Organic matrix laminated composite material with integrated damage detection

Abstract

The invention relates to an organic matrix laminated composite material (3) with integrated damage detection. The composite material (3) is formed from a stack of plies including at least: a probe ply (7), and a conductive ply (9) adjacent to the probe ply (7). The probe ply (7) comprises a substrate and an electrical circuit deposited thereon. The electrical circuit has at least two tracks (15) and at least two electrodes (17). The tracks (15) are electrically insulated from the substrate and each connected to a respective electrode (17). The electrical circuit is in contact with the conductive ply (9) only through the electrodes (17). The composite material (3) is characterized in that the conductive ply (9) covers the electrodes (17). [Fig. 1]

Description

Title of the invention: Organic matrix laminated composite material with integrated damage detection

[0001] The field of the invention relates to the detection of damage within an organic matrix laminated composite material.

[0002] A composite material is an assembly of at least two immiscible materials. Such an assembly makes it possible to give the composite material properties that the materials composing it, taken in isolation, do not possess.

[0003] A composite material is formed of at least a matrix and a reinforcement.

[0004] The matrix ensures the cohesion of the composite material and gives it the desired shape. The matrix surrounds the reinforcement and transfers mechanical stresses to it. The matrix can be organic, metallic, or ceramic. In the case of an organic matrix composite material, the matrix—then called the "resin"—can be thermosetting, for example, epoxy resin, polyester, or polyimide, or thermoplastic, for example, polypropylene (PP), polyamide (PA), or polyetheretherketone (PEEK).

[0005] The reinforcement forms the framework of the composite material and gives it most of its mechanical properties. The reinforcement is generally made of fibers such as carbon fibers, glass fibers, aramid fibers, or even plant fibers. The reinforcement may also include metallic fillers, carbon fillers, silica fillers, or polymer fillers.

[0006] Due to their characteristics, composite materials have applications in various industrial fields such as electronics, automotive, railway, aeronautics and space.

[0007] As an example, it is known in the industry to use a carbon / carbon composite - or C / C - formed of a graphite matrix reinforced with carbon fibers or a carbon fiber reinforced polymer (also known by the English acronym CFRP for "carbon fiber reinforced polymer").

[0008] It is known to layer plies of fibers – generally of similar shapes – to form the reinforcement of a composite material. The composite material made from such a reinforcement is said to be “laminated”.

[0009] Regardless of the field of application, a laminated composite material is susceptible to damage, for example, in the event of impact or exposure to thermal conditions that the laminated composite material is unable to absorb or withstand. The damage to the laminated composite material It generally manifests itself in the form of a crack, delamination, or even a break or rupture of fibers.

[0010] It is known to use a sensor embedded or inserted in the laminated composite material to detect the occurrence of any damage and thus monitor the structural health of the laminated composite material. Such a sensor makes it possible to measure an electrical quantity whose variations can reveal the presence of damage.

[0011] Chinese patent application CN 113752644 A describes an aircraft coating in the form of a laminated composite material in which a damage detection sensor is integrated.

[0012] This sensor comprises a flexible ply, a carbon nanotube film, a protective fiberglass ply, and an insulating fiberglass ply. The flexible ply is formed from a copper-clad polyimide substrate onto which an electrical circuit is etched. The electrical circuit has several electrodes around the periphery of the substrate, which together define a space for the carbon nanotube film. The protective fiberglass ply covers the carbon nanotube film and constitutes the upper ply not only of the sensor but also of the laminated composite material. Finally, the insulating fiberglass ply forms the base of the sensor and isolates the flexible ply and the carbon nanotube film from the other plies of the laminated composite material.

[0013] Such a sensor has several disadvantages.

[0014] First of all, the electrical contact between the electrodes and the carbon nanotube film is made with a conductive substance, which therefore represents a material cost and a production cost.

[0015] In addition, the sensor is formed of several layers - the flexible ply, the carbon nanotube film, the protective glass fiber ply and the insulating glass fiber ply - which increase the dimensions and mass of the laminated composite material, and this solely for the purpose of damage detection.

[0016] Furthermore, the sensor is positioned on the surface and is electrically isolated from the other plies. Consequently, the sensor cannot distinguish between damage that may affect plies of the laminated composite material other than those of the sensor and damage affecting only the sensor, such as abrasion or degradation caused by environmental conditions. Moreover, the detection performed by the sensor is only surface-based, since it concerns the carbon nanotube film and not the plies of the laminated composite material other than those of the sensor.

[0017] Finally, there is a high risk of delamination between the carbon nanotube film and, on the one hand, the flexible ply, and, on the other hand, the protective glass fiber ply.

[0018] The present invention improves the situation.

[0019] In this respect, the invention relates to an organic matrix laminated composite material with integrated damage detection. The composite material is formed from a stack of plies including at least: • a probe fold, and • a conductive fold adjacent to the probe fold.

[0020] The probe fold comprises a substrate and an electrical circuit deposited on the substrate. The electrical circuit has at least two tracks and at least two electrodes, the at least two tracks being electrically insulated from the substrate and each connected to a respective electrode of the at least two electrodes. The electrical circuit is in contact with the conductive fold only via the at least two electrodes.

[0021] The conductive fold covers at least two electrodes.

[0022] In one or more embodiments, the stacking of folds includes an additional conducting fold adjacent to the conducting fold.

[0023] In one or more embodiments, the stack of plies includes at least one additional conductive ply, and each electrode is surmounted by an elongated conductive piece, which elongated conductive piece extends from the electrode and pierces at least part of the stack of plies so as to be in contact with at least one additional conductive ply.

[0024] In one or more embodiments, the stack of folds further includes another conductive fold adjacent to the probe fold. The at least two electrodes are distributed on a first and a second face of the substrate, and the conductive fold and the other conductive fold each cover the electrodes of the at least two electrodes of the first and second faces respectively.

[0025] In one or more embodiments, the at least two tracks each have a respective free end and the free ends are brought together on a connection portion of the probe ply accessible from outside the ply stack.

[0026] Typically, the electrical resistance between the free end of one of the at least two tracks and the electrode to which the track is connected is less than 10 Q, and is preferably less than 1 Q.

[0027] In one or more embodiments, the electrical circuit covers less than half of the surface of the substrate.

[0028] In one or more embodiments, the electrical circuit is formed of a conductive ink, a conductive film precursor coating or a metal deposited by physical vapor deposition.

[0029] In one or more embodiments, the at least two tracks are electrically insulated from the conductive fold by an insulating film, an insulating ink or glass fibers.

[0030] The invention also relates to a damage detection system for organic matrix laminated composite material, characterized in that it comprises: • a composite material as described above, and • an excitation and measurement device electrically connected to at least two tracks and arranged to detect damage within the composite material by the power supply and measurement of electrical quantity of the electrical circuit.

[0031] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analysis of the accompanying drawings on which:

[0032] [Fig. 1] illustrates a damage detection system for an organic matrix laminated composite material according to the invention,

[0033] [Fig.2] illustrates a probe fold of the organic matrix laminated composite material of the system of [Fig. 1], and

[0034] [Fig.3] illustrates the organic matrix laminated composite material of [Fig.1].

[0035] Fig. 1 illustrates a system 1 for detecting damage to organic matrix laminated composite material.

[0036] The system 1 comprises an organic matrix laminated composite material 3 and an excitation and measurement device 5.

[0037] In the following description, the organic matrix laminated composite material 3 is referred to as "composite material 3" for the sake of brevity.

[0038] The organic matrix laminated composite material targeted by damage detection is precisely that integrated into system 1, i.e. composite material 3.

[0039] The composite material 3 is a laminated composite material and therefore takes the form of a stack of plies. More particularly, the composite material 3 is an assembly of an organic matrix and a reinforcement formed by stacking plies.

[0040] The organic matrix is ​​referred to as "resin" in the remainder of the description, except in the expression "organic matrix laminated composite material".

[0041] The resin can be thermosetting, for example epoxy resin, polyester or polyimide, or thermoplastic, for example polypropylene (PP), polyamide (PA) or polyetheretherketone (PEEK).

[0042] The reinforcement formation offers a certain degree of flexibility. Various types of fibers can be used, including carbon fibers, glass fibers, aramid fibers, or even plant fibers. Such fibers can be unidirectional, woven, non-woven, or not oriented in a particular direction. Furthermore, the fibers can be pre-impregnated, that is, already impregnated with resin, or, conversely, dry.

[0043] As illustrated in [Fig.1], the composite material 3 comprises a succession of plies stacked along a vertical direction Z. This succession of plies includes at least one probe ply 7 and one conductive ply 9.

[0044] The probe ply 7 and the conductive ply 9 are adjacent ply, that is to say successive ply in the stack of ply forming the composite material 3.

[0045] The probe fold 7 constitutes, for the device 5, an access to the composite material 3.

[0046] Fig. 2 is a photograph of such a probe fold 7 in top view.

[0047] The probe fold 7 is in the form of a substrate 11 on which an electrical circuit 13 is deposited.

[0048] The substrate 11 supports the electrical circuit 13 and allows it to be in contact -physical and therefore electrical - with the conductive fold 9.

[0049] Typically, the substrate 11 has a thickness between 10 and 500 micrometers (pm).

[0050] The electrical circuit 13 can be formed of a conductive ink, a precursor coating of conductive film or a metal deposited by physical vapor deposition.

[0051] Typically, the thickness of the electrical circuit 13 is less than 100 micrometers (pm).

[0052] The electrical circuit 13 has at least two tracks 15 and at least two electrodes 17. Each track 15 is connected to a respective electrode 17. In the example of [Fig. 2], the electrical circuit 13 has sixteen tracks 15 and sixteen electrodes 17.

[0053] In the example of [Fig.2], the electrical circuit 13 is deposited on only one face of the substrate 11. In other words, the tracks 15 and the electrodes 17 are deposited on the same face of the substrate 11.

[0054] Alternatively, the electrical circuit 13 can be deposited on both faces of the substrate 11. For example, the tracks 15 can be deposited on one face of the substrate 11, while the electrodes 17 are deposited on the other face. Again, by way of example, the tracks 15 can be distributed on both faces of the substrate 11, as can the electrodes 17.

[0055] In particular, in the case where a track 15 deposited on one face of the substrate 11 is connected to an electrode 17 deposited on the other face, this track 15 and this electrode 17 are connected to each other through the substrate 11.

[0056] One or more electrodes 17 may also be "doubled". An electrode 17 is doubled when it is formed of two electrodes arranged respectively on the two faces of the substrate 11 and electrically connected to each other through the substrate 11. The two electrodes of a doubled electrode 17 are located one below the other in the vertical direction Z.

[0057] In the example of [Fig.2], each track 15 has a free end 19 intended to be electrically connected to the device 5.

[0058] Typically, for a given track 15, the electrical resistance between the free end 19 and the electrode 17 is less than 10 ohms (Q), and is preferably less than 1 ohm (Q).

[0059] In the example of [Fig.2], the respective free ends 19 of the tracks 15 are brought together on a connection portion 21 of the probe ply 7. The connection portion 21 is intended to remain accessible from outside the stack of plies when the probe ply 7 is integrated into the composite material 3. The connection portion 21, for example, protrudes from the stack of plies forming the composite material 3.

[0060] By way of illustration, [Fig.3], which is a photograph of the composite material 3 as it comes out of an autoclave curing, shows the connection portion 21 protruding from the stack of plies.

[0061] Alternatively, when the probe ply 7 is intended to be positioned on the surface of the composite material 3, the connection portion 21 can be made directly accessible without protruding from the stack of plies.

[0062] The tracks 15 are electrically isolated from the substrate 11. Such electrical isolation can be achieved by at least two different methods.

[0063] The first method consists of using an insulating—or “non-conductive”—material, that is, a material that impedes the flow of electric current, to create the substrate 11. Electrical insulation is thus achieved by the very nature of the substrate 11. In the example of [Fig. 2], the substrate 11 is formed from a ply of glass fibers. Of course, other insulating materials can be used.

[0064] The second method consists of treating the areas of the substrate 11 intended to receive the tracks 15 to insulate them. Such insulation can be achieved using various techniques, including the liquid deposition of a resin or a dielectric coating, the addition of an insulating layer—for example, an insulating polymer film or a fiberglass ply—or the deposition of an insulating oxide layer. This second method is particularly suitable when the substrate 11 is made from a conductive material; for example, when the substrate 11 is a carbon fiber ply.

[0065] The electrodes 17 may or may not be electrically isolated from the substrate 11. When the substrate 11 is made from an insulating material, the electrodes 17 are electrically isolated from the substrate 11 by the very nature of the substrate 11.

[0066] However, when the substrate 11 is made from a conductive material, it may be advantageous not to insulate the electrodes 17 from the substrate 11. The device 5 is then electrically connected not only to the conductive fold 9 but also to the substrate 11, which therefore allows device 5 to have access to a larger portion of composite material 3.

[0067] With reference again to [Fig.1], the conductive fold 9 is suitable for conducting an electric current.

[0068] The conductive ply 9 is, for example, a ply of carbon fibers. Of course, other conductive materials can be used to make the conductive ply 9. It is also possible to use a ply whose fibers are insulating but coated with a conductive substance.

[0069] The core of the invention lies in the contact - and more precisely the electrical contact - between the electrical circuit 13 and the conductive fold 9.

[0070] The probe ply 7 and the conductive ply 9—which are adjacent plies in the sequence of plies of the composite material 3—are shaped and dimensioned such that the conductive ply 9 covers the electrodes 17 of the electrical circuit 13. In other words, when the conductive ply 9 is positioned on the probe ply 7 along the vertical direction Z, the conductive ply 9 comes into contact with the electrodes 17 and conceals them. Put another way, the contact surface of an electrode 17 and the conductive ply 9 is substantially orthogonal to the vertical direction Z. The electrical circuit 13 is opposite the conductive ply 9 along the vertical direction Z.

[0071] Such a contact allows for direct electrical contact between the electrodes 17 and the conductive fold 9. It is therefore not necessary to use a conductive substance to electrically connect the electrodes 17 and the conductive fold 9, as is the case, for example, in the Chinese patent application CN 113752644 A mentioned previously.

[0072] Furthermore, as mentioned above, the electrodes 17 can be deposited so as to be distributed over both faces of the substrate 11. In such a case, the stack of folds includes another conducting fold—in addition to the conducting fold 9—adjacent to the probe fold 7. In other words, the probe fold 7 is sandwiched between the conducting fold 9 and this other conducting fold to form a sandwich structure. The conducting fold 9 and this other conducting fold then each cover the electrodes 17 on a respective face of the substrate 11.

[0073] It is possible to increase the dimensions of the electrodes 17 to obtain a larger contact area between the electrodes 17 and the conductive fold 9. Typically, the electrodes 17 have a diameter greater than 5 millimeters (mm), which is indeed the case in the example of [Fig. 2]. The electrodes 17 may also be non-circular; other shapes are possible.

[0074] Furthermore, the arrangement of the electrodes 17 on the substrate 11 can be peripheral; but the electrodes 17 can also be arranged regularly on the substrate 11 so as to form a grid. Other arrangements are conceivable.

[0075] It is also possible, during the manufacture of the probe fold 7, to coat the electrodes 17 with a conductive material, for example a conductive ink or a metallic solder to improve the contact between the electrodes 17 and the conductive fold 9. Alternatively, it is possible to weld, glue or fix a metallic piece at the level of an electrode 17.

[0076] Still with a view to improving the contact between the electrodes 17 and the conductive fold 9, it is also possible to place the probe fold 7 and the conductive fold 9 under vacuum to obtain a contact by pressure between the electrodes 17 and the conductive fold 9.

[0077] In order to avoid any short circuit, the electrical circuit 13 is in contact with the conductive fold 9 only through the electrodes 17. In other words, there is no direct contact between the rest of the electrical circuit 13, i.e. the tracks 15, and the conductive fold 9.

[0078] To achieve this, an insulating material can be deposited on the tracks 15. Once the probe fold 7 and the conductive fold 9 are superimposed, the insulating material is interposed between the tracks 15 and the conductive fold 9 such that only the electrodes 17 are in direct contact with the conductive fold 9. Such an insulating material can be a fabric, for example, made of glass fibers. Such a fabric typically has a thickness of less than 500 micrometers (µm). It is also possible to deposit an insulating ink or resin on the tracks 15. Such an insulating ink or resin typically has a thickness of less than 100 micrometers (µm).

[0079] Another possibility for isolating the tracks 15 from the conductive fold 9 is to design the probe fold 7 so that the tracks 15 and the electrodes 17 are not deposited on the same face of the substrate 11. The tracks 15 are then isolated from the conductive fold 9 by the substrate 11 which is interposed between the tracks 15 and the conductive fold 9.

[0080] In the example of [Fig.1], and similarly to [Fig.2], the tracks 15 and the electrodes 17 are deposited on the same face of the substrate 11, that is to say the face intended to be in contact with the conductive ply 9 in the composite material 3. This face of the substrate 11 corresponds to the front face in the vertical direction Z.

[0081] With further reference to [Fig.1], it appears that the stack of plies forming the composite material 3 can include one or more plies in addition to the probe ply 7 and the conductive ply 9. Thus, in the example of [Fig.1], and by way of illustration, the composite material 3 includes an upper ply 23 as well as a lower ply 25.

[0082] Typically, the composite material 3 is made in such a way that the conductive ply 9 is arranged in the middle of the stack of plies, which is the case in [Fig.1].

[0083] However, the composite material 3 can also be made in such a way that the conductive ply 9 is the surface ply of the composite material 3, that is to say the highest ply along the vertical direction Z.

[0084] Furthermore, it is also possible that the probe ply 7 is the surface ply of the composite material 3, in which case the electrodes 17 are deposited on the posterior face of the substrate 11, and not on the anterior face, so as to be in contact with the conductive ply 9, which is then located below the probe ply 7, in the vertical direction Z, in the stack of plies.

[0085] The other plies of the composite material 3 can be conductive plies, for example plies of carbon fibers, or insulating plies, for example plies of glass fibers.

[0086] Advantageously, the composite material 3 includes an additional conductive ply adjacent to the conductive ply 9. In the example of [Fig. 1], such an additional conductive ply is, for example, the upper ply 23. Indeed, due to its contiguity with the conductive ply 9, such an additional conductive ply makes it possible to further extend the detection range of the device 5, which is then able to detect damage such as a crack, delamination or even a break or rupture of fibers at the level of this additional conductive ply.

[0087] In the same spirit, the conductive fold 9 can be in electrical contact with a succession of additional conductive folds, which are successive in the stacking of folds.

[0088] As explained previously, another variant also consists, when the electrodes 17 are distributed on the two faces of the substrate 11, of placing the probe fold 7 between the conductive fold 9 and another conductive fold - which would correspond to the lower fold 25 on the [Fig.1] - which then each cover the electrodes 17 of a respective face of the substrate 11.

[0089] Still with the aim of extending the detection range of the device 5, it is also possible to mount an elongated conductive piece over each electrode 17. Such an elongated conductive piece extends from the electrode 17 on which it is mounted and pierces at least part of the stack of folds so as to be in contact with one or more additional conductive fold(s). Such a conductive piece may be in the form of a metal stud, rod, or punch.

[0090] Such a configuration gives the device 5 access to additional conductive folds that are not directly in contact with the conductive fold 9. For example, an additional conductive fold separated from the conductive fold 9 by one or more insulating fold(s) can thus be reached by each elongated conductive piece extending from an electrode 17. Moreover, it is not necessary for such additional conductive folds, reached by the elongated conductive pieces, to be located above the conductive fold 9 in the vertical direction Z; such conductive folds can instead be below the probe fold 7 in the vertical direction Z.

[0091] The composite material 3 can be manufactured according to various known manufacturing processes.

[0092] In particular, the composite material 3 can be manufactured using a "wet process" or a "dry process". The "wet process" and the "dry process" are two categories of manufacturing processes that differ from each other in the way in which impregnation is carried out.

[0093] Impregnation consists of saturating the reinforcement with resin in a homogeneous manner. A uniform distribution ensures a strong bond between the reinforcement and the resin.

[0094] The wet process encompasses techniques in which the resin impregnates the reinforcement in liquid form. Examples include contact molding, vacuum infusion molding, and resin transfer molding (also known by the English acronym RTM for "resin transfer molding"), which are wet-process manufacturing techniques for organic matrix laminated composite materials.

[0095] The dry process encompasses techniques in which the reinforcement is pre-impregnated. The reinforcement fibers can thus be coated with resin, resulting in a pre-impregnated material. For example, automated fiber placement (AFP) and automated tape laying (ATL) are dry-process composite material manufacturing techniques.

[0096] It should be noted that certain techniques can be implemented using both wet and dry methods, notably filament winding, which consists of winding resin-impregnated fibers onto a mandrel under controlled tension. In particular, when the resin used is thermosetting, the fibers are dipped in a resin bath (wet method); and, when the resin used is thermoplastic, the fibers are pre-impregnated and wound directly onto the mandrel (dry method).

[0097] Manufacturing processes can be categorized in ways other than by the distinction between the "wet process" and the "dry process".

[0098] The presence of the probe fold 7 may lead to some adaptations with regard to impregnation.

[0099] The main issue is that the electrodes 17 must be in contact with the conductive ply 9, and therefore the stack of plies intended to form the composite material 3 must be impregnated without the resin interposing itself between the electrodes 17 and the conductive ply 9.

[0100] It is possible to use only dry folds, that is, folds that are not yet impregnated. To do this, the folds, and in particular the probe fold 7 and the conductive fold 9, are placed under vacuum to obtain contact by pressure. An infusion Vacuum impregnation is then carried out to impregnate all the folds. Pressure contact between the probe fold 7 and the conductive fold 9 prevents the resin from interfering between the electrodes 17 and the conductive fold 9.

[0101] Another possibility is to create a stack of plies in which the plies other than the probe ply 7 and the conductive ply 9 are pre-impregnated. The pre-impregnated plies may have an excess of resin, which can be used to impregnate the probe ply 7 and the conductive ply 9. For example, the plies are draped and placed under vacuum. The stack of plies is then cured in an autoclave. During curing, the resin is distributed homogeneously throughout all the plies, including the probe ply 7 and the conductive ply 9. If the resin in the pre-impregnated plies is insufficient, the missing amount of resin can be infused under vacuum before curing.

[0102] It is also possible to use a pre-impregnated conductive fold 9. In this case, it is necessary to remove the resin from each area of ​​the conductive fold 9 intended to be in contact with the electrodes 17.

[0103] By way of illustration, [Fig.3] is a photograph of a composite material 3 as it comes out of autoclave curing.

[0104] In the case where impregnation is required, it is preferable that the electrical circuit 13 cover less than half of the surface of the substrate 11 so as not to obstruct the diffusion of the resin in the stack of plies.

[0105] The device 5 is arranged to probe the composite material 3 in order to detect the presence of any damage such as a crack, delamination or even breakage or rupture of fibers.

[0106] The device 5 includes a power supply 27, a sensor 29, a memory 31 and a processor 33.

[0107] The source 27 is arranged to be connected to a doublet of tracks 15 and to supply the conductive fold 9 with electrical energy via the electrical circuit 13, and more specifically the electrodes 17 connected respectively to the tracks 15 to which the source 27 is connected.

[0108] In the example of [Fig. 1], the source 27 is a current source arranged to apply a desired intensity to the electric current flowing between the two tracks 15 to which the source 27 is connected. These two tracks 15 are in fact electrically connected to each other via their respective electrodes 17 and the conductive fold 9.

[0109] Alternatively, the source 27 is a voltage source arranged to apply a voltage between the two tracks 15 to which the source 27 is connected.

[0110] The sensor 29 is arranged to be connected to a pair of tracks 15 and to measure an electrical quantity at the level of the electrical circuit 13.

[0111] In the example of [Fig. 1], the sensor 29 is a voltage sensor arranged to measure the voltage between the tracks 15 to which the sensor 29 is connected. Such a voltage sensor can be a voltmeter-type measuring instrument.

[0112] Alternatively, the sensor 29 is a current sensor arranged to measure the intensity of the electric current flowing between the tracks 15 to which the sensor 29 is connected. Such a current sensor is, for example, an ammeter-type measuring instrument.

[0113] In the example of [Fig. 1], the device 5 includes a single source 27 and a single sensor 29.

[0114] Typically, the source 27 and the sensor 29 are both connected to all the tracks 15 by electrical cables. Multiplexers (not shown here) are present and allow selective selection of, on the one hand, the pair of tracks 15 through which the conductive fold 9 is to be supplied with electrical energy and, on the other hand, the pair of tracks 15 by means of which an electrical quantity is measured at the level of the electrical circuit 13.

[0115] The Applicant described, in French patent FR 3 131 372 Bl, an architecture allowing a source - in this case a current source - and a sensor - in this case an analog-to-digital converter - to be connected to electrodes arranged on the periphery of a substrate via multiplexers.

[0116] Alternatively, the source 27 may designate a set of electrical power sources, each arranged to be connected to a respective pair of tracks 15; and the sensor 29 may designate a set of sensors, each arranged to be connected to a respective pair of tracks 15.

[0117] The memory 31 is arranged to store instructions whose implementation by the processor 33 results in the operation of the device 5.

[0118] The memory 31 can be further arranged to store a record of measurements from the sensor 29.

[0119] Memory 31 can refer to any data storage medium designed to receive and store digital data, for example, a hard drive, a solid-state drive (SSD), or more generally any computer hardware that allows data storage on flash memory. Memory 31 can also be random access memory (RAM) or a magneto-optical disk. A combination of several storage media can also be considered.

[0120] The processor 33 is arranged to detect, from the measurements made by the sensor 29, the presence of damage such as a crack, delamination or even breakage or rupture of fibers within the composite material 3.

[0121] The processor 33 can be arranged to operate the device 5 in different modes. For example, the device 5 can operate in a mode of Continuous monitoring in which the source 27 continuously supplies electrical energy to the electrical circuit 13 and in which the sensor 29 performs continuous measurements. The device 5 can also operate in a discrete monitoring mode in which the source 27 supplies electrical energy to the electrical circuit 13 at regular intervals and in which the sensor 29 performs measurements at regular intervals.

[0122] The processor 33 can be implemented in any known form, for example as a microprocessor, a programmable logic device (PLD), a dedicated chip such as a field-programmable gate array (FPGA) or system-on-chip (SoC), a computing resource grid, a microcontroller, or any other suitable form with sufficient computing power for damage detection. One or more of these elements can also be implemented as specialized electronic circuits such as application-specific integrated circuits (ASICs). A combination of processors and electronic circuits is also possible.

[0123] The principle of damage detection is as follows: the source 27 excites a doublet of tracks 15 to supply electrical energy to the conductive ply 9. This supply results in the appearance of an electrical signal, that is, a movement of electrical charge carriers. Such an electrical signal is characterized, at a given instant, by an intensity and a voltage. The source 27 can be a current source, in which case the intensity of the electrical signal is controlled, or a voltage source, in which case the voltage of the electrical signal is controlled. The sensor 29 measures an electrical quantity, and more precisely an intensity or a voltage, at the level of the electrical circuit 13. The processor 33 receives the measurements taken by the sensor 29 and detects, where applicable, the presence of damage such as a crack, delamination, or even a break or rupture of fibers.

[0124] Such damage manifests itself, in the measurements taken by the sensor 29, as an unusual variation or even a discontinuity in the values ​​of the electrical quantity. It should be noted that the analysis performed by the processor 33 may involve monitoring the electrical resistance of the probe fold 7, which can be calculated from the electrical quantity measured by the sensor 29, whether this electrical quantity is the voltage or the current intensity, and with regard to the current or voltage applied by the source 27.

[0125] In such a configuration, the detection performed by the device 5, and more precisely by the processor 33, is binary since the information generated from the measurements performed by the sensor 29 indicates the presence or absence of damage within the composite material 3. However, such detection can be more precise and to allow not only the detection of the presence of possible damage but also, where applicable, to know the location of such damage.

[0126] To this end, the device 5 can implement electrical impedance tomography (EIT).

[0127] Electrical impedance tomography (EIT) is an imaging technique used to determine the electrical properties of a material by measuring the voltage of electrode pairs, usually—but not necessarily—arranged on the surface of the material. The voltage measurements are used to map the electrical charge density or electrical conductivity of the material.

[0128] In the present case, the material may refer to all conductive plies such as the conductive ply 9 which are capable of carrying an electric current through the probe ply 7.

[0129] According to the general principle of TIE, electrode doublets 17 are supplied with electrical energy by the source 27 and the voltage between the two electrodes 17 of different electrode pairs is measured. Several processes have been developed on this principle and differ in their injection pattern – that is, the way in which the electrode doublets 17 to be excited are formed.

[0130] The most commonly used injection pattern is the adjacent pattern - or "adjacent pattern" - described by DC Barber and BH Brown in the article "Applied potential tomography" (Journal of Physics E: Scientific Instruments, vol. 17, no. 9, 1984) and which consists, in the case here, of supplying each pair of electrodes 17 formed of two neighboring electrodes 17 and then measuring the voltage at the level of each pair of electrodes 17 also formed of two neighboring electrodes 17.

[0131] In the field of TIE (Theory of Electrical Imaging), it is known to use inverse problems theory to determine the electrical properties of a material. The principle of this approach is to determine the cause—here, the distribution of the electric charge density or electrical conductivity—from the effects—here, the voltage measurements between the two electrodes of the electrode pairs. Indeed, any change in the distribution of electrical conductivity leads to changes in the potential distribution and, consequently, to changes in the voltage between the electrodes.

[0132] An inverse problem of the TIE is a nonlinear and ill-posed problem. In particular, this inverse problem does not depend continuously on the voltage measurements. As recalled by SR Kupis in the publication "Methods for the Electrical Impedance Tomography Inverse Problem: Deep Leaming and Regularization with Wavelets" (All Theses, 2021), it is possible to use a MIRGN type Gauss-Newton algorithm (English acronym for "modified iteratively regularized Gauss-Newton") or a Tikhonov regularization to solve an inverse TIE problem.

[0133] Furthermore, solvers currently exist that can reconstruct the distribution of electric charge density or electrical conductivity from voltage measurements. For example, the free software EIDORS, which runs under MATLAB (registered trademark), can be used to solve an inverse problem.

[0134] The memory 31 and the processor 33 can thus be configured for solving an inverse TIE problem from the measurements collected by the sensor 29.

Claims

Demands

1. Organic matrix laminated composite material (3) with integrated damage detection, said composite material (3) is formed of a stack of plies including at least: • a probe ply (7), and • a conductive ply (9) adjacent to said probe ply (7), said probe ply (7) comprises a substrate (11) and an electrical circuit (13) deposited on said substrate (11), said electrical circuit (13) having at least two tracks (15) and at least two electrodes (17), said at least two tracks (15) being electrically insulated from said substrate (11) and each connected to an electrode (17) respective to said at least two electrodes (17), said electrical circuit (13) being in contact with the conductive ply (9) only through said at least two electrodes (17), said composite material (3) being characterized in that the conductive ply (9) covers the at least two electrodes (17).

2. Composite material (3) according to claim 1, characterized in that the ply stack includes an additional conductive ply adjacent to the conductive ply (9).

3. Composite material (3) according to claim 1 or 2, characterized in that the ply stack includes at least one additional conductive ply, and each electrode (17) is surmounted by an elongated conductive piece, which elongated conductive piece extends from said electrode (17) and pierces at least a part of the ply stack so as to be in contact with said at least one additional conductive ply.

4. Composite material (3) according to any one of the preceding claims, characterized in that the stack of plies further includes another conductive ply adjacent to the probe ply (7), the at least two electrodes (17) are distributed on a first and a second face of the substrate (11), and the conductive ply (9) and said other conductive ply each cover the electrodes of said at least two electrodes (17) of the first and second faces respectively.

5. Composite material (3) according to any one of the preceding claims, characterized in that the at least two tracks (15) each have a respective free end (19) and said ends free (19) are gathered on a portion of connection (21) of the probe fold (7) accessible from outside the stack of folds.

6. Composite material (3) according to claim 5, characterized in that the electrical resistance between the free end (19) of a track (15) of the at least two tracks (15) and the electrode (17) to which said track (15) is connected is less than 10 Q, and is preferably less than 1 Q.

7. Composite material (3) according to any one of the preceding claims, characterized in that the electrical circuit (13) covers less than half of the surface of the substrate (11).

8. Composite material (3) according to any one of the preceding claims, characterized in that the electrical circuit (13) is formed of a conductive ink, a conductive film precursor coating or a metal deposited by physical vapor deposition.

9. Composite material (3) according to any one of the preceding claims, characterized in that the at least two tracks (15) are electrically insulated from the conductive ply (9) by an insulating film, an insulating ink or glass fibers.

10. System (1) for detecting damage to organic matrix laminated composite material characterized in that it comprises: • a composite material (3) according to any one of the preceding claims, and • an excitation and measurement device (5) electrically connected to at least two tracks (15) and arranged to detect damage within said composite material (3) by the power supply and measurement of electrical quantity of the electrical circuit (13).

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