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    • 专利摘要:
    The present invention relates to a method and a device for the analysis of a structure by diffuse acoustoelastic field tomography and correlation. An optical fiber comprising a plurality of FBG (Fiber Bragg Grating) type measuring points, comprising Bragg grating type sensors, is deployed in or on the structure to be analyzed. The method includes light emission in the optical fiber and correlation measurement for each pair of FBG sensors. In a development, prior imaging of the structure is performed by reconstruction of the propagation speeds. Other developments include: determination of the positions of the FBG sensors, calibration of the tomography, rosette configuration of the sensors constituting the measurement points, the use of a plurality of optical fibers, multiplexers, lasers, optical circulators, omnidirectional optical sensors, active noise sources, such as piezoelectric transducers, integrated or not in the structure.
    公开号:FR3014200A1
    申请号:FR1361916
    申请日:2013-12-02
    公开日:2015-06-05
    发明作者:Bastien Chapuis
    申请人:Commissariat a lEnergie Atomique CEA；Commissariat a lEnergie Atomique et aux Energies Alternatives CEA；
    IPC主号:
    专利说明:

    [0001] FIELD OF THE INVENTION The invention relates to the field of metrology and in particular to the control of health of structures by means of optical fibers. State of the art The control of the integrity of the structures (structures, planes or pipelines for example) during their life is generally done during maintenance operations, with inspection and human intervention. A concrete technical problem consists, for example, in detecting and dimensioning a corroded area on an aircraft fuselage.
    [0002] For these integrity checks, so-called non-destructive (NDT) control methods are generally used according to so-called "conventional" methods (by ultrasound, according to electromagnetic methods, etc.).
    [0003] In recent years there have been research developments aimed at integrating sensors into the structure at key points of the structures, in order to automate the measurements (for example at regular intervals, these intervals being generally close together in time) and to be able to access information on the state of health of certain inaccessible areas, without dismantling or interrupting the functioning of the structure. In general, these developments aim to space maintenance intervals, and thus save money.
    [0004] In particular, some researches envisage the use of guided ultrasonic waves (OG) emitted and detected by piezoelectric transducers (for example PZT type) integrated into the structure. These guided waves propagate over a large distance (a few tens of cm to a few hundred meters in very favorable geometries such as pipelines), so that a limited number of transducers can control a large area. Other technologies can be used to emit and / or detect guided ultrasonic waves (in addition to optical fibers, PVDF films or magnetostrictive sensors for example). A general technical problem lies in finding an acceptable compromise between the number of sensors to integrate (cost, size, weight, etc.) and the quality of the information recoverable by these sensors. A high number of sensors means a high cost and a low number of sensors often implies a lack of reliability of the information, risks of false alarms, or a lack of redundancy in the event of a sensor failure. The multiplication of the sensors, however, poses other specific problems (for example each integrated sensor may constitute an additional point of weakness, which could lead to new defects in the structure). For each sensor, it is also necessary to provide power supply son, which is not always possible. In industrial practice, very few applications provide a satisfactory compromise. As regards the nature of the sensors, the known solutions of the state of the art using lasers as measuring systems are not usable in all circumstances. In particular, lasers can not be integrated into structures.
    [0005] Some known approaches consist in carrying out a reference measurement of the structure in the healthy state in order to notice a difference with a subsequent state to reveal the presence of the defect. In order to make this operation more reliable, different signal processing techniques exist, in particular to compensate for the influence of the temperature, but none is really effective. In all cases the interpretation of the signals remains very delicate. The various aspects of the invention overcome these disadvantages, at least in part. SUMMARY OF THE INVENTION Certain embodiments of the invention advantageously provide for the use of Bragg gratings, in particular "FBG sensors" or "FBG sensors" or "FBG measuring points", "FBG" being the acronym of the English expression "Fiber Bragg Grating". A Bragg grating (or distributed Bragg reflector) is a quality reflector used in waveguides, for example in optical fibers. It is a structure in which layers of two different refractive index materials alternate, which causes a periodic variation of the effective refractive index in the guide. A Bragg grating is a submicronic modulation of the refractive index of the core of the fiber: a network of a few millimeters thus comprises several thousand steps. From a functional point of view, it plays the role of a reflector for a fine spectral band centered at a characteristic wavelength proportional to the pitch and index of the core of the fiber. Thus, any modification of these parameters proportionally shifts the Bragg wavelength. The tracking of its spectral displacements makes it possible to go back to the inductive parameters, such as the temperature or the deformations undergone locally by the optical fiber. These Bragg gratings are made by laser within the core of single-mode fibers. The inscription of these networks may in particular be carried out by transverse exposure with an interference pattern created by two laser beams. According to one embodiment, there is disclosed a method of analyzing a diffuse field correlation structure, an optical fiber having a plurality of measurement points, a measurement point comprising Bragg grating type sensors (Fiber Bragg Grating, FBG), the optical fiber being deployed "in" (eg "posteriorly" or "natively embedded in") or "on" (eg "posed" or "attached" or "associated with") the structure to be analyzed, the method comprising light emission in the optical fiber; and the correlation measurement for at least a portion of the FBG sensor pairs of the acoustoelastic field propagating "in" (or "within", "through", "via") the structure. Sensors or pairs of FBG sensors can be interrogated substantially simultaneously. By the term "substantially", reference is made to the speed of elastic waves and to the fact that metrologically the interrogations occur in a time delta (close time intervals to obtain significant measurements from the point of view of the propagation of elastic waves in the structure). All or some of the sensors can be interrogated, according to various implementations. A subset of sensors may be interrogated substantially simultaneously, while another subset may be subject to a delayed interrogation (for example sequentially or in parallel manner in pairs or even combine these modes of operation. interrogation, rotation, etc.). The acousto-elastic field refers to the field of mechanical waves (sound, ultrasound, etc.) that propagate in a solid medium. Unlike the case of the fluid, there are two types of acoustic waves for a solid material. These waves are better known as elastic waves (shear and compression-traction). The acoustoelastic effect reflects a dependence of the propagation velocity of the acoustic waves as a function of the state of deformation of the solid. The structure to be analyzed does not involve any particular restrictions insofar as any type of structure (particularly industrial) can be analyzed by the methods and systems presently described. In a development, the method further comprises a step of reconstructing the propagation velocities by tomography, the imaging being carried out by inverting all the flight times between the FBG sensors, each flight time for each pair of FBG sensors being deduced from the correlation measurement. This development is optional. It has the advantage of improved subsequent interpretation.
    [0006] In a development, the position in the space of each measuring point is previously and individually measured. This solution has the advantage of its simplicity of implementation.
    [0007] In a development, the temperature of the structure is measured and a variation of flight time induced by a temperature change is compensated. The temperature can indeed affect the flight times and it is appreciable to be able to correct or compensate for the thermal effects. Concretely, a thermocouple can be used but other methods of measurement are possible. In one development, the imaging of the tomographic structure is performed by measuring at least two flight times, a first measurement being made in an initial or reference state and a second measurement being performed in a later state (for the same pairs of measuring points). The subsequent state is called "current", so it corresponds to the present time of the measurement ("second measurement set"). The realization of a tomography (entirely optional) on the data resulting from the first measurement makes it possible in particular to identify certain geometrical particularities of the structure. This correlates with a static measurement made in the initial (or reference) state. The imaging of the structure at rest can be possibly subtracted from subsequent images of the structure (subtraction of pixels, that is to say in terms of image content). In other words, this optional mapping makes it possible to identify certain geometrical features of the structure so as not to confuse them with defects on the mappings obtained on the second measurement set.
    [0008] In one development, the method further comprises a second measurement performed in a later state for the same pairs of measurement points as the first measurement and further comprises a tomography mapping of propagation velocity variations in the structure between the state. initial state and the subsequent state obtained from differences in flight times measured between the two states. In other words, the variation of the volp times (measured for the couples) between the two states is measured. This makes it possible to obtain, by tomography, a mapping of the elastic (elasto-acoustic) wave propagation velocity variations between the moment of the measurement associated with the current state (present) and the moment associated with the reference state. . For example, between the instant t of the measurement and the instant to initial, we can see that the waves go "slower" (respectively "faster") in some places and deduce the identification of defects or damage caused In a development, a measurement point comprises an FBG sensor. The use of a sensor of this type to make diffuse field correlation has not been described a priori. In another development, a measuring point comprises three receivers and directional FBG sensors substantially disposed at 120 ° from each other in a so-called rosette configgation. The rosette configuration is the compromise that minimizes the number of hardware elements while ensuring a good quality of measurement. A measurement point may also include any number of FBG sensors (eg, 5 sensors, 6 sensors, etc.). In one development, correlation measurement includes correlation coda correlation between FBG sensors. This entirely optional development optimizes the device, since it makes easier the arrangement of the optical fiber on the structure. As a result, the time required for implementation can be reduced, the positioning errors of the measurement points minimized, etc. The "correlation coda correlation" consists, for a pair of measuring points A and B, of choosing any measuring point Ci selected from the set of measuring points (except A and B); correlating the measurements for each of the points A and B with this arbitrary measuring point C ,; correlating the coda of these correlations to obtain the correlation between measurement points A and B. It is possible to repeat the operation for some or all of the possible measurement points C and to sum the correlations obtained to obtain a correlation between A and B with better reliability. All this can be applied to all or some of the possible pairs of FBG sensors. In one development, it is disclosed the use of a plurality of optical fibers, each having (at least partially FBG sensors). Implementations in practice may vary. Each sensor or pair or pair of sensors can be interrogated separately. There is also disclosed a system for analyzing a structure, comprising at least one optical fiber having a plurality of measurement points, a measuring point comprising one or more type sensors, Bragg grating (Fiber Bragg Grating, FBG ); a light source coupled to the optical fiber; a photodetector or an optical spectrum analyzer for analyzing the reflected light after it has traveled through the optical fiber; and signal processing means for performing correlation and tomography calculations. In one development, the source of the light source is a wavelength varying laser or a broadband optical source whose reflected optical spectrum is determined. Lasers are now common and the associated measurements are performing well. In one development, the optical fibers can be multiplexed using, for example, optical circulators and / or spectrum analyzers and / or multiplexers. In a development, unidirectional FBG type sensors are complemented or replaced by omnidirectional sensors (Doppler effect-based Fiber Optic (FOD)). The sensors can therefore all be of the FBG type, or all of the FOD type, or else the method can be implemented on a system comprising both types of sensors simultaneously (in variable proportions, versus economic and performance aspects).
    [0009] In a development, the system further comprises one or more active noise sources positioned in or on the structure so as to obtain a diffuse acoustoelastic field, that is to say, best respecting the characteristics of a diffuse field. In one embodiment, said placement or positioning is interactively guided by the measurements in progress. In another embodiment, the location of the noise sources is determined theoretically (i.e., "predetermined"). In another implementation mode is returned an indication as to the adequacy of said positioning (versus the diffuse field hypothesis). In another implementation, the multiplicity of points or sources of noise (combined with randomly placed placements) tends to guarantee the obtaining of a diffuse field (without return loop, i.e. a priori). In other words, there is disclosed a system comprising one or more active noise sources that can be used in addition to or in replacement of the natural sources of noise present in the structure, which may also be advantageous for calibration. These additional sources may be for example piezoelectric transducers judiciously placed in the structure, in order to be able to make measurements when desired (for example, in an airplane if the natural sources are turbulence in flight, it will advantageously make use of additional active sources to be able to perform a measurement on the ground, when there is no more "natural" noise in the structure). These sources will advantageously be placed in such a way as to create an acoustic field that best respects the equi-energy distribution condition. For example, to satisfy this condition, the sources can be placed close to natural diffusers (or even around the area to be controlled). It is possible to use active sources integrated into the structure. It is also possible to use active sources that are not integrated: for example a jet of compressed air whose contact zone randomly sweeps the structure to be examined (so as to satisfy the condition of equi-distribution in energy). The system can therefore comprise at least one source of noise, said source being for example a jet of compressed air whose contact zone randomly sweeps the structure. In a development, an active noise source may be a piezoelectric transducer, possibly of the PZT type. According to one aspect of the invention, a large number of measurement points is advantageously used in combination with a diffuse field correlation measurement, which has never been done with FBG sensors, in order to perform tomography. According to a particular embodiment of the invention, the structure to be examined is "imaged". This imagery allows easier interpretations than those resulting from the analysis of raw signals, since the geometrical singularities of the structure appear in a visual form and are not confused with a defect. In some embodiments, the reference state is no longer necessary. Industrial structures are often very complex geometrically (due to stiffeners, rivets, collages, ...) and consequently appear a multitude of ultrasound echoes. An imagery therefore greatly helps in the interpretation of signals. Moreover, a multiplicity of sensors increases the resolution of the imaging and thus enhances the advantages of the invention. Advantageously, the size of the apparatus according to the invention remains limited, even with many sensors, which allows a relative portability, compatibility and utility with regard to the constraints of an integrated health control system of structures (Structural Health Monitoring, SHM) The measurements are carried out passively, that is to say without emission of acoustic waves. As a result, the energy consumption is reduced and allows embedded solutions (for example on board an aircraft, a boat or at the bottom of the sea). The method produces a map of the area to control easily interpretable (which limits the risk of false alarms). The method is all the more effective when the acoustic field is diffuse, that is to say that geometric elements diffract multiple times the acoustic field, which is particularly true in industrial structures that are never simple plates but include for example stiffeners, rivets or local extra thicknesses that diffract the waves and enhance the diffuse nature of the field.
    [0010] The size is reduced, compatible with an integration of the equipment in the structures to be monitored. For example, for integration into composite materials, whereas the use of piezoelectric transducers generally requires two electrical wires per piezoelectric transducer, a single integrated optical fiber between the composite plies has tens of measuring points. The number of entry points into the structure is therefore very limited, which further limits the potential points of weakness.
    [0011] Thus, the invention will find application for structural health monitoring (SHM) operations, for example for the detection (and characterization) of corrosion on aircraft fuselage, delamination in composite structures The advantages of embodiments and the use of optical fibers include a small footprint, a reduced mass, a large bandwidth, a large offset, an electromagnetic immunity, a good RESISTANCE to severe or ionizing radiation, among others. DESCRIPTION OF THE FIGURES Various aspects and advantages of the invention will appear in support of the description of a preferred embodiment of the invention, but without limitation, with reference to the figures below: FIG. an example of a device according to the invention; FIG. 2 illustrates another example of configuration of the measuring points according to the invention; Figure 3 illustrates an example of amplitude measurement as a function of angle of incidence on a sensor; FIG. 4 illustrates an exemplary configuration of the FBG sensors in a so-called "rosette" form. FIG. 5 illustrates another example of configuration of the optical fiber according to the invention, arranged "in meanders"; Figure 6 illustrates an example for which acoustic rays are impacted by a corroded area; Figure 7 illustrates the possible acoustic paths for an example configuration with 16 circularly arranged sensors. DETAILED DESCRIPTION OF THE INVENTION The invention may advantageously use a large number of measuring points to be able to perform guided-wave tomography. For this, one or more optical fibers on which Bragg gratings (FBG) are etched are embedded in (or glued on) the structure.
    [0012] A single optical fiber may comprise tens of FBGs, and therefore as many measurement points. The size is reduced. However, FBGs can only operate as a guided wave receiver but not as a transmitter. Current SHM systems based on FBGs therefore use additional piezoelectric transducers as transmitters. To make the tomography would be a piezoelectric transducer by FBG, so always a large number of piezoelectric transducers.
    [0013] According to one embodiment of the invention, there is described a technique that allows to provide images of industrial type structures on localized areas and / or of limited thickness (plate or tube type geometry). The images can in particular indicate the propagation speeds of the guided waves. According to some implementations of the invention, this image supply is passive (i.e. without emitting ultrasonic waves by the embedded system). The method comprises in particular: an ultrasonic field measurement passively, by a diffuse field correlation technique, technique resulting from geophysics, and recently studied in the context of integrated health control. This type of measurement has never been performed with optical fibers equipped with Bragg gratings (FBG) as sensors. However, it is found that the Bragg gratings (FBG) advantageously allow 5 to have a large number of measurement points. - a guided wave tomography structure imaging that exploits the presence of this large number of measurement points. Such imaging is known from the state of the art but only by means of "active" measurements, ie measurements requiring the use of ultrasonic wave emitters (for plates or For some pipes, some embodiments of the invention provide for the use of FBG sensors instead of the piezoelectric transducers commonly used in SHM systems. The passage of piezoelectric transducers (for example of the PZT type) to FBG sensors (or measuring points) is not obvious because they are two very different and non-interchangeable technologies. Piezoelectric transducers can be used both as transmitters and as receivers whereas FBGs can only be used as receivers. Moreover, the piezoelectric transducers are omnidirectional while the FBGs are directional. Finally, the montages are specific in both cases (electrical vs. optical). Piezoelectric transducers and FBGs are often presented as complementary (piezoelectric transmitters and FBGs in reception) and current SHM systems based on FBGs all use piezoelectric transducers embedded in or placed on the structure with the optical fiber as a transmitter. acoustic waves. FIG. 1 shows a possible diagram according to an exemplary embodiment of the device according to the invention. In the example, the device consists of an optical fiber 100 integrated into a structure to be studied (or glued to its surface) and which comprises a number of Bragg gratings (Fiber Bragg Grating, FBG), for example the measuring point FBG 101, or FBG 102. The measuring points are represented by small rectangles. Figure 4 details a possible configuration of a measurement point by FBG (so-called "rosette" configuration).
    [0014] A single optical fiber generally comprises a few tens of FBG measurement points per fiber or even a few hundred. The optical fiber is integrated or bonded or laid or attached or associated with the structure According to another embodiment of the invention, a plurality of optical fibers may be used. In this case, these fibers are interrogated separately by means of a multiplexer. The optical fiber 100 is coupled by a coupler 110 to a light source 120 (laser or broadband), which will emit into the fiber, and to a photodetector or an optical spectrum analyzer 130 which will analyze the reflected light after its path in the optical fiber, itself connected to a digital processing unit (140). The different acoustic paths in the zone to be inspected 150 passing through the measuring point FBG 101 and each of the other measuring points is illustrated by the acoustic paths 160. There are at least two possibilities for measuring the guided waves by using measuring points FBG. A first embodiment uses a laser whose wavelength is varied. A second embodiment uses a broadband optical source whose reflected optical spectrum is determined. The first embodiment has the advantage of improved sensitivity. The second embodiment has the advantage of a cost saving. According to alternative embodiments, the coupler 110 may be replaced by an optical circulator ("circulator" in English, not shown) and the spectrum analyzer (passive equipment) or the photo detector 130 by FBGs arranged on fibers multiplexed optics (sometimes referred to as High-Speed Optical Wavelength Interrogation System). Other systems for implementing multiplexed optical fibers exist. FIG. 2 illustrates another example of configuration of the measuring points according to the invention. The measurement points FBG (101, 102, ...) can be arranged in different ways around the area to be inspected 150. FIG. 2 illustrates another spatial configuration of the optical fiber 100 carrying the FBG 101, 102, etc. . The different configurations in terms of layout and number of measurement points are only limited by those resulting from the subsequent reconstruction performance, by means of the tomography algorithm chosen for the defect which one wishes to study. Figure 3 illustrates an example of amplitude measurement as a function of the angle of incidence on a sensor. FBGs per se are directional sensors: the measured amplitude depends on the angle of incidence of the wave on the sensor (FIG. 3a). The fiber 100 comprises an FBG sensor 310 oriented at an angle alpha 304, exposed to a wave in a direction 301 (perpendicular 302): the amplitude is maximum (305) when the FBG is in the direction of propagation of the wave and zero or minimal (306) when it is orthogonal to it (Figure 3b). Since the FBGs are etched in the axis of the optical fiber, if the arrangement shown in FIGS. 1 and 2 corresponds to the actual orientation of the FBGs, the measured amplitude would be practically zero for all the pairs of FBG of interest. that is to say for those whose acoustic path crosses the heart of the area to be inspected. In a particular embodiment of the invention, omnidirectional type optical fiber sensors (for example of the "FOD" Doppler effect-based fiber optic type) are used (instead of or in addition to FBG sensors). embodiment, a configuration called "rosette" is used, illustrated in Figure 4. The figure shows the detail of the arrangement of each measuring point, for example the measuring point FBG 101, the different measurement points being represented by rectangles in FIGS. 1 and 2. Each measurement point comprises three FBG arrays arranged at 120 ° to one another (FBG 1 401, FBG 2 402, FBG 3 403). Because of this spatial configuration, for each pair of measurement points the correlation is made between the two FBGs (one for each measurement point) which are the best aligned. According to a variant illustrated in FIG. 5a, the optical fiber 100 may be arranged in "meanders". In this configuration, fewer paths are then available for tomography (only those for which the FBGs are relatively well aligned can be used, in FIG. 25 the paths marked 501). For each pair (A, B) of measuring points of this network, a correlation of the acoustic field u measured simultaneously over a long period in A and B is performed, for example by applying (there are other possibilities of calculation). : CAB (t) = f (t + -c) .uB (T) dT.
    [0015] It is established that the correlation (in all rigor its derivative) converges towards the function of Green between A and B if the various components of the wave field respect the condition of equi-distribution in energy (the distribution in phase and in amplitude waves is random, so-called "diffuse field" hypothesis). The Green function between A and B is the record that would be obtained in A if a source emitted a Dirac in B.
    [0016] The equi-energy distribution conditions can be obtained when the sources are randomly distributed in the medium or when the number and distribution of sources is limited but the medium is very diffusive. Experimental demonstrations have shown that convergence is achieved in interesting frequency ranges for SHM (from kilohertz to a few megahertz). For example, natural noise sources in industrial structures may be those associated with the turbulent boundary layer in aeronautics, wave impact, engine-induced vibration on a ship, or turbulent flow in a tube. According to a variant illustrated in FIG. 5b, the optical fiber can be arranged without particular meanders (which may be easier or feasible in certain situations). One way to overcome this relatively unfavorable geometry is to carry out the correlation coda correlation which amounts to passing, for each pair (A, B) of measurement points, by at least a third measurement point C, and to make the CAC and CBC correlations and the correlation of the coda of these two signals to obtain CAB, this step can also be repeated for all measurement points C different from A and B and then averaged to improve the signal-to-signal ratio. noise. This implementation requires a simplified arrangement of the fiber, which no longer requires meanders of the fiber to align the FBG with respect to each other (FIG. 5b). In return the signal processing time is more lohg. In practice this is done in the following way: for the pair considered (A, B) is used another measurement point C1 among the set of available points. The signals measured between A and B on the one hand and Ci on the other hand correlate first. Once the correlations C, A and C, B are made, we correlate the coda of these signals to obtain the correlation between A and B. This can be repeated on some or all of the points of measurement Ci, we can sum set of correlations obtained to obtain a better estimate of the Green function between A and B.
    [0017] From the Green function obtained by the correlation is deduced the measurement of the flight time between A and B. Repeated for all the possible receiver pairs, this operation provides a large number of data in time of flight that can be exploited for perform a reconstruction by velocity tomography.
    [0018] FIG. 6 illustrates an example for which the acoustic rays are impacted by a corroded zone 610 on a study zone 150. In the example, certain acoustic rays passing through the measurement points FBG 1 101 (generally FBG n) are impacted. . Of all the possible paths, only those passing through the corroded area 610 (or other damage such as delamination) are impacted (or affected, confer the solid lines in the illustration), the other paths are unchanged (dashed lines). Tomography method according to the invention reverses all measured flight times, in order to reconstruct a map of propagation speeds compatible with all flight times. For guided waves, the propagation velocity depends on the thickness of the structure (by a known relation, ie the dispersion curves), this propagation velocity map can be transposed into a thickness map if one seeks to detect corrosion. This method also works, for example, to detect the delamination of a composite structure (since at the delamination level the speed of the guided waves is also modified).
    [0019] The map that we obtain is an image of the structure. This image is interpretable: the extent of the damaged area is made visible. For corrosion damage, for example, it becomes possible to know the extent and the residual thickness. As a result, the severity of the damage can be assessed, if necessary to take corrective action. Obtaining an image of the structure thus makes it possible to detect one or more defects, without having to subtract the measured signal at a time t from that measured at a time to, a reference state for which the structure is considered to be healthy. The prior provision of this reference state involves many constraints (for example, the need to build a database with measurements at all the temperatures that would have to undergo the structure, problems in case of aging sensors causing false alarms , etc.) Figure 7 illustrates the possible acoustic paths for an example configuration with 16 sensors (type 101) arranged in a circular fashion. The robustness of the method presented here comes from the number of measuring points and therefore the number of possible paths. FIG. 7 shows the multiplicity of the acoustic paths 160 in the case of using 30142Q0 21 of 16 sensors or measuring points. It is possible to use hundreds of sensors. Various embodiments are possible to implement the tomography, particularly as to the calibration of the method. Tomography requires precise knowledge of the position of FBGs. According to one embodiment, the individual positions of the measurement points FBG are measured. According to another embodiment, a calibration is performed just after laying the fiber, at a controlled temperature, in order to measure the flight times between each pair of FBG. If the speed is known, which is not always the case, it is possible to deduce the position of the FBG with very good accuracy. If not, it is possible to measure the flight time for each of the pairs of sensors and to make a mapping of variation of the propagation speed compared to the initial state. Knowing the temperature at the time of calibration, if the temperature of the structure is known by means of an integrated thermocouple at time t, it is also possible to compensate for the variation in temperature-induced flight time. . Otherwise, the temperature generally implies a uniform (although potentially anisotropic) effect whereas a defect will generally have a localized effect. The problems mentioned previously on subtracting the reference state are therefore less critical than in current techniques and above all are offset by a large number of measurement points. According to another embodiment, the structure is mapped to a healthy state (reference state of the structure). In this case, there is no need to subtract the signals. This mapping in the healthy state provides an image that makes it possible to identify certain geometric features (such as rivets for example) within the area to be controlled so as not to identify them as defects during subsequent mappings. According to a completely optional variant, attenuation tomography is performed. The correlation makes it possible to reconstruct not only the phase of Green's function but also its amplitude. Attenuation tomography can then be performed. The convergence of the correlation will be different and the directivity of the FBGs can be compensated. This configuration is advantageous in certain situations, in particular when the defect which one seeks to study has little influence on the speed of propagation of the ultrasonic waves.
    [0020] According to a development of the invention, the correlation between two FBG located on the same fiber can be performed. According to another development, several optical fibers are used, with correlation between two different FBG located on different fibers.
    [0021] The present invention can be implemented from hardware and / or software elements. It may be available as a computer program product on a computer readable medium. The support can be electronic, magnetic, optical or electromagnetic. 30
    权利要求:
    Claims (16)
    [0001]
    REVENDICATIONS1. a method of analyzing a diffuse field correlation structure, an optical fiber having a plurality of measurement points, a measurement point comprising Bragg grating (Fiber Bragg Grating) type sensors, the optical fiber being deployed in or on the structure to be analyzed, the method comprising: - light emission in the optical fiber; the correlation measurement for at least a portion of the FBG sensor pairs 10 of the acoustoelastic field propagating in the structure.
    [0002]
    2. A method further comprising a step of reconstructing the propagation velocities by tomography, the imaging being carried out by inverting all the flight times between the FBG sensors, each flight time for each pair of FBG sensors. being deduced from the correlation measure.
    [0003]
    3. Method according to claim 2, wherein the position in the space of each measuring point is previously and individually measured.
    [0004]
    4. The method of claim 2, wherein the temperature of the structure is measured and a variation in flight time induced by a temperature change is compensated.
    [0005]
    5. A method according to claim 2, comprising a first measurement carried out in an initial or reference state of the structure and comprising an imaging of the structure made by tomography from said first measurement making it possible to identify certain geometrical features of the structure. .
    [0006]
    The method of claim 5, further comprising a second measurement performed in a subsequent state for the same pairs of measurement points as the first measurement and further comprising a tomography mapping of propagation velocity variances in the structure between initial state and the subsequent state obtained from differences in flight times measured between the two states.
    [0007]
    7. Method according to any one of the preceding claims for which a measuring point comprises an FBG sensor.
    [0008]
    8. A method according to any one of the preceding claims wherein a measuring point comprises three receivers and directional FBG sensors substantially disposed 120 ° from each other in a rosette configuration.
    [0009]
    9. The method of claim 1, the correlation measurement comprising a coda correlation correlations between FBG sensors. 20
    [0010]
    A method according to any one of the preceding claims comprising a plurality of optical fibers according to claim 1, each FBG sensor being separately interrogable.
    [0011]
    A system for analyzing a structure, comprising: at least one optical fiber having a plurality of measuring points, a measuring point having one or more Bragg grating (FBG) type sensors; ; a light source coupled to the optical fiber; a photodetector or an optical spectrum analyzer for analyzing the reflected light after it has traveled through the optical fiber; signal processing means for performing correlation and tomography calculations.
    [0012]
    12. System according to the preceding claim, wherein the light source is a laser whose wavelength is varied or a broadband optical source whose reflected optical spectrum is determined.
    [0013]
    13. A system comprising a plurality of optical fibers according to any one of the two preceding claims, the optical fibers being multiplexed by means of at least one optical circulator and / or a spectrum analyzer and / or a multiplexer.
    [0014]
    14. System according to any one of claims 11 to 13 15 for which one or more unidirectional sensors type FBG are complemented by one or more omnidirectional sensors type (Doppler effect-based Fiber Optic (FOD).
    [0015]
    15. System according to any one of claims 11 to 14, further comprising one or more active noise sources positioned in or on the structure so as to obtain a diffuse acoustoelastic field.
    [0016]
    16. System according to the preceding claim, at least one active noise source being a piezoelectric transducer. 30
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    法律状态:
    2015-12-31| PLFP| Fee payment|Year of fee payment: 3 |
    2016-12-29| PLFP| Fee payment|Year of fee payment: 4 |
    2018-01-02| PLFP| Fee payment|Year of fee payment: 5 |
    2019-12-31| PLFP| Fee payment|Year of fee payment: 7 |
    2020-12-28| PLFP| Fee payment|Year of fee payment: 8 |
    优先权:
    申请号 | 申请日 | 专利标题
    FR1361916A|FR3014200B1|2013-12-02|2013-12-02|CONTROL OF INDUSTRIAL STRUCTURE|FR1361916A| FR3014200B1|2013-12-02|2013-12-02|CONTROL OF INDUSTRIAL STRUCTURE|
    PCT/EP2014/075763| WO2015082292A1|2013-12-02|2014-11-27|Testing of an industrial structure|
    EP14803131.3A| EP3077795B1|2013-12-02|2014-11-27|Testing of an industrial structure|
    US15/039,788| US10324026B2|2013-12-02|2014-11-27|Testing of an industrial structure|
    ES14803131T| ES2773545T3|2013-12-02|2014-11-27|Industrial structure control|
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