WO2010055282A1

WO2010055282A1 - Composite sensor

Info

Publication number: WO2010055282A1
Application number: PCT/GB2009/002632
Authority: WO
Inventors: Jonathan Geoffrey Gore; Lisa Fixter; Stuart John Eaton; Michael William Hopkins; Robert West; Leslie G. Stringer
Original assignee: Qinetiq Limited
Priority date: 2008-11-12
Filing date: 2009-11-09
Publication date: 2010-05-20

Abstract

The present invention relates to the measurement of stress using a composite sensor material and more particularly to fibre reinforced polymer (FRP) composite sensors. The composite stress sensor comprises a fibre reinforced composite structure, formed from a polymer matrix which encapsulates at least one ply of a cloth, which has embedded in intimate contact at least one generally elongate magnetically soft amorphous or nanocrystalline electrically resistive element, such that when an AC input current is applied to said element, any stress applied to the composite structure changes the value of the impedance across said element. Typically the cloth is an electrically conducting cloth or made from an electrically conducting material to provide electromagnetic shielding of the magnetically soft amorphous or nanocrystalline electrically resistive element.

Description

Composite sensor

The present invention relates to the measurement of stress using a composite sensor material and more particularly to fibre reinforced polymer (FRP) composite sensors.

FRP composites as a class of materia! are we!! known, and comprise a relatively low modulus polymer matrix phase within which is embedded a relatively high modulus fibrous phase, the fibres typically being of carbon, glass or aramid. Such composites can be formulated to exhibit a high strength to weight ratio and can be moulded to form load-bearing structures of complex curvature, meaning that they are of particular utility in many aerospace applications.

Conventional FRP composites do, however, have relatively poor resistance to impact damage, which in the case of an aircraft structure could be imparted in use e.g. by runway debris or bird strikes, dropped tools in the course of maintenance procedures, or similar collisions. This is due to the lack of plastic deformation mechanisms for absorbing impact energy in such materials. Instead the impact energy is absorbed through various fracture processes, such as matrix cracking, delamination and fibre breakage. Therefore, critical FRP composite structures which are liable to encounter impact risks in service must be subject to stringent and expensive inspection and repair regimes and/or incorporate more material than is required for their principal load-bearing role in an effort to mitigate the problem of impact damage, thereby adding to the weight and cost of the structure.

There are many methods for measuring stress in the prior art. However, few or none fulfil all the requirements of low cost, high robustness and high sensitivity which are the ideal for many applications. Additional constraints may arise when it is also required that the stress to be measured is in a moving part.

Highly sensitive sensors have been developed which employ soft magnetic materials, for example in the form of negative magnetostrictive amorphous or nanocrystalline melt-spun wires and ribbons, and which are based on the GMI effect, such as applicant's WO00/03260.

When an AC drive current is passed through a magnetically soft (normally amorphous or nanocrystalline) electrically resistive conductor, e.g. a wire, ribbon or fibre, the AC voltage thereby developed is highly sensitive to the presence or application of an external magnetic field, particularly when the drive current frequency is greater than 10OkHz, the effect being known as the Giant Magneto- Impedance Effect (GMI). The change in voltage is understood as being a consequence of the dependence of the skin depth of the conductor on the magnetic permeability. Interpretation of the GMI effect was introduced in 1994 simultaneously by Panina and Mohri Appl. Phys. Lett. 65 (1994) 1189 and Beach and Berkowitz Appl. Phys. Lett. 64 (1994) 3652.

In a first aspect there is provided a stress sensor comprising a fibre reinforced composite structure, said composite structure comprising a polymer matrix comprising at least one ply of a cloth, wherein at least one generally elongate magnetically soft amorphous or nanocrystalline electrically resistive element is in intimate contact with said cloth, such that when an AC input current is applied to said element, any stress applied to the composite structure changes the value of the impedance across said element.

In a highly preferred arrangement, the cloth is an electrically conducting cloth and said element is electrically isolated from said cloth.

In a further highly preferred arrangement the elastic modulus of the at least one element and composite structure are substantially matched, such that they have substantially the same value. The incorporation of the at least one element in a composite provides an advantage that it reduces and in most cases removes the requirement for using a magnetic field or stress bias as in prior art devices mentioned above. This provides for a useful stress sensor and in particular an impact sensor.

A highly sensitive stress sensor has been developed that exploits the GMI effect, which requires using amorphous or nanocrystalline soft magnetic materials, for example in the form of negative magnetostrictive amorphous or nanocrystalline melt- spun wires and ribbons. Due to the inverse magnetostriction effect in such materials, the strong skin effect causes the impedance of the element to change with applied stress S, this effect being termed the Giant Stress-Impedance effect (GSI). The physical mechanism of the impedance change is believed to substantially avoid cross-talk problems between orthogonal components of the stress tensor such as can arise with conventional strain gauges.

It is desirable to provide a fibre reinforced composite that has an integral stress sensor, which may be inserted during the manufacture of said composite. This allows for the constant monitoring of the composite during its entire life, such as, for example during transport, handling and possibly during its incorporation into a larger composite structure or article.

The cloth plys used in the invention may be any of the usual types employed in FRP composites, but is preferably one of the group of advanced fibres (typically having a tensile modulus in excess of 50GPa or more preferably in excess of 200GPa) preferably predominantly carbon (including graphite), although other fibres such as, for example, glass, aramid (e.g. Kevlar ®), high modulus polyethylene or boron fibres may be present.

Preferably the cloth is made from an electrically conducting material, so as to provide an electrically conducting cloth. Alternatively, non-conducting cloths may be coated or treated to make them electrically conducting. In one embodiment of a multiple cloth ply fibre reinforced polymer composite, the cloth ply which has located therein at least amorphous wire is preferably conducting, and the other layers may be independently selected from conducting or non conducting cloth plys. Preferably the cloth is a carbon fibre cloth.

An advantage of using an electrically conducting cloth, in particular carbon fibre cloth, is that it provides an electromagnetic shield for the embedded at least one element, thereby allowing for a significant reduction in sensor noise from external electromagnetic inductive (EMI) effects. To achieve particularly effective electromagnetic shielding of the at least one element, it is desirable that the contact between the at least one element and an electrically conducting cloth is intimate.

The integration of an electrically resistive element into a conducting cloth requires good electrical insulation between the element and its host structure; otherwise the sensor's overall sensitivity is severely compromised. The at least one element may be electrically isolated from the conducting cloth by providing the at least one element with an electrically insulating outer sheath. The outer sheath may be made from a glass, ceramic or a non electrically conducting polymer and may be coated onto an element by any known technique. It is important that the outer coating remains electrically isolating and also preferably that it is not too thick such that it does not significantly alter the flexural stiffness of the element, when in the form of a wire or ribbon, as this may hinder the introduction of said wire or ribbon into the weave of the cloth. As mentioned above, certain FRP composite bodies, such as aircraft, boats etc, are liable to encounter impact risks in-service and are currently subject to stringent and expensive inspection and repair regimes and/or incorporate more material than is required for their principal load-bearing role in an effort to mitigate the problem of impact damage, thereby adding to the weight and cost of the structure. Therefore it is extremely desirable to be able to examine the structure of said composite in-situ and in real time. Constant monitoring of the embedded at least one element can reveal problems that occur over a prolonged period of time, as well as unexpected impacts or sudden failures.

It may be desirable to incorporate a stress sensor during the manufacture or assembly phase of a composite structure, such as, for example, a building or bridge. Alternatively, the sensor of the invention may be used in articles where composite materials are already commonplace, such as, for examples vehicles, vessels or aircraft. An integral stress sensor provides the ability to monitor the structural health of the composite structure or article from the outset. The sensors may be used to detect the presence of impacts, which are particularly important in aircraft or to monitor static loadings that occur in building or bridges.

For existing composite bodies, already in use, it may be more desirable to retrofit an applique film comprising the sensor of the invention. The applique film may then be attached directly to the existing composite body to be assessed. This is advantageous because the applique film may be prepared from materials which closely match the existing composite body, i.e. share similar stress properties.

An advantage of using a fibre reinforced composite sensor or the applique film according to the invention, is that fibre reinforced composites undergo very little or no plastic deformation during impact events, therefore all of stress strain encountered by a body is passed onto the fibre reinforced composite sensor or applique film sensor.

Preferably, the at least one element is provided in the form of a wire or ribbon, such that it may be incorporated into the structure of the cloth without causing a significant change in the profile of the cloth. The cloth is typically a woven cloth, preferably it may be provided as a prepreg cloth, which is where the cloth plies are pre- impregnated with resin. Stitching is often used to further reinforce dry cloths, which may then be infused using low cost manufacturing techniques such as resin infusion under flexible tooling (RIFT) or resin transfer moulding (RTM). The at least one element may be incorporated into the cloth in several ways. In one arrangement, the at least one element may be incorporated after the production of the cloth using known stitching methods to position the at least one element, which provides the advantage that the at least one element can be located in the precise and desired arrangement.

In a preferred arrangement, the at least one element is in the form of a wire and may replace part of the tow (i.e. stitching thread), such that said at least one element is incorporated during the actual manufacture of the woven cloth. This provides the advantage that the element can have a very low profile, i.e. does not protrude from the surface of the cloth. Furthermore, a greater number of elements may be inserted into the cloth. A yet further advantage of direct incorporation during the weave stage is that individual amorphous wires are sometimes difficult to handle because the wires tend to slip over one another, thereby distorting the shape of the fabric. It may also be desirable to incorporate elements in the z -direction i.e. through several cloth plys.

A further advantage of incorporating the element into the weave of the cloth is that elements which are thicker than a ply of cloth may cause disruption of said ply. Disruption of the ply or plys may cause distortion of the layered structure, which may in turn have a detrimental effect on the mechanical properties of the final composite. Therefore, the at least one element and the cloth are desirably in intimate contact to reduce the disruption of the cloth layer.

A yet further advantage of weaving the amorphous wire, is when the cloth is selected from an electrically conducting cloth, the intimate contact between the wire and cloth, may further enhance the shielding effect.

A further means of incorporation of the element into a layered structure is to "cut-out" regions in a ply of cloth so as to accommodate the element. Experiments show that cutting the fibres for the cut-out had an adverse effect on tensile strength of the final composite, whilst embedding of the elements without a cut-out did not decrease the strength of the composite. The cost of manufacturing the woven cloth with at least one element should not be significantly more than for a traditional woven cloths, (such as, for example, a carbon cloth or the like), since the amorphous wires can be incorporated with the fibrous tows (the threads) in the same weaving process. Furthermore, the overall manufacturing process for the structure is simplified because the at least one element is already integrated with the fibrous reinforcement and fewer layers and less resin film may be required, thereby saving considerable time and cost. The thickness of the composite can also be reduced in comparison with an example comprising at least one element on top of a woven fibrous ply, since one of the layers (and any necessary matrix interlayer) is effectively eliminated, and this may be particularly advantageous for the production of thin applique films for aerodynamic surfaces such as the exterior surface of a vehicle, vessel or aircraft.

Preferably, the amorphous wires are woven together with at least some of the fibres of cloth to form one or more integral cloth plys. The plys may also be pre-pregs, i.e. pre impregnated with the final resin, which is commonly used in the art to facilitate the lay-up of the final structure.

The polymer matrix material in a structure according to the invention may also be of any of the usual types employed in FRP composites, including both thermosetting and thermoplastic resins. Thermosets are currently preferred due to their lower processing temperatures. Conventional FRP composite fabrication methods can be employed with the cloth, and multiple embodiments may be produced having one or more plys of woven cloth incorporating at least one element together with one or more plys of woven cloths, which do not possess an element.

The fibre reinforced composite sensors of the invention may be made into any two or three dimensional shape, such that they may be configured to substantially adopt the shape of the surface that is to be monitored. The surface of the fibre reinforced materials may be flat, curved so as to form three dimensional configurations, such as those found on aerodynamic structures. Alternatively, the FRP may take the form of a cylindrical structure, which may be achieved by winding reinforcing fibres and amorphous wires around a mandrel and then encapsulating said fibres/wires in a polymer matrix.

It has been found that the optimal drive frequency, i.e. the frequency of the AC input current for GMI elements lies in the kHz region for an element in the form of a wire. An element in the form of a ribbon is more suited to the MHz region. This was tested by providing an AC electrical current through an element at various frequencies for different loading profiles to ascertain to which driving frequency the element in wire and ribbon forms was most sensitive, which is detailed in the experimental section. Preferably, the input frequency needs to be set at its most sensitive, such that a close to linear response is obtained for the voltage output across the at least one element.

The input current may be provided by direct coupling to the at least one element, such as via electrical leads which attach to said at least one element. In an alternative arrangement the current may be provided by inductive or capacitative or RF (radio frequency) coupling to the at least one element. Similarly the analysis of the output voltage, i.e. the change in impedance across the length of the at least one element, which is brought about by the application of stress on the at least one element, may be measured by direct coupling, inductive or capacitative or RF coupling to the at least one element. Alternatively, the change in inductance of the at least one element may be measured as a function of time, using a LCR (inductance- capacitance-resistance) circuit arrangement.

These frequency response ranges permit relatively easy integration into an RF communication system, for example for simple interfacing with a passive RF tag system. In turn, this facilitates remote sensing of stress or related factors in moving parts. It has been found that such elements have only a low power requirement for satisfactory operation, typically less than a watt. It is therefore possible to locate an element within a matrix, for example of plastics or elastic material, for remote interrogation with no external leads or other attachments.

The RF system may be any known system for sensing impedance changes in the at least one element. For example, the wire may form part of a resonant circuit which changes its resonant frequency as the impedance of the at least one element changes. Alternatively the at least one element could be incorporated into a balance bridge providing a frequency modulated output RF signal.

The material of the sensing at least one element may be a cobalt rich amorphous alloy, for example of Co-Si-B, in particular Co_72-5Si_12-SB₁₅. Other alloys containing traces of Mn, Fe, C, Nb, Ni, Cu, Mo and Cr can also be used. Other compositions include Fe₈₁Bi_3-SSi_3-5C₂, Fe_4-9Co_71-8Nb_0-8Si_7-5Bi₅, Co₈₀B₂O, Fe_77-5Si_7-5B₁₅, Ni₈₀Fe₂₀, Fe₆9.₅Cr₄Si7.₅B₁₅, (Co₀.94Feo.o6)72.5 Sii2.5B₁₅ and Fe₇₃^Cu₁Sh_{3 5}B₉. Cobalt rich amorphous or nanocrystalline alloys have extremely high maximum tensile strength values typically of between 1 to 4 GPa. They also have a high elastic modulus typically of around 100 GPa for a Co rich ribbon. In addition they exhibit high corrosion resistance.

Moreover, measurement on a CoSiB wire indicate that an element is generally insensitive to changes in temperature, at least in the range 20 to 150⁰C, making them suitable for use in environments where significantly elevated temperatures are likely to be encountered.

The at least one element may be in the form of a wire or ribbon produced, for example, by melt spinning. Wire and ribbons are typically in the range of from 10 to 125 microns thick (minimum dimension). In manufacture, a quenching step normally follows the melt spinning process, and residual stresses arising therefrom could couple with the magnetostriction to hinder domain rotation and so reduce the GMI effect. It is therefore preferred to anneal the quenched product to increase the sensitivity of the at least one element, for example by furnace annealing, pulse current annealing or direct current annealing.

In one arrangement, the sensor may comprise one or more elements located in one or more cloth plys. Where the elements need to be very long, it may be possible to electrically connect two or more ribbons or wires in series to provide a larger element and hence a large area composite sensor. Such coupling may be by any suitable means such as by direct electrical contact, or by coupling with non-magnetic wires therebetween. Long-line or large area sensors provide an average strain response (a 'global' response) along the entire length of the sensing element.

In a second aspect of the invention there is provided a stress sensor comprising a fibre reinforced composite structure, said composite structure comprising a polymer matrix comprising at least one ply of an electrically conducting cloth, wherein at least one generally elongate magnetically soft amorphous electrically resistive element is in intimate contact with said cloth and wherein said element is electrically isolated from said cloth, such that when an AC input current is applied to said element, any stress applied to the composite structure changes the value of the impedance across said element. According to a third aspect of the invention there is provided a sensing device comprising a sensor according to the invention, a means of providing an input AC current across the at least one element and means of analysing the output voltage from the at least one element.

in a preferred embodiment the at least one generally elongate magnetically soft amorphous electrically resistive element is a single continuous element, more preferably it may be arranged in a loop configuration. The use of a single element per layer or indeed per sensor or applique film provides the advantage of reducing the number of direct electrical connections required in the sensing device. Conveniently there may be multiple loops configured as part of a one resistive connection, thereby providing larger coverage. Preferably, the elements may be configured such that the pass through more than one ply layer.

According to a fourth aspect of the invention, there is provided a method of measuring applied stress on a body or surface using a stress sensor or securing thereto an applique sensor as defined hereinbefore, comprising the steps of providing an input AC current across one or more elements in said sensor or applique sensor and measuring the change in voltage in said elements as a function of time.

According to a fifth aspect of the invention there is provided a structure or vehicle having incorporated therein a stress sensor, applique stress sensor or sensing device according to the invention.

According to a sixth aspect of the invention there is provided the use of at least one generally elongate magnetically soft amorphous electrically resistive element in a fibre reinforced polymer matrix composite as a stress sensor, wherein said element is in intimate contact with at least one ply of an electrically conducting cloth, and wherein said element is electrically isolated from said cloth.

Preferred features of the second, third, fourth, fifth and sixth aspects of the invention are as described above in relation to the first aspect.

The invention will now be more particularly described, by way of example, with reference to the accompanying schematic drawings in which:- Figure 1 illustrates in plan an example of a woven cloth for incorporation in an FRP composite structure according to the invention;

Figure 2 illustrates in plan a further example of a woven cloth for incorporation in an FRP composite structure according to the invention;

Figure 3 illustrates a cross section of a composite structure of the invention;

Figures 4a and 4b show a test CFRP sensor in a top and side view respectively;

Figures 5a to c show voltage time graphs for a range of sensor readings for Experiments 1 to 3;

Figure 6 shows a side view of a composite bridge with sensors embedded therein; and

Figure 7 shows a graph of the change in inductance of the amorphous wire as the bridge in Figure 6 is subjected to increasing loads, as per Experiment 4.

Referring to Figure 1 , there is shown a woven cloth which has been cut from a continuous length of cloth of which the warp direction is indicated by the arrow. The warp comprises a series of combination threads each comprising a flat tow 1 of carbon fibres and a pair of amorphous wires 2, one at each lateral edge of the tow 1. The weft comprises a series of combination threads each comprising a flat tow 3 of carbon fibres and a single amorphous wire 4 at one lateral edge of the tow 3.

The cloth shown in Figure 2 is similar to the Figure 1 embodiment except that in this case there are two amorphous wires per carbon tow in both the warp and the weft. If greater numbers of amorphous wires 2 or 4 per carbon tow 1 or 3 are desired in either direction, the additional wires may be incorporated at regular intervals across the widths of the respective tows.

In each of the illustrated embodiments the type of weave shown is known as "five harness satin", where each weft tow overlies every fifth warp tow, the loops of consecutive tows being displaced by one across the cloth to give the illustrated diagonal pattern, but in principle any conventional weave pattern may be employed. Clearly the number of amorphous wires may be controlled and, likewise, their relative direction i.e. the wires may be incorporated in only the weft or warp. Additional wires will provide for redundancy in the composite sensor, especially in sensors which may be exposed to continued forces or impacts etc.

The invention provides embedded, iong iine sensors for Structural Health Monitoring (SHM) applications, exploiting amorphous wire having a stress-impedance effect as an impact sensor. Advanced composite materials are increasingly being used for various structures, including aerospace applications, due to their high strength to weight ratio. These structures are, however, inherently vulnerable to types of impact, which can be barely visible to the naked eye. Typically, Non-Destructive Evaluation (NDE) methods are used to determine if a structure has been damaged. This technique usually involves hand held ultrasonic probes being used, resulting in a lot of "man hours" being used whilst taking the structure out of service. Structural Health Monitoring provides a means of self monitoring, whereby the structure can determine itself whether it has been damaged or overloaded. Such a structure is often referred to as a "Smart Structure" when the analysis provides a responsive action or indication of action to be taken.

Turning to Figure 3, there is shown a cross section of a fibre reinforced composite structure 11 , which comprises an electrically conducting cloth 13, which contains an amorphous wire/ribbon 10 positioned in intimate contact with the cloth 13. The cloth 13 provides EMI shielding from any stray or external sources. On the outer surface of the wire 10 is an electrically insulating outer sheath (not shown), so as to prevent direct electrical connection between the wire 10 and the cloth 13. The wire 10 may be sewn in place, or incorporated as part of the weave of the cloth, such as, for example, by one of the weave arrangements shown in Fig 1 and Fig 2. The cloth 13 with the embedded wire 10 is encapsulated in a polymer resin 12. In the case where a direct electrical connection is used to drive the input AC current to the element wire 10 and for the analysis of the output voltage, an electrical connector may be required (not shown) to connect said wire 10 to an external power source/detector(not shown). Clearly, no such electrical connector is required where the input current is provided and similarly detected by inductive or coupling means.

Turning to Figure 4a, there is shown a top view of a CRFP test piece 21 , which contains a carbon fibre sheet 23a that has four amorphous wires 20a-20d, as detailed in Experiment 2. Amorphous wires 20c and 2Od (dotted lines) are woven into the carbon fibre 23a, and amorphous wires 20a and 20b are fixed in place using an adhesive. The amorphous wires 20a-20d are connected to power/analysis leads 25, Wa connectors 24. Figures 4a and 4b show the impact sites X, Y and Z, which occur at different locations on the composite structure. Figure 4b, shows 3 plys of carbon fibre 23 and one ply of carbon fibre 23a that possess the amorphous wires 20a-20d. The carbon fibres layers 23, 23b, and amorphous wires 20a-20d, are encapsulated in a polymer resin 32. The power /analysis leads 25 were connected to the detector (not shown). The impact sites X and Y are located on one side of the CRFP test piece 31 , and impact site Z is located on the opposite face.

Figure 5a shows a graph of the electrical voltage response over time for the test piece in Experiment 2. The test piece was subjected to an impact energy of 0.619 J, at impact site Y. The test measurements were taken using amorphous wire 20c, as shown in Figures 4a and 4b, which is a double loop of the amorphous wire that has been woven into the fabric. The three traces show that there is good reproducibility for repeated impact events.

Figure 5b shows the graph of the electrical voltage response over time for Experiment 2, for impact at site Z, i.e. on the opposite face of the test piece. The voltage response is lower when the test piece is subjected to impact at site Z, as the amorphous wire is further away from the impacted upon surface. Again, the three traces show that there is good reproducibility for repeated impact events.

Figure 5c, shows a graph of the electrical voltage response over time for the test piece in experiment 1 , which is where the amorphous wire was merely mounted on the surface of the test sample (i.e. did not form part of the fibre reinforced structure). The test piece in experiment 1 was subjected to the same 0.6129 J impact energy.

The maximum output voltage is marginally higher than compared to that shown in Figure 5a, which is expected. However, there is a significant degree of oscillation (ringing) of the voltage output, which may lead to problems of structural health monitoring of impacts which occur in quick succession, i.e. a number of impacts which occur over a short time period, on the surface to be monitored.

There are a number of advantages of embedding wires into a FRP structure, such as for example, increasing the robustness, such that the amorphous wire is not subject to damage, by a direct impact or a scratch/abrasion against the wire. A yet further advantage is increased reliability compared to surface mounted sensors, because surface mounted sensors the output voltage signal of the sensor is very dependent on the adhesive bond between the external sensor and the structure to be monitored. Over periods of time this bond strength can decrease due to creep, thus affecting signal response or even leading to delamination of the sensor from the structure. Embedded sensors that form an integral part of the structure will remain in place even in the event of a direct impact or repeated surface abrasions. Furthermore, delamination of the CRP will be considerably less likely than the failure of an adhesive bond between a sensor and the structure to be monitored.

Figure 6 shows a side elevation of a test piece built into the shape of a bridge 31 , which has been manufactured from a foam composite material. The composite comprises an outer wall 36 which contains carbon fibre plys 33, one of which has amorphous wires 30a located at 0° to the main axis (length) of the bridge and amorphous wires 30b located at 90° to the same axis, encapsulated in a resin 32. A second CFRP layer forms inner wall 34, without sensors. The inner wall 34 and outer wall 36 sandwiches a layer of expanded foam 35 to create a foam composite bridge structure 31.

Figure 7 shows a graph of the change in inductance of the amorphous wire as the bridge in Figure 6 is subjected to increasing loads, as described in more detail in Experiment 4, below.

Experimental Amorphous wire Experiment 1

The 125 μm amorphous wire was mounted onto the surface of a glass fibre ply and fixed in place using cyan-acrylate adhesive. The wire was laid in a loop configuration and had a length of 470mm, with a parallel spacing between each loop of 20mm. Terminal pads were bonded to the glass fibre ply to enable the end of the amorphous wire to be connected to electrical connecting wires using solder.

A glass test specimen was used as a tensile specimen. The specimen was mounted in a test machine and loaded for three cycles from 0 to 1500 microstrain (ramp rate of 0.5mm/min) at the following input AC current frequencies: 1kHz, 100kHz, 50OkHz, 1MHz, 2MHz, 4MHz, 5MHz, 8 MHz, and 13MHz. The input AC current was provided by direct electrical connection to the amorphous wire. The voltage response from the amorphous wire was measured using a half bridge rectifier. The amorphous wire was a glass coated amorphous wire (Co Fe B Si) provided by MFTI. The strain from 0- 1500 microstrain gave rise to the following minimum and maximum voltage outputs from the sensor. The-Experiments were repeated at varying frequencies, as shown in Table !

Table 1 Summary of Maximum, Minimum and range at various frequencies for 125 μm amorphous wire

The results showed the input currents in the frequency range of 100-500 kHz produced the largest range of output voltages.

Experiment 2

The amorphous wire (125μm) was coated with a non conductive polyamide coating.

The polyamide coating was found to be less fragile than the glass coating layer used in Experiment 1. The polyamide coating provided a more robust coating and still allowed the amorphous wire to be electrically insulated from the conductive carbon fibres.

A small CFRP test piece was constructed using 4 plys of carbon fibre, with the second carbon fibre ply layer possessing the amorphous wires. The CFRP was manufactured using the resin infusion under flexible tooling (RIFT) technique where dry fabrics layers are infused with resin under vacuum. The CFRP test piece was 300 x 300 mm and possessed the following specifications as shown in Table 2, below:

Table 2 Material specifications.

The amorphous wires were arranged at a variety of locations as shown in Fig 4a and 4b. The amorphous wires were either woven into position by hand or fixed into position using cyanate acrylate adhesive.

The woven amorphous wires were woven into a loop configuration by hand and their ends were then pulled through to the surface of the first ply. The ends were soldered to terminal pads to allow for connection to the electronics, via electrical leads, as per Experiment 1. All the amorphous wire ends were protected with PTFE tubing to minimise the stress at the egress points. The positions of the amorphous wires were labelled as shown in Table 3, below, which corresponds to Figures 4a and 4b.

Table 3 Amorphous wire positions and configuration.

In order to impart the energy to the test piece, a metal rod with a flat end was used to carry out the impacts by dropping it at a set height. The rod weighed 360.6g and the height was set to 175mm, which equates to an impact energy of 0.619 J. The height was additionally set to 320mm, resulting in an impact energy of 1.132 J.

As per Experiment 1 , the input AC current was provided by direct electrical connection to the amorphous wire. The input frequency was fixed at 100KHz, as this was found to be an effective input frequency range. The voltage response from the amorphous wire was measured using a half bridge rectifier. Three drop tests were carried out for both impact energy values, for all four of the amorphous wire configurations 20a to 2Od, as shown in Figure 4a.

The response curves, Figure 5a and 5b, for the fully encapsulated embedded wire provide a slightly lower response to the 0.619 J impact compared to when the wire is merely laid δrfthe surface of a specimen, as seen in Experiment 1 (Figure 5c). The embedded wire of Experiment 2 shows a max voltage response of 0.00707V compared to 0.1V in Experiment 1 , which is to be expected as the surface mounted sensor is closer to the impact site.

A 7J impact is usually considered to be the threat level for an aircraft. The sensor of the invention sustained very little damage on the surface of the CFRP panel as a result of the, repeated impacts. The test impact energy of 0.619J is clearly lower than the damage threat level that is required to be detected. Thus the CFRP sensor is capable of detecting an impact energy significantly below that of the 7J threat level, for an aircraft.

Amorphous ribbon Experiment 3

The amorphous ribbon was prepared as a surface mounted ply on the same test sample as for the amorphous wire in Experiment 1. However, in this experiment the glass fibre ply was embedded in a layer of resin, to provide an embedded sensor. The amorphous ribbon was a straight length with no twists or curves as this may have broken the ribbon. The amorphous ribbon was loaded up to 3000μstrain at 100kHz 50OkHz and 8MHz AC input frequencies.

Table 4 Amorphous ribbon loading vs. frequency.

The results in Table 4 show that impedance against force shows a linear trend for this test range, which offers advantages when analysing data.

Experiment 4

The bridge structure as shown in Figure 6 was manufactured using standard lay up techniques. The composite was laid up in a female tool and the final structure was cured using a vacuum bagging technique, to create a foam core encapsulated by inner and outer CFRP walls. The outer CFRP wall contained two sets of amorphous wires, which were orientated at 0° and 90° to main axis of the bridge.

The resin was SP Systems Prime 27 Epoxy resin (ambient cure resin system, with 50° post cure for Iδheurs). The cloth was a carbon cloth 1000gsm/2x2TW7STS 5631 24K. The amorphous wire was 125μm with glass coating, which was woven into the cloth layer.

The bridge structure was subjected to increased 10kg increments of static load and the change in inductance, using an LCR circuit, was measured for each mass increment. The loading (lower line) and unloading (upper line) profile and the change in inductance of the amorphous wire is shown in the graph in Figure 7. The input AC current was applied at a frequency of 100KHz.

In certain arrangements, particularly where use of an LCR circuit may be advantageous, the change in the inductance of the magnetically soft amorphous or nanocrystalline electrically resistive can be measured as a function of time.

The change in inductance of the wire when subjected to loading and unloading give stress profiles which are in close agreement. The fibre reinforced sensors of the invention have been shown to provide use as impact sensors and static load sensors, which may be particularly advantageous for structural health monitoring of composite structures.

Claims

1. A stress sensor comprising a fibre reinforced composite structure, said composite comprising a polymer matrix comprising at least one ply of cloth, wherein at least one generally elongate magnetically soft amorphous or nanocrystalline eiectricaily resistive element is in intimate contact with said cloth.

2. A sensor according to claim 1 , wherein the stress value of the element and composite structure are substantially matched.

3. A sensor according to claim 1 or claim 2, wherein the cloth is an electrically conducting cloth and wherein said element is electrically isolated from said electrically conducting cloth.

4. A sensor according to any one of the preceding claims wherein said element is in the form of a wire or ribbon.

5. A sensor according to any one of claims 3 or 4 wherein said element has an electrically insulating outer sheath, to provide electrical isolation from said cloth.

6. A sensor according to any preceding claim wherein the element is formed of a cobalt rich alloy.

7. A sensor according to any one of the preceding claims wherein one or more elements are embedded in one or more plys of cloth.

8. A sensor according to any preceding claim wherein the element is in the form of a wire and forms an integral part of the weave of the cloth.

9. A sensor according to any preceding claim wherein the element is a single continuous element.

10. A sensor comprising a fibre reinforced composite structure, said composite comprising a polymer matrix comprising at least one ply of an electrically conducting cloth, wherein at least one generally elongate magnetically soft amorphous electrically resistive element is in intimate contact with said cloth and wherein said element is electrically isolated from said cloth.

11. A sensing device comprising a sensor according to any preceding claim, a means of providing an input AC current across the at least one element and means of analysing the output voltage from the at least one element.

12. A device according to claim 11 wherein the output voltage frequency is monitored in the radio-frequency range.

13. A device according to claim 11 or claim 12 wherein said output voltage is analysed by direct coupling to the element.

14. A device according to claim 11 or claim 12 wherein said output voltage is analysed by inductive or capacitative or RF coupling to the element.

15. An applique sensor comprising a sensor according to any one of claims 1 to 10 and a means of attachment to a surface to be monitored.

16. A method of measuring applied stress on a body or surface using a stress sensor according to claim 1 to 10 or securing thereto an applique sensor according to claim 15, comprising the steps of providing an AC input current across one or more elements in said sensor or applique sensor and measuring the change in voltage in said one or more elements as a function of time.

17. The use of at least one generally elongate magnetically soft amorphous electrically resistive element in a fibre reinforced polymer matrix composite as a stress sensor, wherein said element is in intimate contact with at least one ply of an electrically conducting cloth, and wherein said element is electrically isolated from said cloth.

18. A structure, vehicle, vessel or aircraft having incorporated therein a stress sensor according to any one of claims 1 to 10, or a sensing device according to any one of claims 11 to 14, or secured thereto an applique stress sensor according to claim 15.

19. A product, device, method or use as herein described with reference to and/or as illustrated in the accompanying figures.