Structural component incorporating a sensor and method for its monitoring.

Abstract

A structural component (100) comprising a portion made of a composite material having a polymer matrix and reinforcing fibers embedded in the polymer matrix, and at least one sensor (10) embedded in said portion, the sensor (10) having two outer layers (11, 12) and, between the two, a piezoelectric transducer having two electrodes (13, 14) applied against a piezoelectric body (15), the electrodes (13, 14) being connected to electrical signal conductors (16, 17) for transmitting signals to the outside of said portion, the outer layers (11, 12) each being made of a polymer chemically compatible with the polymer matrix of the composite material forming said portion. Corresponding method for monitoring a component. FIGURE IN ABRIDGED: [Fig. 1]

Description

Title of the invention: Structural part incorporating a sensor and method for its monitoring.

[0001] The present invention relates to the field of monitoring the integrity of materials, in particular composite materials constituting parts of mechanical structures.

[0002] BACKGROUND OF THE INVENTION

[0003] Mechanical structures or machines include structural parts, namely parts that support or transmit forces. Since damage to a structural part generally prevents the structural part from performing its function, jeopardizing the overall mechanical structure or causing machine failure, it is important to detect such damage as early as possible, even if the structural part is not visible from outside the mechanical structure or machine, or if access to it is difficult.

[0004] SUBJECT OF THE INVENTION

[0005] The invention aims in particular to enable the monitoring of the integrity of a structural part in real time. Summary of the invention

[0006] For this purpose, according to the invention, a structural part is provided comprising a portion made of a composite material having a polymer matrix and reinforcing fibers embedded in the polymer matrix, and at least one flat sensor which is embedded in said portion, the sensor having two external layers and, between the two, a piezoelectric transducer having two electrodes applied against a piezoelectric body, the electrodes being connected to electrical signal conductors to transmit signals to the outside of said portion, the external layers each being made of a polymer chemically compatible with the polymer matrix of the composite material forming said portion.

[0007] Chemically compatible materials are defined as materials that can form bonds with each other at the molecular or atomic level. The materials can be the same or different materials. Thus, thanks to these characteristics, the presence of the sensor causes little or no degradation in the mechanical performance of the part in which the sensor is embedded. When the structural part near the sensor is damaged, the resulting deformation of the structural part material activates the piezoelectric element, which converts this wave into an electric current proportional to the wave energy. The sensor produces a signal in the form of bursts that can be analyzed to determine the The nature and severity of the damage, such as fiber breakage, interface failure, or delamination, are detected. Such a signal appears before the material completely breaks and therefore constitutes an alarm that can trigger maintenance or even prohibit the use of the part in question. Depending on the composite material and the part's structure (nature and orientation of the fibers), the signal appears as early as 40 to 50% of the maximum stress at which the material breaks. The waves that can be detected by such a sensor are primarily in the high-frequency range, preferably between 300 kHz and 600 kHz. If the structural part incorporates several sensors, it becomes possible to locate the damaged area.

[0008] Preferably, the electrodes and the piezoelectric body are arranged to form a QPS (Quantum Piezoelectric Sensor) type sensor.

[0009] This type of sensor is particularly well suited for the detection of acoustic emission such as that caused by the damage of a structural part.

[0010] According to a particular embodiment, the sensor is formed of a stack of the support, a first of the electrodes, the piezoelectric body, a second of the electrodes, the protective layer.

[0011] Preferably, the stack has a height of less than about 200 pm and the first outer layer has a thickness of less than about 100 pm.

[0012] This low thickness limits the impact of the presence of the sensor on the mechanical performance of the structural part.

[0013] According to a particular feature of the sensor, the piezoelectric body comprises a polymer matrix in which piezoelectric charges are embedded and, preferably, the piezoelectric body advantageously comprises between 20% and 50% by volume of piezoelectric charges and preferably at least 30% of charges.

[0014] This aims to ensure that the sensor provides signals that are detectable by the processing unit to which the sensor is connected and allows for relatively easy manufacturing of the sensor.

[0015] The fillers are for example PZT or BaTiO3 type ceramics having a sensitivity d33 between 300 and 600 pC.N 1 approximately.

[0016] A ceramic with a Curie temperature above 150°C or even 200°C is preferably chosen because this temperature allows the charges to retain their polarization despite the thermal stresses to which the sensor is subjected during its manufacture (for example during the baking of the composite).

[0017] The polymer of the piezoelectric body and its packaging is preferably a material chemically compatible with the polymer matrix of the composite material forming said part and belongs, for example, to the following group of technical and thermostable polymers: polyurethane, polyepoxide, polyester, polyamide, poly(phenylene ether-ether-ketone), polyphthalamide, polyethylene, polyaramide, po- lynorbonene, formophenolic, bakelite.

[0018] The invention also relates to a method for monitoring such a structural component. The method comprises the following steps: - during the manufacture of the structural part, integrate into a part of the structural part a sensor which has a rigidity at most equal to a final rigidity of said part and which includes a support, a protective layer and between the two a piezoelectric transducer comprising two electrodes applied against a piezoelectric body, the electrodes being connected to electrical signal conductors to transmit signals to the outside of said part (in wired mode or wirelessly via an antenna), the support and the protective layer each being made of a polymer chemically compatible with the polymer matrix of the composite material forming said part; - when using the structural part, capture signals transmitted by the conductors and monitor a change in the signals.

[0019] The process of the invention therefore makes it easy to monitor the integrity of a structural part from its development to the end of its life.

[0020] Preferably, the sensor is manufactured by screen printing.

[0021] This manufacturing method is particularly inexpensive and well suited to mass production.

[0022] Advantageously, the piezoelectric body comprises a polymer matrix in which piezoelectric charges are embedded and the process includes, during the manufacture of the sensor, a step of polarizing the piezoelectric charges to orient the dipoles of the charges according to a polarization direction relative to the sensor and, during the integration of the sensor into the structural part, the step of orienting the sensor to position the polarization direction relative to a predetermined direction of stress on the structural part.

[0023] The piezoelectric body is then positioned to be more sensitive in the predetermined direction of excitation.

[0024] Other features and advantages of the invention will become apparent from the following description of a particular, non-limiting embodiment of the invention. Brief description of the drawings

[0025] Reference will be made to the attached drawings, among which:

[0026] [Fig-1] [Fig.1] is a schematic top view of a sensor according to the invention;

[0027] [Fig.2] [Fig.2] is a schematic view of this sensor, in cross-section along line II-II of the [Fig.l];

[0028] [Fig. 3] [Fig. 3] is a schematic view of a structural part incorporating such sensor, in cross-section, with a monitoring device for the structural part.

[0029] [Fig.4] [Fig.4] is an example of the sensor's response as a function of the load exerted on the part until the composite material of the part is damaged;

[0030] [Fig. 5] [Fig. 5] illustrates the decrease in amplitude of the sensor response as a function of the distance during a Hsu Nielsen test. DETAILED DESCRIPTION OF THE INVENTION

[0031] With reference to the figures, the invention is described herein in application to a structural part 100 such as a wind turbine blade, a vehicle chassis, a pump or turbine housing, a building structure... As can be seen, the applications of the invention are numerous and varied.

[0032] The structural part 100 is made of a composite material comprising a polymer matrix and reinforcing fibers embedded in the polymer matrix (some reinforcing fibers have been symbolized in [Fig. 3], with magnification to make them visible). The fibers are, for example, made of glass, carbon, or polyaramid.

[0033] According to the invention, at least one sensor, generally designated as 10, is embedded in a portion of the structural part 100. The sensor 10 is flat, meaning that its thickness is small compared to its other dimensions. For example, a sensor with a main surface area of ​​20 mm by 60 mm (parallel to the main plane of the sensor) has a thickness of 200 µm.

[0034] The sensor 10 comprising two external layers - namely an electrically insulating support 11 and an electrical insulation layer 12 - and, between the two, a piezoelectric transducer comprising two electrodes 13, 14 applied against a piezoelectric body 15. The sensor 10 is here formed from a stack of the support 11, the electrode 13, the piezoelectric body 15, the electrode 14, the electrical insulation layer 12. The electrodes 13, 14 and the piezoelectric body 15 are arranged to form a QPS type sensor. The electrodes 13, 14 are each connected to an electrical conductor 16, 17 formed by a conductive track extending over the support 11 until it opens outside the sensor 10 to transmit electrical signals produced by the transducer to the outside of the structural part 100. The conductive tracks are for example made of an ink based on particles of silver, carbon or any other conductive material.

[0035] The piezoelectric body 15 comprises a polymer matrix in which piezoelectric charges are embedded (some piezoelectric charges, of micro or nanometric size, have been symbolized in [Fig. 2], with magnification to make them visible). Preferably, the piezoelectric body comprises at least 30% of charges. The charges are for example ceramics of type PZT or BaTiO3 having a sensitivity d33 (the sensitivity d33 is the sensitivity along the transverse direction, i.e. in the direction of the thickness, perpendicular to the main plane of the sensor) of between 300 and 600 pC.N 1 approximately.

[0036] The support 11, the electrical insulation layer 12, and the polymer forming the matrix of the piezoelectric body 15 are each made of a polymer chemically compatible with the polymer matrix of the composite material forming said part. By chemically compatible materials, it is understood that the materials can form bonds with each other at the molecular or atomic level. The materials may be the same or different materials. The polymer forming the matrix of the piezoelectric body 15 belongs, for example, to the following group: polyurethane, polyepoxide, unsaturated polyester, polyamide 11, poly(phenylene ether-ether-ketone), polyphthalamide, polyethylene, or any other material compatible with that of the part.

[0037] Preferably, the stack has a height of less than about 200 pm and the support has a thickness of less than about 100 pm.

[0038] The materials used and this relatively low height ensure that the sensor 10 has a rigidity at most equal to the rigidity of said part of the structural part 100 in which it is embedded.

[0039] The sensor 10 is here manufactured by screen printing and integrated into the structural part 100 during the manufacture of the structural part 100. The support 11 is preferably made in the form of a sheet on which the electrode 13, the piezoelectric body 15, the electrode 14, the electrically insulating layer 12 will be successively deposited.

[0040] Advantageously, during the manufacture of the sensor 10, a step is performed to polarize the piezoelectric charges of the piezoelectric body 15 in order to orient the charge dipoles along a polarization direction relative to the support 11 of the sensor 10 (the dipoles are generally oriented along their thickness, i.e., aligned with a direction perpendicular to the principal plane of the sensor). During the integration of the sensor 10 into the structural part 100, the sensor 10 is oriented to position the polarization direction relative to a predetermined direction of stress on the structural part 100. The piezoelectric body 15 is then positioned to be more sensitive in the predetermined stress direction. This is particularly useful, for example, when the structural part 100 has a preferred stress path. The sensor 10 is then positioned in the vicinity of this stress path and oriented accordingly.

[0041] The method for monitoring such a structural part includes the steps, during the use of the structural part 100, of capturing signals transmitted by the conductors and monitoring an evolution of the signals in order to deduce a state of the structural part 100.

[0042] To do this, the conductors 16 are connected to a connector 17 allowing a processing circuit 110 to be connected to the sensor 10. Alternatively, the conductors 16 are connected to an antenna itself carried by the structural part 10 and the processing circuit 110 is connected to a radio transmission antenna.

[0043] When structural part 100 is damaged in the vicinity of sensor 10, the wave resulting from the deformation of the material of structural part 100 stimulates the piezoelectric element 15, which converts this wave into an electrical current proportional to the wave energy. Sensor 10 produces an electrical signal in the form of bursts that can be analyzed (shape, frequency, maximum amplitude, and integral of the wave, etc.) by the processing circuit 110 to determine the nature and severity of the damage. If structural part 100 incorporates several sensors 10, the transmitted signals vary according to the distance of the sensors 10 from the damaged area, making it possible to locate the damaged area.

[0044] The processing circuit 110 can be configured to continuously process the signals output from the sensor. Alternatively, the processing circuit 110 can be configured to periodically process the signals output from the sensor. The periodicity can be fixed or variable, and will be set by an operator, for example, according to the operating conditions of the structural component.

[0045] Figures 4 and 5 show an example of the response of a sensor according to the invention.

[0046] Preferably, the device is arranged to have a background noise of 40 dB and a dynamic range of 60 dB. Preferably, the device is arranged to ensure a detection distance of at least 250 mm.

[0047] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0048] In particular, the sensor may have a different structure from that described.

[0049] It can comprise a different number of layers. The electrodes can be side by side. The sensor can comprise a support, a first electrically insulating layer, a first electrode, the piezoelectric body, a second electrode, a second electrically insulating layer, and an outer layer.

[0050] The thicknesses are given as an indication and may be different: the stack may in particular have a height greater than 200 pm.

[0051] The piezoelectric charge ratio can be greater than or less than 30%.

[0052] The materials may differ from those indicated as long as the compatibility condition is met. The polymer used is a material with high mechanical and thermal strength, such as a technical polymer preferably having a Young's modulus greater than 5 GPa and a temperature resistance above 200°C. The ceramic must have a flexibility similar to that of the composite material or at least sufficient flexibility so as not to hinder the shaping of the composite material.

[0053] A piezoresistive transducer can be placed on the part equipped with the sensor of the invention. The piezoresistive transducer makes it possible to detect an overall decline in the properties of the composite material, revealing in particular local deformations indicative of damage to the part. The piezoelectric transducer generally has a higher sensitivity than the piezoresistive transducer, which theoretically allows it to reveal damage earlier, but also makes it more sensitive to external disturbances (shocks, vibrations, etc.). The piezoresistive transducer thus makes it possible to confirm or refute the damage revealed by the piezoelectric transducer.

[0054] The invention may be manufactured by any suitable manufacturing method and in particular those mentioned in document EP-A-3320306.

[0055] It is possible to change the range of frequencies to which the sensor is sensitive, for example by modifying the arrangement of the electrodes of the piezoelectric transducer.

[0056] The invention can be implemented using a conventional system for the production and processing of acoustic emissions.

Claims

Claims

1. Structural part (100) comprising a part made of a composite material comprising a polymer matrix and reinforcing fibers embedded in the polymer matrix, and at least one sensor (10) which is embedded in said part, the sensor (10) comprising two external layers (11, 12) and, between the two, a piezoelectric transducer comprising two electrodes (13, 14) applied against a piezoelectric body (15), the electrodes (13, 14) being connected to electrical signal conductors (16, 17) for transmitting signals to the outside of said part, the external layers (11, 12) each being made of a polymer chemically compatible with the polymer matrix of the composite material forming said part.

2. Part according to claim 1, in which the electrodes (13, 14) and the piezoelectric body (15) are arranged to form a QPS type sensor.

3. Part according to claim 2, in which the sensor is formed from a stack of a first (11) of the external layers (11, 12), a first (13) of the electrodes (13, 14), the piezoelectric body (15), a second (14) of the electrodes (13, 14), a second (12) of the external layers (11, 12).

4. A part according to claim 3, wherein the stack has a height of less than approximately 200 pm.

5. A part according to claim 4, wherein the first outer layer (11) has a thickness of less than approximately 100 μm.

6. Part according to any one of the preceding claims, in which the piezoelectric body (15) comprises a polymer matrix in which piezoelectric charges are embedded.

7. Part according to claim 6, in which the piezoelectric body (15) comprises between 20% and 50% by volume of fillers and preferably at least 30% of fillers.

8. Part according to claim 6 or 7, in which the fillers are PZT or BaTiO3 type ceramics having a sensitivity d33 of between 300 and 600 pC.N 1 approximately.

9. Part according to claim 6, in which the polymer of the piezoelectric body (15) is made of a material chemically compatible with the polymer matrix of the composite material forming said part.

10. A part according to claim 6, wherein the polymer of the piezo-body electric (15) is a polymer from the following group: polyurethane, polyepoxide, unsaturated polyester, polyamide 11, poly(phenylene ether-ether-ketone), polyphthalamide, polyethylene.

11. Method for monitoring a structural part (100) comprising a part made of a composite material comprising a polymer matrix and reinforcing fibers embedded in the polymer matrix, comprising the steps of: - during the manufacture of the structural part (100), integrating into a part of the structural part (100) a sensor (10) which comprises two external layers (11, 12), and between the two a piezoelectric transducer comprising two electrodes (13, 14) applied against a piezoelectric body (15), the electrodes (13, 14) being connected to electrical signal conductors (16, 17) for transmitting signals to the outside of said part, the external layers (11, 12) each being made of a polymer chemically compatible with the polymer matrix of the composite material forming said part;- when using the structural part, capturing signals transmitted by the conductors (16, 17) and monitoring a change in the signals.;

12. The method of claim 11, wherein the sensor (10) is manufactured by screen printing.

13. Method according to claim 11, in which the piezoelectric body (15) comprises a polymer matrix in which piezoelectric charges are embedded and the method comprises, during the manufacture of the sensor (10), a step of polarizing the piezoelectric charges to orient the charges in a polarization direction relative to the sensor (10) and, during the integration of the sensor (10) in the structural part (100), the step of orienting the sensor (10) to position the polarization direction relative to a predetermined direction of stress on the structural part (100).

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