THREE-DIMENSIONAL DEVICE FOR MEASURING LOCAL DEFORMATIONS

Description

[0001] The present invention generally relates to a three-dimensional device for measuring local deformations, which is embedded inside a structure, and more particularly to determine the deformation tensor undergone by this structure. Technical background

[0002] Structures that are very commonly used in architecture, civil engineering works and generally in fields related to mechanics are subject to mechanical stresses that can change over time. These stresses cause deformations in the volume of these structures which can lead to altering their mechanical characteristics. To ensure monitoring of the properties of these structures it is necessary to be able to measure the evolution of the deformations that occur in the volume of these structures.

[0003] The evolution of the deformations of the structures and the estimation of the stresses exerted on them can be monitored by devices placed on the external surface of the structures such as visual indicators for the deformations or mechanical sensors sensitive to the deformation on the surface of these structures (FR2855210). The measurement of the deformation within the volume itself can be estimated by integrating a uniaxial sensor or "measuring fiber" such as an optical fiber in the volume of the structure itself (WO2006127034). However, these methods only allow at best to estimate an average along the axis of the sensor of the components of the stresses or deformations exerted in a fiber placed in the monitored volume.Finally, the direct integration of a sensor into the monitored volume can lead to weakening of the structure which can be due to the difference in mechanical properties of the materials composing the sensor, or to modify the distribution of stresses in the volume of the structure near the sensor. In the latter case the particular shape of the sensor can lead to local concentrations of stresses conducive to the initiation of cracks in the vicinity of this sensor. Furthermore, the modification of the distribution of stresses in the vicinity of the sensor is detrimental to the quality of the measurement, which risks not ideally reflecting the quantities that one wishes to measure, which can lead to using these sensors as threshold alerts rather than as systems for carrying out quantitative measurements.

[0004] To improve the quality and reliability of the measurement of stresses and strains occurring locally within a volume, the applicant then developed a three-dimensional sensor embedded in the heart of a host structure to locally measure the 6 components of the strain tensor of this structure. This three-dimensional sensor was the subject of an international application (WO2014140496). It is based on the principle of using a test body of ellipsoidal shape, and preferably spherical, made of homogeneous elastic material with known mechanical properties and intended to be included in said host structure. As means of measuring strain, WO2014140496 describes the use of a strain measuring fiber attached to the inside of the test body, to transmit strains from the test body to the measuring fiber.The strain sensor of WO2014140496 is based on the Eshelby inclusion problem [1, 2]< which proposes a complete theoretical framework for evaluating the deformations of a host medium (or matrix) that would exist in the matrix in the absence of the sensor, from the measurement of the deformation field in the elastic sphere (the inclusion). WO2014140496 describes more particularly radial strain sensors based on the assembly of optical fibers passing through a spherical test body. The sensors used and developed by the Applicant for the invention that is the subject of international application WO2014140496 are fiber Bragg gratings that have an active length typically of the order of 10 mm for a diameter of 250 µm. They must be glued on either side of the fiber Bragg grating, typically over a length of approximately 15 mm.Thus, with through-fiber technology, it is practically impossible to reduce the diameter of the test body to less than 40 mm. This de facto prohibits the use of the sensor for all applications to living organisms, or for the study of flow in fluids, two areas for which multi-axis deformation measurement is of major interest. On the other hand, this also limits its use to areas requiring the use of a larger sensor (for example in soil mechanics). It is indeed very complex to machine a hole for housing a fiber (here 250 µm) for which the slenderness (i.e. the length to diameter ratio) is important.

[0005] In order to overcome these drawbacks, the Applicant has developed, as an alternative to the three-dimensional sensor of WO2014140496 based on radial strain sensors, and to the three-dimensional sensor proposed in the article "Whispering gallery modes for in-situ measurement of strain tensor" by Corentin Guigot et al.,implementing gallery modes, a three-dimensional device for measuring local deformations of a host structure. This three-dimensional device is also embedded inside a structure and based on the principle of using a spherical test body made of homogeneous elastic material with known mechanical properties provided with deformation sensors. Similarly to WO2014140496, the three-dimensional device which is the subject of the present invention is also based on the Eshelby inclusion problem 1, 2< . Unlike the three-dimensional sensor of WO2014140496, the deformation sensors are assembled tangentially, being wound on the surface of a sphere to measure variations of six perimeters formed by the intersection between the sphere (test body of the three-dimensional device) and six different planes (in the sense of vector planes).With the three-dimensional measuring device according to the invention, the measurement of the deformation of the test body sphere is carried out by measuring the variations in the perimeters, and not by measuring deformations using unidirectional sensors. Summary of the invention

[0006] More particularly, the present invention relates to a three-dimensional device for locally measuring the deformations of a host structure made of a solid, liquid, viscous or gaseous material, said device comprising a test body having the shape of a sphere S with center O made of a homogeneous elastic material with known mechanical properties, said test body being intended to be included in said host structure, - deformation measuring means attached to said test body so that the deformations undergone by said test body are transmitted to said deformation measuring means, a measuring system adapted to, on the one hand, detect signals coming from the measuring means which are representative of the deformations of the deformation measuring means, and on the other hand to determine the local deformations of said host structure from the detected signals and the known mechanical properties of the materials of the test body, and possibly of the host structure, means for connecting the deformation measuring means to said measuring system, said device being characterized in that said deformation measuring means are constituted by a three-dimensional deformation sensor comprising at least six tangential sensors which describe circles of perimeters Li defined by the intersection between planes Pi and said sphere, said planes Pi being distinct (in the sense of vector planes), i being an integer varying between 1 and 6, said tangential sensors being physical rings constituted by optical fibers, or electric cables or piezoelectric cables, said physical rings being connected by connections to said measuring system, said tangential sensors being capable of giving a measurement of the variations ΔLi of the perimeter Li when said test body undergoes a homogeneous deformation field, the deformation tensor ε undergone by said test body being a symmetrical tensor determined by the matrix relation {ε}=|Q|{ΔL},with {ε} defining the 6-dimensional vector grouping the six unknown terms of the strain tensor ε, Q defining an invertible 6 x 6 square matrix, dependent only on the orientation of the planes Pi and therefore independent of said host structure and its state of deformation ε 0 and {ΔL} being a 6-dimensional vector having as components the variations ΔLi, said device also being characterized in that said system is adapted to analytically calculate the strain tensor ε 0 undergone by said host structure from the strain tensor ε. ,

[0007] As for the host structure, it is made of a solid, liquid, viscous or gaseous material, in which the three-dimensional sensor is inserted (if it is solid) or embedded (if it is liquid, viscous or gaseous). If the host structure is a liquid or a gas, it is not necessary to know its properties to measure the pressure field. If the host structure is a solid, it will be possible to analytically calculate the strain tensor ε 0 undergone by the host structure ε with accuracy as long as the mechanical properties of the host structure are known.

[0008] As for the test body, it is made of a homogeneous elastic material with known mechanical properties. Examples of materials that can be used for the test body include metallic materials (such as steel, aluminum and its alloys), glass, polymers and concrete.

[0009] To determine the strain tensor ε undergone by the spherical-shaped test body of the three-dimensional device according to the invention, it is considered that the latter undergoes a homogeneous strain field, according to Ehselby's theory 1, 3< . The test body is surrounded by at least 6 tangential sensors (in the form of circles) numbered i, i being a natural integer varying from 1 to 6, which are each capable of giving a measurement of the variations ΔLi of the perimeter Li when said test body undergoes a homogeneous strain field, from which the relative variations ΔLi / Li are calculated. The circles are defined by the intersection between the sphere S and distinct planes Pi, which can pass through the center O of the sphere (test body). To obtain the six components of the strain tensor ε, it is necessary to carry out a minimum of 6 perimeter measurements Li. The choice of the planes Pi must be made in such a way as to obtain an invertible 6 x 6 square matrix.Among all the possible combinations, we will preferably opt for a distribution of the tangential sensors such that they are arranged in planes perpendicular to the normals of the faces of a regular dodecahedron with the same center O as the sphere S.

[0010] For this first embodiment, the physical rings can be connected to each other at a single point U by the connections, which are gathered in a central sheath connected to the measurement system.

[0011] For this first embodiment, a suitable measurement system can be used to stimulate the tangential sensors.

[0012] Advantageously, the test body may consist of a solid sphere, as illustrated in figures 3 to 6 commented below in the detailed description of the figures.

[0013] According to an advantageous embodiment of the device according to the invention, the latter can be configured as follows: the host structure is made of a cold-formed material, and the three-dimensional sensor is made up of 6 rings arranged so as to form a structure defining the sphere S with center O, each ring being provided with a tangential sensor made up of an optical fiber, or an electric cable or a piezoelectric cable and the structure of the three-dimensional sensor acts as a test body. This embodiment can in particular be used for the study of cold-formed materials (concrete, geopolymer, resins).

[0014] In this embodiment, materials chosen from concretes and polymers, which are materials that can be cold molded, may be used for the host structure. Brief description of the figures

[0015] Other advantages and particularities of the present invention will result from the description which follows, given by way of non-limiting example and made with reference to the accompanying drawings in which: [ Fig. 1 ] : there figure 1 comprises a diagram a) schematically showing the insertion of a spherical test body (called inclusion) into a homogeneous elastic medium or matrix (called host structure), and a diagram b) of the principle of the three-dimensional sensor of the device according to the invention; [ Fig. 2 ] : there figure 2 schematically illustrates a first embodiment of the device according to the invention; [ Fig. 3 ] : there figure 3 schematically illustrates the three-dimensional deformation sensor of the device according to the invention according to the first embodiment illustrated in figure 2 , in which the three-dimensional strain sensor consists of six tangential sensors in the form of physical rings; [ Fig. 4 ] : there figure 4schematically illustrates two unclaimed variants of the three-dimensional deformation sensor of the device: the photograph on the left shows a first variant embodiment according to which the tangential sensors are replaced by the measurement of optical waves of gallery mode type, while the photograph on the right shows a second variant embodiment according to which the tangential sensors are replaced by the measurement of elastic waves of gallery mode type which are each generated by a piezoelectric piezoelectric line; [ Fig. 5 ] : there Figure 5 more particularly illustrates the first variant embodiment of the unclaimed example illustrated in the figure 4 , showing in particular a device for injecting and receiving optical waves; [ Fig. 6 ] : there figure 6 more particularly illustrates the second variant embodiment of the unclaimed example illustrated in the figure 4, showing the measurement of elastic waves generated by a piezoelectric line 11 piezoelectric (only one of which is shown on the figure 6 for the sake of clarity); [ Fig. 7 ] : there figure 7 schematically illustrates the three-dimensional deformation sensor of the device according to the invention according to one embodiment, in which the three-dimensional sensor is constituted by 6 rings made of the host structure material and arranged so as to form a sphere. Detailed description of the invention

[0016] [ Fig. 1 ] represents, in 1a, schematically a homogeneous elastic medium or matrix (called host structure) 2 of which we wish to determine locally the state of deformation. The structure 1 is for example a rigid structure, such as a civil engineering structure, in particular made of concrete, where appropriate hollowed out and / or reinforced in places. As visible on the figure 1, the host structure 2 comprises a spherical-shaped recess, adapted to receive a test body 5 as will be described later. The principle of the device according to the invention is the determination of the deformation tensor ε 0 of the host medium 2 from the deformation tensor ε in the elastic sphere of the test body 5. [ Fig. 1 ] represents, in 1b, a schematic diagram of the three-dimensional sensor of the device according to the invention, showing in particular a spherical test body inside which the faces of a regular (fictitious) dodecahedron with the same center O as the sphere S (fictitious dodecahedron) have been represented, the normals of which are perpendicular to the planes Pi containing the sensors (this is a preferential mode of distribution of the tangential sensors which will be described later in relation to the figure 3 .

[0017] [ Fig. 2 ] completes the schematic representation of [ Fig. 1] at the level of diagram 1a showing the entire three-dimensional device 1 according to the invention, in which the spherical test body provided with a three-dimensional deformation sensor 7 comprising six tangential sensors 71, 72, 73, 74, 75, 76 connected to the measuring system 9 (or instrumentation zone).

[0018] [ Fig. 3 ] shows in more detail the three-dimensional deformation sensor of the figure 2 ; in which the six tangential sensors 71, 72, 73, 74, 75, 76, are in the form of physical rings made up of optical fibers, or electric cables or piezoelectric cables. These physical rings are grouped at the same point U using connection sections 80 to be assembled in a central sheath 81 connected to the instrumentation zone 9.

[0019] The first version of the 3D deformation sensor consists of gluing 6 unidirectional sensors along the perimeters of a sphere. These sensors, which are in fact 6 cables (optical, electrical or piezoelectric), are grouped together at the same point using connection sections to be assembled in a single sheath. Each of these sensors is contained in a plane Pi passing through the center of the sphere S and perpendicular to the normal (designated by i = 1, 2, 3, 4, 5 or 6 on the figure 3 ) of a regular dodecahedron with the same center O as the sphere S.

[0020] An unclaimed alternative to using sensors in the form of cables glued to the surface of a sphere is illustrated by [ Fig. 4 ]: this alternative consists of injecting waves into the spherical test body 5 to make them describe the perimeters of length Li: these can be optical waves 71 as illustrated in the diagram on the left of [ Fig. 4] and described in the scientific publication of S. Arnold et al. 4< , or it can be mechanical waves 71, as illustrated in the right diagram of [ Fig. 4 ] and described in the scientific publication by D. Clorennec and D. Royer 5< . The expected gain is mainly the miniaturization of the device according to the invention 1.

[0021] In the case where optical waves are injected, optical wave injection-reception devices of the optical fiber sensor type 10 are used, as illustrated in [ Fig. 5 ]. On the Figure 5 , the optical fiber sensors 10 are coupled to the test body 5, which is coated with a buffer layer 50 fixing the optical fiber 10 and whose thickness is greater than the wavelength λ, the buffer layer 50 having mechanical properties identical to those of the test body 5 and a known refractive index Several geometries of optical transmitters and receivers are given: on the left diagram of the Figure 5 , the coupling with the test body 5 is carried out by the end of the optical fiber 10, while in the central diagram, it is carried out using a tapered optical fiber 10. The coupling can also be carried out by network (right diagram of the Figure 5 ).

[0022] [ Fig. 6 ] shows an unclaimed three-dimensional strain sensor 7 consisting of tangential sensors (only one being shown in the figure 6 for the sake of clarity) which are based on the measurement of elastic waves generated by a piezoelectric line 11. The expected gain with this architecture is mainly the improvement of the metrological performance of the device. These piezoelectric sensors can be of similar technology to those described on the figures 2 to 6 , the test body consists of a solid sphere.

[0023] An alternative according to the invention, illustrated by [ Fig. 7] consists of no longer using a solid sphere as a test body 5, but only elastic rings. This solution can be used for cold-molded materials as host structures 2, such as concretes, geopolymers, or resins. In such an embodiment, the three-dimensional sensor 7 is constituted by 6 rings 51, 52, 53, 54, 55, 56 arranged so as to form a structure 5' defining the sphere S with center O and acting as a test body. Each ring 51, 52, 53, 54, 55, 56 is provided with a tangential sensor 71, 72, 73, 74, 75, 76 constituted by an optical fiber, or an electric cable or a piezoelectric cable. LIST OF REFERENCES

[0024] 1. " The elastic field outside an ellipsoidal inclusion", by Eshelby John Douglas and Peierls Rudolf Ernst, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 252(1271):561-569, October 1959. 2. "An Embedded 3D Strain Tensor Sensor Based on the Eshelby's Inclusion" by M. L. M. François, Y. Lecieux, D. Leduc, C. Lupi, and E. Rozière, Experimental Mechanics, 57(5):801-811, June 2017. 3. " The détermination of the elastic field of an ellipsoïdal inclusion and related problems" by J. D. Eshelby, Proceedings of the Royal Society of London, Vol. 241, (1957), pp. 376- 396. 4. " Shift of whispering-gallery modes in microspheres by protein adsorption" by S. Arnold, M. Khoshsima, I. Teraoka, S. Holler, and F. Vollmer, Opt. Lett. 28, 272-274 (2003). 5 " Investigation of surface acoustic wave propagation on a sphere using laser ultrasonics" by D. Clorennec et D. Royer, Applied physics letters 85 (2004), p. 2435-2437.

Claims

1. Three-dimensional device (1) for locally measuring the deformations of a host structure (2) which is made of a solid, liquid, viscous or gaseous material, said device (1) comprising: - a test body (5) in the form of a sphere (S) with center (O) made of a homogeneous resilient material known mechanical properties, said test body (5) being intended to be included in said host structure (2), - deformation measurement means (7) attached to said test body (5) so that the deformations undergone by said test body (5) are transmitted to said deformation measurement means (7), and - a measurement system (9) suitable for both detecting signals from said measurement means (7) which are representative of deformations of said deformation measurement means (7), and determining the local deformations of said host structure (2) from the detected signals and the known mechanical properties of the materials of the test body (5) and optionally of the host structure (1), - means (8, 80, 81) for connecting said deformation measurement means (7) to said measurement system (9), said device (1) being characterized in that said deformation measurement means (7) consist of a three-dimensional deformation sensor comprising at least six tangential sensors (71, 72, 73, 74, 75, 76) which describe circles of perimeters Li defined by the intersection between planes (Pi) and said sphere (S), said planes (Pi) being separate, i being an integer varying between 1 and 6, said tangential sensors (71, 72, 73, 74, 75, 76) being physical rings consisting of optical fibers, or electric cables or piezoelectric cables, said physical rings (71, 72, 73, 74, 75, 76) being linked by connections (80) to said measurement system (9), said tangential sensors (71, 72, 73, 74, 75, 76) being capable of giving a measurement of the variations ΔLi of the perimeter Li when said test body (5) undergoes a homogeneous deformation field, the deformation tensor ε undergone by said test body (5) being a symmetric tensor determined by the matrix relation {ε}= |Q| {ΔL}, with {ε} defining the vector of dimension 6 grouping the six unknown terms of the deformation tensor ε, Q defining an invertible 6 x 6 square matrix, depending solely on the orientation of the planes (Pi) and therefore independent of said host structure (2) and its state of deformation ε0 and {ΔL} being a vector of dimension 6 having as its components the variations ΔLi, said device (1) also being characterized in that said measurement system (9) is suitable for analytically calculating the deformation tensor ε0 undergone by said host structure (2) from the deformation tensor ε.

2. Three-dimensional device (1) according to claim 1, wherein said planes (Pi) pass through the center (O) of said test body (5).

3. Three-dimensional device (1) according to claim 2, wherein said tangential sensors (71, 72, 73, 74, 75, 76) are arranged in planes perpendicular to the normals of the faces of a regular dodecahedron with the same center (O) as said sphere (S).

4. Three-dimensional device (1) according to any one of claims 1 to 3, wherein said physical rings (71, 72, 73, 74, 75, 76) are interconnected at a single point (U) by said connections (80), said connections (80) being gathered into a central sheath (81) which is connected to said measurement system (9).

5. Three-dimensional device (1) according to any one of claims 1 to 4, wherein said measurement system (9) is suitable for stimulating said tangential sensors (71, 72, 73, 74, 75, 76).

6. Three-dimensional device (1) according to any one of claims 1 to 5, wherein said test body (5) is a solid sphere.

7. Three-dimensional device (1) according to any one of claims 1 to 5, wherein said host structure (2) is made of a cold-formed material, the test body (5) consisting of 6 rings (51, 52, 53, 54, 55, 56) arranged so as to form a structure (5') defining the sphere (S) of center (0), each ring (51, 52, 53, 54, 55, 56) being provided with a tangential sensor (71, 72, 73, 74, 75, 76) consisting of an optical fiber, or an electric cable or a piezoelectric cable, said structure (5') serving as the test body (5).