Method for monitoring the state of health of a composite material by electrical impedance measurement, corresponding device, corresponding computer program product and corresponding storage medium

Abstract

The invention relates to a technique for monitoring the state of health of a portion of composite material during a phase of processing the material using pairs of excitation (3a) and measurement (3b) electrodes, a measurement unit (1) and a processing unit (2). The measurement unit (1) comprises an electrical impedance calculator (10) configured to deliver an input excitation signal and receive an output measurement signal, and to determine data representative of the complex electrical impedance (two-point configuration), and a signal conditioning module (12) connected between the impedance calculator (10) and the sensing portion of the material and configured to deliver a conditioned measurement signal on the basis of the raw measurement signal produced at the output of the measurement electrode in response to the excitation signal applied to the excitation electrode (extension to a four-point configuration). The processing unit (2) is configured to track the state of health of the material by comparing the state of health with a reference impedance signature characteristic of correct or incorrect behaviour of the material during the processing phase.

Description

[0001] Method for monitoring the health status of a composite material by electrical impedance analysis, device, product, computer program and corresponding storage medium

[0002] technical field

[0003] The invention falls within the field of non-destructive testing of materials, and in particular composite materials. More specifically, the invention relates to a technique for monitoring the condition of a composite material during its manufacture or after its manufacture (in service), in order to detect any anomalies, such as the appearance of structural defects.

[0004] The invention relates in particular, but not exclusively, to the manufacture of CFRP (for "Carbon Fibres Reinforced Polymer") and GFRP (for "Glass Fibres Reinforced Polymer") composite parts, and in particular to monitoring the polymerization process by thermal means (on the composite and / or its unreinforced matrix).

[0005] Previous art

[0006] The remainder of this document focuses specifically on describing the problem encountered by the inventors of this invention in the context of inspecting composite structures for aeronautical applications. The invention is not limited to this particular application, but is relevant to any technique for monitoring composite materials or parts facing a similar or related problem.

[0007] CFRP and GFRP composites are among the materials increasingly used in numerous industries, particularly in aerospace and wind energy, due to their mechanical and thermal properties combined with their low weight. For example, CFRP consists of a polymer matrix reinforced with carbon fibers whose orientation is optimized for mechanical loading. After manufacturing, structural components made of composite material undergo regulatory non-destructive testing to guarantee their quality.

[0008] The manufacturing process itself, for composites, includes a thermal manufacturing cycle phase (polymerization cycle for thermosetting matrices, consolidation cycle for thermoplastic matrices, or both for vitrimeric matrices). During this phase, the temperature distribution within the produced part can be monitored, as well as pressure and vacuum levels, depending on the manufacturing processes. During manufacturing, this type of material can be subject to various defects: material heterogeneities, dimensional instability, etc. It is therefore important to monitor all the manufacturing processes for composite parts, as the performance of the resulting products depends on controlling the various manufacturing parameters and conditions. However, the control steps currently offered are primarily carried out in post-production.They are particularly costly and time-consuming to implement. Furthermore, they require that the entire part be completed before the necessary inspection can be carried out.

[0009] Furthermore, during their service life, aircraft components are subjected to various types of stress, both expected and accidental, leading to varying degrees of alteration in their structural integrity. Thus, as they are used, wear, malfunctions, or damage can occur and must be monitored. Planned maintenance strategies are currently mandated by regulations in the relevant sectors. These strategies include, for example, conducting regular visual inspections of critical components and, at less frequent intervals, disassembling them for a complete verification of their structural integrity.These maintenance strategies are based on the statistical lifespan of the aircraft's various components, which is not optimal because they require, on the one hand, grounding the aircraft and, on the other hand, the systematic dismantling of the part or component in question. This does not eliminate the risk of unnecessary dismantling of parts or components and the creation of additional damage during this critical phase.

[0010] Finally, to ensure the qualification of composite structures, it is necessary to use non-destructive quality control based on specific techniques whose purchase and use costs may be incompatible with current economic and competitive challenges.

[0011] In this context, there is therefore a real need to propose a technique for monitoring composite parts that is non-intrusive, simpler and less expensive to implement, compatible with most industrial processes for the production of composite structures and that can be used throughout the life cycle of the structure concerned.

[0012] Objectives of the invention

[0013] The invention, in at least one embodiment, aims in particular to overcome these various drawbacks of the prior art. More specifically, one objective of the invention in at least one of its embodiments is to provide:

[0014] a solution that allows us to do away with traditional post-production quality control, and therefore offer a solution that is more economically attractive;

[0015] a solution that allows for a higher level of monitoring;

[0016] a solution that is versatile, that is to say compatible with treatments of different kinds, and easily industrializable;

[0017] a solution that allows for real-time readjustment of manufacturing parameters;

[0018] A solution that enables the early detection of potentially defective parts; a solution that offers the possibility of predictive maintenance of parts. Presentation of the invention

[0019] The present invention provides a solution to overcome the drawbacks of the prior art.

[0020] According to a first aspect, the present technique relates to a device for monitoring the health status of at least a portion of a composite material during a material processing phase using at least one pair of electrodes comprising an excitation electrode and a measuring electrode, the excitation and measuring electrodes being connected to a measuring unit of the device and arranged in a geometric configuration defining a portion of electrical impedance capture of the material, the measuring unit comprising, for a given capture portion of the material:

[0021] an electrical impedance calculator configured to deliver an input excitation signal and receive an output measurement signal (two-point measurement configuration), and to determine representative data of the complex electrical impedance of said sensing portion resulting from a comparison between the input signal and the output signal,

[0022] a signal conditioning module connected between the impedance calculator and said sensing portion, and configured to apply a conditioned excitation signal to the excitation electrode from the input excitation signal and to deliver a conditioned measurement signal from the conditioned excitation signal and a raw measurement signal produced at the output of the measurement electrode in reaction to the conditioned excitation signal, the output measurement signal received by the calculator corresponding to the conditioned measurement signal delivered by the module (extension to a four-point measurement configuration for the calculator),

[0023] the monitoring device further comprising a processing unit configured to determine a current impedance signature of said capture portion during the processing phase from the electrical impedance data determined by the measuring unit and to compare the current impedance signature with a reference impedance signature characteristic of a reference behavior of the material during the processing phase according to said predefined configuration.

[0024] Thus, the principle of electrical impedance measurement relies on the use of a signal conditioning module which, by decoupling the excitation points from the measurement points between the material and the impedance calculator, enables electrical impedance measurements and the monitoring of their variations, for high-level in-situ monitoring of the material during processing. This approach allows for simple and cost-effective real-time monitoring of composite materials, adaptable to a wide range of composite structures and most industrial processes for production or post-production quality control of these parts. Decoupling the excitation points from the measurement points allows for impedance measurements of low, even very low, amplitudes with significantly improved accuracy (typically less than 1 Ohm).Furthermore, this approach eliminates the need for dedicated sensors by considering the composite material itself as a sensor (defined here as the sensing portion) when placed between two electrically conductive electrodes. The first electrode excites the material, while the second captures the measurement signal produced in response to the excitation. Thus, the processing phase encompasses both the material manufacturing phase (deposition, draping, polymerization (if the composite has a thermosetting or vitrimer matrix), consolidation (if the composite has a thermoplastic matrix), etc.) and post-manufacturing phases such as qualification after commissioning, in-service quality control, maintenance, or structural health monitoring (SHM).The processing unit is therefore configured to monitor the health status of the material by comparing it with a reference impedance signature characteristic of correct behavior (considered without anomalies) or incorrect behavior (considered with anomalies) of the material during the processing phase.

[0025] According to a particular implementation, the signal conditioning module comprises: - a first input connected to the output of the electrical impedance calculator to receive the input excitation signal delivered by the electrical impedance calculator in the form of an alternating electrical voltage,

[0026] - a first output connected to the excitation electrode to apply the conditioned excitation signal in the form of an alternating current,

[0027] - a second input connected to the measuring electrode to retrieve the raw measurement signal produced at the output of the measuring electrode in the form of an alternating current,

[0028] - a third input connected to the input of the excitation electrode to receive the conditioned excitation signal applied to the excitation electrode,

[0029] - a second output connected to the input of the electrical impedance calculator to deliver the measurement signal conditioned as an alternating voltage to the impedance calculator.

[0030] More specifically, the signal conditioning module includes:

[0031] - a first electrically connected conditioning stage between the first input and the first output of said module, and itself comprising an excitation signal regulator and a voltage-current converter, - a second electrically connected conditioning stage between the second and third inputs and the second output of said module, and itself comprising a differential measurement amplifier and a current-voltage converter.

[0032] In this way, the material's excitation point and the impedance measurement point can be separated (a so-called four-point measurement configuration), ensuring greater measurement sensitivity, especially for low electrical impedance values. The first stage converts the input excitation voltage signal into a regulated excitation current signal, while the second stage converts the raw, i.e., noisy, measurement current signal into a clean (i.e., noise-free) measurement voltage signal, resulting in a much more accurate reading of the material's impedance behavior during the processing phase.

[0033] According to a particular characteristic, at least one pair of electrodes is arranged in a geometric configuration belonging to the group comprising: a longitudinal configuration, a transverse configuration, a thickness configuration, a transverse diagonal configuration, or a longitudinal diagonal configuration. This list is not exhaustive. The device allows for the measurement of the electrical impedance of the composite material in its entirety or in a localized area, depending on the positioning of the electrode pairs.

[0034] According to a particular feature, the device further comprises at least one temperature sensor arranged to measure the temperature of at least one electrical impedance capture portion of the material and / or the material's processing temperature. The processing unit is configured to compare the current impedance signature with said at least one measured temperature. The temperature sensor can be arranged on the surface or within the core of the composite material and / or at a distance from it. The invention thus relies on an instrumentation architecture that couples electrical impedance sensors with temperature sensors, enabling the material's impedance signatures to be determined as a function of both time and temperature, thereby increasing the accuracy of the data.For example, this allows for a more precise determination of the critical transition points of the composite material during manufacturing (typically during the polymerization or consolidation cycle).

[0035] In a particularly interesting implementation, the device comprises several pairs of excitation electrodes, each pair arranged in a distinct geometric configuration defining a separate electrical impedance sensing portion of the material and associated with a separate measurement channel of the measurement unit. Each measurement channel includes an impedance calculator and a signal conditioning module connected between the impedance calculator and the sensing portion associated with that measurement channel. The processing unit is configured to determine a current impedance signature for each of the material's electrical impedance sensing portions. These measurement channels are independent and can be activated simultaneously or sequentially to perform synchronous or asynchronous measurements as required.This implementation therefore allows for simultaneous or sequential impedance measurements at different points and directions within the material. A graphical representation (in 2D or 3D) of the material's electrical impedance can then be established based on the various measurements collected by the measuring unit.

[0036] In another aspect, the present technique relates to a method for monitoring the health status of at least a portion of a composite material during a material processing phase using at least one pair of electrodes comprising an excitation electrode and a measuring electrode, the excitation and measuring electrodes being arranged in a predefined configuration to define a portion for capturing electrical impedance of the material, the method comprising the following steps, for a given portion of the material:

[0037] - apply an excitation signal to the excitation electrode of said capture portion of the material which produces in reaction a raw measurement signal produced at the output of the measurement electrode; - determine a current impedance signature of said capture portion from data representative of the complex electrical impedance of said capture portion, said data being determined from a conditioned measurement signal obtained from the raw measurement signal and the excitation signal;

[0038] - compare the current impedance signature with a reference impedance signature characteristic of a reference behavior of the material during the processing phase according to said predefined configuration.

[0039] Thus, it is possible to accurately detect any manufacturing or operational defects through simple electrical impedance analysis. Electrical impedance analysis provides a picture of the composite's condition during processing and allows for corrective actions to be planned if necessary (adjustment of manufacturing parameters, early detection of anomalies (structural anomalies, functional anomalies, premature wear, etc.)).

[0040] According to a particular implementation, the process further includes the following steps: estimation of a level of similarity between the reference impedance signature and the current impedance signature;

[0041] Verification that the estimated similarity level is greater than a predetermined similarity threshold; in case of a negative verification, an anomaly is detected during the processing phase and an indicator of its presence is generated. The similarity level quantifies the degree of resemblance between the current signature and the reference signature, this degree of resemblance (or similarity) being maximum when the two signatures are identical. Thus, a similarity level below a certain threshold indicates the presence of an anomaly. The similarity threshold is sized to define a certain tolerance in the comparison of the two signatures. Indeed, they may not be identical in every respect but are very close to each other, at least in their main characteristics.

[0042] According to a specific implementation, the process also includes the following steps, in the event of an anomaly being detected:

[0043] comparison of the current impedance signature with at least one signature belonging to a set of reference anomaly signatures, each characteristic of a predetermined type of material anomaly during the processing phase according to said predefined configuration, identification of the anomaly type based on the reference anomaly signature for which the anomaly is detected.

[0044] This allows the nature of the anomaly to be automatically identified, and, if necessary, an appropriate action to be triggered based on the diagnosis established.

[0045] According to a specific implementation, the process further includes, upon determination of an alert level associated with the type of anomaly identified, a decision step for at least one of the following actions:

[0046] material processing will stop depending on the determined alert level;

[0047] continuation of material processing according to the determined alert level; modification of at least one material processing parameter according to the type of anomaly identified.

[0048] Thus, a procedure adapted to the nature of the anomaly can be implemented: for example, a first level of alert allowing the continuation of the current processing, a second level of alert allowing the continuation of the current processing with a modification of the processing parameter by means of a feedback loop, a third level of alert triggering a stoppage of the current processing.

[0049] According to another aspect, the present technique relates to a computer program product which includes program code instructions for implementing the aforementioned process (in any of its various embodiments), when said program is executed on a computer.

[0050] According to another aspect, the present technique relates to a computer-readable and non-transient storage means, storing a computer program comprising a set of instructions executable by a computer to implement the aforementioned process (in any of its various embodiments).

[0051] Figures

[0052] Other features and advantages of the invention will become apparent from reading the following description, given by way of indicative and non-limiting example, and the accompanying drawings, in which: - Figure 1 is a simplified schematic diagram of a monitoring system according to a particular embodiment of the invention;

[0053] - Figure 2 illustrates, in the form of functional blocks, the operating principle of the signal conditioning module according to a particular embodiment of the invention;

[0054] - Figure 3 presents, in the form of a flowchart, a particular embodiment of the monitoring process according to the invention;

[0055] - Figure 4 represents the simplified structure of a processing unit implementing the process according to a particular embodiment of the invention;

[0056] - Figures 5a, Fig 5b, Fig 5c and Fig 5d illustrate four examples of electrode configurations implemented to monitor the health status of the material in different directions of space;

[0057] - Figure 6 is a graphical representation illustrating a first example of impedance signatures of a composite material during a thermal manufacturing cycle;

[0058] - Figure 7 is a graphical representation illustrating a second example of impedance signatures of a composite material during a thermal manufacturing cycle.

[0059] Detailed description of the invention

[0060] In the figures in this document, identical elements are designated by the same numerical reference. Furthermore, the elements are not necessarily to scale, as the emphasis is on illustrating the principle of the invention.

[0061] The general principle of the invention is based on an innovative instrumentation architecture to monitor the health status of a composite material by electrical impedance measurement during manufacturing or after its implementation.

[0062] The remainder of this document focuses more specifically on describing the invention within the framework of a protocol for monitoring the polymerization and / or thermal consolidation phase in the manufacture of an aircraft airframe component, for example, made of composite material. Indeed, this manufacturing phase has a decisive influence on the structural properties of the composite parts used. The invention is, of course, not limited to this particular field of application, but is of interest for any type of composite part whose structural characteristics can be characterized by electrical impedance analysis. A simplified diagram of a monitoring system 100 according to a particular embodiment of the invention is presented in relation to Figure 1.The 100 monitoring system is configured to monitor the health status of a composite structure 3 during manufacturing, intended for production, such as an aircraft airframe component. The composite structure 3, used here as an example, is a stack of CFRP (carbon fiber reinforced polymer matrix) composite plies intended to undergo vacuum heat treatment to activate the material's polymerization process. It is understood that any other type of composite structure could be considered. It is therefore an intermediate product that can be polymerized by heat treatment.

[0063] The composite structure 3 is represented in simplified form by a parallelepiped-shaped block placed in a vacuum chamber 5, which is equipped with heating, vacuum, and possibly pressure systems adapted to the polymerization process. In this example, the composite structure 3 consists of a material based on epoxy resin and carbon fibers.

[0064] In this particular embodiment, the purpose of the monitoring system 100 is to monitor in real time, by electrical impedance measurement and temperature, the different phases of polymerization and / or consolidation of the material during its manufacturing cycle but also during its overall life cycle, therefore in service or even end of life.

[0065] The monitoring system 100 comprises, more specifically, a measuring unit 1, a processing unit 2, a plurality of electrode pairs 3a-3b, and a set of thermocouples 41-42. Electrode 3a represents the electrical excitation electrode and electrode 3b represents the electrical measuring electrode of the system. Electrodes 3a and 3b are arranged on the surface of the composite structure 3 in a coplanar configuration along the longitudinal axis L, but other electrode configurations (surface or core) are of course possible without departing from the scope of the invention.

[0066] The monitoring system 100 shown in Figure 1 contains only one pair of electrodes. The number of electrodes shown is intentionally limited for purely pedagogical purposes, so as not to overload the figure and its associated description. To obtain representative health monitoring of the composite structure, a larger number of excitation-measurement electrode pairs can be used. For example, an architecture based on an implementation of eight excitation-measurement electrode pairs, each arranged in a distinct geometric configuration in three dimensions, provides a faithful and accurate representation of the structural health of the material in the measurement area. Figures 5a to 5d illustrate different electrode configurations that can be implemented to monitor the health of a composite structure according to the invention.Each electrode pair comprises an excitation electrode and a measurement electrode arranged in a geometric configuration that defines a portion of the material's electrical impedance for sensing. Indeed, due to the known electrical resistivity and capacitance properties of CFRP composites, the portion of material between the electrodes of an excitation-measurement electrode pair is considered to act as the equivalent of an electrical impedance sensor for monitoring the material's state at the time of measurement. This functionalization of the composite material eliminates the need for embedded sensors such as dielectric sensors, or those using other transduction methods (piezoelectric, optical fibers, etc.).Thus, an implementation of eight pairs of excitation-measurement electrodes leads to the functionalization of eight electrical impedance capture portions operating on eight independent measurement channels, which can be activated in a synchronized manner or in a switched manner at ultra-short time intervals.

[0067] The measuring unit 1 is electrically connected to the various electrode pairs for the purpose of measuring the complex electrical impedance Z of different portions of the material. Such measurements make it possible to monitor the evolution over time of the magnitudes |z| and arguments 0 of this complex impedance Z and to deduce, if necessary, the associated resistance, capacitance, and inductance values. The measuring unit 1 is also electrically connected to the thermocouples 41-42 for the purpose of measuring the temperature (T) of the material and / or the thermal chamber. Typically, the presence of a first thermocouple located on or in the material and a second thermocouple located in the chamber at a distance from the material (dedicated to measuring the chamber) makes it possible to verify the temperature differences between the setpoint temperature and the actual temperature of the material, thus enabling the control of the chamber's setpoint temperature.

[0068] The measuring unit 1 comprises an electrical impedance calculator 10 and a signal conditioning module 12 located between the impedance calculator 10 and the composite structure 3. The signal conditioning module 12 is electrically connected to the impedance calculator 10 and to the excitation-measurement electrodes 3a-3b, which constitute the electrical impedance sensing portion CP. The impedance calculator 10 is configured to determine the electrical impedance of the material from an analog measurement signal. The calculator's impedance measurement range extends from 0.1 Ohm to 1 MegaOhm, including the extended range provided by the signal conditioning module 12.

[0069] An example of a conditioning module is described later in relation to Figure 2. The measuring unit 1 further includes a thermal measuring module connected to the thermocouple set 41-42 for temperature monitoring of the material. The processing unit 2 is electrically connected to the measuring unit 1 for the purpose of controlling the measuring unit 1. The processing unit 2, the principle of which is described later in relation to Figure 4, includes means for implementing the steps of the process of the invention described below. In particular, it has program code instructions enabling the implementation of the process of the present invention.The processing unit 2 is configured, on the one hand, to apply an electric field between each of the different pairs of electrodes implemented on / in the composite structure 3 via the measuring unit 1 and, on the other hand, to receive, store and process the electrical impedance data and temperature data delivered by the measuring unit 1. A human-machine interface is provided to allow the operator to execute the process of the invention for the purpose of monitoring the health status of the material being manufactured and its life cycle.

[0070] The 100 monitoring system described above relies on a new instrumentation architecture combining a measurement unit with a portion of electrical impedance capture and two thermocouples to monitor in real time the structural state of the composite and the phase transitions of the organic matrix during its life cycle.

[0071] We will now describe, with reference to Figure 2, an example of a signal conditioning module 12 according to the invention. The signal conditioning module 12 comprises a set of analog processing circuits whose role is to deliver a high-level measurement signal representative of the material's typical structural behavior, particularly—but not exclusively—for low electrical impedance values ​​(typically less than 10 Ohms). Such a module allows for a so-called "four-point" measurement implementation, decoupling the two electrical excitation points (electrical contacts w1 and w2 connected to electrode 3a) from the two electrical measurement points (electrical contacts w3 and w4 connected to electrode 3b). This approach enables low-amplitude impedance measurements without the effects of potential noise related to the environment and the measurement system.

[0072] The conditioning module 12 includes, in particular:

[0073] - a first input el connected to the output of the electrical impedance calculator 10 to receive an excitation voltage signal SI delivered by the impedance calculator 10;

[0074] - a first output if connected to the input of the excitation electrode 3a to deliver an excitation current signal S2 from the excitation voltage signal SI;

[0075] - a second input e2 connected to the output of the measuring electrode 3b to receive the raw measurement signal S3 delivered by the measuring electrode 3b;

[0076] - a third input e3 connected to the input of the excitation electrode 3a to receive the excitation current signal S2 applied to the excitation electrode 3a (referenced as signal S2' in the figure); - a second output s2 connected to the input of the impedance calculator 10 to deliver the conditioned measurement output signal S4 to the impedance calculator 10;

[0077] - a voltage regulator Cl to stabilize the power supply to the components to which the regulator is connected;

[0078] - a voltage-to-current converter C2;

[0079] - a C3 measurement amplifier;

[0080] - a C4 current-to-voltage converter.

[0081] The voltage regulator C1 is electrically connected to converter C2, amplifier C3, and converter C4. Its function is to maintain a continuous power supply to the components it powers (i.e., the components to which it is connected), regardless of variations in input voltage or component load. The presence of such a regulator within the conditioning module protects the components from fluctuations that could damage the electronic equipment or disrupt its operation, thus ensuring optimal and safe operation of sensitive electronic components. If the load (e.g., the current drawn) changes, the voltage regulator readjusts the power supply to maintain a stable voltage.If the input voltage varies (for example, due to variations in mains power or other electrical disturbances), the regulator adjusts the output to compensate for these fluctuations and maintain a constant output voltage.

[0082] The voltage-to-current converter C2 is electrically connected on one side to the first input el of the conditioning module to receive the excitation voltage signal SI, and on the other side to the first output si of the conditioning module to deliver, by voltage-to-current conversion, the excitation current signal S2. This voltage-to-current converter is configured to allow reliable transmission of the excitation signal, adapted to the electrical impedance of the material and minimizing losses.

[0083] The measurement amplifier C3 is electrically connected to the second e2 and third e3 inputs of the conditioning module. It is a differential electronic amplifier whose output signal is proportional to the difference between the two input signals. This amplifier is configured to amplify the voltage difference measured across the electrically excited material (the voltage differences being of very small amplitudes, typically less than one volt, or even a hundred millivolts), while suppressing noise, so as to deliver, after processing, a low-amplitude current signal S3'. Thus, by separating the electrical excitation source from the measurement amplifier, the arrangement of electronic components C2-C3 enables a four-point measurement implementation, particularly well-suited for measuring low electrical impedances.The current-voltage converter C4 is connected on one side to the output of the measuring amplifier C3 to receive the low amplitude current signal S3' and on the other side to the second output s2 of the conditioning module, to deliver by current-voltage conversion the conditioned measuring voltage signal S4, to the impedance calculator 10.

[0084] The excitation signals SI (Vout) and measurement signals S4 (Vin) have a voltage amplitude generally between 0 and 5V (while taking care not to reach a certain saturation threshold).

[0085] The electrical impedance calculator 10 is therefore configured to output the input excitation signal SI and receive the output measurement signal S4 in order to determine representative data of the complex electrical impedance of the CP sensing portion by comparing the input signal SI and the output signal S4 of the conditioning module. The signal conditioning module 12 is configured to apply the conditioned excitation signal (S2) to the excitation electrode 3a from the input excitation signal SI and to output the conditioned measurement signal S4 from the conditioned excitation signal S2' and the raw measurement signal S3 produced at the output of the measurement electrode (3b) in response to the conditioned excitation signal.

[0086] Thus, to refine the reading and eliminate noise related to the system's electronics, the proposed architecture separates the measurement from the material's electrical excitation using a four-point measurement configuration. This function is achieved by separating the excitation (blocks C1 and C2) from the measurement (blocks C3 and C4) using two compensation stages, labeled 12-1 and 12-2 in Figure 2. The first stage comprises the regulator C1 and the converter C2. The second stage comprises the differential amplifier C3 and the converter C4. The voltage signal S1 is converted into a current signal S2. The measurement is therefore performed via the current signals S2' and S3. The current signal S3' is an amplification of the clean (i.e., noise-free) measurement signal via C3, and S4 is the clean measurement signal converted into a voltage via C4.

[0087] Figure 3 generically represents a flowchart of a particular embodiment of the process according to the invention. This flowchart illustrates the main steps in implementing the process, labeled E10 to E40. These steps are carried out, for example, by the processing unit 2 described above and aim to provide information, for instance, on the structural health of the CFRP composite material during its thermal manufacturing cycle (the so-called "polymerization" phase for thermosetting matrices or the "consolidation" phase for thermoplastic matrices). This is, of course, an illustrative example, and the principle described below can be applied to other material processing phases, such as during its lifecycle after commissioning (for implementing quality control, for example).Furthermore, for the sake of simplicity in this description, only the electrode pair 3a-3b is considered to be used to monitor the structural behavior of the material. Thus, the focus is on monitoring the condition of the portion of material located between the excitation electrode 3a and the measurement electrode 3b, referred to as the portion of interest. Of course, the principle described below can be applied to each of the electrode pairs implemented on / in the material to monitor the structural behavior of the entire composite structure in different spatial directions (such as those illustrated, for example, in Figures 5a-5d).

[0088] After placing the pre-polymerized composite material (curable by heat treatment) in the thermal chamber, the operator initiates a heat treatment cycle via the human-machine interface. The human-machine interface is the control panel that allows the user to communicate with the measuring unit 1 and the thermal chamber 5, providing them with instructions based on the desired treatment cycle and responding to instructions sent to the microprocessor. These instructions relate in particular to the operational parameters of the thermal cycle (vacuum, pressure, and temperature) and to material monitoring (excitation electric field). Once the configuration is complete, the user can initiate the process of the invention via the human-machine interface.

[0089] Step E10 (referenced as "APP") consists of applying an alternating electric field between the excitation electrode 3a and the measuring electrode 3b, which are positioned on the area of ​​interest in the material. To achieve this, the calculator 10 of the measuring unit 1 generates an alternating voltage signal SI, designed to bring the electrodes 3a and 3b to different electrical potentials at a given frequency. Typically, a sinusoidal alternating voltage is applied by the calculator 10. This voltage has an RMS value V ou t adjustable (for example between 1 and 5 V) and an excitation frequency f eadjustable (for example, between 1 kHz and 100 kHz). The electric field thus generated in the material between the excitation and measurement electrodes produces in response a raw measurement signal S3, which is received by the measurement electrode 3b. This signal has the same frequency but is time-shifted. To perform a four-point measurement, the signal conditioning module 12 is activated.

[0090] The computer 10 retrieves the measurement signal S4 during the material tracking phase and determines the complex electrical impedance Z of the material's area of ​​interest (the CP sensing area) by comparing it to the input excitation signal SI, generated from the measurement signal Vin alone (conventional two-point measurement) or, after conditioning, from the measurement signal V4 delivered by module 12 (four-point measurement according to the invention). In the case of an RC system, the real part provides information on the resistive evolution, and the imaginary part provides information on the capacitive evolution of the CP sensing area. In parallel, the computer 10 also retrieves the thermal measurement signals from the two thermocouples 41-42 collected during the thermal cycle of material manufacturing, in this example.

[0091] At step E20 (referenced as "DET"), processing unit 2 performs a frequency analysis of the electrical impedance data collected for the CP capture portion to determine a current impedance signature for the material's region of interest. This current impedance signature reflects the real-time structural state of the material for a given capture portion. The resulting current impedance signature is stored in a local memory or table within processing unit 2. Thermal measurements provide information on the temperatures of the material and / or the chamber during the material's thermal manufacturing cycle.

[0092] At step E30 (referenced "COM"), the processing unit 2 compares the current impedance signature previously obtained with a reference impedance signature characteristic of a reference behavior of the material during the thermal manufacturing cycle for the same electrode configuration, in order to detect any anomaly during the cycle.

[0093] A reference impedance signature is a material impedance signature that has been previously defined by impedance measurement for in-situ monitoring of a given manufacturing cycle or the material lifecycle for a given electrode configuration. This signature can also be defined during cycle monitoring as part of a control step, allowing the link between this signature and a specific state of the composite structure (reference health or failure state at a given time). The reference impedance signature is stored in the memory or local table of the processing unit 2. A first example of a current signature (SCI) and a reference signature (SRI) for a CFRP of formulation NC66 / NA1808 is shown in Figure 6. A second example of current signatures (SC2-SC6) and a reference signature (SR2) for a CFRP of formulation T700 / M21 is shown in Figure 7.The temperature ramp is referenced RT1 and RT2 in Figures 6 and 7 respectively. The references DI, D2 and D3 refer to defects identified during the thermal cycle by comparison to the reference signature.

[0094] Thanks to the collected temperature measurements and electrical impedance analysis, the current impedance signature can be defined as a function of both time and temperature, making it possible to accurately determine the critical transition points of the material. The points labeled PL, PG, PV, and PF in Figure 6 correspond respectively to the liquefaction, gelation, vitrification, and reaction termination points characteristic of CFRP.

[0095] The comparative impedance analysis implemented in step E30 relies on two substeps of the process. Processing unit 2 first (first substep) estimates the level of similarity between the current impedance signature (which has just been determined) and the reference impedance signature stored in memory. The level of similarity quantifies the degree of resemblance between the current signature and the reference signature, this degree of resemblance (or similarity) being maximum when the two signatures are identical. The level of similarity estimated by the processing unit can be expressed as a percentage, for example. The estimation method can be based on or inspired by techniques well known to those skilled in the art. Processing unit 2 then verifies (second substep) that the estimated level of similarity is greater than a predetermined similarity threshold. The similarity threshold is, for example, set at 80%.This threshold is sized to define a certain tolerance for comparison between the current signature and the relevant reference signature. A similarity level below this threshold indicates that a manufacturing anomaly is occurring or has occurred for the CP capture portion, while a similarity level above this threshold indicates normal material behavior during the cycle.

[0096] At step E40 (referenced "ANO"), processing unit 2 generates information indicating the presence or absence of a manufacturing anomaly based on the results of the previous comparison step.

[0097] In the event of a negative check, processing unit 2 considers a manufacturing anomaly to be present in the CP capture portion and generates information indicating the presence of an anomaly associated with this material portion of interest, for example, a visual and / or audible alert via a system monitoring console (human-machine interface). The information displayed on the monitoring console may include explicit notification that it is a manufacturing anomaly in the material portion of interest and data relating to the nature of this anomaly. The data relating to the nature of the anomaly is obtained by comparing the current signature with at least one signature belonging to a set of reference anomaly signatures, each characteristic of a predetermined type of material anomaly during the cycle, according to the relevant electrode configuration.Indeed, since reference anomaly signatures may have been previously stored in the local memory of the processing unit associated with a known anomaly type in the material being manufactured, the device can easily trace the malfunction back to its cause and inform the user for diagnostic and preventive maintenance purposes. This is particularly valuable in other application contexts, notably for monitoring the health status of materials in service. Explicit data on the estimated similarity level can also be provided to the user (in addition to the alert) via the monitoring console for diagnostic purposes. A reliability level for the information provided can also be generated.The processing unit 2 according to the invention is further configured, upon determination of an alert level associated with the type of anomaly identified, with a decision step for at least one of the following actions: a) stopping the material processing based on the determined alert level; b) continuing the material processing based on the determined alert level; c) modifying at least one material processing parameter (cycle temperature, pressure, vacuum, etc.) based on the type of anomaly identified. Thus, depending on the type of anomaly identified, it is possible to control the processing means, for example, the temperature, pressure, or vacuum within the chamber, based on the results of electrical impedance measurements obtained during the cycle. The process according to the invention therefore offers the possibility, depending on the nature of the anomaly, of real-time reconfiguration of certain material processing parameters during manufacturing.

[0098] If the verification is successful, processing unit 2 considers that no anomaly occurred during the material manufacturing cycle and informs the operator by generating explicit information via the system's monitoring console. Explicit data on the estimated similarity level can also be provided to the user, as well as the reliability level of the information provided.

[0099] Figure 4 shows the simplified structure of a device 400 implementing the monitoring method according to the invention, for example, the particular embodiment described above in relation to Figures 1 to 2. This device 400 comprises a random access memory 430 (for example, RAM), a processing block or unit 410, equipped, for example, with a processor, and controlled by a computer program stored in a read-only memory 420 (for example, ROM or a hard drive). At initialization, the code instructions of the computer program are, for example, loaded into the random access memory 430 before being executed by the processor of the processing unit 410. The processing unit 410 receives as input the conditioned measurement signal SM according to the invention acquired by the measurement unit for a given portion of interest of the material.The processor of the processing unit 410 processes the measurement signal SM according to the instructions of the computer program and generates as output information indicating the presence or absence of an anomaly SI, possibly accompanied by data on the nature of the anomaly and explicit data on the level of similarity estimated by the device. This Figure 4 illustrates only one particular way, among several possible ways, of implementing the algorithm detailed above, in relation to the flowchart in Figure 3. Indeed, the method according to the invention can be carried out interchangeably:

[0100] - on a reprogrammable computing machine (a PC, a DSP processor or a microcontroller) running a program comprising a sequence of instructions, or - on a dedicated computing machine (for example a set of logic gates such as an FPGA or an ASIC, or any other hardware module).

[0101] In the case where the invention is implemented on a reprogrammable computing machine, the corresponding program (i.e. the sequence of instructions) may be stored in a removable or non-removable storage medium, this storage medium being readable partially or totally by a computer or a processor.

[0102] Figures 5a, 5b, 5c, and 5d illustrate examples, by way of non-limiting, of excitation-measurement electrode configurations in an orthonormal coordinate system with L, T, and Z axes for the case of a unidirectional composite. It is understood that other excitation-measurement electrode configurations are possible. The configuration in Figure 5a represents a longitudinal geometric configuration suitable for applying an electric field E1 between the excitation electrode 3a and the measurement electrode 3b along the L-axis of the material and on its surface. The configuration in Figure 5b represents a transverse geometric configuration suitable for applying an electric field E2 between the excitation electrode 3c and the measurement electrode 3d along the T-axis of the material and on its surface.The configuration in Figure 5c represents a diagonal-longitudinal geometric configuration suitable for applying an electric field E3 between the excitation electrode 3e and the measurement electrode 3f along the Z and L axes of the material, at its core. The configuration in Figure 5d represents a diagonal-transverse geometric configuration suitable for applying an electric field E4 between the excitation electrode 3g and the measurement electrode 3h along the Z and T axes of the material, at its core. Another configuration, not shown, could be based on a thickness-dependent geometric configuration along the Z axis only.

[0103] All these excitation / measurement electrode pair configurations allow each to define a distinct impedance sensing area and can be combined on the same material. The electrical impedance measurements from each of these impedance sensing areas are collected and processed independently by the measurement unit. They can then be concatenated to provide a three-dimensional representation of the material's condition during the processing phase. The excitation / measurement electrode pairs can be placed on and / or in the material temporarily (only for monitoring a specific processing phase) or permanently installed.In the second case, the electrode pairs can be reused for material monitoring during a subsequent manufacturing step and / or after manufacturing, in service (in real time) or for maintenance checks, for example, after a predefined operating time of the material. In this case, the electrodes must be shaped to be electrically accessible from outside the material. The embodiment described above is used for monitoring the health status of a CFRP composite during its polymerization phase by heat treatment (on the composite and / or its unreinforced matrix).This is of course an example given by way of illustration and the invention can very well be applied to other materials of interest (reinforced or unreinforced matrix composite), as well as to other phases of material processing both during its manufacture and in post-production or after its commissioning during a maintenance check for example.

[0104] Thus, other materials of interest may be used without departing from the scope of the invention, including materials belonging to the non-exhaustive list which includes, for example:

[0105] CFRP (“Carbon Fiber Reinforced Polymer”) composite comprising a matrix based on epoxy, polyester, phenolic or thermoplastic, reinforced with carbon fibers;

[0106] GFRP (“Glass Fiber Reinforced Polymer”) composite comprising a matrix based on epoxy, polyester, vinyl ester or phenolic, reinforced with glass fibers;

[0107] KFRP (“Kevlar Fibre Reinforced Polymer”) composite comprising an epoxy or thermoplastic matrix reinforced with aramid fibers;

[0108] BFRP (“Basalt Fiber Reinforced Polymer”) composite comprising a thermosetting or thermoplastic-based matrix reinforced with basalt fibers;

[0109] carbon / glass hybrid fiber composite comprising an epoxy or polyester-based matrix reinforced with glass and carbon fibers;

[0110] hybrid fiber nanocomposite comprising an epoxy or thermoplastic matrix reinforced with carbon, glass, aramid or basalt fibers and nanometric fillers; natural fiber composite comprising a biopolymer or epoxy matrix reinforced with natural fibers (such as flax, jute, hemp or coconut fibers, for example); matrix composite belonging to the polyimide family, such as phenolic or polybismaleimide resins (also known as BMI resins);

[0111] metal fiber composite (Aluminium, Titanium, ...);

[0112] composite with semiconducting fibers (Boron);

[0113] ceramic matrix or fiber composite.

[0114] These materials of interest can also incorporate spot additives (in powder form, for example) to modify the surface characteristics of the part to be manufactured (pigment, adhesion, UV treatment). Reinforcing fibers can be in point form or in a predefined woven form, depending on the desired properties.

Claims

DEMANDS 1. Device for monitoring the health status of at least a portion of a composite material during a material processing phase using at least one pair of electrodes comprising an excitation electrode (3a) and a measuring electrode (3b), the excitation and measuring electrodes being connected to a measuring unit (1) of the device and arranged in a geometric configuration defining a portion of electrical impedance capture (CP) of the material, the measuring unit (1) comprising, for a given portion of the material: - an electrical impedance calculator (10) configured to deliver an input excitation signal (SI) and receive an output measurement signal (S4), and to determine data representative of the complex electrical impedance of said capture portion resulting from a comparison between the input signal (SI) and the output signal (S4), - a signal conditioning module (12) connected between the impedance calculator (10) and said sensing portion (CP), and configured to apply a conditioned excitation signal (S2) to the excitation electrode (3a) from the input excitation signal (SI) and to deliver a conditioned measurement signal (S4) from the conditioned excitation signal (S2) and a raw measurement signal (S3) produced at the output of the measurement electrode (3b) in reaction to the conditioned excitation signal, the output measurement signal received by the calculator corresponding to the conditioned measurement signal delivered by the module, the monitoring device further comprising a processing unit (2) configured to determine a current impedance signature of said capture portion during the processing phase from the electrical impedance data determined by the measuring unit (1) and to compare the current impedance signature with a reference impedance signature characteristic of a reference behavior of the material during the processing phase according to said predefined configuration.

2. A monitoring device according to claim 1, wherein the signal conditioning module (12) comprises: - a first input (el) connected to the output of the electrical impedance calculator to receive the input excitation signal (SI) delivered by the electrical impedance calculator in the form of an alternating electrical voltage, - a first output (S1) connected to the excitation electrode to apply the conditioned excitation signal (S2) in the form of an alternating current, - a second input (E2) connected to the measuring electrode to recover the raw measurement signal produced at the output of the measuring electrode in the form of an alternating current, - a third input (e3) connected to the input of the excitation electrode to receive the conditioned excitation signal (S2) applied to the excitation electrode, - a second output (s2) connected to the input of the electrical impedance calculator to deliver the conditioned measurement signal (S4) in the form of an alternating voltage to the electrical impedance calculator.

3. Monitoring device according to claim 2, wherein the signal conditioning module (12) comprises: a first conditioning stage (12-1) electrically connected between the first input and the first output of said module, and itself comprising an excitation signal regulator (Cl) and a voltage-current converter (C2), a second conditioning stage (12-2) electrically connected between the second and third inputs and the second output of said module, and itself comprising a differential measuring amplifier (C3) and a current-voltage converter (C4).

4. Monitoring device according to any one of claims 1 to 3, wherein said at least one pair of electrodes is arranged in a geometric configuration belonging to the group comprising: a longitudinal configuration, a transverse configuration, a thickness configuration, a transverse diagonal configuration, a longitudinal diagonal configuration.

5. Monitoring device according to any one of claims 1 to 4, comprising at least one temperature sensor arranged to measure the temperature of at least one electrical impedance sensing portion of the material and / or the processing temperature of the material, the processing unit (2) being configured to compare the current impedance signature taking into account said at least one measured temperature.

6. A monitoring device according to any one of claims 1 to 5, comprising several pairs of excitation electrodes, each pair being arranged in a distinct geometric configuration defining a distinct electrical impedance sensing portion (CP) of the material and associated with a measurement channel distinct from the measurement unit (1), each measurement channel comprising an impedance calculator (10) and a signal conditioning module (12) connected between the impedance calculator (10) and the sensing portion (CP) associated with said measurement channel, the processing unit (2) being configured to determine a current impedance signature for each of the electrical impedance sensing portions of the material. 7.Method for monitoring the health status of at least a portion of a composite material during a material processing phase using at least one pair of electrodes comprising an excitation electrode (3a) and a measurement electrode (3b), the excitation and measurement electrodes being arranged according to a predefined configuration to define an electrical impedance capture portion of the material, the method comprising the following steps, for a given capture portion of the material: - apply (E10) an excitation signal to the excitation electrode (3a) of said material capture portion which in reaction produces a raw measurement signal (S3) produced at the output of the measurement electrode (3b); - determine (E20) a current impedance signature of said capture portion from data representative of the complex electrical impedance of said capture portion, said data being determined from a conditioned measurement signal (S4) obtained from the raw measurement signal (S3) and the excitation signal; - compare (E30) the current impedance signature with a reference impedance signature characteristic of a reference behavior of the material during the processing phase according to said predefined configuration.

8. Monitoring method according to claim 7, further comprising the following steps: - estimation of a level of similarity between the reference impedance signature and the current impedance signature; - verification that the estimated similarity level is greater than a predetermined similarity threshold; - in case of negative verification, detection (400) of an anomaly during the processing phase and generation of information indicating the presence of an anomaly.

9. A monitoring method according to claim 8, further comprising the following steps, in the event of detection of an anomaly: - comparison of the current impedance signature with at least one signature belonging to a set of reference anomaly signatures, each characteristic of a predetermined type of material anomaly during the processing phase according to said predefined configuration, - identification of the type of anomaly based on the reference anomaly signature for which the anomaly is detected.

10. A monitoring method according to claim 9, further comprising, upon determination of an alert level associated with the type of anomaly identified, a decision step for at least one of the following actions: - Stop processing of the material according to the determined alert level; - Continue processing of the material according to the determined alert level; - modification of at least one material processing parameter depending on the type of anomaly identified.

11. Product computer program, comprising program code instructions for implementing the method according to at least one of claims 7 to 10, when said program is executed on a computer.

12. A computer-readable and non-transient storage medium, storing a computer program product according to claim 11.

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