sensor

Abstract

There is disclosed a sensor for detecting an overload condition, comprising: at least one sensing layer including a brittle material; at least insulating layer affixed to a side of the one or more sensing layer, wherein a characteristic of the sensing layer is dependent upon a strain applied to the sensor.

Description

[0001] SENSOR

[0002] Field

[0003] There is provided a sensor for monitoring an overload condition in a structure, which overload condition may be caused by strain, and which may cause damage to the structure if the strain goes beyond a certain level.

[0004] Background

[0005] Composite materials, or structures formed from composite or other materials, are used in many industries.

[0006] Polymer composites, for example, have complex laminate structures. Detecting damage in the laminate structure is challenging. Flaws, ranging from micro-transverse cracks to catastrophic delamination, may be difficult to detect by visual inspection, and may require the use of sophisticated scanning techniques that are slow, expensive to operate, and inefficient for a structure while it is in operation.

[0007] An objective is to provide a sensor, which addresses the problem of the prior art. Summary

[0008] There is disclosed a sensor for detecting an overload condition, comprising: at least one sensing layer including a brittle material; at least insulating layer affixed to a side of the one or more sensing layer, wherein a characteristic of the sensing layer is dependent upon a strain applied to the sensor.

[0009] The at least one sensing layer may include brittle material includes a plurality of conductive paths, each formed of brittle material. The at least one sensing layer may be a carbon fibre layer, the plurality of conductive paths formed of carbon fibres. The at least one sensing layer may be an elongated layer, in which current flows in a direction of a length of the sensing layer. The elongated layer may be one of an irregular shape, a linear shape, a serpentine shape, a helical shape, or an S-shape. The sensing layer may be an elongated layer retained in a planar layer, the at least one pair of insulating layers being planar layers. The elongated layer may be a helical shape, and the at least one pair of insulating layers are cylindrical. The at least one sensing layer may be a planar layer, in which current flows in a direction of a length and a width of the sensing layer.The characteristic may change when an overload condition is applied to the sensor. At least one of the conductive paths may be broken when an overload condition is applied to the sensor.

[0010] The characteristic may be an electrical characteristic. The characteristic may be an electric resistance. The electric characteristic may be dependent upon measured current flow in the one or more sensing layers. The electrical characteristic may be measured resistance in the one or more sensing layers.

[0011] The at least one sensing layer may be provided with at least two electrodes, the characteristic of the at least one sensing layer being detected by taking a measurement at the at least two electrodes. The at least one sensing layer may be provided with at least four electrodes, the characteristic of the at least one sensing layer being detected by taking a measurement at the at least four electrodes. The planar carbon fibre layer may be of a rectangular shape, and the at least four electrodes are provided one at each corner.

[0012] The characteristic may be a heat characteristic. A heat generated by the at least one sensing layer generates heat. The heat generated by the at least one sensing may change when an overload condition is applied to the sensor.

[0013] The characteristic may be a visual characteristic.

[0014] The at least one sensing layer may comprise a tape or tow of a carbon fibre material. The at least one pair of insulating layers may have a greater strain to failure characteristic than the at least one sensing layer, such that the insulating layers will not fail when an applied strain causes the sensing layer to fail in part.

[0015] The sensor may comprise at least two sensing layers, a thickness and / or a configuration of the at least two sensing layers being different.

[0016] The at least one sensing layer may generate heat, and the generated heat is used for a further purpose. The further purpose may be to heat an apparatus to which the sensor is mounted.

[0017] The position of the overload may be additionally sensed.

[0018] A heat map may be generated, the heat map indicating the location of the overload.The one of a plurality of sensing layers which detects the overload condition may be indicative of the location of the overload.

[0019] A sensing layer of each sensor may be configured to exhibit a change in the characteristic of that sensing layer in dependence on a different level of strain applied to the sensor.

[0020] The two or more sensors may be elongated sensors disposed in parallel.

[0021] The two or more sensors may be disposed in a single tape.

[0022] The sensing layers of the two or more sensors may be the same layer.

[0023] A sensing arrangement may include at least two sensors, a sensing layer of each sensor being substantially orthogonal, wherein a characteristic between orthogonal sensing layers is measured at a location at which the sensing layers crossover. The characteristic may be capacitance.

[0024] The characteristic may provide an indication of the location of a strain characteristic identified in one of the orthogonal sensor layers, the orthogonal sensors layers being elongated sensor layers.

[0025] There may be provided a plurality of sensing layers orthogonally disposed with respect to a further plurality of sensor layers, a characteristic between the respective orthogonal sensors layer being measured at any node where there is a crossover of sensing layers for the respective pluralities.

[0026] A sensing arrangement or sensors as described may be provided in any combination.

[0027] A structure may include one or more sensors as defined.

[0028] The sensor may comprise a pair of insulating layers affixed to either side of the one or more sensing layer.

[0029] The other side of the one or more insulating layer may be affixed to a surface of a structure under test.There is provided a method of forming a sensor, comprising: forming at least one brittle sensing layer; and forming at least one pair of insulating layers either side of the at least one brittle sensing layer.

[0030] The method may further comprise gluing the sensor to a structure for overload testing of the structure.

[0031] The method may further comprise forming the structure in combination with forming the sensor.

[0032] There is provided a method of operating a sensor, the sensor comprising at least one sensing layer including a brittle material, and at least one pair of insulating layers affixed either side of the one or more sensing layer, wherein a characteristic of the sensing layer is dependent upon a strain applied to the sensor, the method comprising: applying an electrical current to the sensing layer and monitoring the characteristic to detect an overload condition.

[0033] The characteristic may be an electrical characteristic, the method comprising monitoring the electrical characteristic.

[0034] The method may comprise monitoring an electric resistance.

[0035] The characteristic may be a heat characteristic, the method comprising monitoring the heat characteristic.

[0036] The method may comprise monitoring a heat emitted by the sensor.

[0037] Description of Figures

[0038] For a better understanding, and by way of example only, reference is made to the accompanying drawings in which:

[0039] FIG. 1(a) illustrates an exemplary sensor;

[0040] FIG. 1(b) illustrates an exemplary electrical resistance versus strain plot before an overload condition for the sensor of FIG. 1(A);

[0041] FIG. 2(a) illustrates an exemplary embodiment ofa sensorafter an overload condition; FIG. 2(b) illustrates an exemplary electrical resistance versus strain plot after an overload condition for the sensor of FIG. 2(a);

[0042] FIG. 2(c) illustrates an exemplary electrical resistance versus axial strain plots for loading and unloading strain for a sensor according to FIG.1(a);FIG. 2(d) illustrates multiple exemplary electrical resistance versus axial strain plots for loading and unloading strain for a sensor according to FIG.1(a);

[0043] FIG. 3(a) illustrates a first example embodiment of the sensor of FIG. 1(a);

[0044] FIGS. 3(b) and 3(c) illustrate performance of the first example embodiment of the sensor according to FIG. 3(A) before overload and after overload respectively;

[0045] FIG. 4 illustrates a second example embodiment of the sensor of FIG. 1(A);

[0046] FIG. 5 illustrates a third example embodiment of the sensor of FIG. 1(a) in an example implementation;

[0047] FIGS. 6(a) to 6(c) illustrate example embodiments of a multi-layered senor;

[0048] FIG. 7 illustrates an example test circuit using the exemplary sensor of FIG. 1(a); FIG. 8 illustrates an exemplary method of detection using the exemplary sensor; FIGS. 9(a) to 9(c) illustrate an example implementation of multiple substantially parallel example elongated sensors on a tape;

[0049] FIGS. 10(a) to 10(d) illustrate an example implementation of at least two orthogonally disposed example elongated sensors; and

[0050] FIG. 11 illustrates an example test circuit using exemplary sensors for determining at strain and capacitance characteristics.

[0051] Description of Preferred Embodiments

[0052] The sensor is now described with reference to examples, embodiments and variations. The sensor is for sensing an overload and may be referred to as an overload sensor. The sensor, as will be described, is formed of multiple different layers, and may also be referred to as a laminate sensor, a composite sensor or a hybrid sensor. The sensor may be referred to by any combination of these names.

[0053] In the following discussion of examples, the exemplary sensor comprises a carbon fiber layer as a sensing layer. This is for purposes of describing examples. In general the sensing layer is a layer of brittle material. A carbon fibre material is an example of a brittle material. A brittle material is a material which is hard, but is liable to break easily.

[0054] The sensing layer may include other material in addition to the brittle material.With reference to FIG. 1(a), an exemplary sensor 10 may comprise a hybrid laminate structure of a carbon fibre layer 4 positioned between two insulator layers 2 and 6, such as glass layers. A characteristic of the carbon fibre layer 4 changes when the carbon fibre layer is overloaded, and the stress at which the carbon fibre layer is overloaded is regulated, at least in part, by the two insulator layers. The characteristic which changes may be an electrical characteristic. The electrical characteristic may be a piezoresistive characteristic of the carbon fibre layer.

[0055] It should be noted that while the sensor may comprise a sensing layer positioned between two insulator layers, in alternative arrangements an insulator layer may be provided on only one side of the sensing layer. The sensing layer may then be affixed, on the other side, to a surface of the structure under test, that surface acting as an insulator layer for the sensor.

[0056] Electrical current in the carbon fibres of the carbon fibre layer is determined by the thickness and width of the carbon fibre layer, which may determine the number (or volume) of carbon fibres through which current conducts. An electrical characteristic of the carbon fibre layer changes when electrical current in the carbon fibres of the carbon fibre layer is restricted through fragmentation of these individual fibres, reducing the electrical current in the carbon fibres of the carbon fibre layer.

[0057] It can be noted that the carbon fibre layer 10 is actually formed of a plurality of carbon fibres. For the sake of convenience, the layer is referred to as a carbon fibre layer, rather than a layer of a plurality of carbon fibres. It should be understood that reference to a carbon fibre layer is reference to a plurality of carbon fibres.

[0058] The layers of the sensor 10 can be bonded while the carbon layer 4 is manufactured, i.e. during a "co-curing" technique, as both the insulator layers 2,6 and the carbon fibre layer 4 are cured together. Alternatively, the layers can simply be glued together separately after each layer is manufactured individually.

[0059] With reference to FIGS. 1(a) and (b) and 2(a) to 2(d), there is illustrated the concept of the change to a characteristic of the example sensor 10, in the example an electrical characteristic of the piezo-resistivity of the example sensor 10, before and after overload.

[0060] The sensor 10 is not piezoresistive if it is not overloaded. The electrical resistance of the carbon fibre layer 4 stays at a relatively constant level at different strain values applied tothe hybrid laminate sensor 10 if it is not overloaded. Strictly speaking, the resistance changes a small amount due to weak individual carbon fibres breaking at low strains, but this is not represented in the graph of Figs. 2(b) to 2(d) (discussed below) given its insignificance.

[0061] FIG. 1(b) illustrates a plot 21 of electrical resistance 20 against strain 24 for the sensor 10 of Fig. 1(a). As illustrated, as the applied strain increases, the electrical resistance of the carbon fibre layer 4 remains constant. The carbon fibre layer 4, and the sensor 10, does not exhibit piezoelectric behaviour.

[0062] However, if the carbon fibre layer 4 is overloaded, it becomes piezoresistive, so change of strain causes change of electrical resistance. With reference to FIG. 2(a) there is shown the sensor 10, with the carbon fibre layer 4 overloaded. As illustrated, the carbon fibre layer 4 has areas of fragmentation (but not complete fragmentation), denoted by reference numeral 12.

[0063] FIG. 2(b) illustrates a plot 23 of electrical resistance 20 against strain 24 for the sensor 10 in such an overload condition. As illustrated, as the applied strain increases, the electrical resistance of the carbon fibre layer increases. The carbon fibre layer 4, and the sensor 10, now exhibits piezoelectric behaviour.

[0064] The difference between the two cases is distinctive, so the occurrence of overload in the load history of the structure can be detected. The level of plateau in the response is indicative of the extent of overload and damage.

[0065] As per FIG. 2(b), as the strain increases the electrical resistance increases. As the strain increases, individual fibres of the carbon layer fragment, so the effective cross-section of the carbon-layer through which electrical current passes (width and height) reduces. As will be discussed with respect to embodiments later, it may be advantageous to limit the number of carbon fibres in the sensing layer, in order to avoid the effect of fragmentation being less noticeable, due to the electrical current being able to bypass individual fragmented fibres via a large volume of non-fragmented fibres around them.

[0066] Referring again to FIG. 1(A), the hybrid laminate sensor 10 may be made from sandwiching thin-ply carbon fibre 4 between insulating layers 2 and 6, such as glass fibre layers. The carbon fibre layer 4 is electrically conductive, and the glass layers 2 and 6 are electrically insulating materials. If the applied overload is beyond the failure strain of thecarbon fibre layer 4 (which may also be referred to as the sensing layer), this will cause fragmentation of the carbon layer. Glass layers 2 and 4 have higher failure strain and will not develop damage when the carbon fibre layer 4 is overloaded, so they keep the fragmented carbon layer constrained within the hybrid laminate sensor 10. As per described examples, the constrained fragmented carbon layer, such as layer 4 in FIG. 2(a) positioned between the intact glass layers 2 and 6, generates piezo resistivity with high sensitivity to strain once overloaded.

[0067] If the sensor is not overloaded, the path of resistance versus strain will be the same when strain is applied and when it is removed. Thus for Fig. 1(a) for example, the resistance is approximately or substantially the same when strain is applied and removed. Similarly in Fig. 2(a), if the sensor has been overloaded as shown, then on further operation (provided a new overload is not applied) the path of resistance is the same when strain is applied versus when strain is removed.

[0068] With reference to FIGS. 2(c) and 2(d), there are illustrated two examples of experimental results on a test sensor, such as sensor 10 of FIG. 1(a).

[0069] FIG. 2(c) illustrates a plot of resistance (measured in Ohms) against axial strain (%) for a hybrid composite sensor 10 being loaded under tension beyond the failure strain of the carbon fibre layer 4. As illustrated by line 20 in FIG. 2(c), the electrical resistance of the hybrid composite stays constant until the carbon fibre layer 4 starts to fragment. After initiation of the carbon fibre layer fragmentation, the electrical resistance starts to increase significantly, from about 16Q to about 1000Q. When the sensor 10 is unloaded, the unloading path 22 is entirely different from the loading one, which is a sign of a new overload having been applied to the sensor.

[0070] After the sensor has been altered by the overload as shown in Fig. 6, the next time a strain is applied the resistance will follow the path 22 during loading and unloading (provided a new overload is not applied).

[0071] If there is no damage to the carbon fibre layer when loading, then no new damage occurs to the carbon fibre layer when loading, and the electrical characteristics of the loading path and the unloading path will be the same.If there is damage to the carbon fibre layer when loading, then new damage occurs to the carbon fibre layer when loading, and then the electrical characteristic of the unloading path will be different to the electrical characteristic of the loading path. The structure is therefore overloaded if the sensor shows a variable resistance measurement while in operation.

[0072] FIG. 2(d) illustrates the response of the sensor 10 under four consecutive load cycles, slightly beyond the failure strain of the carbon fibres in the carbon fibre layer 4 in each load cycle. In each cycle the sensor is extended slightly more than the previous cycle, so a small number of new fragmentations are generated in each successive cycle.

[0073] A first strain plot 30 shows a first strain applied, which as it increases is associated with an approximately or substantially constant level of electric resistance. At an axial strain approaching 2%, a first overload condition occurs and the electric resistance increases due to fragmentation. As the first strain is removed, a first release plot 31 of the first overload shows the variation of the electric resistance. As overload has occurred, the constant electric resistance no longer occurs for this first release plot as the strain is removed.

[0074] A second strain plot 32 shows a second strain applied, which as it increases is associated with a level of electric resistance which was followed when the first level of strain was released after the first overload. This does not follow a constant electric resistance, as the sensor has been overloaded previously. As long as the sensor is not further overloaded, the relationship between electric resistance and strain will substantially follow the release plot 31 of the first strain. At an axial strain approaching 2% but greater than the peak of the first axial strain, a second overload condition occurs and the electric resistance increases due to further fragmentation. As the second strain is removed, plot 33 shows the variation of the electric resistance. As a second overload condition has been applied, not only is the constant electric resistance no longer occurring, but the second release plot 33 does not follow the second strain plot 32, because of a new overload.

[0075] A third strain plot 34 shows a third strain applied, which as it increases is associated with a level of electric resistance which was followed when the second level of strain was released after the second overload. This does not follow a constant electric resistance, as the sensor has been overloaded previously. As long as the sensor is not further overloaded, the relationship between electric resistance and strain will substantially follow the release plot33 of the second strain. At an axial strain approaching 2% but greater than the peak of the second axial strain, a third overload condition occurs and the electric resistance increases due to further fragmentation. As the third strain is removed, plot 35 shows the variation of the electric resistance. As a third overload condition has been applied, not only is the constant electric resistance no longer occurring, but the third release plot 35 does not follow the third strain plot 32, because of a new overload.

[0076] A fourth strain plot 36 shows a fourth strain applied, which as it increases is associated with a level of electric resistance which was followed when the third level of strain was released after the third overload. This does not follow a constant electric resistance, as the sensor has been overloaded previously. As long as the sensor is not further overloaded, the relationship between electric resistance and strain will substantially follow the release plot 35 of the third strain. At an axial strain approaching 2% but greater than the peak of the third axial strain, a fourth overload condition occurs and the electric resistance increases due to further fragmentation. As the fourth strain is removed, plot 37 shows the variation of the electric resistance. As a fourth overload condition has been applied, not only is the constant electric resistance no longer occurring, but the fourth release plot 37 does not follow the fourth strain plot 34, because of a new overload.

[0077] A subsequent strain applied will follow the behaviour of the plot 34, unless a higher overload occurs again.

[0078] The unloading / reloading plots of the consecutive cycles match, indicating that the piezoresistive response is repeatable and reversible as long as there is no further fragmentation. However, when new fragmentations are generated at the end of each loading, the electrical resistance has a significant sudden increase in the electrical resistance.

[0079] The detection does not require identification of how many overloads have taken place. The piezo resistivity is due to damage in the sensing layer. New damage simply makes the sensor more piezoresistive.

[0080] Several hybrid composite configurations have been experimentally tested, and similar behaviour has been observed for different loading scenarios, e.g. tension, bending and indentation.With reference to Figs. 3(a) to 3(c), 4 and 5, example sensor implementations are illustrated.

[0081] With reference to FIG. 3(a), there is illustrated a first exemplary implementation 41 of the sensor 10. This first exemplary implementation may be referred to as a serpentine configuration.

[0082] In this first exemplary implementation, the sensor 41 comprises a sensing carbon fibre layer 40 positioned between two insulating layers 42 and 44, such as glass layers..

[0083] The sensing carbon fibre layer 40 is a serpentine shape, to provide an elongated trace which is routed around the available plane between the insulating layers 42 and 44. Each end of the carbon fibre layer is provided with an electrode, denoted 46a and 46b.

[0084] With reference to FIGS. 3(b) and 3(c) there is illustrated operation of the hybrid laminate sensor 41 of FIG. 3(a). An Ohm meter 48 is connected between the electrodes 46a, 46b. One-dimensional electrical current flows in carbon fibre layer 40 between the electrodes, along the elongated length of the sensor layer 40.

[0085] In Fig. 3(b) the hybrid laminate sensor 41 of FIG. 3(a) is not overloaded, and as current flows the Ohm meter 48 will reflect that the carbon fibre layer is exhibiting a steady piezoelectric resistance characteristic. Reference numeral 50 denotes the one-dimensional current flow through the elongate carbon fibre layer.

[0086] With reference to Fig. 3(c), due to a strain applied to the sensor 41 of Fig. 3(a) the carbon fibre layer 40 is damaged, and becomes fractured, as denoted by broken connection 54. The Ohm meter 48 will now exhibit a varying piezoelectric resistance of the fibre layer 40. Electrical resistance will depend on the damage status and strain, as exemplified above with reference to FIG. 2(b). Reference numeral 52 denotes the one-dimensional current flow through the elongate carbon fibre layer, which is reduced compared to the current flow 50 due to the fragmentation 54.

[0087] Note in Fig. 3(c) the fragmentation 54 shows the carbon fibre layer as fully broken. This is for illustrative purposes, to emphasize the fragmentation. In practice, the location at which the carbon fibres break may not be exactly at the same plane and therefore, some of the carbon fibres may stay unbroken at such plane. However, existence of discontinuity in the broken carbon fibres means that the electrical resistance changes under variable strainElectrical current along the elongated length of the carbon fibre layer is restricted through the thickness and width of the serpentine-shaped carbon fibre layer. In-plane, onedimensional (ID) electrical current is preferably produced, by which it is meant that current flows in one direction across the length of the carbon fibre layer, through individual carbon fibre strand. As the strands become fragmented, due to overload, the number of carbon fibres though which one-dimensional current flows reduce, changing the electric resistance.

[0088] The sensing carbon layer will be fragmented if an overload occurs, as shown in Figure 3(c). This causes fragmented carbon fibre ends sandwiched between the insulating layers to have a resistive response or show sensitive electrical resistance to variable strain. This allows a determination of damage in the structure. A damaged sensing layer results in a varying resistive electrical response since the sensing layers are sandwiched between glass layers and are held in place.

[0089] The serpentine configuration, providing the carbon fibre layer sandwiched between two planar layers, may have applicability for implementations where the integrity of a planar structure is tested, with the sensor 41 being affixed to the planar structure.

[0090] With reference to FIG. 4, there is illustrated a second exemplary implementation 61 of a hybrid laminate sensor 10. This second exemplary implementation may be referred to as a helical configuration.

[0091] In this second exemplary implementation, a helical sensing carbon fibre layer 60 is positioned between two cylindrical insulating layers 62 and 64, such as cylindrical glass layers. The insulating and fibre layer composite provide the sensor 61.

[0092] The sensing carbon fibre layer 60 is a helical shape, to provide a trace which is routed around between the cylindrical insulating layers 62 and 64. Each end of the helical carbon fibre layer is provided with an electrode, denoted 66a and 66b. The insulating cylinders 62, 64 have different diameters, so that one may be placed inside the other, with the helicalshaped carbon fibre layer in-between.

[0093] It will be apparent to one skilled in the art that the carbon fibre layer 60 of the helical configuration operates in accordance with the same principles as the carbon fibre layer of the serpentine configuration described with reference to FIGS. 3(b) and 3(c).The helical configuration 61, providing the carbon fibre layer sandwiched between two cylinders, may have applicability for implementations where the integrity of a cylindrical structure is tested, such as a pipe, with the sensor 61 being positioned around the pipe, such as being slid along the pipe or manufactured around the pipe.

[0094] In the helical configuration 61a, the electrical current effectively flows in one dimension, confined by the shape of the carbon fibre layer 60, with the elongated helical layer 60 operating similarly to the elongated layer 40 of FIG. 3(a).

[0095] The use of an elongated carbon fibre layer 40 or 60 in each of the examples of FIG.

[0096] 3(a) or FIG. 4 which is formed of a strip-shape, is advantageous, When fragmented due to overload, the local increase in the electrical resistance caused due to the damage in the carbon layer cannot be by-passed, and so the piezoelectric behaviour may manifest more readily.

[0097] The serpentine and helical examples illustrate that the sensor and the fibre layer may be shaped in any way as necessary according to an implementation. For example the insulating layers of the serpentine arrangement may be curved rather than planar, to affix to a curved surface. The serpentine configuration may be laminated between curved insulating layers, and be curved itself.

[0098] Whilst the implementation of the carbon fibre layer as a strip as shown in the serpentine and helical arrangement may be advantageous, it may be implemented in other shapes. For example in the serpentine arrangement, the carbon fibre layer may instead be a planar carbon fibre layer, corresponding in planar size and shape to the insulating layers. Similarly in the helical configuration, the carbon fibre layer may be a cylindrical layer, positioned between the two insulating cylindrical layers. The carbon fibre layer may also be a straight strip of carbon fibre between two electrodes, having the same width and depth as the carbon fibre layers shown in the serpentine and helical configurations. Other shapes of the carbon fibre layer between two electrodes may be provided.

[0099] Where the carbon fibre layer is provided as a larger surface, it may still be appropriate to implement the layer with two electrodes, or more than two electrodes may be suitable for the implementation.With reference to FIG. 5, there is illustrated a third example embodiment. In this third example embodiment, two sensors 71 and 73 are affixed either side of a substrate 70. Each of the sensors 71 and 73 comprises a planar carbon fibre layer positioned between two planar insulating layers, which detail is not shown, but each sensor is generally consistent with the structure of Fig. 1(a).

[0100] In this example, four electrodes are provided, respectively at the four corners of each rectangular, planar carbon fibre sensing layer. As shown, the sensor 71 has electrodes 80a, 80b, 80c, 80d. Due to the planar configuration of this arrangement, electrical current flows in two-dimensions in each sensor, and each sensor configuration may be considered a two-dimensional sensor.

[0101] In general, one-dimensional current flow may be considered point-to-point or flowing in the direction of a length of an elongated conductor, and two-dimensional current flow may be considered as flowing in the direction of a length and width of a conductor. The conductor is placed between two insulators (as described above), and current is not considered to flow in a height dimension, across the height between the two insulator layers.

[0102] Each sensor may be considered a cell.

[0103] Each sensing layer of each sensor 71, 73 could typically be a uni-directional or a multidirectional composite e.g. a cross-ply [0 / 90] laminate.

[0104] A frame 74, 76 is respectively provided around the border of each sensor 71, 73 respectively, to retain the integrity of the laminated layers of each sensor. The sensors of Fig.

[0105] 5 may be referred to as full-plate sensing layers.

[0106] Electrical current in the sensing carbon fibres is restricted through the thickness and is in-plane, and a two-dimensional (2D) electrical current is produced.

[0107] The arrangement of FIG. 5 may be termed a unit-cell. An array of many unit-cells may be provided next to each other, with any arbitrary shape or arrangement of each unit-cell.

[0108] There has been described a sensor for sensing overload conditions, in broad terms with reference to FIGS. 1 and 2, and as applicable to a selection of examples with reference to FIGS. 3 to 5. Without limitation to any example, there is thus provided a sensor for sensing overloads on a composite structure, preferably with restricted one-dimensional carbon fibreconfigurations, with restricted electrical current in the thickness and in-plane directions, oriented along any direction along which strain due to overload is to be monitored, e.g. the first principal strains, variation of the sensor's pattern in places with more than a principal strain direction.

[0109] In implementations the sensor may comprise a plurality of sensing layers, and may comprise a plurality of pairs of insulating layers - or insulating layers interleaved with sensing layers- each provided either side of a sensing layer. Thus a single sensor may comprise multiple structures as shown in FIG. 1(a), layered on top of each other. In general, the sensor may comprise one or more sensing layers, and one or more respective insulating layers either side of each sensing layer.

[0110] This is further illustrated with respect to FIGs. 6(a) and 6(b).

[0111] FIG. 6(a) shows a sensor 110 formed of three sensing layers 114a, 114b, 114c with four insulator layers 112a, 112b, 112c, 112d in between, such that each carbon layer is sandwiched between two insulator layers, insulator layers being provided as inner and outer layers of the sensor 110.

[0112] Each sensing layer 114a, 114b, 114c may be driven independently by a current which is independently monitored, such that when there is an overload the one of the sensing layers which has fragmented is detected.

[0113] FIG. 6(b) shows two of the sensing layers 114a, 114b of the sensor 110, and the associated insulator layers 112a, 112b, 112c, 112d. In FIG. 6(b), the sensing layers are encapsulated by the insulating layers, such that the ends of the sensing layers are surrounded by insulating material, as denoted by insulator areas 116a, 116b, 116c, 116d.

[0114] In addition, the top and bottom of the structure is provided with grounds planes 118 and 120. A better electromagnetic (EM) field containment is provided by the arrangement of Fig. 6(b) with the ground planes.

[0115] As with Figs. 6(a), in the arrangement of Fig. 6(b) the sensing layers may be driven independently by a current which is independently monitored, such that when there is an overload the one of the sensing layers which has fragmented is detected.In each of FIGS. 6(a) and 6(b) the number of sensing layers shown is illustrative, and more (or less) sensing layers may be provided.

[0116] The sensing layers may not directly overlap. The lengths of the sensing layers may differ, such that in some regions along the length of the sensing layers, there may be more or less sensing layers. FIG. 6(c) shows a cross-section through an elongated section of the sensor of FIG. 6(b). As can be seen the sensing layers 114a and 114b overlap for a section, but there are also sections where only one of the sensing layers is present. This may help identify the location along the length of the sensor when an overload condition is detected by either sensing layer. If one sensing layer detects an overload condition but the other does not, then the overload damage can be identified as being associated with a portion of the sensor, and therefore a portion of an apparatus to which the sensor is attached, where that sensing layer is provided.

[0117] Where multiple sensors are provided, the thickness and / or configuration of the sensing layer may vary for each sensor. With reference to FIG. 7, there is illustrated an example in-use test set-up incorporating a sensor. The sensor of FIG. 1(A) is illustrated as having electrode connections 9 to the carbon fibre layer 4, and with a battery or power source 5 and a resistance meter 7 (such as an Ohm meter) connected in series with the carbon fibre layer 4.

[0118] The battery or power source 5 and resistance meter 7 are each connected to a controller 11, which may include a processor 27 and a memory 29, for controlling the battery or power source 5, and monitoring the resistance meter 7. The battery or power source 5 generates an electrical current which pushes through the carbon fibre sensing layer 4, and the resistance meter monitors the electric resistance provided by the carbon fibre sensing layer 4. This is an example configuration, and other configurations may be utilised.

[0119] As will be discussed further hereinbelow, the sensor may provide a dual-function, i.e. an overload sensing function as described, and a further function.

[0120] FIG. 8 illustrates an exemplary method utilising the sensor.

[0121] In a step 100, a determination is made as to whether the electrical resistance of the sensor is changing over time, and for this purpose a threshold is set for the determination. The threshold may be 10%, i.e. whether the electrical resistance changes by 10%.If the electric resistance has not changed by 10%, then it is determined in step 102 that the sensor - and the structure which it is sensing - has been overloaded.

[0122] If the electric resistance has changed less than 10%, then in a step 104 it is determined whether the sensor is under variable load. Step 54 is assessing if the object the sensor is attached to is under operation or not.

[0123] If the sensor is under variable load, then in a step 106 it is determined that the structure is healthy and not damaged.

[0124] If the sensor is not under variable load, then in a step 108 design loads are applied to the structure, and the method returns to step 50 to repeat.

[0125] If the object which the sensor is attached to is not in operation, e.g. a composite of a plane which is sitting and not flying, then the sensor reports back constant amplitude resistances. However, this does not mean there is no damage in the structure as the sensor is not under variable load. The way to determine if the sensor is under variable load is either by use of typical strain gauges or other similar methods, or simply by the operator informing the system that the sensor is in operation.

[0126] In step 108, the process simply takes the structure to an operational loading scenario because unless the sensor is in such loading, the outcome from the sensor is not reliable.

[0127] The sensor is a self-sensing hybrid composite, made out of insulation / carbon fibre / insulation laminates. Insulating layers may be glass fibre layers, which are electrically insulating materials. The insulating layers preferably separate the carbon fibre of the sensing layer from a substrate the sensor is bonded to.

[0128] The sensor can have different forms, e.g. serpentine, helical or 2D cross-ply laminate according to the described example embodiments, but are not limited to these example embodiments.

[0129] In example implementations, the carbon fibre layer is formed of tapes or tows of carbon fibres, consistent with the serpentine and helical elongated configurations. This means that a large area can be covered with tapes with gaps between them. This reduces the number of required electrodes. The direction of the tapes may be along the direction along which it is important to monitor strain, e.g. maximum strain, at a typical design loading scenario. Thewidth of the tapes are preferably smaller than a critical crack size that is needed to be detected. If the tape is too wide, the change of electrical resistance will be significantly smaller. Thus tape configurations may be used on flat panels such as on s-shape or serpentine configurations, as well as on tubes.

[0130] An advantageous deployment of the sensor layer is as part of a tape. An elongated sensor layer can be provided as part of a tape, and the tape can be deployed in any desired implementation. As will be discussed below in respect of an improvement, a tape may provide multiple, elongated, parallel sensor layers.

[0131] When deployed in a tape implementation, the sensor layer can be used in automated composite manufacturing. The tape can be formed on a composite structure during manufacturing. This may be a robotic manufacturing process, which makes composite wraps.

[0132] Other sensors may be provided with the sensor layer, monitoring other parameters such as temperature. When deployed in a tape implementation, the tape may be equipped with such additional sensors, for example temperature sensors.

[0133] A thickness range of the carbon fibre layer 4 may be below 0.025mm, and preferably under 0.05mm.

[0134] In example implementations, the insulating layers are preferably as thin as possible without risking a short circuit of the sensing layer and a substrate to which the sensor is bonded. Having glass or other non-conductive fibres in the form of thin plies or preforms are preferred choices.

[0135] Athickness range of the insulating layers 6 may be O.lto 0.5mm, preferably to 0.2mm. In example implementations, the electrodes give low-contact stable connection between an external electrical source and the carbon fibres embedded in a matrix forming the carbon fibre sensing layer. A "co-curing" technique may be used to connect the electrodes, in which thin copper foils are positioned next to uncured conductive layer e.g. carbon fibre reinforced polymer (CFRP) and the foils are cured together. Perforated foils may also be used. Copper meshes may also be used. Other materials may be used.

[0136] The plots of FIGS. 5 are from tensile tests. The best results are achieved when the damaged carbon fibres undergo tension. Other loading scenarios are impact and indentation.In example implementations dry tows are used instead of cut prepregs. The cost of manufacture of the sensors can be lowered, and the manufacturing process may be automatised more easily. Rather than cutting prepregs and laying it down, templates can be made and dry carbon fibre tows placed there. Insulating layers can be applied on either side. Sensors can be made and sold separately, and do not have to be made within the structure that they are testing, but can be.

[0137] As noted above, the sensing layer including a carbon fibre material is an example, and in general the sensing layer includes a brittle material. In examples, the brittle material may be a carbon fibre material.

[0138] The hybrid composite sensor provides significant benefits, including:

[0139] • Through-thickness electrical insulation of sensing carbon fibre composites to inhibit electrical current through the thickness around the fragmented carbon fibres.

[0140] • In-plane ID patterns e.g. serpentine or helical configurations to constrain the electrical current in one-dimension to minimise the use of electrodes required.

[0141] • Piezo-resistivity of the hybrids and dependence of electrical resistance on both damage and strain.

[0142] It is proposed to add a sensor, made out of a sandwich of insulation / sensing carbon / insulation to any substrate, rather than trying to measure the change of electrical resistance of carbon fibre composite substrates themselves. A technical outcome is that electrical resistance is significantly magnified, from a maximum of 5-20% to 2-3 orders of magnitude difference. This significant change of resistance means that it is possible to detect damage in practice using this method using inexpensive electronics. Also, it is possible to apply the sensor to any other material substrate, e.g. glass fibre composites or others.

[0143] Damage can be remotely detected based on a change of electrical resistance. The technique can be applied to painted structures and structures not accessible visually.

[0144] The hybrid overload sensors can be used for damage detection in three ways:

[0145] • Placed at critical locations, on or inside composite laminates, where damage is more likely to occur.

[0146] • Cover the whole structure so any external overload can be detected.

[0147] • Used as a "self-sensing" material to build the entire structure.The sensory technique may be manufactured using commercially available carbon and glass fibre composite layers, so their exploitation does not require investment in raw material production.

[0148] There is provided a composite structure health monitoring technique. Forecasted applications range from energy transfer composite pipes, and pressurised storage vessels to continuously growing structural components in the aerospace and automotive sectors. In-service monitoring enhances reliability, safety of the current products and help designing significantly higher performance products by avoiding over-designed structures.

[0149] A reliable monitoring technique alleviates composite inspection difficulties for several industrial sectors. Significant benefits are achieved, including reduced operational costs, enhanced safety, less inspection downtime, and more optimal structural designs.

[0150] It will improve sustainability by allowing a more efficient use of materials, whilst producing safer and more environmentally friendly products at lower costs.

[0151] The sensor proposed may be used in multiple implementations, in industries and applications including:

[0152] • oil and gas pipes, and gas cylinders.

[0153] • automotive and wind energy.

[0154] • defence and aerospace.

[0155] • in-service structural health monitoring, such as X-ray CT, ultrasound, or Eddy current etc.

[0156] A sensor may be manufactured at the same time as a substrate of the structure to be tested is manufactured (co-curing), so as to effectively be factory fitted.

[0157] A sensor may be retrofitted or bonded as an additional item to an already manufactured substrate using glue.

[0158] The sensor can provide a dual-function, providing the overload sensing as described, and in addition providing a second function within an apparatus in which it is installed. For example, the carbon fibre layer may be connected to electrical power to generate heat, and can be used for a wide range of applications, e.g. deterrent of bio-fouling in air or in water environment, de-icing or heating of the apparatus to which it is connected.

[0159] The second function may be to use the carbon fibre for heat generation.Temperature changes may be detected by pulses of heat generated by the carbon layer, to distinguish a damaged sensor from an undamaged one. Referring to FIG. 7, the thermal imager 13 may detect generated heat denoted by reference numeral 15, and is connected to the controller 11. Thermography may thus be used to determine a damaged (overloaded) sensor. This may be in addition to, or instead of, the sensing od change in an electrical characteristic using the resistance meter 7.

[0160] Thus a change to resistance, or a change to emitted temperature, may be used individually or together to indicate a change to a characteristic of the sensing layer, which indicates an overload status.

[0161] In arrangements, the sensor is not only able to sense overload, but is able to sense the location of the overload.

[0162] In a first arrangement, where overload is sensed using a change to a thermal characteristic, the location of the thermal characteristic can be sensed based on the heat map generated, which shows the location of changed heat. Hence location is identified.

[0163] In a second arrangement, where overload is sensed with a sensor having multiple sensing layers, than the location can be determined based on which of the these layers has overloaded -the overload must be in a location where the layer is present. The multiple layers may be important, because a sensing terminal can be placed at the same end of the sensor, such that two layers are then needed.

[0164] Based on electric transmission line theory, the transmissions lines can be long or short. The characteristic length of the line is measured in wavelengths of the transmitted electric signal, so the same line (the same physical length) can be both short and long for different frequencies of the electric signal.

[0165] In a third arrangement, transmission line theory and AC current with different frequencies may be used to determine location. The equivalent circuit properties of a set of sensors (two sensing layers separated by an insulating layer) can be determined. Equivalent circuit properties may include real and imaginary values of the impedance, e.g. using a Nyquist plot. In a simple example, two carbon fibre tapes separated by an insulating layer may form a capacitor, and the properties of the capacitor depend on the length of the tapes. If the tapes are broken, then the capacity of the capacitance will reduce. In this way, it is possible toidentify a location of damage. This third arrangement may use low frequency, e.g. 100 Hz. A typical sensor with a 1 km length will be actually short for such signals and changes in the capacitance of the sensor can be sensed if one of the layers is broken.

[0166] In a fourth arrangement, reflectometry may be used to locate damage. Reflectometry is based on sending a signal to one of the sensing layers while the other one is grounded. The length of the sensor affects the resonating frequency of the reflected signal when an AC current is applied. By finding the resonating frequency, it is possible to estimate the length of the sensor. This fourth arrangement uses traditional time domain reflectometry, working by sending electric pulses (high frequency signals) to the sensor and recording the returned (reflected) signal.

[0167] In general the described sensor is for detecting an overload condition where a characteristic of a sensing layer is dependent upon a strain applied to the sensor. That characteristic may be an electrical characteristic, a thermal characteristic, or a visual characteristic.

[0168] For example, where the overload sensor is attached to a structure without a layer on the sensor, or with a transparent layer on the sensor, fragmentation and damage to the sensing layer (e.g. carbon fibre tapes) may be detectable with the naked eye. An example implementation of a sensor without a layer on the sensor is where the overload sensor is attached to an outer surface of a pipe. A visual characteristic is thus dependent on the strain applied to the sensor.

[0169] One or more of these characteristics may be detected at any instant.

[0170] Identifying the location of damage identifies the location of failure.

[0171] In a further improvement, a plurality of sensor layers may be provided together, monitoring a same area of a structure or proximate positions within a common area of a structure, to provide an improvement. With reference to FIG. 9, there is illustrated an example of this improvement in which multiple sensor layers are provided.

[0172] In this example an elongated tape includes three elongated sensor layers 126, 128 and 130. Three is an example number, and in general two or more sensor layers may be provided in this improvement. The tape comprises an upper tape surface 122 and a lower tape surface 124. The three elongated sensor layers 126, 128 and 130 are laminated between the upperand lower tape surfaces 122 and 124, and then the laminated tape structure may be deployed in any desired application. The tape can be fixed to any structure to be tested.

[0173] According to this improvement, the elongated sensor layers are each configured to have a different tolerance. This permits, when monitored, each sensor layer to signify a different degree of failure. For example each sensor fibre may have a different percentage rating for a stress failure.

[0174] Fig. 9(a) shows a perspective view of the three elongated sensor layers during formation of a laminated tape, with the upper and lower layers 122, 124. Fig. 9(b) shows a cross-section of the laminated tape. Fig. 9(c) shows a top-down view of the laminated tape.

[0175] The deployment of sensor layers according to this improvement is not limited to deployment using a tape, and provides two or more sensor layers attached to a structure in relatively close proximity, the sensor layers having different strain characteristics to be able to sense different levels of strain.

[0176] In this example the multiple sensor layers are illustrated as provided in a same substrate, being parallel within the plane of the substrate. In alternative, the multiple layers may be provided in different layers of a substrate.

[0177] In a still further improvement two sensor layers may be overlaid in order to provide additional sensing. In addition to an individual sensor layer providing strain sensing, two overlaid sensor layers may provide additional sensing. This improvement is described with reference to Fig. 10.

[0178] Fig. 10(a) illustrates two sensor layers 134 and 136, with one overlaying the other, and disposed in a substantially orthogonal relationship relative to each other. Each sensor layer may be configured to sense strain, as disclosed herein.

[0179] Fig. 10(b) illustrates a cross-section through A-A in Fig. 10(a), being a cross section at the locations of the intersection of the respective orthogonal sensor layers 134 and 135. Although not shown in Fig. 10(a) for simplicity, in Fig. 10(b) it can be seen that the respective sensor layers 134 and 136 are encapsulated within respective substrates 138 and 142, with an additional intervening substrate layer 140 being provide between these overlaid substrates 138 and 142. The substrates 138 and 142 may be placed on top of each otherwithout the intervening substrate layer 140, the requirement being to ensure insulation between the overlaid layers.

[0180] As can be further seen from Fig. 10(b), in the cross-section a portion of sensor layer 134 overlies a portion of sensor layer 136. It will be appreciated that in other cross-sections of the overlaid structure, the two substantially orthogonal sensor layers will not have an overlaid relationship.

[0181] At the point where the two sensor layers cross, and therefore are overlaid, a capacitance value can be measured between the two sensor layers. This provide an additional measurement metric, which can be used to sense / measure another condition.

[0182] This is further illustrated in Fig. 1(c), where an equivalent circuit 144 shows a capacitor 146, representing the capacitance between the overlapping sensor layers. This equivalent circuit is completed by a battery or power cell 150 and a measurement instrument 148 for measuring the value of the capacitance. Any change in the relationship between the sensor layers in the overlapping area, will be detected by a change in the measured capacitance value.

[0183] Fig. 10(d) further extends this improvement to an arrangement where a plurality of substantially parallel sensor layers 152a, 152b, 152c are provided to overlay a plurality of substantially parallel sensor layers 154a, 154b, 154c. The plurality of sensor layers 152a, 152b, 152c are disposed substantially orthogonal to the plurality of sensor layers 154a, 154b, 154c, and the plurality of sensor layers 152a, 152b, 152c are provided in a layer above the plurality sensor layers 154a, 154b, 154c, as per Fig. 10(b).

[0184] As denoted in Fig. 10(d) there are a plurality of nodes 156a to 516i at which one sensor layer crosses an underlying sensor layer, with the sensor layers being substantially orthogonal. At each of these nodes a capacitance value can be measured, as per with Fig.

[0185] 10(c), and a characteristic measured in addition to strain.

[0186] With reference to Fig.11 there is illustrated exemplary circuitry for monitoring two characteristics, expanding on Fig. 10. A sensor 160 includes a sensor layer 164, and is formed on top of a sensor 162 including a sensing layer 166. The sensing layers 164 and 166 are orthogonal to each other, so sensing layer 164 is illustrated as extending across the page, and sensing layer 177 is illustrated as extending into the page.Sensor contacts 172 and 174 connect to respective end points of the sensor layer 164, and connect the sensor layer, in series, with a power source 180 and a resistance meter 182. In this way, a strain characteristics can be measured as described above.

[0187] Sensors contacts 168 and 170 are positioned on the exposed surface of the substrates 160 and 162, aligned with the crossover points of the respective substantially orthogonal sensor layers 164, 166. The sensor contacts are connected in series with a power source 176 and a capacitance meter 178. In this way, a capacitance characteristic can be measured as described above.

[0188] In this improvement, a strain characteristic and a capacitance characteristic can be measured by two fibre sensors provided in a substantially orthogonal overlapping relationship.

[0189] The measured capacitance may provide an indication of the behaviour between layers, and any change in the capacitance value indicates a change in the behaviour between payers.

[0190] An example use of the measured capacitance is to assist in determining location of strain damage in a sensor layer, such as an engaged sensor layer. The sensor layer itself may provide an indication of strain damage. Measuring the capacitance may then provide an indication of where the strain damage has occurred on the fibre. For example, a change in capacitance at an individual node in Fig 10(d), together with an indication of strain on a particular fibre, may provide an indication that the strain has occurred in the vicinity of that changed capacitance. A location of strain damage may, for example, be indicated by comparing a ratio of a capacitance between adjacent nodes, with a variation in the ratio indicating a location of strain damage.

[0191] In any described improvement, as in any described example, the sensor layer may be a fibre.

[0192] The above has set out various examples. One skilled in the art will appreciate that various details of the examples may be omitted without deviation from the invention. One skilled in the art will appreciate that various details of the examples may be combined without deviation from the invention.The above examples set out examples of sensors, sensor arrangements, systems incorporating sensors and / or sensor arrangements, methods of sensing, methods of sensing using sensors, sensors arrangements and / r sensor systems, and methods of controlling sensors, sensors arrangements and / or sensor systems.

[0193] The term "substantially" were used herein may be considered to mean that a feature is within a tolerance of an absolute value.

[0194] In addition to any modifications explicitly mentioned above, it will be evident to a person skilled in the art that various other modifications of the described embodiment may be made withing the scope of the invention.

[0195] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features.

[0196] In general, the foregoing description has set out examples, embodiment and modification. Aspects of any example, embodiment and modification may be combined individually or in combination.

Claims

CLAIMS1. A sensor for detecting an overload condition, comprising:at least one sensing layer including a brittle material;at least insulating layer affixed to a side of the one or more sensing layer, wherein a characteristic of the sensing layer is dependent upon a strain applied to the sensor.

2. A sensor according to claim 1, wherein the at least one sensing layer including brittle material includes a plurality of conductive paths, each formed of brittle material.

3. A sensor according to claim 2, wherein the at least one sensing layer is a carbon fibre layer, the plurality of conductive paths formed of carbon fibres.

4. A sensor according to any one of claims 1 to 3 wherein the at least one sensing layer is an elongated layer, in which current flows in a direction of a length of the sensing layer.

5. A sensor according to claim 4 wherein the elongated layer is one of an irregular shape, a linear shape, a serpentine shape, a helical shape, or an S-shape.

6. A sensor according to any one of claims 1 to 5 wherein the sensing layer being an elongated layer is retained in a planar layer, the at least one pair of insulating layers being planar layers.

7. A sensor according to claim 5, wherein the elongated layer is a helical shape, and the at least one pair of insulating layers are cylindrical.

8. A sensor according to any one of claims 1 to 3 wherein the at least one sensing layer is a planar layer, in which current flows in a direction of a length and a width of the sensing layer.

9. A sensor according to any one of claims 1 to 8 wherein the characteristic changes when an overload condition is applied to the sensor.

10. A sensor according to claim 2 to 9 wherein at least one of the conductive paths is broken when an overload condition is applied to the sensor.

11. A sensor according to any one of claims 1 to 10, wherein the characteristic is an electrical characteristic.

12. A sensor according to claim 11, wherein the characteristic is electric resistance.

13. A sensor according to claim 11 or claim 12, wherein the electric characteristic is dependent upon measured current flow in the one or more sensing layers.

14. A sensor according to any one of claims 11 to 13 wherein the electrical characteristic is measured resistance in the one or more sensing layers.

15. The sensor of any one of claim 11 to 14 wherein the at least one sensing layer is provided with at least two electrodes, the characteristic of the at least one sensing layer being detected by taking a measurement at the at least two electrodes.

16. The sensor of any one of claim 11 to 14 when dependent on claim 8, wherein the at least one sensing layer is provided with at least four electrodes, the characteristic of the at least one sensing layer being detected by taking a measurement at the at least four electrodes.

17. The sensor of claim 16 wherein the planar carbon fibre layer is of a rectangular shape, and the at least four electrodes are provided one at each corner.

18. The sensor of any one of claims 1 to 8 wherein the characteristic is a heat characteristic.

19. The sensor of claim 14 wherein a heat generated by the at least one sensing layer generates heat.

20. The sensor of claim 18 or claim 19 wherein the heat generated by the at least one sensing changes when an overload condition is applied to the sensor.

21. The sensor of any one of claims 1 to 20 wherein the characteristic is a visual characteristic.

22. The sensor of any one of claims 1 to 21 wherein the at least one sensing layer comprises a tape or tow of a carbon fibre material.

23. The sensor of any one of claims 1 to 22 wherein the at least one pair of insulating layers have a greater strain to failure characteristic than the at least one sensing layer, such that the insulating layers will not fail when an applied strain causes the sensing layer to fail in part.

24. The sensor of any one of claims 1 to 23 comprising at least two sensing layers, a thickness and / or a configuration of the at 23 two sensing layers being different.

25. The sensor of any one of claims 1 to 24 wherein the at least one sensing layer generates heat, and the generated heat is used for a further purpose.

26. The sensor of claim 25, wherein the further purpose is to heat an apparatus to which the sensor is mounted.

27. The sensor of any one of claims 1 to 26, wherein the position of the overload is additionally sensed.

28. The sensor according to claim 27 when dependent on claims 18 to 26, wherein a heat map is generated, the heat map indicating the location of the overload.

29. The sensor according to claim 27, wherein the one of a plurality of sensing layers which detects the overload condition is indicative of the location of the overload.

30. A sensing arrangement including two or more sensors according to any preceding claim, a sensing layer of each sensor being configured to exhibit a change in the characteristic of that sensing layer in dependence on a different level of strain applied to the sensor.

31. The sensing arrangement of claim 30, in which the two or more sensors are elongated sensors disposed in parallel.

32. The sensing arrangement of claim 30 or claim 31 in which the two or more sensors are disposed in a single tape.

33. The sensing arrangement of any one of claims 30 to 32 in which the sensing layers of the two or more sensors are the same layer.

34. A sensing arrangement including at least two sensors according to any one of claims 1 to 29, a sensing layer of each sensor being substantially orthogonal, wherein a characteristic between orthogonal sensing layers is measured at a location at which the sensing layers crossover.

35. A sensing arrangement according to claim 34, in which the characteristic is capacitance.

36. A sensing arrangement according to claim 34 or claim 35 in which the characteristic provides an indication of the location of a strain characteristic identified in one of the orthogonal sensor layers, the orthogonal sensors layers being elongated sensor layers.

37. A sensing arrangement according to any one of claims 34 to 36 in which there is provided a plurality of sensing layers orthogonally disposed with respect to a further plurality of sensor layers, a characteristic between the respective orthogonal sensors layer being measured at any node where there is a crossover of sensing layers for the respective pluralities.

38. A sensing arrangement according to any one of claims 34 to 37 in combination with the sensing arrangement of any one of claims 30 to 33.

39. A structure including one or more sensors according to any preceding claim.

40. The sensor of any one of claims 1 to 30 wherein the sensor comprises a pair of insulating layers affixed to either side of the one or more sensing layer.

41. The sensor of any one of claims 1 to 31 wherein the other side of the one or more insulating layer is affixed to a surface of a structure under test.

42. A method of forming a sensor, comprising:forming at least one brittle sensing layer; andforming at least one pair of insulating layers either side of the at least one brittle sensing layer.

43. The method of claim 42, further comprising gluing the sensor to a structure for overload testing of the structure.

44. The method of claim 42 or claim 43, further comprising forming the structure in combination with forming the sensor.

45. A method of operating a sensor, the sensor comprising at least one sensing layer including a brittle material, and at least one pair of insulating layers affixed either side of the one or more sensing layer, wherein a characteristic of the sensing layer is dependent upon a strain applied to the sensor, the method comprising: applying an electrical current to the sensing layer and monitoring the characteristic to detect an overload condition.

46. The method according to claim 45, wherein the characteristic is an electrical characteristic, the method comprising monitoring the electrical characteristic.

47. The method according to claim 46, the method comprising monitoring an electric resistance.

48. The method according to claim 47, wherein the characteristic is a heat characteristic, the method comprising monitoring the heat characteristic.

49. The method according to claim 48, the method comprising monitoring a heat emitted by the sensor.31

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