/or the thickness of the coating is dependent on the resistance per unit length of the underlying carbon fibre, since either or both of these coating parameters are intended to at least closely match the corresponding parameter of the carbon fibre.

[0041] Thermal Coefficient of Resistance (TCR)

[0042] The thermal coefficient of resistance is an inherent property of electrically conductive materials, and refers to their relative change of electrical resistance as their temperature changes. A positive ˜TCR refers to materials that experience an increase in resistance when their temperature is raised, and a negative TCR refers to materials that experience a decrease in electrical resistance when their temperature is raised. Some known carbon fibres, for example, have been found to have a TCR of about −0.03%/° C., indicating that for each increase in temperature by 1° C., there is a resultant decrease in electrical resistance of about 0.03%.

[0043] Resistance per Unit Length

[0044] Resistance per unit length of a material is dependent primarily on the resistivity ρ of the material, and its cross-sectional area. Resistivity is an intrinsic property of a material that quantifies how strongly it opposes the flow of electric current.

[0045] Referring to FIG. 3 of the drawings, a carbon fibre filament 10 having a conductive coating 12 is shown schematically in cross-section.

[00001]$\mathrm{Resistance}.Math..Math.\mathrm{per}.Math..Math.\mathrm{unit}.Math..Math.\mathrm{length}=\frac{\rho }{A}$

[0046] where ρ=resistivity (in Ω.Math.m) and A=cross-sectional area (in m.sup.2). Thus, if a specific resistance per unit is required to be achieved for the coating, then, given its intrinsic resistivity, the cross sectional area can be selected accordingly. It can be seen from FIG. 3 that the cross-sectional area is dependent on the thickness r.sub.2 of the coating 12. If the radius of the coated fibre is R and the radius of the carbon fibre filament is r.sub.1 (which is known), then:

A.sub.coating=πR.sup.2−πr.sub.1.sup.2

where:

R=r.sub.1+r.sub.2

[0047] Thus, given that r.sub.1 is known, it is possible to select the thickness r.sub.2 of the coating 12 (which has a known resistivity p) to give a desired cross-sectional area A, thereby resulting in a coating of a desired resistance per unit length.

[0048] Referring to FIG. 6 of the drawings, there is illustrated graphically the predicted strain errors across a typical air platform temperature range for conventional and thermally compensated carbon fibre tows, wherein a thermally compensated sensor employs a coating with substantially equal and opposite TCR and substantially matching resistance per unit length. As shown, the maximum strain error can be reduced from +/−15000 microstrain to less than +/−400 microstrain.

[0049] Specific examples will now be provided by way of illustration only, but it will be appreciated by a person skilled in the art that numerous modifications and variations in coating materials, coating methods and coating thicknesses are possible, according to the claimed invention, and the present invention is not in any way intended to be limited hereby.

EXAMPLE 1

[0050] Carbon fibre filament with TCR ˜+0.03%/° C. and diameter ˜6 μm;
Coating material: ˜75% copper/25% nickel with a TCR of ˜+0.03%/° C., and ρ ˜3×10.sup.−8 Ωm;

Method of Coating: Electroplating

[0051] FIG. 4 illustrates graphically the TCR for copper/nickel alloys, showing that 75/25 Cu/Ni gives a TCR of ˜+0.03%/° C.

[0052] As discussed above, the thickness of the coating is selected such that the resistance per unit length is substantially the same, or at least closely matched, to that of the underlying carbon fibre. Some or all of the carbon fibres may be coated to give a range of compensation profiles.

EXAMPLE 2

[0053] Carbon fibre filament with TCR ˜+0.03%/° C. and diameter ˜6 μm;
Coating material: medium-phosphorous (7-9%) nickel with a TCR of ˜+0.03%/° C.;

Method of Coating: Electroless Plating

[0054] Such nickel-coated fibres are, as mentioned above, known, but are typically used for their electrical properties (e.g. screening). Their use for thermal compensation in strain sensing has not been proposed or suggested. The inventors have performed tests to prove the basic concept, and for one such test, a mixed fibre tow was prepared based on a typical conventional carbon fibre tow of approximately 3000 fibres to which was added a small number (˜20) of nickel coated carbon fibres to create a single hybrid tow. The TCR of the resultant tow was measured and compared to the response of an unmodified tow, and the results are illustrated in FIG. 5 of the drawings. It can be seen that the addition of even a small number of metallised fibres in this manner has reversed the TCR response of the mixed fibre tow, and illustrates clearly the underlying principle of various embodiments of the present invention.

[0055] Exemplary embodiments of the invention include the use of carbon fibre tows including or comprising a plurality of individually metallised carbon fibres, and any known method of achieving such coating can be employed, as described above. However, as explained above, metallised carbon fibres have previously been suggested for use in respect of their electrical properties and, therefore, the metal coating would in that case be as thick as practically possible, whereas in the present invention, the thickness of the coating is dictated by the desired resistance per unit area, which is likely to be significantly less than in known metallised fibres. In the case of both of the specific examples given above, it will be appreciated by a person skilled in the art that the thickness of the conductive coating will ideally be of the order to 10-100 nm but, again, the present invention is not necessarily intended to be limited in any way in this regard. Modifications and variations to the described embodiments can be made without departing from the scope of the invention as claimed.