Fiber optic strain detector, and method for calibrating such a detector

Abstract

A deformation detector in a structure, comprising: - an elastic textile web (1) including a sensitive area (10) formed by alternating side-emitting optical fibers (2) and side-receiving optical fibers (3), woven parallel to each other with elastic fibers allowing reversible displacement of the optical fibers (2, 3) relative to each other along a sensitive axis (X) contained in the principal plane of the web; - a light source (4) for injecting a light signal at one end of the bundle of side-emitting optical fibers (2); - an optical sensor (5) configured to generate a signal representative of a variation in a physical quantity of the light captured by the side-receiving optical fibers (3) induced by the displacement of the optical fibers along the sensitive axis. Figure for the abbreviation: Fig 1

Description

Title of the invention: Optical fiber-based strain detector, and method for calibrating such a detector. Technical field

[0001] The invention relates to the field of detecting, using optical fibers, deformation of a civil engineering structure to be inspected, such as forming cracks. More particularly, the invention relates to a deformation detector based on optical fibers and a method for calibrating such a detector.

[0002] The invention finds its application in particular in the detection of cracks on a surface of a work of art or building. State of the art

[0003] Structures such as engineering works, buildings, civil engineering works, historical monuments, etc., are subject to seismic, mechanical and climatic hazards which combine with the natural aging of the materials which constitute them, and accelerate their deterioration.

[0004] In particular, concrete and reinforced concrete structures are subject to cracking and material degradation over time. To prevent the risk of accidents related to this deterioration, considerable attention is now paid to monitoring the condition of these structures. Data relating to the monitoring of these structures can be used to analyze and understand the behavior of a structure and to optimize maintenance costs.

[0005] There are various techniques for inspecting and monitoring structures, such as visual inspection, sampling, but also non-destructive techniques based on measuring instruments, including displacement sensors or strain gauges for detecting cracks.

[0006] With regard to concrete and reinforced concrete structures, crack detection can be carried out using fiber optic sensors, such as fiber optic extensometers, installed on or within a concrete structure for in-situ monitoring. For example, document WO 02 / 082010 describes a fiber optic extensometer comprising Bragg gratings, with readings obtained by spectral analysis of the beams. Description of the invention

[0007] The present invention proposes an alternative solution for strain detection based on optical fibers, less expensive than existing optical fiber-based solutions, offering good measurement sensitivity and suitable for monitoring or detecting cracks present or forming in concrete and reinforced concrete structures.

[0008] The invention thus relates to a deformation detector in a structure, comprising: - an elastic textile sheet comprising a sensitive area with a sensitive axis contained in the principal plane of the sheet, and an anchoring area of ​​the textile sheet to the structure; . the sensitive area being formed of an alternation of side-emitting optical fibers and side-receiving optical fibers, woven parallel to each other with elastic fibers allowing reversible displacement of the optical fibers relative to each other along at least the sensitive axis, the side-emitting optical fibers being grouped into a bundle at the level of a first edge of the textile sheet, and the side-receiving optical fibers being grouped into a bundle at the level of a second edge of the textile sheet; - a light source arranged opposite one end of the side-emitting optical fiber bundle, and configured to emit a light signal inside said bundle; - an optical sensor arranged opposite one end of the side-receiving optical fiber bundle, and configured to generate a signal representative of a variation of a physical quantity of the light captured by the side-receiving optical fibers induced by the displacement of the optical fibers in the sensitive axis.

[0009] The sensitive axis is advantageously substantially perpendicular to the main axis of the optical fibers. The sensitive area is intended to be positioned in a zone where a crack is likely to form or develop. The anchoring zone may be the sensitive area itself, or be at least on the periphery of the sensitive area, for example, only on the periphery, or on the periphery and on the sensitive area.

[0010] According to one embodiment, the light source can be a light-emitting diode LED configured to emit light in the visible spectrum, for example white light, or in the near-visible spectrum, for example a near-infrared LED.

[0011] Advantageously, the optical sensor can be a color sensor (or spectral detector) configured to generate at least one signal representative of the variation of the correlated color temperature (CCT) of the light captured by the optical fibers with lateral reception.

[0012] Lateral emission and reception of light rays by an optical fiber can be achieved through the presence of asperities or alterations on the peripheral surface of the optical fiber. These asperities or alterations, which result in imperfections on the surface of the optical fiber cladding and at the interface between the core and the cladding, allow the lateral scattering of light rays in all directions. directions, and the lateral reception of at least some of these rays by neighboring optical fibers. These alterations can be achieved by removing material from the thickness of the optical fiber cladding, for example, by roughening the outer surface of each fiber, or by creating incisions through mechanical, thermal, or chemical treatment applied to the optical fibers. Both the position and shape of these imperfections are generally random, and the penetration of light rays into the neighboring fiber is purely probabilistic. Consequently, the probability that more rays will penetrate is higher if the neighboring fiber is closer. The farther away the fiber is, the more rays will be lost to the external environment.

[0013] Due to the specific configuration of the optical fibers in the sensitive area and their positioning in a medium subject to change, there are factors that can interfere with the measurements. CCT measurements offer the advantage of providing optical information to discriminate or detect potential physical phenomena, such as changes in the light propagation medium due to the presence of fluids that alter the light signal (muddy water, rusty water, turbid gas, etc.), which may be superimposed on the deformation or appear independently of any deformation phenomenon, and which are likely to introduce deformation detection errors.In other words, since the sensor can be sensitive to other physical phenomena (temperature, humidity), its response could differ for each phenomenon, having a non-uniform impact on the evolution of each colorimetric component. The phenomenon could then be identified based on the behavior of the different colors. These curves could be combined with physical solutions to eliminate the effects caused by these other phenomena and isolate the measurements related to distance variation.

[0014] Thus, according to one embodiment, the color sensor can be configured to further provide information relating to the colorimetric composition of the light captured by the side-receiving optical fibers, for example, the CIE RGB or XYZ or Lab or du'v trichromatic coordinates. For example, the evolution or variation of the colorimetric composition can be analyzed to deduce the variation of the distance and / or physical phenomena appearing in the environment of the detector.

[0015] Indeed, the XYZ values ​​change depending on the measurement distance, and therefore on the distance between two adjacent optical fibers, although the ratio between the XYZ values ​​for all distances remains essentially the same when dealing with the same light. The present invention thus exploits this variation in the values ​​of the chromatic components for the detection of crack-type deformation.

[0016] For example, the color sensor may comprise different optical cells, for example RGB optical cells each formed of a photodiode coupled to a filter corresponding to a chromaticity coordinate of the XYZ coordinate system, and a C optical cell formed of a photodiode coupled to a transparent filter. The correlated color temperature (CCT) can be calculated from the XYZ values.

[0017] The optical sensor may include at least: - an optical cell corresponding to "C" (or "clear" in English): corresponds to a filter called "clear" (or transparent), that is to say that it does not filter any color or wavelength; - an optical cell corresponding to "R" (for "Red:" in English) or "X": red filter, that is to say, mainly captures wavelengths related to the color red; - an optical cell corresponding to "G" (for green in English) or "Y": green filter, that is to say, mainly captures wavelengths related to the color green; - an optical cell corresponding to "B" (for blue in English) or "Z": blue filter, that is to say mainly captures wavelengths related to the color blue.

[0018] The normalized values ​​from cells R, G and B are therefore naturally lower than the values ​​from cell C. The C values ​​are a good indication of the overall luminous intensity of the light captured.

[0019] According to another variant, the optical sensor can be configured to generate a signal representative of the variation in the light intensity (for example, measurements in lux) captured by the side-emitting optical fibers.

[0020] In other words, the physical quantity of the light captured by the side-receiving optical fibers can be the correlated color temperature CCT, or the luminous intensity of the light captured by the side-receiving optical fibers.

[0021] Thus, a system for detecting physical phenomena such as deformation or displacement may include: - the deformation detector as presented above; - a communication module configured to send information from the detector to a module for analyzing and processing this information.

[0022] In practice, the textile mat can be fixed to the surface of a structure in an active area to be monitored or can be embedded in the structure, for example, cast in concrete during construction. When the textile mat is intended to be positioned on the surface, it may include a protective layer, for example, in the form of a coating or a film laminated to the surface, covering the area not intended to be in contact with the surface of the structure, so as to limit ambient light interference. This protection may also be rigid. This protective layer may The textile sheet has filtering optical properties on one side to prevent all or part of the ambient light from interfering with measurements, and reflective properties on the other side to optimize the useful light flux received by the optical fibers with side reception. The textile sheet can be attached to the surface by bonding the anchoring areas, for example, with epoxy adhesive. The textile sheet thus takes the form of an adhesive patch, and at least the sensitive portion can have, at rest (i.e., without stretching), a length between 3 cm and 30 cm and a width between 3 cm and 30 cm.

[0023] In practice, side-emitting and side-receiving optical fibers can be bundled together at the same edge of the textile layer or at separate edges of the textile layer.

[0024] The invention also relates to a method for calibrating the deformation detector described above, comprising: - fixing the anchoring zones between two jaws in a predefined initial position; - activation of the light source to inject light into the side-emitting optical fibers; - the movement of at least one of the two jaws between an initial position and a final position and following a constant and predefined speed of movement, so as to carry out at least one deformation cycle of the sensitive area of ​​the textile sheet, the deformation cycle comprising a controlled stretching phase followed by a controlled release phase, the stretching phase inducing the optical fibers to move away from each other in the direction of the sensitive axis, and the release phase inducing the optical fibers to move closer together in the direction of the sensitive axis; - the measurement by the optical sensor, throughout the deformation cycle, of the variation in the physical quantity of the light captured by the optical fibers with lateral reception; - the determination of a polynomial equation of order between 2 and 4, giving, as a function of the variation of the measurement of the physical quantity of the light captured by the optical fibers with lateral reception, the variation in the displacement of the jaw or of the optical fibers relative to each other.

[0025] In other words, this calibration method makes it possible to determine a model that gives the relationship between the variation in the measurement of the physical quantity of the captured light and the variation in the displacement of the optical fibers. In particular, the calibration method makes it possible to generate a polynomial equation of order between 2 and 4, which gives, as a function of the variation in the measurement of the physical quantity of the captured light, the variation in the displacement of the optical fibers.

[0026] In practice, the determination of the polynomial equation is obtained via a polynomial regression, which improves the detection sensitivity, making it possible in particular to detect very small variations in displacement between two optical fibers.

[0027] According to one embodiment, the displacement step comprises performing a series of deformation cycles of the sensitive area, the first deformation cycle being used to determine said polynomial equation, and the subsequent deformation cycles being used to verify the consistency of the information relating to the variation of the displacement determined from the polynomial equation. In practice, when the consistency is poor, the textile mat and / or the paired electronic system are discarded.

[0028] In practice, due to the specific characteristics of the textile fabric manufacturing process, including weaving and alterations, a textile fabric does not exhibit exactly the same optical characteristics or behavior as another textile fabric produced using the same manufacturing process. Therefore, the polynomial equation determined via the calibration method of the present invention is specific to each pair formed by the textile fabric and the optical sensor, so that the calibration method is performed for each textile fabric paired with the optical sensor, intended for use in the detection and monitoring of structures.

[0029] Advantageously, said physical quantity of the light captured by the side-receiving optical fibers can be the correlated color temperature CCT.

[0030] The physical quantity of the light captured by the optical fibers with side reception can also be a spectral or luminous intensity quantity, such as Lux, candelas, lumens or any other optical quantity.

[0031] Advantageously, the predefined initial position corresponds to a pre-tensioned configuration imposed on the sensing area. In other words, in the initial position, the optical fibers are spaced in the direction of the sensing axis by a predefined minimum distance, for example, 0.2 mm. Thus, the polynomial equation determined via the calibration method of the invention is specific not only to the textile mat used, but also to the imposed pre-tensioned configuration. Indeed, due to the specific manufacturing characteristics of the textile mat, the sensing area does not respond in exactly the same way depending on the initial spacing between the optical fibers. In practice, the textile mat is therefore preferably positioned with the same pre-tensioned configuration on or within the structure to be monitored. Installation methods for ensuring the pre-tensioning of the textile mat on or within the structure to be monitored can be used.

[0032] The displacement step may consist of moving not just one of the two jaws, but of moving both jaws symmetrically with respect to each other, the jaws being those of a traction machine.

[0033] The maximum travel distance of the jaw can be between 2 mm and 10 mm, depending on the critical dimensions of the crack to be detected or monitored. The travel speed can be between 0.1 mm / min and 1 mm / min, for example 0.5 mm / min or 0.3 mm / min.

[0034] In an embodiment in which the optical sensor is a color sensor configured to provide several response curves, each relating to the evolution of a colorimetric component of the light captured by the side-receiving optical fibers, the polynomial equation can be determined from the response curve exhibiting the largest amplitudes or intensities. For example, if the optical sensor provides the evolution of the R, G, B, and Clear components, the curve relating to Clear is used.

[0035] In another embodiment, the optical sensor can be configured to generate a signal representative of the variation in the luminous intensity of the light (for example, measurements in candela) captured by the optical fibers with lateral reception.

[0036] Preferably, the characteristics of the calibration carried out according to the calibration method of the invention, such as the polynomial equation and the pretension distance, can be encoded in the form of a barcode or a QR code affixed to the strain detector, or stored in an RFID tag.

[0037] The optical sensor thus calibrated makes it possible to detect a crack opening of the order of 0.1 mm, and also makes it possible to monitor the evolution of the crack over time.

[0038] This calibration thus makes it possible to detect deformations of 0.1% for a luminous area 100mm wide.

[0039] In practice, measurements are taken periodically at regular intervals, and the sensor can be configured to send an alert signal outside of these periodic measurements when the formation of a crack is detected. Brief description of the drawings

[0040] The present invention and its advantages will become more apparent from the following description of several embodiments given by way of non-limiting examples, with reference to the accompanying drawings, in which:

[0041] [Fig.1] is a schematic representation of the strain detector according to one embodiment.

[0042] [Fig.2] is a schematic representation of the strain sensor calibration process according to one embodiment.

[0043] [Fig.3] is another schematic representation of the strain sensor calibration process according to one embodiment.

[0044] [Fig.4] is a graphical representation of the evolution over time of the values ​​from the R, G, B and C cells (in Kelvin) of an optical sensor during deformation phases of the textile sheet, according to one embodiment.

[0045] [Fig.5] is an illustration of an example of a polynomial equation.

[0046] [Fig.6a] is a graphical representation of the evolution of the measurements from the detector of the invention and the actual measurements of crack width by the DIC method, on a test specimen with dimensions of 10 cm x 10 cm x 40 cm.

[0047] [Fig.6b] is a graphical representation of the evolution of the measurements from the detector of the invention and the actual measurements of crack width by the DIC method, on a specimen with dimensions of 15 cm x 15 cm x 60 cm.

[0048] [Fig.6c] is a graphical representation of the evolution of the measurements from the detector of the invention and the actual measurements of crack width by the DIC method, on a test specimen with dimensions of 15 cm x 24 cm x 330 cm.

[0049] [Fig. 7 is a photo of a part of the specimen subjected to deformation, on which a crack is created. Description of the implementation methods

[0050] A deformation or displacement detector according to one embodiment is schematically illustrated in [Fig. 1]. This detector is intended for monitoring an active area of ​​a concrete or reinforced concrete structure. The deformation may correspond to the appearance of a crack or the evolution of a crack.

[0051] The strain detector illustrated in [Fig. 1] comprises: - a textile tablecloth with 1 elastic band; - a light source 4; and - an optical sensor 5.

[0052] The textile layer 1 includes a sensitive area 10 with sensitive axis X contained in the main plane of the layer 1, and an anchoring area 11 at the periphery of the sensitive area 10.

[0053] The sensitive area 10 is formed by alternating side-emitting optical fibers 2, hereafter referred to as the "emitting optical fiber," and side-receiving optical fibers 3, hereafter referred to as the "receiving optical fiber." These emitting optical fibers 2 and receiving optical fibers 3 extend substantially perpendicularly to the sensitive axis X and are positioned alternately along the direction of the sensitive axis X. These optical fibers 2 and 3 are woven with elastic fibers allowing movement reversible optical fibers 2, 3 relative to each other along the sensitive axis X.

[0054] The transmitting optical fibers 2 are grouped into a bundle 20 at the level of an edge of the textile layer, and the receiving optical fibers 3 are grouped into a bundle 30 at the level of the same edge of the textile layer.

[0055] The anchoring zone 11 can be located on either side of the sensitive zone 10 in the direction of the sensitive axis X. In practice, the anchoring zone 11 can also be the sensitive zone 10, particularly when the sensitive zone 10 is intended to be bonded or fixed in concrete. The anchoring zone can be made by weaving non-stretchable or non-deformable fibers, so that only the sensitive zone deforms in order to limit the absorption of part of the overall deformation by the anchoring zone.

[0056] Lateral emission and reception of light rays by an optical fiber can be achieved through alterations made to the peripheral surface of the optical fiber. In other words, optical fibers have alterations or physical defects on their peripheral surface that allow for the lateral emission and reception of light rays. These alterations can be achieved by removing material from the thickness of the optical fiber cladding. The alterations can be achieved in various ways, for example, by roughening the outer surface of each fiber, or by creating incisions through mechanical, thermal, or chemical treatment applied to the optical fibers. The manufacturing process described in document EP3303667 can be implemented.

[0057] The textile tablecloth is thus in the form of an elastic patch and can be composed of elastane warp yarns or any other elastic deformable textile material, and non-elastic weft yarns (for example polyester, cotton, etc.), as well as emitting optical fibers 2 and receiving optical fibers 3.

[0058] A protective layer for the sensitive area 10 against external physical phenomena (rain, frost, sun, UV radiation, etc.) that could disrupt the measurements may be provided. For example, a coating layer or a pocket made by thermal welding or ultrasound may be used to encapsulate the textile layer. An inner face of the protective layer may be white or have reflective properties to optimize the light flux captured by the receiving optical fibers 3.

[0059] By way of example, the textile sheet may exhibit an elasticity in the sensible axis between 10% and 20%.

[0060] The coating layer may have the following properties: - Opacity: the textile tablecloth remains 100% opaque even after stretching; - Waterproofing.

[0061] For example, the protective layer can be made of a double-layered, deformable silicone, allowing the patch to stretch along the cracks. The first silicone layer can be transparent and fill the gaps between the optical fibers while allowing light to propagate between them. The second surface layer does not penetrate between the fibers and can be deformable and opaque. Both layers are hydrophobic and prevent water from interfering with the light scattered by the optical fibers.

[0062] The light source 4, for example a white LED, is arranged opposite one end 200 of the bundle 20 of emitting optical fibers 2, and is configured to emit a light signal inside the emitting optical fibers 2. In practice, the ends of the optical fibers are bundled together in a ferrule or a crimped or glued connector.

[0063] The optical sensor 5 is arranged opposite one end 300 of the receiving optical fiber bundle 30 3.

[0064] In one embodiment, the optical sensor 5 is a color sensor configured to: - provide a signal representative of the variation in the correlated color temperature (CCT) of the light captured by the receiving optical fibers 3, and / or - provide signals that are representative of each of the variation of RGB or XYZ chromatic values.

[0065] In another embodiment, the optical sensor 5 can be a photosensitive cell configured to generate an electrical signal representative of the variation in light intensity captured by the receiving optical fibers 3, or of the variation of any other physical quantity representative of a variation in light intensity such as Lux or Candelas.

[0066] For example, the color sensor may include optical cells in the form of photodiodes, each coupled to an optical filter: - an optical cell corresponding to "C" (or "clear" in English): coupled to a filter called "clear", meaning that it does not filter any color or wavelength; - an optical cell corresponding to "R" (for "Red:" in English) or "X": red filter, that is to say, mainly captures wavelengths related to the color red; - an optical cell corresponding to "G" (for green in English) or "Y": green filter, that is to say, mainly captures wavelengths related to the color green; - an optical cell corresponding to "B" (for blue in English) or "Z": blue filter, that is to say mainly captures wavelengths related to the color blue.

[0067] The color sensor thus provides the normalized values ​​of the R, G and B (or X, Y and Z) components, the values ​​from the "C" cell, and the correlated color temperature CCT calculated from the XYZ values.

[0068] Due to the specific manufacturing processes of the textile sheet, including weaving and alterations, one textile sheet does not exhibit exactly the same optical characteristics or behavior as another textile sheet produced using the same manufacturing process. Therefore, each textile sheet requires calibration to determine a relationship between the variation in the measured physical quantity of the captured light and the variation in the displacement of the optical fibers.

[0069] The detector calibration method according to one embodiment comprises, with reference to Figures 2 and 3: - the fixing 60, 60a, 60b of the anchoring zones 11 between two jaws 70, 71 in predefined initial position.

[0070] This predefined initial position corresponds to a pre-tensioned configuration imposed on the sensitive area 11, in which the adjacent optical fibers 2, 3 are spaced in the direction of the sensitive axis X by a predefined minimum distance, for example 0.2 mm. The sensitive portion 10 therefore has an initial length of L0 in the sensitive axis.

[0071] The calibration process then comprises: - the activation 61 of the light source 4 to inject light into the emitting optical fibers 2; - the displacement 62 of a jaw 70 along the sensitive axis X, between an initial position and a final position at a constant speed of displacement, for example 0.5 mm / min, so as to achieve at least one deformation cycle, for example four deformation cycles, of the sensitive area 10 of the textile sheet.

[0072] Each deformation cycle consists of a controlled stretching phase, followed by a controlled relaxation phase.

[0073] The stretching phase (modeled by arrow 72 in [Fig. 3]) corresponds to the progressive movement of the movable jaw 70 away from the fixed jaw 71 at the speed of travel, from the initial position to the final position, and results in an increase in the spacing between the optical fibers. The length of the sensitive portion 10 increases, going from L0 to L1.

[0074] The release phase (modeled by arrow 73 on [Fig.3]) corresponds to a progressive approach of the movable jaw 70 relative to the fixed jaw 71 at the speed of movement, from the final position to the initial position, and results in a decrease in the spacing between the optical fibers.

[0075] The calibration process further comprises: - the measurement 63 by the optical sensor 5, during all deformation cycles, of the variation of the correlated color temperature of the light captured by the receiving optical fibers 3, or of the variation of each of the values ​​of the components R, G and B (or X, Y and Z) and C of the light captured by the receiving optical fibers 3, or of the variation of the luminous intensity through a physical quantity such as the Lux or the Candela for example, depending on the nature of the optical sensor 5.

[0076] Fig. 4 illustrates the evolution of the values ​​of the components R, G, B and C (Curves R, G, B and C respectively) of the light captured by the receiving optical fibers 3, according to the calibration process above.

[0077] The normalized values ​​from cells R, G and B are naturally lower than the values ​​from cell C. Thus, the values ​​from the Clear cell are used for the rest of the calibration process which consists of determining 64 a polynomial equation of order between 2 and 4, giving, as a function of the variation of the value from the Clear cell, the variation in the displacement of the jaw or the optical fibers relative to each other, by carrying out for example a polynomial regression on the values ​​of the Clear cell.

[0078] Figure 5 illustrates an example of a polynomial equation acquired based on measurements obtained during the stretching phase of the calibration process. The x-axis represents the measurements of the Clear cell, and the y-axis represents the displacements between the jaws (in mm). The continuous curve C represents the actual evolution of the measurements from the Clear cell, and the PR points correspond to those of the polynomial regression.

[0079] Thus, the polynomial equation determined via the calibration process of the invention is not only specific to the textile sheet used, but also to the imposed pre-tension configuration.

[0080] Experimental result:

[0081] To verify the consistency of the deformation measurements, measurements are obtained using the detector of the invention and using the Digital Image Correlation (or DIC for "Digital Image Correlation" in English) method.

[0082] The evolution curves of the measurements from the detector of the invention incorporating the calibration presented above (C1 curves) and the evolution curves of the actual measurements of crack width (C2 curves) are shown in Figures 6a to 6c.

[0083] With the detector of the invention, the textile sheet is fixed to an active area of ​​a concrete structure, and a mechanical force is applied to the structure so as to cause a crack to appear and to change the width of this crack. The measurements were taken with a sensor having a sensitive area of ​​7 cm x 7 cm, and test specimens of different dimensions.

[0084] [Fig. 6a]: Dimensions of the test specimen: 10 cm x 10 cm x 40 cm

[0085] [Fig. 6b]: Dimensions of the test specimen: 15 cm x 15 cm x 60 cm

[0086] [Fig. 6c]: Dimensions of the test specimen: 15 cm x 24 cm x 330 cm

[0087] Using the DIC method, for each test specimen, one face of the concrete beam is painted white, and a black paint speckling is applied to the area to be deformed. A camera on a tripod is placed facing the beam, taking photographs every 5 seconds of the speckling as the specimen deforms. The crack is formed under the same conditions as for the detector of the invention. After deformation, the photographs are analyzed using software that measures the evolution of the spacing over time of the points of the speckling on either side of the crack. Figure 7 shows a photograph of a portion of the specimen E subjected to deformation, on which a crack F is created.

[0088] A good correlation of the measurements is observed.

[0089] The same principle can be applied to a configured optical sensor 5 to measure a variation in the light intensity captured by the receiving optical fibers, using a physical quantity such as the Lux or the Candela, for example. Thus, the measurements from the optical sensor 5 are used to determine a polynomial equation of order between 2 and 4, which gives, as a function of the variation in the light intensity captured by the receiving optical fibers, the variation in the displacement of the jaw(s) or of the optical fibers relative to each other.

[0090] The strain sensor of the invention, thus calibrated, can be used for the detection and monitoring of cracks or crack development. The textile mat can be fixed either by attaching the sensitive area to the area to be monitored, for example, by gluing the sensitive area directly to the surface of the structure, or by adhering an anchoring zone around the periphery of the sensitive area to the surface of the structure. The textile mat can also be anchored by embedding it in the concrete. The textile mat is fixed with an initial pretension corresponding to the calibration conditions.

Claims

Demands

1. A strain detector in a structure, comprising: - an elastic textile web (1) including a sensitive area (10) with sensitive axis (X) contained in the principal plane of the web, and an anchoring area (11) at least on the periphery of the sensitive area (10); . the sensitive area (10) being formed of an alternation of side-emitting optical fibers (2) and side-receiving optical fibers (3), woven parallel to each other with elastic fibers allowing reversible displacement of the optical fibers (2, 3) relative to each other along at least the sensitive axis, the side-emitting optical fibers (2) being grouped into a bundle (20) at the level of a first border of the textile web, and the side-receiving optical fibers (3) being grouped into a bundle (30) at the level of a second border of the textile web;- a light source (4) arranged opposite one end (200) of the bundle (20) of side-emitting optical fibers (2), and configured to emit a light signal inside said bundle (20); - an optical sensor (5) arranged opposite one end (300) of the bundle (30) of side-receiving optical fibers (3), and configured to generate a signal representative of a variation of a physical quantity of the light captured by the side-receiving optical fibers (3) induced by the displacement of the optical fibers in the sensitive axis.

2. Strain detector according to claim 1, wherein the light source is a light-emitting diode LED configured to emit light in the visible or near-infrared spectrum.

3. Deformation detector according to claim 1 or 2, wherein the optical sensor 5 is a color sensor configured to generate at least one signal representative of the variation in the correlated color temperature CCT of the light captured by the side-receiving optical fibers (3), or a sensor configured to generate a signal representative of the variation in the luminous intensity of the light captured by the side-receiving optical fibers.

4. A deformation detector according to claim 3, wherein the color sensor is configured to further provide information relating to the colorimetric composition of the light captured by the optical fibers with side reception, in the form of CIE RGB or XYZ or Lab or du'v trichromatic coordinates.

5. Deformation detector according to claim 3, wherein the color sensor comprises RGB optical cells each formed of a photodiode coupled to a filter corresponding to a chromatic coordinate of the XYZ frame, and an optical cell C formed of a photodiode coupled to a transparent filter.

6. A method for calibrating a strain detector according to any one of claims 1 to 5, comprising: - fixing (60) the anchoring areas between two jaws in a predefined initial position; - activating (61) the light source to inject light into the side-emitting optical fibers; - moving (62) at least one of the two jaws between an initial position and a final position and at a constant and predefined speed of movement, so as to carry out at least one strain cycle of the sensitive area of ​​the textile web, the strain cycle comprising a controlled stretching phase followed by a controlled release phase, the stretching phase inducing the optical fibers to move away from each other in the direction of the sensitive axis, and the release phase inducing the optical fibers to move closer together in the direction of the sensitive axis;- the measurement (63) by the optical sensor, throughout the deformation cycle, of the variation of the physical quantity of the light captured by the optical fibers with lateral reception; - the determination (64) of a polynomial equation of order between 2 and 4, giving, as a function of the variation of the measurement of the physical quantity of the light captured by the optical fibers with lateral reception, the variation in the displacement of the jaw or of the optical fibers relative to each other.

7. Calibration method according to claim 6, wherein the determination of the polynomial equation is obtained via a polynomial regression.

8. A calibration method according to claim 6 or 7, wherein the displacement step (62) comprises performing a succession of deformation cycles of the sensitive area, the first cycle of deformation being used for the determination (64) of said polynomial equation, and other deformation cycles being used to verify the consistency of the information relating to the variation of the displacement determined from the polynomial equation.

9. Calibration method according to any one of claims 6 to 8, wherein said physical quantity of the light captured by the side-receiving optical fibers is the correlated color temperature CCT, or the luminous intensity of the light captured by the side-receiving optical fibers.

10. Calibration method according to any one of claims 6 to 9, wherein the predefined initial position corresponds to a pre-tension configuration imposed on the sensitive area.

11. Calibration method according to any one of claims 6 to 9, wherein the optical sensor is a color sensor configured to provide several response curves, each relating to the evolution of a colorimetric component or the intensity of the light captured by the optical fibers with side reception, and the determination (64) of the polynomial equation is obtained from the response curve exhibiting the largest amplitudes or intensities.

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