Field

The present invention relates to an apparatus for monitoring physical characteristics of a component. The apparatus has applications in structural health monitoring of components, for example, composite component such as for aircraft. The present invention also relates to a method of manufacturing said apparatus.

Background

It is often desirable to monitor the integrity of metallic or composite components or structures. One approach to such monitoring is‘structural health monitoring’ (SHM), which can provide a remote assessment of the structural integrity of a component or structure based on recorded sensor data and appropriate data processing and/ or interpretation. This can provide a user with a prediction of the remaining lifetime of the component, for example, or help determined maintenance schedules.

Structural health monitoring can be performed on both metallic and composite components, but such monitoring is of particular importance for composite

components; composites absorb the energy of impact events, which can result in delamination, and ultimately failure, of the composite.

Previous techniques for structural health monitoring utilise various sensor technologies (for example, piezoelectric, fibre optic, or strain based sensors) to monitor the condition of the structure or component. Typically systems consist of a network of transducers and a controller for acquiring and/ or processing the data from the transducers. The transducers remain fixed to the component of structure to be monitored, and the controller can be located locally or remotely, depending on the application. Such continual monitoring can facilitate the condition based maintenance of a component, which in turn can significantly reduce maintenance costs by reducing the frequency and costs of maintenance (since pre-emptive maintenance does not need to be carried out on the component). Moreover, by facilitating precise identification of where, and what, maintenance is required, both the amount of materials used, and the time taken to carry out the maintenance, can be reduced. Such structural health monitoring has particular application in fields such as aviation, where it is desirable to reduce the time taken for maintenance (since this must be performed whilst the aircraft is grounded, during which time the aircraft is not in operation and therefore not generating revenue). However, for structural health monitoring to be feasible for applications such as aviation, the cost and reliability of the system must provide significant advantages over the traditional approach of regular maintenance schedules and conventional, non- destructive, testing of components. The reliability of SHM systems depend directly on the integrity of the on-board equipment over the lifetime of the structure, and the possibility of repair or replacing the faulty components. Since any sensors would need to be permanently located on an aircraft, it is therefore necessary that the sensors employed for the structural health monitoring can perform effectively in the

operational environment of the aircraft (for example, that the sensors can withstand the humidity, the extreme temperature changes, and the vibrations experienced on board the aircraft). Moreover, the sensors must be light, since any additional weight affects fuel efficiency.

Efforts have been made towards the development of reliable SHM systems. For example, US 2013/0044155 describes a system for monitoring the structural health of a structure using a distributed network of nanoparticle ink based piezoelectric assemblies deposited directly onto the structure. Such printing can be used in place of the more traditional approach of etching wires and/or sensors. However, the manufacture of such a directly printed system is complex, time consuming and costly, and does not provide the desired characteristics of flexibility, scalability, and reliability. For example, the assemblies of US 2013/0044155 are not replaceable or repairable, which is desirable when trying to provide a reliable SHM system.

Summary

In a first aspect, an apparatus is provided in accordance with appended independent claim 1. In a second aspect, a method of manufacturing such an apparatus is provided in accordance with appended method claim 15. In a third aspect, a method of embedding an apparatus within a composite structure is provided in accordance with appended independent claim 25. Optional features are provided in the appended dependent claims. In the following description, there is provided an apparatus for monitoring physical characteristics of a component. The apparatus comprises: a polyimide film and at least one transducer provided on the polyimide film. The apparatus also comprises, for each transducer, at least one flexible, ink based, conductive wire for providing power and/or communication connectivity to the transducer.

By utilising ink based, conductive wires for providing power and/or communication connectivity to the transducer (i.e. wires that have been printed or deposited using another additive manufacturing technique), a lighter and more flexible apparatus can be provided, whilst still providing good signal quality. Moreover, since the conductive wires are printed rather than, for example, etched, a thinner polyimide film may be used, since no polyimide substrate material is removed during the manufacturing process. This further reduces the weight of the apparatus as compared to those manufactured using traditional techniques. The apparatus can be used to monitor the physical characteristics of components for use in, for example, structural health monitoring. Such apparatus have particular utility in applications where weight is a concern, for example in aviation.

In some groups of embodiments, the apparatus further comprises a thermoplastic film for mounting the apparatus on the component. For example, the thermoplastic film maybe for surface mounting the apparatus on a surface of the component. The thermoplastic film allows for more flexible application of the apparatus to the component than traditional adhesives, such as epoxy, since these can be difficult to apply with a uniform thickness. Since the thickness of the adhesive used can affect signal quality, uniformity of the adhesive is important. Use of a pre-ap plied thermoplastic film, which can be thinner and have a more uniform thickness than adhesives applied directly onto one of the surfaces to be adhered by an end user, can therefore improve the reliability of the signals from the transducers. Optionally, the thermoplastic film has a melting point of 200 degrees Celsius or less, optionally a melting point of 170 degrees Celsius or less, optionally a melting point of 150 degrees Celsius or less. Optionally the thermoplastic film has a melting point of between 90 degrees Celsius and 150 degrees Celsius, optionally a melting point of between 120 and 130 degrees Celsius. This range of melting points or melting temperatures can provide for easier removal of the apparatus from the composite surface, since the thermoplastic film can be heated to the appropriate melting point without damage to the transducers and other electronics of the apparatus. Once the thermoplastic film has melted, the polyimide film can be removed from the composite surface quickly and easily, and then a new polyimide film can be applied by re-heating the thermoplastic film to the necessary temperature (or, if desired, the previously removed film can be re-applied).

Optionally the thermoplastic film is formed from a thermoplastic material having a heat deflection temperature greater than or equal to 70 degrees Celsius, optionally a heat deflection temperature greater than or equal to 80 degrees Celsius. Optionally the thermoplastic film has a heat deflection temperature of between 80 and 120 degrees Celsius, optionally a heat deflection temperature of between 90 and 110 degrees Celsius. Optionally, the thermoplastic film has a density between 0.80 g/cm ³ and 0.10 g/cms, optionally a density between 0.90 g/ cm ³ and 0.95 g/cm ³ . Optionally, the thermoplastic film has a density of 0.92 g/cm ³ .

In particular, the properties of the thermoplastic film must be suitable for the specific application of the apparatus. For example, when applied the heat deflection

temperature of the film has to be higher than the maximum service temperature of a structural health monitoring system (which can be 80 degrees Celsius). The heat deflection temperature (or heat distortion temperature) is the temperature at which a polymer or plastic (including a thermoplastic) deforms under a specified load. The heat deflection temperature can be determined using the test procedure outlined in ASTM D648, which is similar to that defined in the ISO 75 standard. The melting temperature of the thermoplastic film of the apparatus should be lower than the maximum service temperature of the sensors used within the apparatus (for PZT sensors, this is about 150 degrees Celsius) and lower than the maximum service temperature of the host material (this is about 170 degrees Celsius for a thermoset composite). This can help to ensure that the thermoplastic film can be applied as an adhesive without damaging the rest of the apparatus or the underlying component, which can facilitate the repair and replacement of the apparatus. Such material properties also provide sufficient adhesion under the environmental loads expected on an aircraft, which can help to avoid fatigue and thus failure of the apparatus. Optionally, the thermoplastic film has a thickness between 30 microns and 80 microns, optionally a thickness between 40 microns and 70 microns. Optionally, the thermoplastic film has a thickness between 40 g/ m ² (approximately 44 microns or micrometres) and 60 g/m ² (approximately 66 microns or micrometres). Such thicknesses can be advantageous, since they can provide for the requisite adhesion of the apparatus without acting to dampen vibrations from the component, or in anyway insulate said vibrations from the transducer. As such, more reliable and accurate results of the physical characteristics of the underlying component can be provided with the present apparatus.

Optionally, the apparatus comprises a plurality of (i.e. at least two) transducers. At least two of the plurality of transducers have a common conductive wire, which conductive wire acts as a ground wire. By providing at least two transducers having a common conductive ground wire, cross-talk between transducers can be minimised. In particular, the wires can be designed to provide shielding and thus reduce cross-talk simply by the configuration of the conductive wires. Optionally the distance between each two adjacent flexible conductive wires is greater than or equal to 0.4 mm, optionally the distance is greater than or equal to 0.5 mm. In other words, adjacent wires are separated by at least 0.4 mm, optionally, by at least 0.5 mm). Such distances between adjacent ones of the conductive wires can further reduce cross-talk. The use of printing processes to manufacture the conductive wires can be particularly beneficial in such applications, since it can facilitate more flexible conductive wire network design, without imposing the constraints of chemical etching processes, for example.

Optionally, the apparatus further comprises a protective layer arranged on top of the conductive wires. Optionally, the protective layer is a dielectric layer which may be ink jet printer, or otherwise disposed, on top of the conductive wires. This protective layer can shield and protect the printed conductive wires from thermal shock and moisture corrosive materials. Optionally, the dielectric protective layer can be polymerised through UV radiation and/ or conventional thermal heating in order to form a more resilient dielectric layer. Such a dielectric layer can be used instead of, or in

combination with, another polyimide film arranged on top of the transducers and conductive wires.

Optionally, the conductive wires are silver nanoparticle ink based conductive wires. Silver-based inks can be beneficial in applications such as printed electronics, due to their high conductivity (silver-based inks have the highest electrical conductivity per unit volume of any metal), the ability for high-volume manufacturing, and their performance stability. Copper is relatively reactive so copper based conductive wires tend to exhibit rapidly formed, and poorly conducting, oxide layers on the surface; since silver is relatively unreactive, these oxide layers form very slowly so the electrical properties of silver based conductive wires can be more stable over time.

Optionally, the one or more conductive wires each have a thickness of between 2 and 10 microns, optionally a thickness between 3 and 9 microns, optionally a thickness between 4 and 8 microns. The thickness and the width of the printed wires can be designed to meet the requirements of the application and keep the weight addition at minimum; this is of particular consideration when the apparatus is employed in structural health monitoring of an aircraft, for example. Moreover, the conductivity of the printed wires can be increased by increasing the thickness and/or the width of the printed lines. Thus, based on the application requirements these two dimensions can be modified in order to achieve a balance between the desired electrical conductivity and the weight of the apparatus. The wire thickness can be increased by printing consecutive layers of ink (e.g. silver based ink) on top of each other.

Optionally, the conductive wires of the apparatus are formed by a printing or other additive manufacturing process, for example by inkjet printing. Inkjet printing the conductive wires can provide a relatively simple and efficient method of manufacture.

In particular, the simplicity of the inkjet printing process can facilitate repair of previously deployed apparatus, since new conductive wiring networks can be printed and easily combined with existing printed networks already situated on a component surface. In particular, the combination of thermoplastic adhesive as described above and printed conductive wires can provide a simple apparatus which can be easily repaired or replaced, at a relatively low cost. Printing of the conductive wires can also help to minimise waste material, in particular as compared to equivalent wires manufactured by etching. As such, environmental waste and manufacturing costs can be reduced.

Optionally, the polyimide film has a thickness of 100 microns or less, optionally a thickness of 50 microns or less, optionally a thickness of 30 microns or less. Preferably, the polyimide film has a thickness of 25.4 microns, which thickness, when in

combination with the above-described flexible conductive wires, can facilitate the provision of a flexible and lightweight apparatus for monitoring physical characteristics of components. Characteristics of the polyimide film used in the apparatus which should be considered when choosing a polyimide material include: dielectric strength, flexibility, shrinkage at elevated temperatures, resistance to moisture, gas and vapour transmission, as well as the thermal and mechanical stability. For printing of conductive wires, the surface energy of the polyimide film should also be considered to ensure a suitable substrate is provided. The polyimide film may be formed from Kapton® B(iooB) (DuPont), or any other polyimide film that exhibits similar material characteristics. For example, Kapton® B(iooB) has a surface energy suitable for such printing applications, and can form a suitable substrate for almost every commercially available ink.

Optionally, the polyimide film has a dielectric strength (tested using ASTM D-149-94) of between 2000 and 4000 Volts/mil (about 79 V/pm to 157 V/um), optionally a dielectric strength of between 2500 and 3500 Volts/mil (about 98 V/pm to 138 V/pm), optionally a dielectric strength of 2800 Volts/mil (about 110 V/pm). Optionally, the polyimide film has a tensile strength (tested using ASTM D-882-91) of between 200 and 300 MPa, optionally a tensile strength of between 230 and 250 MPa, optionally a tensile strength of 241 MPa. Optionally the polyimide film has a modulus (tested using ASTM D-882-91) of between 2000 to 4000 MPa, optionally a modulus of between 2500 MPa and 3500 MPa, optionally a modulus of 3034 MPa. Optionally, the percentage shrinkage at 200 degrees Celsius (tested using IPC-TM-650) is less than 0.1%. Optionally, the percentage elongation (tested using ASTM D-882-91) is between 60% and 70%, optionally the percentage elongation is 65%. Optionally, a single connector is electrically coupled or connected to the one or more conductive wires for connecting each transducer to an electrical power source and/or a controller. Using a single connector can provide a lighter apparatus. Moreover, the use of printing or additive manufacturing to form the conductive wire networks can facilitate the simple and easy connection of the conductive wires to said single connector. Optionally, the connector is a surface mounted connector. Alternatively, more than one connector may be used. In some alternative arrangements, each of the one or more conductive wires is electrically coupled to a different connector for connecting each transducer to an electrical power source and/or a controller.

Optionally, the transducers comprise piezoelectric transducers. In some arrangements, the transducers maybe sensors, which sensors may optionally be printed or additively manufactured in a similar manner to the manufacturing of the conductive wires. Such sensors may provide signals indicative of impacts or vibrations experienced by the component to a controller. In other arrangements, the transducers may be actuators and maybe actively controlled by a controller coupled to the transducers by the one or more connectors. In such an arrangement, the actuators may be controlled to apply vibrations to the component in order to monitor the response, for example.

Optionally, the transducers are formed of a different material than the conductive wires. For example, the transducers maybe printed or additively manufactured from a different material than the conductive wire. Alternatively, the transducers may be otherwise manufactured from a different material and provided on the polyimide film independently of the manufacture of the conductive wires. In some arrangements, the transducers are provided on the polyimide film and are ceramic transducers, optionally the transducers provided on the polyimide film are ceramic piezoelectric transducers.

In the following description, a method of manufacturing an apparatus for monitoring physical characteristics of a component is described. The method comprises additively manufacturing, on a polyimide film, one or more flexible, ink based, conductive wires for providing power and/or communication connectivity to a transducer; and providing at least one transducer on the polyimide film, each transducer associated with at least one flexible conductive wire.

Additive manufacturing may comprise any suitable process for forming ink based conductive wires. For example, any direct write printing process maybe used, such as: a jetted atomized deposition process, an inkjet printing process, an aerosol printing process, a pulsed laser evaporation process, a flexography printing process, a micro- spray printing process, a flat bed silk screen printing process, a rotary silk screen printing process, a gravure printing process, or another suitable direct write printing process. In some groups of embodiments, additively manufacturing comprises inkjet printing the one or more flexible conductive wires to form flexible, ink-based, conductive wires.

When the conductive wires are formed by inkjet printing, the method may optionally further comprise sintering the one or more printed flexible wires in order to provide a cohesive conductive wire network which facilitates the provision of electrical power and/ or communication connectivity to/ from the transducers. The parameters of the sintering process depend on the ink type used to form the flexible conductive wires, the selection of which depends on the demands of the specific application of the apparatus. Sintering can affect the conductivity and the quality of the printed circuits, so the parameters should be carefully chosen. For example, a variety of low-temperature sintering inks have been introduced to the market over the last few years, which address issues regarding the relatively high temperatures required for the sintering process and the corresponding impact on conductivity etc. Therefore, the sintering temperature can be chosen based on the selection of ink. It is well known that different ink formulations require different sintering conditions. The parameters for sintering should therefore be chosen accordingly. Optionally, sintering comprises sintering at a temperature between 100 and 200 degrees Celsius, optionally 150 degrees Celsius, for a time period between 20 minutes and 40 minutes, optionally 30 minutes. For example, the sintering process can occur for 30 minutes at 150 degrees Celsius. Such parameters provide for said cohesive conductive wire network, without compromising the structural integrity of the polyimide film, or any transducers that may have already been provided on the polyimide film. It will be understood that other forms of additive manufacturing besides printing may also require a step of sintering and the sintering parameters may be adjusted as required. In some cases, sintering can also be achieved using infrared radiation or a photonic curing system, rather than heating in an oven, for example.

Optionally, the one or more printed conductive wires are formed from silver nanoparticle ink. Optionally, the silver nanoparticles have a particle diameter of less than 50 nanometres. Optionally, the silver nanoparticle ink comprises 30 to 35 % by weight of silver nanoparticles. Such ink formulations can be beneficial since they provide a good balance between a high conductivity and a low sintering temperature (between 120 and 150 °C). Moreover the viscosity and the surface tension of such inks enable the printing of silver wires on a variety of polymeric substrates, using conventional inkjet printers. However, any other inks with similar properties can be employed for the development of the apparatus described herein; the ink selection should be based on the application demands.

Optionally, the method further comprises mounting the polyimide layer on a thermoplastic film (or otherwise applying a thermoplastic film to the polyimide layer). The application of a thermoplastic film to the polyimide layer of the apparatus can facilitate the simple adhesion of the apparatus to the surface of a component for monitoring physical characteristics of said component. In other arrangements, no thermoplastic layer is applied to the polyimide film and the apparatus may be embedded within a composite component, rather than being applied to the surface of said composite. Alternatively, it will be understood that the apparatus can be mounted to a component with another adhesive other than the thermoplastic film.

Optionally, the method further comprises additively manufacturing the transducer in addition to the conductive wires. Optionally, additively manufacturing the transducer comprises printing the transducer. Such an approach is particularly applicable when the transducers are implemented as sensors, since such passive transducers can be relatively easily printed during the same manufacturing step as the conductive wire. In this arrangement, the transducer may be formed from the same material as the one or more conductive wires.

Alternatively, the transducer is formed from a different material to the one or more conductive wires. For example, the transducer may be additively manufactured from a different material than the conductive wires, or the transducer may be manufactured in any other suitable way, independently of the conductive wires, and subsequently provided onto the polyimide film for electrical connection to the conductive wires. Optionally, the transducers provided on the polyimide film are piezoelectric transducers, optionally the transducers are ceramic transducers, optionally the transducers are piezoelectric ceramic transducers. When the transducers are not additively manufactured, or when they are additively manufactured in a separate manufacturing step, the method may optionally further comprise applying, for each transducer, conductive paste to the polyimide film to electrically connect the transducer and the one or more conductive wires. The above described apparatus is a more robust apparatus for monitoring the physical characteristics of a component. The use of a thermoplastic film as an adhesive can provide an apparatus which is both repairable and replaceable, due to the possibility of non-permanently surface mounting the apparatus on a component to be monitored. The specific ink based conductive wires used, in combination with the polyimide film, provide a lighter and more flexible apparatus which can be applied to a variety of components, regardless of their geometry, and can provide further advantages in terms of the ability to repair the apparatus, as discussed above. The flexibility of the apparatus is further enhanced by the thermoplastic film, which can allow for easy application of said apparatus to a component surface. In the following description, a method of embedding an apparatus (for monitoring physical characteristics of a component) into a composite component is provided. The method comprises additively manufacturing, on a polyimide film, one or more flexible ink based conductive wires for providing power and/ or communication connectivity to a transducer; providing at least one transducer on the polyimide film, each transducer associated with at least one of the one or more flexible conductive wires; providing a second polyimide film to cover the one or more conductive wires and the at least one transducer to form the apparatus; subsequently embedding the apparatus between layers of composite fibres, optionally, p re-impregnated composited fibres; and curing the layers of composite fibres to form the composite component.

By embedding the apparatus into a component during the manufacture of said component, there is no need for additional adhesive layers, which can provide for improved signals from the transducers. However, it will be understood that such an apparatus will not be repairable or replaceable, and thus must have a lifetime equivalent to that of the component itself. As discussed above, the above apparatus can provide the requisite robustness and reliability for such an application.

In any of the above-described first, second, or third aspects, the apparatus or method may be for monitoring physical characteristics of the components for use in structural health monitoring. The component may be a composite component, optionally a composite component for use in an aircraft. It will be understood that, due to the operational environment of an aircraft, apparatus used in such applications must be sufficiently reliable and robust. As discussed above, the apparatus of the first aspect can provide the necessary characteristics.

Any of the above described optional or preferable features may be combined with any of the other above mentioned features, provided there is no contradiction in such a combination. Moreover, any of the features of the first aspect can be combined or applied to the second and third aspects, and vice versa. Brief description of the drawings

The below description is with reference to the following Figures:

Figures lA and lB provide schematic cross sectional illustrations of two different implementations of an apparatus for monitoring physical characteristics of component;

Figure 2 illustrates example conductive wire geometries for the apparatus of Figures lA and lB;

Figure 3A illustrates application of the apparatus of Figure lA to a composite component;

Figure 3B illustrates embedding of the apparatus of Figure lB within a composite component;

Figure 4 provides experimental results illustrating the effect of mechanical load on transducer output for the apparatus of Figure lA; and

Figures 5A and 5B provide experimental results illustrating the effect of thermal loading on transducer output for the apparatus of Figure lA.

Detailed description

With reference to Figures lA and lB, an apparatus 100 for monitoring physical characteristics of the component is described. The apparatus comprises a polyimide film 102 with at least one transducer 104 provided on the polyimide film 102. It is to be understood that that cross sections illustrated in Figures lA and lB are not to scale.

Polyimide film 102 may formed from Kapton® B(iooB) (DuPont) film, for example, which polyimide film has the following properties: a thickness of 25.4 microns, a dielectric strength of 2800 Volts/mil (about 110 V/pm), a tensile strength of 241 MPa, a modulus of 3034 MPa, and a percentage elongation of 65%. Alternatively, any other form of polyimide film may be used, provided the polyimide film has the appropriate material and electrical characteristics, as described above. Transducer 104 can be a sensor or an actuator, depending on the application of the apparatus 100. Transducer 104 is a piezoelectric transducer, but any other suitable form of transducer may be used. In some embodiments, transducer 104 is a ceramic piezoelectric transducer. For example, transducer 104 maybe a DuraAct™ (Physik Instrumente, PI) transducer comprising piezoelectric ceramic PIC255 (PI Ceramic). PIC255 has a density of 7.8 g/cms and a Curie temperature of 350 degrees Celsius. Alternatively, transducer 104 may be any other suitable form of transducer formed from any suitable piezoceramic or piezoelectric material.

The apparatus 100 further comprises, for each transducer, at least one flexible conductive wire 106. The conductive wire 106 can be an ink based conductive wire, that is, the conductive wire 106 is formed from an ink deposited on the polyimide film 102 (i.e. by printing or by another suitable additive manufacturing process other than printing). Conductive wire 106 is arranged to provide power and/or communication connectivity to transducer 104. For example, conductive wire 106 maybe arranged to provide electrical signals from the transducer 104 to a data-logger or controller, for example controller 110 of apparatus 100. Conductive wire 106 may also be arranged to receive power from the controller 110 or from an additional external power source (not shown) and to provide said power to the transducer 104. Conductive wire 106 may be formed of silver based ink, for example silver nanoparticle based ink. The ink may comprise nanoparticles of silver of a diameter of 100

nanometres or less, optionally 50 nanometres or less, suspended in a solvent. Such conductive wires can exhibit a weight reduction of 50% as compared to the weight of etched wires, and a weight reduction of 70% as compared to conventional conductive wires. Moreover, due to the versatile character of the inkjet printing technology, the developed conductive network can be purposely designed to allow for accurate sensor placement and thus better meet application demands.

In some arrangements, conductive wire 106 has a thickness of between 4 and 8 micrometres, which thickness of wire can be more easily manufactured using printing or additive manufacturing processes than with traditional etching techniques. The resulting apparatus therefore has flexible conductive wires 106 on a flexible substrate (polyimide film 102), which provides for easier application of the apparatus to a component for the monitoring of physical characteristics, since the flexibility allows the apparatus to be applied to a variety of component geometries.

The apparatus 100 further comprises a connector 108 for connecting the conductive wire 106 to controller 110. In this arrangement, controller 110 also provides power to the transducer 104, but it will be understood that in other arrangements, an additional power supply or power source may be provided in addition to controller 110. Therefore, connector 108 may also be suitable for providing power, in addition to communication connectivity, to the transducer 104. Connector 108 may be a surface mounted connector, or any other suitable form of connector. In some examples, a single surface mounted connector is provided for the plurality of conductive wires 106 on the polyimide film 102, although one or more connectors may be used depending on the application.

The above described arrangement results in an apparatus 100 with decreased weight as compared to a conventional apparatus for structural health monitoring. In particular, conventional SHM apparatus or systems comprise a coaxial cable and a BNC connector per transducer, meaning the weight of the SHM system is proportional to the number of transducers used. By replacing the conventional coaxial cable with the above described lightweight, inkjet printed (or ink based), circuits and using a single surface mounted connector instead of a number of BNC connectors, the additional components required for handling of coaxial cables (i.e. cable clips, bundlers, protective layers) are also no longer required. As such, apparatus 100 represents a significant decrease in the overall weight of the system. In particular, the weight of a single BNC connector with a i5cm-long coaxial cable is 7 grams, giving an approximate weight of 35 grams for a system with four transducers (excluding the transducers and any controller). However, the weight of apparatus 100 for the same size and same number of transducers can be reduced by 67%, i.e. is about 12 grams (again, excluding the transducers and any controller). Moreover, the improvements in weight reduction become more significant as the number of transducers increases, since inkjet printing of longer circuits has a negligible effect on the weight of the overall system. Optionally, as described with reference to Figure lA, in some groups of embodiments the apparatus 100 further comprises a thermoplastic film 112. Thermoplastic film 112 is applied to the polyimide film 102 in order to adhere said film to a composite

component when the apparatus is in use. Thermoplastic film 112 is of particular benefit in applications where the apparatus is to be disposed on a surface of a composite component (or any other component, e.g. a metallic component), rather than embedded within it (since when the apparatus is embedded the epoxy resin used to cure the composite provides any necessary adhesion).

The provision of a thermoplastic film 112 on the apparatus as an adhesive can minimise the risk that the apparatus is adhered to a composite surface using another adhesive, for example epoxy paste, by an end user of the apparatus. The thickness of the adhesive used to surface mount the apparatus can affect the signals produced by the transducers of the apparatus. Inconsistent thickness of adhesive therefore can lead to unreliable results. The provision of a thermoplastic film as part of the apparatus obviates these risks, since a uniform film thickness across the apparatus (and across multiple apparatus deployed on the same structure) can be more easily provided. A more reliable and robust apparatus can therefore be provided.

The thermoplastic film 112 optionally has a melting temperature of between 90 degrees Celsius and 150 degrees Celsius, preferably a melting temperature of 120 to 130 degrees Celsius. Optionally, the thermoplastic film has a heat deflection temperature of between 80 and 120 degrees Celsius, preferably a heat deflection temperature of between 90 and 110 degrees Celsius. Optionally, the thermoplastic film has a thickness between 30 microns and 80 microns, preferably a thickness between 44 microns and 66 microns.

These material characteristics can facilitate the easy replacement and repair of the apparatus 100, since the thermoplastic film 112 can be heated to within this

temperature range (i.e. between 70 and 170 degrees Celsius, preferably between 80 and 150 degrees Celsius) easily, and without damage to the electronics of the apparatus. Heating of the thermoplastic film 112 to these temperatures without damage of the apparatus, or the underlying component, is also facilitated by the relatively thin nature of the thermoplastic film. Once the thermoplastic film 112 is softened, the apparatus can be easily removed from any component it is attached to. A replaceable/repairable apparatus can therefore be provided.

In other embodiments, as described with reference to Figure lB, the apparatus 100 can comprise a second polyimide film 102b covering transducer 104 and conductive wire 106. In this arrangement, the transducers and conductive wires are in essence sandwiched between two polyimide films 102a, 102b. This arrangement can prevent a short circuit occurring between the conductive printed wires 106 and fibres of an associated composite component in which the apparatus 100 is to be embedded. It will be understood that apparatus 100 shown in Figure lA may also be provided with a second polyimide film 102B, as shown in Figure lB. Similarly, apparatus 100 of Figure lB may be additionally provided with a thermoplastic film 112, as in Figure lA. Apparatus 100 is constructed by printing at least one conductive wire 106 on polyimide film 102. The printing process described herein comprises inkjet printing, but any other form of printing or additive manufacturing process can be used (for example, aerosol printing, jetted atomised deposition, pulse laser evaporation, etc). Conductive wires 106 can be formed by the printing of silver-based paint, for example silver nanoparticle ink. An example of such inkjet printing is described below in the

Experimental Data section.

The versatile character of the printing process enables the precise and cost-effective creation of complex conductive wire network geometries and patterns on flexible substrates (e.g. polyimide film 102). Examples of such resulting networks of conductive wires 106 are illustrated in Figure 2 and, in particular, Figure 2C illustrates the flexibility of the resulting network of conductive wires 106. The printing process can facilitate the provision of thinner and more flexible conductive wires than can be achieved with traditional manufacturing techniques, such as etching, and the printing of ink based conductive wires means such wires can be provided in a simple and efficient manner. Moreover, etching techniques typically utilise copper to form the conductive wires. The use of conductive ink based wires, in particular silver nanoparticle based ink, can therefore also lead to a lighter network of conductive wires than traditional products without any reduction signal quality.

After printing, the deposited ink based conductive wires 106 are preferably sintered in order to remove any remaining traces of solvents and to fuse the conductive particles into a cohesive conductive trace. In one example, sintering took place took place in a laboratory oven for 30 min at 150 degrees Celsius, but it will be understood that any other appropriate sintering process maybe applied. For example, the conductive wires maybe sintered at a temperature of between 120 and 180 degrees Celsius, for a period of time of between 20 and 40 minutes, depending on the particular properties of the ink used to form the conductive wires. When other additive manufacturing techniques are used, different sintering parameters may be employed, depending on the particular manufacturing technique.

During the sintering process, the thermal expansion of the conductive particles, along with the evaporation of any solvent used to suspend the conductive particles, can result in the formation of a continuous and cohesive conductive layer. This provides for an improved channel for conduction electrons to flow. In particular, the conductive wires 106 formed as a result of the printing and sintering process can consist of a smooth and dense network of conductive particles (e.g. silver) that can enhance the conductivity of the printed wire.

In some examples, transducer 104 is also formed by printing or additive manufacturing processes, and may be printed at the same time as, or at a different time to, conductive wire 106. Transducer 104 maybe printed from the same material as the conductive wires, or from a different material. In such an arrangement, an electrical connection between the transducers and the conductive wires can be formed by overlapping the components during printing. Alternatively, the electrical connection can be formed by applying a conductive adhesive to join the transducer and the conductive wire, for example, a silver loaded epoxy adhesive may be used. In other examples, transducer 104 is not printed and is formed from a different material to the conductive wires.

When the transducer is not printed, the transducer is typically provided on the polyimide film 102 after the conductive wires 106 are printed (and preferably after the conductive wires are sintered); the electrical connection between the components may again be formed by applying a conductive adhesive, or the electrical connection may be formed in any other suitable way. Such transducers may be ceramic transducers, or any other suitable form of transducer.

The apparatus 100 described above can be surface mounted onto a component through application of the thermoplastic film 112. This arrangement can be advantageous, since it can facilitate the replacement and/ or repair of the apparatus, thereby prolonging the overall lifetime of a structural health monitoring system incorporating said apparatus. Since the thermoplastic film 112 used to adhere the transducers to the components has a melting point of (preferably) around 120-130 °C (optionally 90-i50°C), it is possible to heat the thermoplastic film and peel off the polyimide film in order to replace some or all of the transducers. Such replacement/ repair can be further facilitated by the ink based nature of the conductive wires, since new conductive wires can be printed and electrically connected to the old network.

Alternatively, the apparatus 100 maybe embedded within a composite structure in order to monitor the physical characteristics of said composite. To embed the apparatus 100, a second polyimide film 102b is provided, as shown in Figure lB. The apparatus is then provided between layers of composite fibres. Preferably, the layers of composite fibres are pre-impregnated composite fibres, i.e., fibres that are already pre- impregnated with an epoxy resin or other thermoset polymer matrix material; this arrangement means that no additional adhesive needs to be provided with the apparatus. In other words, no thermoplastic layer is required.

Once the apparatus is provided between layers of composite fibres, or embedded between said layers, the layers of composite fibres can be cured, for example in an autoclave, in order to form a composite component embedded with said apparatus. In this arrangement, the apparatus is not replaceable or repairable, and thus needs to reliably monitor the physical characteristics of the composite for the lifetime of the composite component. Such reliability can be provided by the apparatus described herein. It will be understood that the method of manufacture of an apparatus for embedding in a composite component maybe identical to the method of manufacture of an apparatus for surface mounting on a component, although without the addition of the

thermoplastic film 112. However, in some instances the thermoplastic film 112 may also be provided when the apparatus is to be embedded within the composite, for example when the carbon fibres are not pre-impregnated.

Experimental data

In one example of the above-described apparatus, conductive wires 106 were formed by the inkjet printing of a silver nanoparticle suspension in a solvent of tri ethylene glycol monomethyl (here silver ink from Sigma-Aldrich was used). The concentration of silver nanoparticles was 30-35 % by weight and the particle diameter was less than or equal to 50 nanometres (nm). The viscosity of the example suspension ranged from 10 to 18 mPa seconds, and the surface tension was between 35 and 40 mN per metre. The resistivity of the ink used was iΐmW cm.

The substrate used for printing was a 25 pm-thick polyimide film (Kapton™, see above described properties). DuraAct™ piezoelectric transducers were used (see above properties), while the connections between the printed circuits and the transducers were created using silver epoxy adhesive. A durable surface-mounted connector with an operating temperature range of -40 °C to +85 °C was used. The inkjet printing of the conductive wires 106 was performed using a piezoelectric Dimatix DMP 2850 printer. The piezo voltage was set at 20 V, and a customized wave form with a maximum jetting frequency of 5 kHz employed to provide suitable drop formation. During printing, the substrate temperature was maintained at 60 °C. A drop spacing of 30 micrometres (pm) was selected and the printing width set at 1 millimetre (mm). After printing, sintering took place took place in a laboratory oven for 30 minutes, at 150 °C.

In order to decrease the electrical resistivity of the resulting printed wires, three layers of silver-based ink were printed on top of each other. The resistivity of the printed silver nanoparticle wires was calculated at approximately 8 mW cm, which is

approximately five times the bulk silver resistivity (1.59 mW cm).

The resulting apparatus was assessed under thermal and mechanical loading conditions in order to determine the suitability of the apparatus for use in aeronautical

applications. This assessment is described with reference to Figures 4 and 5A and 5B.

Mechanical fatigue

In order to examine the effect of multiple fatigue cycles on the durability and robustness of the apparatus, and specifically the durability of the inkjet-printed conductive wires, the apparatus was subjected to tensile-tensile fatigue testing for 10 ⁶ cycles. For the fatigue testing, a hydraulic INSTRON universal testing machine equipped with a 100 kN load cell was used. The selected cyclic frequency was 5 Hz, the maximum tensile load was 5 kN and the stress ratio was R = 0.1. The signal maximum amplitude of two transducers was recorded at distinctive load cycles of the fatigue test in order to monitor any change in the maximum amplitude due to mechanical loading.

The mechanical fatigue results for the above apparatus, surface-mounted onto a carbon fibre reinforced polymer (CFRP) component with a thermoplastic film having a melting point of 150 °C, are presented in Figure 4. The percentage change in the maximum amplitude of the propagated wave was calculated using the initial maximum amplitude as reference. As can be seen from Figure 4, the maximum amplitude of the signals exhibit an initial increase of approximately 2% after the first 10 cycles of the mechanical fatigue load, followed by a relatively stable period. This effect can be attributed to the stabilization or settlement of the system during the initial stage of the test, which resulted in a relatively low change in the amplitude of the recorded signals. At the final stages of the fatigue testing, at approximately io ⁶ cycles, the recorded signal maximum amplitude of both sensors showed a second percentage change of almost 4%. Ignoring the initial percentage change due to the stabilization of the SHM system, the total percentage change during the mechanical testing was approximately 2%, showing a satisfying performance after 10 ⁶ cycles of fatigue loading.

Thermal load

The apparatus, again mounted onto the surface of the CFRP component, was exposed to the environmental test profile depicted in Figure 5A. Tests were performed using a TAS Series 3 temperature and climatic chamber (Temperature Applied Sciences, Worthing, UK). The chamber temperature was measured with a PRT platinum resistance thermometer probe, and K-type thermocouples were in contact with the apparatus to monitor temperature of the sensors. At the end of the thermal loading test, the effect of thermal treatment on electrical impedance was studied.

The evolution of the impedance magnitude spectrum for both transducers before and after the thermal loading is presented in Figure 5B. It is evident that the impedance magnitude, as well as the resonance frequencies of both sensors, remained unaltered after the thermal loading testing. Before the thermal loading, the resonance frequency, and the impedance magnitude at this frequency, of sensor 1 were 275.72 kHz and

316.85 W, respectively. After the thermal treatment, the resonance frequency remained exactly the same at 275.72 kHz, while the magnitude of impedance showed a minor increase at 318.49 W. Similar behaviour was observed for sensor 2. For the reference state, the resonance frequency was found to be 278.72 kHz, and the impedance magnitude at this point was 308.00 W. At the end of the thermal loading test, the resonance frequency was slightly lower at 276.89 kHz, while the impedance magnitude showed a minor increase at 313.85 W.

The above described apparatus provides for more reliable and robust monitoring of physical characteristics of a component, such as a composite component of an aircraft. The combination of ink based conductive wires and polyimide film substrate provides for a more flexible apparatus which can be applied to a variety of composite geometries and can be both replaceable and repairable. It is noted herein that while the above describes various examples, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which maybe made without departing from the scope of the present invention as defined in the appended claims.

A polyimide is a polymer of imide monomers. An imide is a functional group consisting of two acyl groups bound to nitrogen. Polyimide materials are lightweight, flexible, resistant to heat and chemicals, and therefore are useful in a diverse range of applications. For example, polyimide films are often used for insulation and passivation purposes. However, it will be understood that the polyimide film described above may be replaced with any other flexible film having the desired thickness and/or physical

characteristics. For example, the flexible film may be formed of polyamide-imide or any other suitable polymer. In a further aspect, there is provided: an apparatus for monitoring physical characteristics of a component, the apparatus comprising: a flexible film; at least one transducer provided on the flexible film; and for each transducer, at least one flexible, ink based, conductive wire on the flexible film for providing power and/or communication connectivity to the transducer.