CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from UK Patent Application No. 20 20 490.5, filed on 22 December 2020.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus for measuring an electrical property of an electrically conductive material.

Materials made from fibres, including but not limited to woven, stitched and non-woven materials, may be used in the manufacture of fibre-reinforced composite materials, such as carbon fibre reinforced polymer (CFRP) glass fibre reinforced polymer (GFRP) and other combinations of other fibres and polymers. The fibre material is typically laid up in a mould and impregnated with a thermoset resin (matrix). After curing, sometimes with the addition of heat, a rigid composite material is produced. The fibres in the material may all be carbon or other electrically conductive fibres, and may be mixed with other fibres such as glass fibre. If the proportion of electrically conductive fibres is high enough, then the fibre material and thus the composite material will be electrically conductive.

A composite material can be incorporated into a laminate stack for reinforcement. Typically, a laminate stack includes layers of reinforcement material such as glass fibre around a layer of fibre material. Resin is then added to create the final component. This component may be integrated into a final composite structure, or the process of forming the component may be part of the manufacture of a composite structure.

Increasingly such components, in addition to providing reinforcement, are used as heating elements in composite structures, for instance, for aircraft wings, wind turbine blades and radar equipment. They can prevent, in cold weather, ice accumulating on the surface. Snow or ice accumulation on an aircraft wing or wind turbine blade will affect its stability, and on a radome will impair the transmission of microwaves. By passing an electrical current through a layer of electrically conductive fibre material it is possible to heat the surface of a composite structure.

Fibre materials used for this purpose are frequently veils or mats, made using mixtures of conductive and non-conductive fibres to meet specific resistive/conductive properties. A heating element is laid into a laminate stack close to the surface, but insulated from other conductive fibres. The electrical power is fed to the veil element by way of a busbar or electrode at each end, and for a given level of electrical power the heat generated depends upon the resistivity of the veil material. For optimum efficiency and efficacy, therefore, it is necessary to know the resistivity (or, alternatively expressed, the conductivity) of the material comprising the heating element.

Non-woven reinforcement materials such as veils and mats are made by depositing chopped fibres comprising of conductive fibre such as carbon fibre, either on their own or combined with an insulating fibre such as glass fibre, together with a binder onto a conveyorized platform in selected proportions to achieve the required weight. The process may involve a wet lay process or the fibres and binder may be deposited dry in a dry process. Either way a mat or veil is produced has an even deposit of fibre.

The relative proportions of carbon fibre and glass fibre are adjusted to obtain a target surface resistivity, measured in ohms per square. The amount of carbon added is adjusted to allow for the fact that the conductivity of carbon fibre varies from batch to batch: the higher the conductivity of the carbon fibre in a given batch, the more conductive the veil will be, and therefore the weight of the veil or mat will have to be reduced to increase the surface resistivity. Conversely, the lower the conductivity of the carbon fibre for a given batch, the more the weight will have to be increased to meet the target resistivity. These adjustments comprise the standard way of controlling surface resistivity values of the veil.

Methods of measuring the surface resistivity of a veil are known. However, it has been noted by the applicant that for a particular fibre material, its surface resistivity inaccurately predicts its heating effect when part of a component. The heating effect varies even between batches of material made using the same raw materials. It is therefore difficult to predict, using known methods, the electrical power that should be applied to a heating element once it is incorporated into a structure, and a degree of trial and error needs to be used. It is an object of the invention to overcome this difficulty.

According to a first aspect of the invention, there is provided apparatus for measuring an electrical property of a sheet of electrically conductive material according to claim 1. According to a second aspect of the invention, there is provided a method of measuring an electrical property of a sheet of electrically conductive material according to claim 16. According to a third aspect of the invention, there is provided a method of measuring an electrical property of a sheet of electrically conductive material according to claim 32.

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventor and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. Components and processes distinguished by ordinal phrases such as “first” and “second” do not necessarily define an order or ranking of any sort.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 illustrates a first embodiment of an apparatus for measuring an electrical property of an electrically conductive material;

Figure 2 illustrates the apparatus shown in Figure 1 after a sheet of fibre material has been placed on it;

Figure 3 is a plan view of the apparatus shown in Figure 2;

Figure 4 illustrates the apparatus shown in Figure 1 after a lid has been closed;

Figure 5 is a diagrammatic cross section of the apparatus shown in Figure 3;

Figure 6 is a diagrammatic cross section of the apparatus shown in Figure 3 after compression has been applied; Figure 7 is a diagrammatic cross section of a second embodiment of an apparatus for measuring an electrical property of an electrically conductive material;

Figure 8 is a diagrammatic plan view of the apparatus shown in Figure 7;

Figure 9 is a diagrammatic cross section of the apparatus shown in Figure 7 after compression has been applied; and

Figure 10 details steps carried out to measure an electrical property of a fibre material.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Composite structures are most commonly manufactured by laminating layers of fibre reinforced material into a mould. The layers are impregnated with resin, typically by resin infusion, prepreg or hand layup. Using vacuum, there are broadly two ways of infusing the resin into the mould: under pressure (a process widely known as resin transfer moulding) or under vacuum (a process widely known as vacuum assisted resin transfer moulding or simply vacuum infusion). For example, in the vacuum infusion process reinforcement fabrics are laid in a mould, a resin distribution medium (RDM) is positioned, and a vacuum bag is placed over them and sealed to or around the mould so as to be airtight. Then a vacuum is created within the mould by drawing off air from it, causing the vacuum bag to be pressed down onto the fibres by atmospheric pressure, and low-viscosity resin admitted to the mould to be drawn in by the effect of the vacuum, to surround the fibres and then set (possibly with the application of heat) to form a matrix in which the fibres are embedded.

In either process, the fibre material used as a heating element is compressed when manufacturing a composite structure. The applicant has realised that this compression changes the resistivity of the material, as it causes more of the conductive fibres to be in contact with each other. Further, the fibre used may have differing properties from fibre used in other batches of the same material; for example, filaments of carbon fibre may have differing conductivity. If a batch of carbon fibre filaments has a low conductivity, then a fibre material made from it may need to have more carbon fibre by weight in order to provide the same surface resistivity as previous batches. However, when the material is compressed, the greater amount of carbon fibre leads to an unexpected increase in conductivity.

The applicant has therefore realised that to accurately predict the heating effect of a fibre material when incorporated into a composite structure, it is necessary to measure its resistivity under the same compression it will be subjected to when in that structure. Two embodiments of apparatus suitable for this measurement are herein described, the first with reference to Figures 1 to 6 and the second with reference to Figures 7 to 9. Further, by measuring the resistivity of a sheet of fibre material both when it is compressed and when it is in the final composite form, it is possible to create a calibration formula that will allow an accurate prediction of the material’s heating effect, and this will be described with reference to Figure 10.

Carbon fibre sheets are discussed herein, but the electrically conductive fibre material may be made from nickel fibre, metal-coated carbon, or any other suitable fibre. Further, the sheet need not be a fibre material, but may be any electrically conductive compressible material.

The apparatus and method described herein may also be used to measure other electrical properties of sheets of electrically conductive material under compression.

Figure 1

Figure 1 illustrates apparatus forming a first embodiment of the invention. Measuring apparatus 101 comprises a rigid housing 102, made from aluminium although any suitable metal or other stable, rigid material could be used. A lid 103 is closed during operation. The apparatus is controlled using control panel 104, which includes a power switch 105 for an ohmmeter 122, and terminals 106 and 107 for connection to external measuring equipment if necessary, for example for calibration of ohmmeter 122. Power switch 123 and taps 108 and 109 may be used to control a vacuum pump, within housing 102, and dial 110 is an indicator for the vacuum pump.

Top surface 111 of housing supports a seal 113 and a substrate 114. A sheet of breather fabric 112 is placed between the substrate 114 and top surface 111 , but in other embodiments could be omitted. Thus the top surface 111 functions as a base for the seal and substrate.

Substrate 114 is in this example a raised platform with sloping sides, made from plastic, although any stable, rigid non-conductive material could be used. In the top of substrate 114 are defined two linear parallel recesses, into which electrodes or busbars 115 and 116 are placed. Wires providing connections to these electrodes pass through substrate 114 and housing 102, both to ohmmeter 122 and to terminals 106 and 107.

Electrodes 115 and 116 are laid lengthwise on the top of surface of substrate 114, so that the longest edges of the electrodes are parallel to the top of surface of substrate 114. This creates a rectangular area (which may be a square) on the top surface, defined by the electrodes, as will be further described with reference to Figure 3. In other embodiments, if the electrodes had considerable depth or width such that one of the two shorter edges of the electrodes could form the side of the square, it might be that the longest edge was orthogonal to the top surface of the substrate.

Substrate 114 further defines a plurality of air holes 117, which lead to the base of the substrate and provide air flow for a vacuum pump inside housing 102, as will be described with reference to Figure 5.

Lid 103 is connected to housing 102 using hinges 118 and 119, although it could be unconnected and placed on top when required. It comprises a rigid surround 120, made from aluminium although any suitable metal or other material could be used, supporting a flexible membrane 121. Membrane 121 is made from silicone, although any flexible non-conductive material could be used.

In this example the size of the housing is around 400 mm wide, 400 mm long and 300 mm deep, but this would vary depending on the size of the sheet to be tested. Power is supplied to apparatus 101 via one or more power supplies (not shown). The ohmmeter and the vacuum pump may have individual power supplies or be connected to a single supply.

Figure 2

In order to measure the resistivity of a sheet 201 of fibre material, it is placed on the top surface 202 of substrate 114. The sheet does not have to be placed centrally on the substrate, but must completely cover electrodes 115 and 116. In this example it does not cover air holes 117, but this would be permissible.

Electrodes 115 and 116 are sunk into top surface 202 of substrate 114, so that their tops are flush with the top surface, in order that sheet 201 is compressed evenly. However, in other embodiments this may not be the case, particularly if very thin electrodes are used relative to the thickness of the sheet.

Figure 3

Figure 3 is a plan view of top surface 111 of housing 102 supporting seal 113 and substrate 114. Sheet 201 of electrically conductive fibre material has been placed on the top surface 202 of substrate 114. Electrodes 115 and 116 are indicated below sheet 201 in dashed lines.

In this example, sheet 201 is a square with sides of around 100 mm. Electrodes 115 and 116 are substantially parallel and of the same length, indicated by arrow 301 , which in this example is around 75 mm. Their inside edges are a distance apart indicated by arrow 302, which in this example is also around 75 mm. Thus length 301 is the same as distance 302, meaning that the area enclosed by the electrodes (ie the edges of the area are defined by the electrodes) is a square, indicated by dotted lines 303. Alternatively, the area enclosed by the electrodes could be a non-square rectangle, in which case a calculation would be required to convert the measurement into ohms per square.

The surface resistivity of the fibre material inside square 303 can be measured by applying a current to one electrode, measuring the voltage from the other, and calculating the resistivity from the voltage drop. The measurement is in ohms per square, and should be the same whatever size of square is measured. Therefore, the length of the electrodes is not important as long as the distance between them is equal to it and as long as the electrodes are fully covered by sheet 201. In other embodiments the measurement might be taken differently, and therefore the layout of the electrodes might be different.

However, the applicant has realised that measuring the surface resistivity of the uncompressed sheet does not give an accurate prediction of the heating capability of the fibre material when incorporated into a composite material.

Figure 4

After sheet 201 has been placed on the top surface 202 of substrate 114, lid 103 is closed as shown in Figure 4. This causes membrane 121 to be placed above top surface 114, creating a space between membrane 121 and substrate 114.

A vacuum is then applied using a vacuum pump inside housing 102, controlled by power switch 123 and taps 108 and 109. The vacuum pump causes the air within the space to be sucked out, and the membrane is pulled tightly against substrate 114, the shape 401 of which can be seen underneath the membrane. The membrane is also pulled tightly against seal 113, the shape 402 of which can be seen underneath the membrane. The seal ensures that air from outside apparatus 101 does not leak into the space. Thus, pressure is applied to top surface 202 of substrate 114, compressing sheet 201.

Power switch 105 is operated to turn on ohmmeter 122, and the resistivity of sheet 201 is measured. This measurement is its bulk resistivity rather than surface resistivity because the sheet has been compressed.

In other embodiments, pressure could be applied to substrate 114 in other ways. Rather than a membrane sealing against a base, the substrate with the sheet on top could be placed inside a vacuum bag, in which case the substrate would not need to be a raised platform with air holes, and this type of embodiment will be described with reference to Figure 7. Another means for creating a vacuum could be used rather than a pump; alternatively a press could be used rather than a vacuum, as long as even pressure across the sheet could be applied.

Figure 5

Figure 5 is a diagrammatic cross section across the line A-A in Figure 3, after membrane 121 has been placed above the substrate and before compression has been applied to sheet 201.

Electrodes 115 and 116 are on the top surface 202 of substrate 114, such that the top of each electrode is flush with top surface 202. Sheet 201 is placed on top surface 202, and membrane 121 is placed above that, in this embodiment by closing lid 103. When lid 103 is closed, if board 112 and its supported elements were not present, membrane 121 would be located just above the top surface 111 of housing 102. Therefore, as shown in Figure 5, membrane 121 is distorted upwards by seal 113 and substrate 114. This creates an airtight space 500 between board 112 and membrane 121 , bounded by seal 113.

Wire 501 is connected to electrode 115, and wire 502 is connected to electrode 116. The wires pass through substrate 114 and board 112 to ohmmeter 122. Terminals 108 and 109 are also connected to the electrodes but these connections are not shown. Seals 504 and 505 seal the holes where wires 501 and 502 exit board 112 respectively, to ensure space 500 is airtight.

Substrate 114 defines a recess 506 in its lower surface 507, and air holes 117 lead from top surface 202 to the recess 506. Board 112 defines an aperture 508 beneath recess 506, thus creating a flow route, indicated by arrow 509, for air to be removed from space 500: through air holes 117, through recess 506, through sheet of breather fabric 112 and through aperture 508. Air can be removed from space 500 by vacuum pump 509, which is connected to aperture 508. Breather fabric 112 ensures the vacuum is applied evenly, but in other embodiments is is omitted. In this embodiment recess 506 is empty, but in other embodiments it may contain a breathable fabric or other breathable material, instead of breather fabric 112 being present.ln other embodiments, a flow route may be created in a different way, and the vacuum pump may be connected in a different place. For example, substrate 114 may be flat rather than raised, and the vacuum pump may be connected to an aperture in base 112 adjacent to the substrate.

Figure 6

Figure 6 is the same diagrammatic cross section as in Figure 5, after compression has been applied to sheet 201. Vacuum pump 509 is operated to remove air from space 500, thus pulling membrane 121 towards substrate 114 and applying pressure to sheet 201 , which is compressed. In this diagram gaps are shown between membrane 121 and other elements of the apparatus, but this is for ease of illustration only and it will be understood that when a vacuum is applied there will be no gaps. Seal 113 provides a seal around the space 500.

After sheet 201 has been compressed, its resistivity is measured using meter 112, via wires 501 and 501 and electrodes 115 and 116.

Thus there is herein described a first embodiment of an apparatus for measuring an electrical property of a sheet of electrically conductive compressible material, comprising a substrate 114 of non-conductive material having a top surface 202 on which a sheet may be placed, two electrodes 115 and 116 on the top surface, means 509 for applying pressure to the top surface of the substrate such that the sheet is compressed, and means 122 for measuring an electrical property of the sheet, attached to the electrodes. The two electrodes are placed on the top surface such that they define two edges of a rectangular space on the top surface.

Figure 7

Figure 7 is a diagrammatic cross section of an apparatus 701 forming a second embodiment of the invention. The cross section is along the line B-B shown in Figure 8.

In this embodiment, a sheet 702 of electrically conductive fibre material is layered between a top sheet of glass fibre 703 and a bottom sheet of glass fibre 704 to form a laminate stack. In this embodiment bottom glass fibre sheet 704 forms the substrate, onto the top surface 715 of which fibre material 702 sheet is placed. Electrodes 705 and 706 are in this embodiment on top of fibre material sheet 702, although they could also be beneath it.

A vacuum bag 707 made of plastic is placed around the laminate stack, and a vacuum pump 708 is connected to an aperture 709 in the bag. In this embodiment the bag is formed from a top and bottom plastic sheet 710 and 711 respectively. The laminate stack is placed on bottom plastic sheet 711 , top plastic sheet 710 is placed on top of the laminate stack, and the edges are sealed. Alternatively the bag may be a pre-created bag into which the laminate stack is placed. In either case top plastic sheet 710 of the bag 707 forms the membrane that is placed above the top surface of the substrate, which in this example is glass fibre sheet 704. Thus a space 712 is created in bag 707, including between top plastic sheet 710 and bottom glass fibre sheet 704, and vacuum pump 708 may be used to create a vacuum in this space.

The apparatus 701 further comprises inlet resin pipe 713 and outlet resin pipe 714. These are used to feed resin into the vacuum bag when it is under vacuum, in order to create a matrix around the fibres of sheet 702. This mimics the formation of a composite material. Any type of suitable resin could be used, such as a polymer resin, ceramic resin or organic resin, dependent upon what is required for the composite material.

Another means for creating a vacuum could be used; alternatively a press could be used rather than a vacuum, as long as even pressure across sheet 702 could be applied.

Figure 8

Figure 8 is a diagrammatic plan view of apparatus 701. Top sheet 710 of vacuum bag 707 overlays the laminate stack formed by glass fibre sheets 703 and 704 and electrically conductive sheet 702, and resin pipes 713 and 714. Aperture 801 provides a sealable opening for vacuum pump 708 (not shown).

Electrodes 705 and 706 in this embodiment overlay sheet 702 and are long enough to extend beyond vacuum bag 707. Their exact length is not important, and it can be seen that they are not the same length. However, the length of each electrode that is in contact with sheet 702 (shown by arrow 802) must be equal to the distance between the electrodes (shown by arrow 803) such that the area of the sheet to be measured is a square.

A resistivity meter 804 is directly connected to the electrodes using wires 805 and 806, and can be used to measure the resistivity of sheet 702 by applying a current to one electrode, measuring the voltage from the other, and calculating the resistivity from the voltage drop. Again, the measurement is in ohms per square, and should be the same whatever size of square is measured. Thus the size of sheet 702, and thus of the whole apparatus 701 , is not important.

Vacuum bag 707 is sealed around the points where resin pipes 712 and 713 and electrodes 705 and 706 enter the bag.

Figure 9

Figure 9 is the same diagrammatic cross section as in Figure 7, after compression has been applied to sheet 702. Vacuum pump 708 is operated to remove air from space 712 within bag 707, causing top sheet 710 to move towards the sheet 702 and substrate 704. Glass fibre sheets 703 and 704 are incompressible, and therefore the pressure applied by top sheet 710 compresses sheet 702 of fibre material. The pressure also causes electrodes 705 and 706 to sink into sheet 702. Using resistivity meter 804, a first measurement of the bulk resistivity of the sheet 702 is taken via electrodes 705 and 706 using meter 804.

In this diagram gaps are shown between the plastic sheets of bag 707 and other elements of the apparatus, but this is for ease of illustration only and it will be understood that when a vacuum is applied there will be no gaps. Next, resin is admitted to space 712 via pipe 713. Vacuum pump 708 is still being operated and therefore the resin is pulled through the space to outlet resin pipe 714. A resin trap (not shown) may be used to ensure that no resin is sucked into the outlet 801 for the pump. The resin flowing through the space impregnates sheet 702 to surround the fibres, and cures to form a matrix. Heat may be applied at this stage to facilitate the curing process.

After the resin has cured, the laminate stack has been formed into an example of a composite component including a composite material and two sheets of glass fibre. A second measurement of the bulk resistivity of sheet

702 is measured via electrodes 705 and 706 using meter 804. This is likely to be a lower value compared with the first measurement.

Therefore it is possible, using this apparatus, to measure the bulk resistivity of a sheet 702 of electrically conductive compressible material, both under basic compression and then as part of a component. Any number of layers made from any type of material may be used in the laminate stack, chosen based on the composite material and final component that the fibre material will be used for.

As an alternative method, sheet 702 may be tested alone using this embodiment. In such a method, the sheet and electrodes are placed alone into bag 707. The bottom plastic sheet 711 of the bag may be rigid enough to withstand the vacuum, in which case it can be considered as the substrate. Alternatively a rigid substrate may be placed into the bag. The method then proceeds as described above to obtain a first measurement, but without glass layers 703 and 704. This may be desirable if the pressure of top glass layer

703 compresses sheet 702 even before the vacuum is applied. If it is then desired to obtain a second measurement, for the fibre material when part of a final component, then the sheet and electrodes can be removed from the bag or the bag dismantled, and the layers added to form a laminate stack as previously described. The resistivity may also be measured during intermediate stages, such as after the laminate stack has been formed but before the resin is admitted. As a further alternative, the sheet may be tested when impregnated with resin as part of a composite material, but without the additional layers forming the laminate stack.

Thus there is herein described a second embodiment of a method for measuring an electrical property of a sheet of electrically conductive compressible material, comprising the steps of placing a sheet 702 of electrically conductive material on the top surface 715 of a substrate 704 (or 711 ) of non-conductive material and in contact with two electrodes 705 and 706, applying pressure to the top of the substrate such that the sheet is compressed, and measuring, using the electrodes, an electrical property of the sheet while it is compressed.

Figure 10

Two embodiments of the invention have been described herein. In the first embodiment, the apparatus 101 is quick and easy to use: a sheet of material is placed on the substrate, the lid is closed, the vacuum pump is operated, and a measurement is taken. In the second embodiment, the apparatus 701 is not as quick to use, as the position of the electrodes needs to be measured and the vacuum bag needs to be sealed around the various elements; however, this embodiment allows a composite material to be created using the measured sheet.

A method of measuring resistivity may use both embodiments. The second embodiment can be used to determine a calibration formula for use with the first embodiment, and such a method is disclosed in Figure 10.

At step 1001 , a first measurement of the resistivity under compression of a sheet of the material is taken using apparatus 701. The measurement taken is the first measurement described with reference to Figure 8.

At step 1002, the sheet is incorporated into a composite material or final component, as in the second embodiment, including admitting resin into the space under compression. A second measurement of the resistivity of the same sheet is taken.

Steps 1001 and 1002 may be repeated as necessary with sheets taken from different batches of similar fibre material until a sufficient number of pairs of measurements have been taken, and at step 1003 these are used to determine a calibration formula. This may be as simple as the average percentage reduction in resistivity, or it may be more complex.

In addition to steps 1001 and 1002, further intermediate measurements may be taken. For example the laminate stack may include more layers, and measurements may be taken when each is added. Measurements may be taken when the resin is first added and during the curing process. These measurements may be used to improve the accuracy of the calibration formula.

Once a calibration formula has been determined it is possible to estimate the resistivity of an eventual composite material or component by measuring the resistivity under pressure of the fibre material it comprises. Thus at step 1004 a measurement of the resistivity under compression of a sheet of another batch of similar fibre material is taken using apparatus 101 , and at step 1005 its resistivity in a composite material or final component is estimated using the calibration formula.

The method described with respect to Figure 10 may be carried out using any embodiment of the invention. While in this description steps 1001- 1002 and step 1004 have been carried out using different embodiments, this may not be the case.