System for detecting damage to a composite material

Abstract

The invention relates to a system (1) for detecting damage to a composite material, which system comprises: - a fibre layer (3) suitable for being integrated into a composite material, - a conductive part (5) in contact with the fibre layer (3), and - a stimulation and measurement device (7) comprising one or more conductive wires (9, 11) each incorporated into the fibre layer (3) and in contact with the conductive part (5) and a detector (13) arranged to power a conductive wire (9) and to detect, by measuring electrical resistance, an interruption of the current flow between the powered conductive wire and the conductive part (5) or, optionally, between the powered conductive wire (9) and a third-party conductive wire (11) electrically connected to each other via the conductive part (5).

Description

[0001] Description

[0002] Title: Composite material damage detection system

[0003] The field of the invention relates to the detection of damage to a composite material, in particular a crack or degradation at the interface with a conductive part.

[0004] A composite material is an assembly of at least two immiscible materials. Such an assembly allows the composite material to have properties that the materials that compose it, taken individually, do not possess. Due to their characteristics, composite materials have applications in various industrial fields such as electronics, automotive, railway, aeronautics and space.

[0005] A composite material is formed of at least a matrix and a reinforcement.

[0006] The matrix ensures the cohesion of the composite material and allows it to be given the desired shape. The matrix surrounds the reinforcement and transfers mechanical forces to it. The matrix is ​​typically organic, metallic, or ceramic. In particular, in the case of an organic matrix composite material, the matrix—then called resin—can be thermosetting, for example epoxy, polyester, or polyimide resin, or thermoplastic, for example polypropylene (PP), polyamide (PA), or polyetheretherketone (PEEK).

[0007] Reinforcement forms the backbone of the composite material and gives it most of its mechanical properties. Reinforcement can increase the strength of the matrix or make it lighter. Reinforcement is generally made of fibers such as carbon fibers, glass fibers, aramid fibers, or plant fibers. Reinforcement can also include metal fillers, carbon fillers, silica fillers, or polymer fillers. For example, it is known in the industry to use a carbon / carbon composite - or C / C - formed from a graphite matrix reinforced with carbon fibers or a carbon fiber reinforced polymer (also known by the acronym CFRP for "carbon filter reinforced polymer").

[0008] Regardless of the field of application, a composite material is susceptible to damage, for example in the event of impact or exposure to thermal conditions that the composite material is unable to absorb and withstand. Damage to the composite material generally appears in the form of cracking or delamination.

[0009] Furthermore, the damage suffered may correspond to degradation at the interface - which may take the form of contact by pressure, bonding or welding - between the composite material and a part belonging to a structure into which the composite material is integrated. Such a part may in particular be a means of fastening, for example an insert, a screw or a rivet, used for the assembly of the structure and for the integration of the composite material.

[0010] It is known, to detect possible damage, to use a sensor taken or inserted into the composite material and whose electrical resistance is measured to monitor the state of the composite material in order to detect possible degradation. Such degradation then manifests itself by a change or even a discontinuity in the electrical resistance.

[0011] International application WO 2020 / 245564 A1 describes a sacrificial sensor in the form of a strand and which is integrated into a non-conductive composite material. The sacrificial sensor is in contact with several plies of the composite material such that damage to the latter modifies the electrical properties of the sacrificial sensor, in particular its electrical resistance.

[0012] US patent US 9,329,021 B1 relates to monitoring the condition of a composite material using strips of piezoresistive material inserted between the plies of the composite material. Each strip is equipped, at its ends, with electrodes connected to a voltage or current source. A detector, also connected to the electrodes, measures the electrical resistance of the strip to identify possible damage. European patent application EP 2 950 085 A1 proposes to use together a thermal element - for example a metal wire - and a detection element contained in separate plies of a composite material. More precisely, the thermal element and the detection element are electrically insulated and separated from each other by a ply of fibers. The thermal element is stimulated to heat the composite material and then the electrical resistance of the detection element is continuously measured to determine the thermal profile of the thermal element.The thermal profile is then compared to a reference profile to determine the condition of the composite material and detect the presence of a crack or delamination.

[0013] However, none of the existing solutions take advantage of the environment in which the composite material is used, i.e. the structure or technical system in which the composite material is integrated. Indeed, as explained previously, the composite material is likely to be in contact with a part, for example a means of fastening, and the potential damage may therefore concern the contact between the composite material and the part. However, the solutions described previously focus on the composite material as such and do not take into account its possible interactions with the structure or technical system of which it is a part.

[0014] The present invention improves the situation.

[0015] In this respect, the invention relates to a system for detecting damage to a composite material comprising:

[0016] - a layer of fibers suitable for integration into a composite material, and

[0017] - a conductive part in contact with the fiber layer.

[0018] The system further comprises a stimulation and measurement device comprising one or more conductive wires each incorporated into the layer of fibers and in contact with the conductive part and a detector arranged to supply a conductive wire and to detect, by measuring electrical resistance, an interruption in the flow of current between the supplied conductive wire and the conductive part or, where appropriate, between the supplied conductive wire and a third conductive wire electrically connected to each other via the conductive part. The layer of fibers is for example composed of glass fibers.

[0019] Alternatively, the fiber layer is composed of carbon fibers, and each conductive wire is electrically insulated from the fiber layer.

[0020] In one or more embodiments, at least one conductive yarn is a textile yarn.

[0021] In one or more embodiments, at least one conductive wire is carbon.

[0022] In one or more embodiments, the detector comprises a voltage source arranged to apply a voltage between the powered conductive wire and the conductive part or, where appropriate, between the powered conductive wire and the third-party conductive wire, and a current sensor arranged to measure the intensity of the current flowing between the powered conductive wire and the conductive part or, where appropriate, between the powered conductive wire and the third-party conductive wire.

[0023] Alternatively, the detector comprises a current source arranged to cause a current to flow between the powered conductive wire and the conductive part or, where appropriate, between the powered conductive wire and the third-party conductive wire, and a voltage sensor arranged to measure the voltage between the powered conductive wire and the conductive part or, where appropriate, between the powered conductive wire and the third-party conductive wire.

[0024] In one or more embodiments, at least one conductive yarn is incorporated into the fiber layer by weaving, knitting, braiding, or bonding.

[0025] In one or more embodiments, at least one conductive wire is in contact with the conductive part by pressure, gluing or welding.

[0026] The invention further relates to a composite material comprising a plurality of plies and the system described above, one of the plies being formed by the layer of fibers of the system.

[0027] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the appended drawings in which: [Fig. 1] schematically illustrates a first embodiment of a system for detecting damage to a composite material according to the invention comprising a layer of fibers, a conductive part as well as a stimulation and measurement device with at least one pair of conductive wires,

[0028] [Fig. 2] is a photograph of part of the system, namely a conductive part and two pairs of conductive wires,

[0029] [Fig. 3] illustrates the variations in the electrical resistance of a pair of conductive wires of the stimulation and measurement device of the system of [Fig. 1], and

[0030] [Fig. 4] schematically illustrates a second embodiment of a system for detecting damage to a composite material according to the invention comprising a layer of fibers, a conductive part and a stimulation and measurement device with a single conductive wire.

[0031] [Fig. 1] illustrates a system 1 for detecting damage to a composite material.

[0032] A composite material is an assembly of at least two immiscible materials: a matrix and a reinforcement. The matrix can be organic, metallic, or ceramic—these are referred to as organic matrix composite (OMC), metal matrix composite (MMC), and ceramic matrix composite (CMC), respectively. The reinforcement is usually made of fibers, such as carbon fibers, glass fibers, aramid fibers, or plant fibers. The reinforcement can also include fillers such as metal fillers, carbon fillers, silica fillers, or polymer fillers.

[0033] The composite material can be assembled by stacking several layers - then called plies - each corresponding to the matrix or the reinforcement. For example, it is known to arrange the plies so as to alternate those corresponding to the matrix and those corresponding to the reinforcement. It is also possible to form the matrix by molding and to embed the reinforcements therein. In the context of the invention, the composite material is part of a structure or a technical system in an industrial field such as electronics, automotive, railway, aeronautics or even space. The composite material is thus likely to undergo mechanical stresses or to be exposed to external conditions, for example temperature and pressure, which can cause ruptures such as cracking or delamination.

[0034] The system 1 comprises a layer of fibers 3, a conductive part 5 and a stimulation and measurement device 7.

[0035] The layer of fibers 3 is suitable for being integrated into a composite material, and more precisely here the composite material whose system 1 is intended to detect any damage. The layer of fibers 3 then forms a reinforcement within the composite material.

[0036] The layer of fibers 3 may advantageously be composed of non-conductive fibers, i.e. fibers which oppose the flow of current such as glass fibers. Alternatively, the layer of fibers 3 may be composed of conductive fibers, i.e. fibers which conduct current such as carbon fibers, by means of adjustments to the device 7 which are explained in the remainder of the description.

[0037] The fiber layer 3 here designates a conventional reinforcing ply, the manufacture of which is well known to those skilled in the art.

[0038] For example, a conventional process for manufacturing glass fibers involves heating a glass composition to a temperature of approximately 1500°C and then forming molten glass filaments by drawing. A size is then applied to the filaments to gather them into a yarn, protect them from the risk of abrasion that can occur during ply formation, increase the stiffness of the filaments and facilitate subsequent impregnation by the matrix. The glass fiber reinforcement ply is then formed by weaving.

[0039] In general, the fiber layer 3 can be formed by weaving, knitting, braiding or bonding. The particularity of the system 1 is to exploit the environment of the composite material. The conductive part 5 is thus part of the structure or technical system within which the composite material is used. For example, the conductive part 5 can be a fastening means such as an insert, a screw or a rivet used for the assembly of the structure or technical system and for the integration of the composite material. More particularly, the conductive part 5 is in contact with the fiber layer 3. Such contact can result from pressure, therefore mechanical stress, or be obtained by bonding.

[0040] The conductive part 5 may be made of metal, for example copper, aluminum, tin, zinc or even iron. The conductive part 5 may also be a metal alloy, for example steel. Still by way of example, the conductive part 5 may also be made of carbon.

[0041] The device 7 is arranged to be installed on the composite material, and more generally on the structure or technical system of which the composite material is part, in order to detect any damage.

[0042] Damage refers in particular to a crack or loss of contact between the fiber layer 3 and the conductive part 5.

[0043] The device 7 comprises at least one conductive wire and a detector 13.

[0044] In the embodiment illustrated in [Fig. 1], the device 7 comprises a pair of conductive wires 9 and 11.

[0045] Each conductive thread 9, 11 is incorporated into the fiber layer 3. The incorporation of the pairs of conductive threads 9 and 11 into the fiber layer 3 can be carried out at the time of formation thereof, for example by weaving, knitting, braiding or bonding. Alternatively, the conductive threads 9 and 11 can be incorporated into the fiber layer 3 after formation thereof, and more precisely at the time of integration of the conductive part 5.

[0046] The conductive wires 9 and 11 are furthermore electrically connected to each other via the conductive part 5 with which they are in direct contact, by pressure, gluing or welding. The conductive wires 9 and 11 may be textile wires and therefore be formed by spinning, that is to say the assembly or agglutination of textile fibers. Furthermore, the ability of the conductive wires 9 and 11 to conduct current may be given to them by the material from which they are composed, for example carbon. The conductive wires 9 and 11 may also be made of copper or made from an alloy. Alternatively, such an ability may be obtained by means of a conductive coating, for example a metallic coating.

[0047] Advantageously, the conductive wires 9 and 11 have a breaking strength - also called "strain to failure" in the English-speaking literature - less than or equal to that of the layer of fibers 3. Such a characteristic makes it possible to increase the probability that a crack propagating in the layer of fibers 3 will cause the rupture of at least one of the conductive wires 9 and 11.

[0048] It is further important that current cannot flow between the pair of conductive wires 9 and 11 and the layer of fibers 3. To achieve this, the layer of fibers 3 may simply be composed of non-conductive fibers - typically glass fibers -, in which case the electrical insulation is achieved by the very nature of the layer of fibers 3 which cannot conduct current. However, as mentioned above, it is also possible for the layer of fibers 3 to be composed of conductive fibers - typically carbon fibers -, in which case it is necessary to electrically isolate the conductive wires 9 and 11 from the layer of fibers 3. The conductive wires 9 and 11 may be insulated using an insulating sheath or coating, for example a polymeric insulator such as polyurethane, polyvinyl chloride (better known by the English acronym PVC for "polyvinyl chloride") or polytetrafluoroethylene (PTFE).

[0049] In the example of [Fig. 1], the device 7 comprises a single pair of conductive wires 9 and 11. It should however be understood here that the device 7 may comprise a plurality of pairs of conductive wires such as the pair of conductive wires 9 and 11.

[0050] For illustration purposes, [Fig. 2] is a photograph of the system 1, and more precisely of a part of the system 1 in an embodiment in which the device 7 comprises, in addition to the pair of conductive wires 9 and 11, an additional pair of conductive wires 15 and 17.

[0051] In [Fig. 2], the conductive part 5 is an instrumented metal part whose visible face - in contact with the pair of conductive wires 9 and 11 as well as with the pair of conductive wires 15 and 17 - is intended to form an interface with the layer of fibers 3 (absent from the photograph) of the system 1.

[0052] The use of several pairs of conductive wires makes it possible to increase the chances of detecting damage to the fiber layer 3 or degradation at the interface between the fiber layer 3 and the conductive part 5 which is manifested by a progressive loss of contact.

[0053] Furthermore, as explained in the remainder of the description with reference to [Fig. 4], the device 7 may also comprise a single conductive wire or several conductive wires each treated individually.

[0054] When the device 7 comprises several conductive wires - typically several pairs of conductive wires - the device 7 may further comprise one or more multiplexers (not shown in the figures) to be able to use the same electronic resources, in particular the same detector 13 described below, for all of the conductive wires.

[0055] The detector 13 is arranged to detect an interruption in the flow of current within the pair of conductive wires 9 and 11 by measuring electrical resistance.

[0056] Referring again to [Fig. 1], the detector 13 comprises a power source 19, a sensor 21, a memory 23, a processor 25 and a communication module 27.

[0057] The source 19 is arranged to be connected to the pair of conductive wires 9 and 11. More particularly, the source 19 is arranged to supply the pair of conductive wires 9 and 11.

[0058] The source 19 may be a voltage source, in which case the source 19 applies a voltage between the conductive wires 9 and 11. Alternatively, the source 19 is a current source, in which case the source 19 generates a current that flows from one conductive wire to the other of the pair of conductive wires 9 and 11.

[0059] The sensor 21 is arranged to be connected to the pair of conductive wires 9 and 11. More particularly, the sensor 21 is arranged to measure an electrical quantity, namely the intensity of the current flowing in the pair of conductive wires 9 and 11 or the voltage between the conductive wires 9 and 11. The measurement may correspond to the determination of the value of the electrical quantity or quite simply to the detection of the crossing of a threshold.

[0060] Several implementation methods are thus possible.

[0061] In the embodiment illustrated in [Fig. 1], the source 19 is a voltage source and the sensor 21 is a current sensor - for example an ammeter-type measuring instrument - arranged to measure the intensity of the current flowing in the pair of conductive wires 9 and 11. The detector 13 is then arranged to determine the value of the electrical resistance from the measured current intensity and the voltage imposed by the source 19.

[0062] Alternatively, the source 19 is a current source and the sensor 21 is a voltage sensor - for example a voltmeter-type measuring instrument - arranged to measure the voltage between the conductive wires 9 and 11. The detector 13 is then arranged to determine the value of the electrical resistance from the measured voltage and the intensity of the current imposed by the source 19. Such a configuration is also illustrated in [Fig. 4] described below.

[0063] As mentioned previously, damage to the composite material may result in a crack in the fiber layer 3 or in a loss of contact between the fiber layer 3 and the conductive part 5. Damage to the fiber layer 3 is likely to have an impact on the pair of conductive wires 9 and 11 and on the conductive part 5.

[0064] Thus, the propagation of a crack can result in the rupture of at least one of the conductive wires 9 and 11. The conductive wires 9 and 11 are then no longer electrically connected and the current no longer flows from one to the other. The interruption of the flow of current is manifested by an increase in the electrical resistance detected by the detector 13. Furthermore, the loss of contact between the layer of fibers 3 and the conductive part 5 also interrupts the flow of current between the conductive wires 9 and 11 since these are electrically connected via the conductive part 5. Here again, the increase in the electrical resistance is detected by the detector 13.

[0065] A reading of electrical resistance measurements taken by detector 13 is shown in [Fig. 3]. The curve shown more precisely illustrates the percentage variation of electrical resistance as a function of time expressed in seconds (s).

[0066] The analysis of this curve makes it possible to observe the effects of a loss of contact - in this case a detachment - between the layer of fibers 3 and the conductive part 5 on the electrical resistance of a pair of conductive wires such as the pair of conductive wires 9 and 11.

[0067] In particular, the absence of variation in the electrical resistance from 0 to 320 seconds (s) indicates an absence of damage. The gradual increase in the electrical resistance from 320 to 675 seconds (s) corresponds to the start of detachment of the fiber layer 3. The sudden increase in the electrical resistance from 675 to 750 seconds (s) corresponds to the end of detachment, i.e. the total loss of contact between the fiber layer 3 and the conductive part 5.

[0068] The memory 23 is arranged to store instructions whose implementation, by the processor 25, results in the operation of the detector 13.

[0069] The memory 23 may further be arranged to store a record of electrical resistance measurements - or values ​​of equivalent quantities obtained by processing the electrical resistance measurements - and / or to keep a history of the damage suffered by the composite material, and more precisely here by the layer of fibers 3. Such a history may be used in real time to prevent damage and / or subsequently for the purposes of maintenance, consolidation or reinforcement of the structure or technical system of which the analyzed composite material is a part.

[0070] The memory 23 may designate any data storage medium arranged to receive and store digital data, for example a hard disk, a solid-state drive (SSD) or more generally any computer hardware allowing the storage of data on flash memory. The memory 23 may also be a random access memory or a magneto-optical disk. A combination of several storage media may also be envisaged.

[0071] The processor 25 may be arranged to operate the detector 13 in different modes. For example, the detector 13 may operate in a continuous monitoring mode in which the source 19 continuously supplies the pair of conductive wires 9 and 11 and in which the sensor 21 continuously measures, as appropriate, the intensity of the current flowing within the pair of conductive wires 9 and 11 or the voltage between the conductive wires 9 and 11. The detector 13 may also operate in a discrete monitoring mode in which the source 19 supplies the pair of conductive wires 9 and 11 at regular intervals and in which the sensor 21 measures at regular intervals, as appropriate, the intensity of the current flowing within the pair of conductive wires 9 and 11 or the voltage between the conductive wires 9 and 11.

[0072] The processor 25 can be produced in any known manner, for example in the form of a microprocessor, a programmable logic circuit (better known by the acronym PLD for “Programmable Logical Device”) or a dedicated chip of the FPGA type (acronym for “Field Programmable Gate Array”) or SoC (acronym for “System on Chip”), a grid of computing resources, a microcontroller or any other specific form having the computing power necessary to detect damage to the composite material on which the system 1 is installed. One or more of these elements can also be produced in the form of specialized electronic circuits of the ASIC type (acronym for “Application-Specific Integrated Circuit”). A combination of processors and electronic circuits can also be envisaged.

[0073] The communication module 27 is arranged to transmit data stored in the memory 23, for example the reading of electrical resistance measurements or the history of damage suffered by the composite material, to an operator's terminal.

[0074] [Fig. 4] illustrates an embodiment in which the device 7 comprises a single conductive wire 29. In this embodiment, the detector 13 is arranged to detect an interruption in the flow of current between the conductive wire 29 and the conductive part 5 by measuring electrical resistance.

[0075] It may be noted that the shape of the conductive part 5 in [Fig. 4] is different from that in [Fig. 1], which has no effect on the general operation of the device 7.

[0076] The source 19 is then arranged to be connected to the conductive wire 29 and to the conductive part 5. More particularly, the source 19 is arranged to supply the conductive wire 29. Furthermore, the conductive part 5 can be connected to ground.

[0077] Likewise, the sensor 21 is arranged to be connected to the conductive wire 29 and to the conductive part 5. More particularly, the sensor 21 is arranged to measure the intensity of the current flowing between the conductive wire 29 and the conductive part 5 or the voltage between the conductive wire 29 and the conductive part 5.

[0078] Unlike the example of [Fig. 1] in which the source 19 is a voltage source and in which the sensor 21 is a current sensor, [Fig. 4] illustrates an example in which the source 19 is a current source and in which the sensor 21 is a voltage sensor. It is understood that, as for the embodiment of [Fig. 1], it is also possible to use a voltage source for the source 19 and a current sensor for the sensor 21.

[0079] Here again, damage to the composite material may result in a crack, the propagation of which may result in the breakage of the conductive wire 29 or in a loss of contact between the conductive wire 29 and the conductive part 5. Indeed, in either case, the conductive wire 29 is then no longer electrically connected to the conductive part 5 and the current no longer flows between the two. The interruption of the flow of current is manifested by an increase in the electrical resistance detected by the detector 13. It should be noted that the device 7 may comprise several conductive wires each treated individually in the same way as the conductive wire 29 shown in [Fig. 4]. Generally speaking, the device 7 may comprise one or more pairs of conductive wires such as the pair of conductive wires 9 and 11 and / or one or more individual conductive wires such as the conductive wire

[0080] 29.

[0081] The respective operations of the other components of the detector 13, in particular the memory 23, the processor 25 and the communication module 27, are similar to those described previously with reference to [Fig. 1],

Claims

Claims

1. System (1) for detecting damage to a composite material comprising: - a layer of fibers (3) suitable for being integrated into a composite material, and - a conductive part (5) in contact with said layer of fibers (3), said system (1) being characterized in that it further comprises a stimulation and measurement device (7) comprising one or more conductive wires (9, 11, 15, 17, 29) each incorporated in said layer of fibers (3) and in contact with the conductive part (5) and a detector (13) arranged to supply a conductive wire (9, 15, 29) and to detect, by electrical resistance measurement, an interruption in the flow of current between said supplied conductive wire (29) and the conductive part (5) or, where appropriate, between said supplied conductive wire (9, 15) and a third conductive wire (11, 17) electrically connected to each other via the conductive part (5).

2. System (1) according to claim 1, characterized in that the layer of fibers (3) is composed of glass fibers.

3. System (1) according to claim 1, characterized in that the layer of fibers (3) is composed of carbon fibers, and in that each conductive wire (9, 11, 15, 17, 29) is electrically insulated from said layer of fibers (3).

4. System (1) according to one of the preceding claims, characterized in that at least one conductive thread (9, 11, 15, 17, 29) is a textile thread.

5. System (1) according to one of the preceding claims, characterized in that at least one conductive thread (9, 11, 15, 17, 29) is made of carbon.

6. System (1) according to one of the preceding claims, characterized in that the detector (13) comprises a voltage source (19) arranged to apply a voltage between the supplied conductive wire (29) and the conductive part (5) or, where appropriate, between the supplied conductive wire (9, 15) and the third conductive wire (11, 17), and a current sensor (21) arranged to measure the intensity of the current flowing between the supplied conductive wire (29) and the conductive part (5) or, where appropriate, between the supplied conductive wire (9, 15) and the third conductive wire (11, 17).

7. System (1) according to one of claims 1 to 5, characterized in that the detector (13) comprises a current source (19) arranged to cause a current to flow between the supplied conductive wire (29) and the conductive part (5) or, where appropriate, between the supplied conductive wire (9, 15) and the third conductive wire (11, 17), and a voltage sensor (21) arranged to measure the voltage between the supplied conductive wire (29) and the conductive part (5) or, where appropriate, between the supplied conductive wire (9, 15) and the third conductive wire (11, 17).

8. System (1) according to one of the preceding claims, characterized in that at least one conductive thread (9, 11, 15, 17, 29) is incorporated into the layer of fibers (3) by weaving, knitting, braiding or gluing.

9. System (1) according to one of the preceding claims, characterized in that at least one conductive wire (9, 11, 15, 17, 29) is in contact with the conductive part (5) by pressure, gluing or welding.

10. Composite material comprising a plurality of plies and the system (1) according to one of the preceding claims, characterized in that one of the plies is formed by the layer of fibers (3) of said system (1).

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