Long-gauge fiber Bragg grating strain sensor and damage monitoring method

Abstract

The present invention discloses a long-gauge fiber Bragg grating strain sensor and a damage monitoring method. The sensor includes a plurality of long-gauge strain sensing units connected in series by rigid anchoring ends. Within the long-gauge strain sensing units, a plurality of flexibly anchored regions arranged at intervals divide the sensing fiber core of the long-gauge strain sensing unit into multiple small-gauge sub-units, and each small-gauge sub-unit contains a fiber Bragg grating sensing element; the flexibly anchored regions generate radial anchoring forces on the sensing fiber core, and the sensing fiber core can relatively slip under the condition of overcoming the radial anchoring forces in the flexibly anchored regions, and the radial anchoring forces generated by the flexibly anchored regions reduce the sensitivity of the local deformation of the sensor outer sheath to the sensing fiber core. The strain sensor provided in this application can accurately identify and measure the local damage events of the structure located inside the long-gauge unit, and provide high-precision deformation information perception, refined identification of the local damage state within the long gauge, and accurate measurement for the health monitoring of various structures.

Description

Technical Field

[0001] The present invention relates to the technical field of intelligent sensing and monitoring, and in particular to a long-gauge fiber Bragg grating strain sensor with a built-in flexible subunit. Background Art

[0002] In the field of fiber Bragg grating sensing, there are mainly two types of fiber Bragg grating strain sensors: point-type fiber Bragg grating strain sensors and long-gauge fiber Bragg grating strain sensors. The point-type fiber Bragg grating strain sensor fully pastes or encapsulates the fiber Bragg grating sensing element with a shorter gauge. In engineering monitoring applications for large-scale infrastructure structures, it presents a point-type configuration. Long-term engineering practice shows that since such sensors can only be arranged at different parts of the structure in a discrete form, they can only be used to measure the local strain of the structure and it is difficult to effectively cover the key areas of the structure and obtain effective data reflecting the overall deformation and damage state of the structure. For the long-gauge fiber Bragg grating strain sensor, the effective sensing characteristic scale is determined according to the characteristics of the monitoring object and application scenario. By long-gauge encapsulating the fiber Bragg grating element (the gauge is generally between dozens of centimeters and several meters), that is, through a fixed-point anchoring method with a certain gauge, the fiber Bragg grating sensing element is deformation-coupled with the outer sheath of the sensor. The measured deformation is the average strain between long gauges, enabling the sensing system to effectively cover the key areas of the measured structure with a small number of sensing units. Further combining relevant mechanical theories, it is possible to achieve a comprehensive direct mapping of macro and micro characteristics such as the deformation angle, load, internal force, and dynamic parameters of the structure, as well as the effective identification of early damage to the structure.

[0003] Although the existing long-gauge encapsulated fiber Bragg grating strain sensors can provide effective data for the macro and micro dynamic and static state monitoring of various structures, since the structural damage of general infrastructure occurs at the material scale, only the overall characterization of structural damage within a certain spatial range can be obtained through the existing long-gauge sensing system. It is still difficult to achieve more in-depth and refined damage information within the long gauge, such as the specific location, quantity, and quantitative measurement of damage. Obtaining this information is of great significance for aspects such as improving the durability of the structure and refined maintenance assessment.

[0004] In view of the above problems, in the field of health monitoring of various civil engineering infrastructure structures, there is a need for a strain sensor with a simple structure, convenient for large-scale manufacturing, capable of simultaneously achieving accurate monitoring of long-gauge strain information and structural refined damage information, so as to lay an information foundation for realizing a low-cost, high-precision, dynamic and static, multi-level structural health accurate monitoring system, as well as an infrastructure refined predictive maintenance intelligent management and maintenance system. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a long-gauge fiber Bragg grating strain sensor with built-in flexible sub-units and a monitoring method in view of the deficiencies of the above-mentioned prior art, which is convenient for achieving comprehensive coverage of key areas of the structure, accurate static and dynamic perception of long-gauge strain, and identification and accurate measurement of local damage states within the long gauge.

[0006] To solve the above technical problem, the technical solution adopted by the present invention is:

[0007] The present invention first provides a long-gauge fiber Bragg grating strain sensor, which includes a plurality of long-gauge strain sensing units connected in series by rigid anchoring ends. Inside the long-gauge strain sensing unit, a plurality of flexibly spaced anchoring areas divide the sensing fiber core of the long-gauge strain sensing unit into multiple small-gauge sub-units, and each small-gauge sub-unit contains a fiber Bragg grating sensing element; the flexible anchoring area generates a radial anchoring force on the sensing fiber core, and the sensing fiber core can relatively slip in the flexible anchoring area by overcoming the radial anchoring force. The radial anchoring force generated by the flexible anchoring area reduces the sensitivity of the deformation of the local part of the sensor outer sheath and transfers it to the sensing fiber core.

[0008] The flexible anchoring area inside the long-gauge strain sensing unit is composed of flexible anchoring clamps arranged outside the inner sheath of the sensor; the inner diameter of the inner sheath of the sensor is larger than the outer diameter of the sensing fiber core, and the inner sheath of the sensor is sleeved outside the sensing fiber core, so that there is a sliding space between the sensing fiber core and the inner sheath of the sensor; the flexible anchoring clamp squeezes the inner sheath of the sensor, causing the diameter of the inner sheath of the sensor to shrink until it contacts the outside of the sensing fiber core, thereby generating frictional force at the interface of the flexible anchoring area, and transferring the deformation of the sensor outer sheath to the sensing fiber core in the small-gauge sub-unit through the action of frictional force, either completely or partially.

[0009] The flexible anchoring clamp is an elastic member or an elastic-rigid composite member composed of an inner elastic layer and an outer rigid layer bonded together; the elastic member squeezes the inner sheath of the sensor, causing the inner sheath of the sensor to radially contract; the outer rigid layer of the elastic-rigid composite member undergoes a contraction deformation under the action of external force extrusion or physical self-excitation, driving the inner elastic layer to squeeze the inner sheath of the sensor, causing the inner sheath of the sensor to radially contract.

[0010] As a further preference of the present invention, there are two or more fiber Bragg grating grating areas in the long gauge between the rigid anchoring ends of the sensing fiber core, forming a fiber Bragg grating string with evenly distributed grating intervals in the long gauge. The grating string sensing element is an ultra-weak grating or a strong grating and their combination;

[0011] As a further preference of the present invention, the gratings in the sensing fiber core are processed by the multi-point dense femtosecond grating writing technology, and the integrated grating writing is carried out synchronously with the wire drawing process during the wire drawing forming stage of the optical fiber core, so that the grating grating areas have superior characteristics such as spatially dense arrangement and large measurement range;

[0012] As a further preference of the present invention, according to the different principles of the demodulation device, the fiber grating element in the sensing fiber core may have a grating reflectivity within the range of strong grating or ultra-weak grating reflectivity;

[0013] As a further preference of the present invention, the sensing fiber core is processed by different high-performance coating processes or cabling processes according to specific application scenarios. The sensing fiber core is in the form of a bare fiber string of fiber gratings or a tight-buffered fiber string of fiber gratings. The coating material used in the above processing process has excellent elasticity, harsh environment tolerance and long-term stability, and tightly wraps the internal fiber core, effectively avoiding interlayer slip between different materials, so that the deformation of the anchoring area can be accurately and reliably transmitted to the fiber core;

[0014] As a further preference of the present invention, if the sensing fiber core adopts the form of a tight-buffered fiber string of fiber gratings as the sensing fiber core, the sensing fiber core is composed of a bare fiber string of fiber gratings and an outer fiber sheath sleeved outside it. The outer fiber sheath is tightly coupled with the bare fiber, and can effectively transfer external deformation to the fiber core;

[0015] As a further preference of the present invention, the rigid anchoring end of the long-gauge fiber grating strain sensor is composed of a high-toughness and high-elasticity anchoring adhesive and a high-performance fiber composite, which can firmly composite the sensing fiber core with the sensor outer sheath, so that the sensing fiber core in the rigid anchoring area and the sensor outer sheath are deformed and coupled under long-term large strain and harsh environmental conditions, effectively preventing interface slip and creep;

[0016] As a further preference of the present invention, the rigid anchoring end of the long-gauge fiber grating strain sensor is composed of a special resin with high toughness, high elasticity and high durability and a high-performance basalt fiber braided material. This anchoring resin also has strong permeability to the polymer layer; during the anchoring forming process, through a strict temperature control process, the resin penetrates the fiber coating layer and reaches the fiber glass core directly and forms a direct composite with it. The additional stress on the fiber core in the anchoring area during the anchoring forming process is mild and controllable. The cured anchoring area has excellent stiffness and flexibility, so as to realize the anti-slip anchoring of the fiber and the stress control of the anchoring area;

[0017] As a further preference of the present invention, a sensor inner sheath is provided outside the sensing fiber core. The inner diameter of the sensor inner sheath is larger than the outer diameter of the sensing fiber core. The sensor inner sheath is sleeved outside the sensing fiber core, so that there is a sliding space between the sensing fiber core and the sensor inner sheath;

[0018] As a further preference of the present invention, the inner sheath of the sensor is made of a material with a low elastic modulus and self-lubricating properties, so that the frictional force between it and the sensing fiber core is very small at normal dimensions; while under the action of the external circumferential pressure, it is prone to radial contraction, generating a uniform circumferential pressure on the internal sensing fiber core;

[0019] As a further preference of the present invention, the material of the inner sheath of the sensor has good anti-seepage and glue isolation properties, which can ensure that the gap between the inner sheath and the sensing fiber core is not penetrated by the colloid during the processing and forming of the outer sheath; during the long-term use of the sensor, it can effectively isolate environmental water vapor, corrosive elements, etc. from having an adverse impact on the sensing element;

[0020] As a further preference of the present invention, a flexible anchoring fixture is sleeved outside the inner sheath of the sensor. During the preparation of the sensor, the flexible anchoring fixture reduces its own size and squeezes the inner sheath of the sensor, so that its diameter shrinks to contact the outside of the sensing fiber core, thereby generating a certain frictional force at the interface of the anchoring area. Thus, the deformation of the outer sheath of the sensor can be transmitted to the sensing fiber core in the small gauge sub-unit entirely or partially through the frictional force;

[0021] As a further preference of the present invention, the flexible anchoring fixture is composed of an inner elastic layer and an outer rigid layer bonded together. Under the action of external extrusion or specific physical self-excitation, the outer rigid layer can undergo precisely controllable shrinkage deformation, driving the inner elastic layer of the fixture to squeeze the inner sheath of the sensor, causing the inner sheath to generate uniform radial contraction;

[0022] As a further preference of the present invention, for the flexible anchoring fixture, if the external extrusion strategy is adopted to cause the fixture to undergo shrinkage deformation, the outer rigid layer of the flexible anchoring fixture is made of a material with a certain stiffness, good plastic deformation ability and corrosion resistance, and a special precision extrusion device is used to make it shrink and deform strictly according to the designed dimensions;

[0023] As a further preference of the present invention, for the flexible anchoring fixture, if the specific physical self-excitation strategy is adopted to cause the fixture to undergo shrinkage deformation, the outer rigid layer of the flexible anchoring fixture is made of a shape memory material with a certain stiffness and good corrosion resistance. Through the excitation of specific physical conditions such as temperature change, the anchoring fixture undergoes precise shrinkage deformation according to the pre-designed dimensions;

[0024] As a further preference of the present invention, the inner elastic layer of the flexible anchoring fixture is made of a high-elasticity and high-durability polymer material, so that when it shrinks, it can generate uniform extrusion stress within the range of the fixture and can maintain stable mechanical properties under long-term harsh environments;

[0025] As a further preference of the present invention, for the long - gauge fiber Bragg grating strain sensor, a sensor outer sheath is sleeved outside the inner sheath, the rigid anchoring area and the flexible anchoring area, and the outer sheath is tightly combined with the above - mentioned structure to form an integral structure with deformation coupling;

[0026] As a further preference of the present invention, in the long - gauge fiber Bragg grating strain sensor, there is a uniform pre - strain in the sensing fiber core within the long gauge due to the pulling of the rigid anchoring area, that is, the sensing fiber cores at each flexible sub - unit have the same pre - strain in the free state. This pre - strain is formed during the sensor processing, which enables the sensor to measure not only tensile strain but also compressive strain;

[0027] As a further preference of the present invention, the sensor outer sheath presents as a flexible rod with a certain stiffness, having a certain structural strength and stiffness, which can provide a stable and reliable structural support for the pre - strain of the sensor fiber core and at the same time effectively protect the internal structure of the sensor;

[0028] As a further preference of the present invention, the sensor outer sheath adopts different materials and laminated structures according to specific application scenarios, so that the sensor has good flexibility, constructability, durability and tolerance to harsh environments, thus ensuring the long - term stability of the sensor performance while deforming synergistically with the structure;

[0029] As a further preference of the present invention, the sensor outer sheath can adopt a high - performance composite material composed of basalt fiber and epoxy resin or thermoplastic resin, forming different forms of composite configurations such as rod materials, rib materials, and plate materials, so that the sensor is suitable for the embedding, inlaying, and external pasting layout of different application scenarios, and at the same time has superior characteristics such as light weight, high strength, flexibility, durability, and green low - carbon;

[0030] The present invention also provides a damage monitoring method based on the long - gauge fiber Bragg grating strain sensor. By comparing the strain differences between each small - gauge sub - unit and to judge the generation and evolution of damage and conduct quantitative monitoring on it, where is the critical anchoring slip strain.

[0031] The present invention has the following beneficial effects:

[0032] By adding flexible anchoring areas in the long gauge between the rigid anchoring ends of the long - gauge sensor and dividing the sensing fiber core into several small - gauge sub - units, the present invention enables the long - gauge sensor to have the ability to finely identify and measure internal damage in the long gauge, thus solving the deficiency that existing long - gauge strain sensors are difficult to distinguish fine damage information within the gauge.

[0033] The flexible anchoring design of the sensor in the present invention makes it difficult for local damage deformation of the structure to cause large local deformation of the optical fiber, resulting in fracture failure of the sensing system. The concentrated damage deformation will be jointly borne by several sub-units, realizing desensitization of local optical fiber sensing (that is, the optical fiber at the local damage actually bears less deformation than the local deformation increment caused by the damage, and the sensed strain is smaller than that of rigid anchoring). On the premise of realizing refined identification and accurate measurement of damage characteristics, the safety and stability of the sensing system can be greatly improved.

[0034] The design of the flexible anchoring area of the sensor in the present invention enables the grating sensing strain of each sub-unit to be the average strain of the sub-unit. Based on this, accurate strain-displacement conversion and long gauge length strain conversion can be carried out, so as to realize high-precision measurement of the refined damage deformation of the structure, and greatly improve the efficiency of the long gauge length strain sensing system in the refined and accurate monitoring and evaluation of the structure.

[0035] Explanation of increased economy: The rigid anchoring end of the sensor is difficult to process and has a high cost, while the flexible anchoring area is easy to process and has a low cost. The combination of the two not only improves the functionality of the sensing system, but also has good economy.

[0036] The sensor design of the present invention has a simple structure, is convenient to process, has a low cost, is strong, tough and stable, and is convenient for large-scale sensor production and industrial application. It is of great significance for the refined identification and measurement of local damage of various structures and the accurate assessment of the macro and micro dynamic and static states of the structure.

[0037] In summary, the sensor proposed in the present invention is in the shape of a flexible rod, slender and delicate, which is convenient for realizing full coverage of key areas of the structure and accurate perception of multi-level deformation. Brief Description of the Drawings

[0038] Figure 1 is the concept and structure schematic diagram of a long gauge length fiber Bragg grating strain sensor with an internal flexible sub-unit in the present invention;

[0039] Figure 2 is Figure 1 the structure schematic diagram of the long gauge length strain sensing unit in;

[0040] Figure 3 is Figure 1 the structure sectional view of the long gauge length strain sensing unit in;

[0041] Figure 4 is Figure 3 the A-A sectional view of;

[0042] Figure 5 is the schematic diagram of different packaging forms of the sensing fiber core of a long gauge length fiber Bragg grating strain sensor with an internal flexible sub-unit in the present invention;

[0043] Figure 6 is a schematic structural diagram of a flexible anchoring fixture for a long - gauge fiber - Bragg - grating strain sensor with an internal flexible subunit according to the present invention;

[0044] Figure 7 is a schematic diagram of the forming mode of a flexible anchoring fixture for a long - gauge fiber - Bragg - grating strain sensor with an internal flexible subunit according to the present invention;

[0045] Figure 8 is a schematic layout diagram of the application of a long - gauge fiber - Bragg - grating strain sensor with an internal flexible subunit in an engineering structure according to the present invention;

[0046] Figure 9 is a schematic diagram of different forms of a long - gauge fiber - Bragg - grating strain - sensing intelligent composite material with an internal flexible subunit according to the present invention;

[0047] Figure 10 is a schematic diagram of the principle and application of a long - gauge fiber - Bragg - grating strain sensor with an internal flexible subunit in structural deformation and crack monitoring according to the present invention.

[0048] In the figure: 1. Rigid anchoring end; 2. Sensing fiber core; 3. Fiber Bragg grating; 4. Inner sheath of the sensor; 5. Outer sheath of the sensor; 6. Flexible anchoring fixture; 7. Flexible anchoring area; 8. Glass fiber core of the fiber Bragg grating; 9. Coating layer; 10. Outer fiber sheath; 11. Elastic layer; 12. Rigid layer; 13. Long - gauge fiber - Bragg - grating strain sensor; 14. Cable tie; 15. Groove; 16. Steel bar; 17. Structural adhesive. Specific embodiments

[0049] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

[0050] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "left side", "right side", "upper part", "lower part", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second", etc. do not represent the importance of the components, so it cannot be understood as a limitation to the present invention. The specific dimensions adopted in this embodiment are only for illustrating the technical solution and do not limit the protection scope of the present invention.

[0051] As Figure 1 shown, a long - gauge fiber - Bragg - grating strain sensor with an internal flexible subunit is formed by connecting a plurality of long - gauge sensing units containing subunit in series through a rigid anchoring end 1.

[0052] The structure of the long - gauge sensing unit is as Figure 2As shown in the figure, it includes a sensing fiber core 2, an inner sheath 4 of the sensor, a flexible anchoring area 7, and an outer sheath 5 of the sensor. The sensing fiber core 2 is an array of fiber Bragg grating strings. The rigid anchoring end 1 of the sensor firmly and durably composits the sensing fiber core 2 and the outer sheath 5 of the sensor to form a long gauge length strain sensing unit. Within the long gauge length between the rigid anchoring ends, several flexible anchoring areas 7 divide the sensing fiber core 2 into multiple small gauge length sub-units, and each small gauge length sub-unit contains a fiber Bragg grating sensing element. The flexible anchoring area 7 can desensitize and transfer the significant deformation occurring locally in the outer sheath 5 of the sensor to the sensing fiber core 2.

[0053] As Figure 2 shown, generally there are two or more fiber Bragg grating regions within the long gauge length between the rigid anchoring ends of the sensing fiber core 2, forming a fiber Bragg grating string with evenly distributed intervals between the grating regions within the long gauge length. The number of grating regions is determined according to the refinement degree of the perception of structural damage information in the application scenario. The gratings in the sensing fiber core are processed by the multi-point dense femtosecond grating writing technology, and the integrated grating writing is synchronized with the wire drawing process during the wire drawing forming stage of the optical fiber core, so that the grating regions have superior characteristics such as spatially dense arrangement and large measurement range.

[0054] According to different principles of the demodulation device, the reflectivity of the fiber Bragg grating element in the sensing fiber core can be in the range of strong grating or ultra-weak grating reflectivity, and the demodulation principles include but are not limited to the wavelength division multiplexing principle and the time division multiplexing principle.

[0055] The sensing fiber core can select different packaging structures according to specific application scenarios, and is processed by different high-performance coating processes or cabling processes according to the requirements of the application environment. The sensing fiber core can be in the form of a bare fiber of the fiber Bragg grating string or a tight-buffered fiber of the fiber Bragg grating string. The coating material used in the processing technology has excellent elasticity, harsh environment tolerance, and long-term stability, and tightly wraps the internal optical fiber core, effectively avoiding the interlayer slip between different materials, so that the deformation of the anchoring area can be accurately and reliably transmitted to the optical fiber core.

[0056] If the sensing fiber core adopts the form of a tight-buffered fiber of the fiber Bragg grating string as the sensing fiber core, see Figure 5 Figure, the sensing fiber core 2 is composed of a bare fiber of the fiber Bragg grating string and an outer optical fiber sheath 10 sleeved outside it. The bare fiber of the fiber Bragg grating string is composed of a fiber Bragg grating glass fiber core 8 and an externally provided coating layer 9. The outer optical fiber sheath 10 is tightly coupled with the bare fiber of the fiber Bragg grating string, and can effectively transfer the external deformation to the fiber core. For example, preferably, a tight-buffered fiber Bragg grating string is adopted. The fiber elements of the grating string can adopt G652D single-mode optical fiber, and the glass fiber core and the cladding of the optical fiber are 9.5μm and 125μm in diameter respectively. The coating layer and the outer sheath of the tight-buffered fiber are preferably made of acrylate and nylon materials respectively.

[0057] The rigid anchoring end of the long-gauge fiber Bragg grating strain sensor is composed of a highly tough and elastic anchoring adhesive and a high-performance fiber composite, which can firmly composite the sensing fiber core with the outer sheath of the sensor, and can couple the deformation of the sensing fiber core and the outer sheath of the sensor in the rigid anchoring area under long-term large strain and harsh environmental conditions, effectively preventing interface slip and creep. Preferably, the rigid anchoring end is composed of a special resin with high toughness, high elasticity and high durability and a high-performance basalt fiber braided material. The anchoring resin also has strong permeability to the polymer layer; during the anchoring forming process, through a strict temperature control program, the resin penetrates the fiber coating layer and reaches the fiber glass core directly and forms a direct composite with it. The additional stress on the fiber core in the anchoring area during the anchoring forming process is mild and controllable. The cured anchoring area has excellent stiffness and flexibility, so as to realize the anti-slip anchoring of the optical fiber and the stress control in the anchoring area. The length of the rigid anchoring area needs to be determined comprehensively according to resin characteristics and the type of sensing fiber core, etc., generally about 2-5 cm long.

[0058] The inner sheath of the sensor located outside the sensing fiber core has an inner diameter slightly larger than the outer diameter of the selected sensing fiber core. The inner sheath of the sensor is sleeved outside the sensing fiber core, so that there is a sliding space between the sensing fiber core and the inner sheath of the sensor; preferably, the inner sheath of the sensor is made of a material with a low elastic modulus and self-lubricating characteristics, so that the friction between it and the sensing fiber core is very small when it is in normal size; and when it is subjected to the external circumferential pressure, it is easy to generate radial contraction, generating a uniform circumferential pressure on the inner sensing fiber core. Preferably, the inner sheath material should also have good anti-seepage and glue isolation properties, which can ensure that the gap between the inner sheath and the sensing fiber core is not alternately penetrated during the processing and forming of the outer sheath; effectively avoid the adverse effects of environmental water vapor, corrosive elements, etc. on the sensing element during the long-term use of the sensor.

[0059] The flexible anchoring fixture 6 is one of the core components of the long-gauge fiber Bragg grating strain sensor, and it is also one of the features of this application that differentiates it from the prior art methods. The flexible anchoring fixture 6 is sleeved outside the inner sheath of the sensor. During the preparation of the sensor, the flexible anchoring fixture 6 can reduce its own size and squeeze the inner sheath of the sensor, so that its diameter shrinks to contact the outside of the sensing fiber core, and then a certain frictional force is generated at the interface of the anchoring area, so that the deformation of the outer sheath of the sensor can be transmitted to the sensing fiber core in the small-gauge sub-unit entirely or partially through the frictional force. The length of the flexible anchoring area needs to be determined comprehensively according to the fixture style, inner sheath characteristics, sensing fiber core type, etc., generally about 1-5 cm long.

[0060] Such as Figure 6 and Figure 7As shown, the flexible anchoring fixture can adopt a variety of different design styles, but generally consists of an inner elastic layer 11 and an outer rigid layer 12 bonded together. Under the action of external extrusion or specific physical self-excitation, the outer rigid layer can undergo precisely controllable shrinkage deformation, driving the inner elastic layer of the fixture to extrude the inner sheath of the sensor, causing the inner sheath to generate uniform radial shrinkage. Figure 6 and Figure 7 preferably shows two different forms of the flexible anchoring fixture.

[0061] For the flexible anchoring fixture, if the external extrusion strategy is adopted to cause the fixture to shrink and deform, the outer rigid layer is made of a material with a certain stiffness, good plastic deformation ability and corrosion resistance, and a special precision extrusion device is used to make it shrink and deform strictly according to the design dimensions; if the specific physical self-excitation strategy is adopted to cause the fixture to shrink and deform, the outer rigid layer of the flexible anchoring fixture is made of a shape memory material with a certain stiffness and good corrosion resistance, such as memory metal or memory polymer, and is excited by specific physical conditions such as temperature change, so that the anchoring fixture undergoes precise shrinkage deformation according to the pre-designed dimensions. Preferably, the inner elastic layer of the flexible anchoring fixture is made of a high-elastic and high-durable polymer material, so that when it shrinks, it can generate uniform extrusion stress within the range of the fixture and maintain stable mechanical properties in a long-term harsh environment.

[0062] As Figure 1 shown, for the long-gauge fiber Bragg grating strain sensor of the present invention, a sensor outer sheath is sleeved outside the inner sheath of the sensor, the rigid anchoring area and the flexible anchoring area, and the sensor outer sheath is closely combined with the above structures to form an integral structure with deformation coupling. In the long-gauge fiber Bragg grating strain sensor of the present invention, there is a uniform pre-strain in the sensing fiber core within the long gauge due to the traction of the rigid anchoring area, that is, the sensing fiber cores at each flexible sub-unit have the same pre-strain in the free state. This pre-strain is formed during the sensor processing, which enables the sensor to measure not only tensile strain but also compressive strain. Preferably, for general deformation monitoring, the sensing fiber core is pre-tensioned to about 3000 με throughout its length.

[0063] Preferably, the sensor outer sheath of the present invention presents as a flexible rod with a certain stiffness, having a certain structural strength and stiffness, capable of providing stable and reliable structural support for the pre-strain of the sensor fiber core, and at the same time effectively protecting the internal structure of the sensor to prevent potential mechanical impacts. The sensor outer sheath of the present invention can adopt different materials and laminated structures according to specific application scenarios, so that the sensor has good flexibility, constructability, durability and tolerance to harsh environments, thereby ensuring the long-term stability of the sensor performance while deforming in coordination with the structure.

[0064] Preferably, the outer sheath of the sensor can be made of a high-performance composite material composed of basalt fiber and epoxy resin or thermoplastic resin to form composite configurations in different forms such as rods, ribs, and plates, so that the sensor is suitable for pre-embedded, embedded, and external deployment in different application scenarios, while having superior characteristics such as light weight and high strength, flexibility and durability, and green and low-carbon. Figure 8 The schematic diagram shows the specific methods and types of sensor deployment. Figure 9 The schematic diagram shows different forms of smart composites formed by combining sensors with high-performance composite materials.

[0065] Figure 10 This is an example of the application of the sensor of the present invention in the deformation and crack monitoring of concrete components, which illustrates the principle and effect of the sensor in accurately sensing structural deformation and realizing the identification and quantification of damage within a long gauge. The long gauge fiber Bragg grating strain sensor 13 provided by the present invention can be directly tied to the steel bars 16 in the reinforced concrete structure through a cable tie 14, or the long gauge fiber Bragg grating strain sensor 13 provided by the present invention can be pasted on the surface of the component through a structural adhesive 17, and the long gauge fiber Bragg grating strain sensor 13 provided by the present invention can also be pre-buried in the groove 15 inside the component. In this example, the long gauge sensor is fully coupled with the monitoring structure through the aforementioned layout method. Preferably, the long gauge fiber Bragg grating sensor in this example is divided into four flexible subunits within the long gauge. To simplify the discussion, we number these four subunits and the related anchoring areas according to the figure, that is, the first subunit to the fourth subunit are numbered U1-U4 in sequence; the rigid anchoring ends are numbered G1 and G2, and the flexible anchoring areas are numbered R1-R3 in sequence.

[0066] Figure 10 The examples show the structural strain information sensed by the sensor of the present invention at different times. It should be noted that Figure 10 The strain distribution shown in is the relative strain distribution relative to the sensor in a free state, rather than the absolute strain of the sensing core, that is, it does not include the pre-strain part that the sensor has acquired during the processing. According to the description of the sensor structure in the previous article, since this sensor adopts a rigid fixed-point anchoring combined with a flexible fixed-point anchoring method to transfer the structural deformation to the sensing core, this design can ensure that the sensing core of each subunit actually senses the strain and is evenly distributed inside the subunit, that is, the spatial distribution of the sensing core strain within the long gauge length will be stepped; this strain distribution feature makes the fiber Bragg grating strain measurement value in each sensing subunit equal to the average strain of the subunit, so the axial displacement deformation of the sensing core of each subunit can be directly inferred based on the strain measurement value; since the two ends of the long gauge length are rigidly anchored, the subunit strain and the long gauge length strain have the following precise corresponding relationship:

[0067] in, is the long - gauge strain of the i - th long - gauge unit, is the strain of the j - th sub - unit of the i - th long - gauge unit, and n is the number of sub - units. Through this formula, the long - gauge strain can be calculated based on the sensor of the present invention, and combined with relevant mechanical analysis theories, the macro - micro, dynamic and static monitoring and evaluation objectives of various structures can be achieved. It should be noted that since there is the following relationship between the long - gauge strain and the relative displacement between long - gauges:

[0068] where GL is the long - gauge of the sensor, is the relative displacement between long - gauges of the i - th unit.

[0069] To illustrate the role of the sensor of the present invention in damage perception and quantification within the long - gauge, in this embodiment, when the sensor is tested at time T2, a structural crack appears within the range of sub - unit U3, and the crack width gradually expands over time. At time T1, the structure has not yet cracked. It can be noted that the difference in strain between each sub - unit is relatively small, reflecting the general distribution of the structure strain at that moment. Between time T1 and time T2, the strain within U3 suddenly increases, while the strains of its adjacent U2 and U4 remain relatively stable, which can reflect that concentrated damage deformation has occurred within U3, indicating the specific location of the structural damage.

[0070] As Figure 10 shown, since the flexible anchoring design is adopted between the sub - units of the sensor of the present invention, when the difference in strain between adjacent sub - units reaches a certain level, the sensing fiber core of the key sub - unit with a larger strain will have a relative slip with it at the flexible anchoring area, so that there is an upper limit value for the difference in strain between the key sub - unit and its adjacent sub - unit - that is Figure 10 the critical anchoring slip strain shown in . The specific magnitude of is related to various factors such as the specific materials and dimensions of each part of the sensor, the design of the flexible anchoring area, etc.; generally speaking, the stronger the anchoring effect of the flexible anchoring area on the sensing fiber core, the larger the value. It should be noted that since the present invention adopts a precisely controllable flexible anchoring area forming method, the anchoring effect of different flexible anchoring areas on the sensing fiber core can be made approximately the same, that is, the values of different anchoring areas are also approximately the same; therefore, the generation and evolution of damage can be judged and quantitatively monitored by comparing the strain difference between each sub - unit with

[0071] In this embodiment, since a crack occurs within the range of U3, U3 is the key sub - unit. At time T2, the value between U3 and its adjacent U2 reaches , and then reaches with U4.。Since the strain difference between adjacent units cannot be greater than , as the crack continues to expand, the strains of U2 and U4 also increase suddenly. It can be seen that when the structure undergoes local damage deformation, the key sub-units will drive their adjacent units to deform together. As the degree of damage increases, more adjacent units will participate in the common deformation and share the strain increment caused by the damage deformation together. This avoids the drawback that general local strain sensors often have a large strain and the sensor breaks and fails when facing the discontinuous deformation of structural damage. Through the flexible anchoring design, the strain of the key sub-units of this sensor can be effectively controlled, significantly reducing the failure risk of the sensor. On the other hand, as the damage progresses, the strain sequences sensed by each sub-unit show a distinct pattern in the spatio-temporal distribution, that is, they show an arithmetic stepped arrangement centered on the key sub-units, and the stepped range gradually expands as the damage progresses; therefore, the damage can be located and measured by observing this phenomenon and based on the relevant calculations of the strains of each sub-unit. Based on the above observations, by comparing time and time of the strain distribution of the sub-units, a metric formula for the damage deformation within the key sub-units can be given: Wherein, is the deformation amount of the structural damage at the key unit at , is the gauge length of the sub-unit, n is the number of sub-units within the long gauge, is the strain at the sub-unit at at time, is the active sub-unit determination function of the sub-unit at time, defined as: Wherein, is a constant related to the deformation mode of the monitored structure, the accuracy of the sensing system, and the monitoring frequency, and can generally be selected according to experience. Specifically, should be selected to be significantly less than , but should also have a certain magnitude to avoid misjudging the active sub-units caused by monitoring noise.

[0072] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all belong to the protection scope of the present invention.

Claims

1. A long gauge fiber Bragg grating strain sensor, comprising a plurality of long gauge strain sensing units connected in series by rigid anchoring ends, characterized in that: In the long gauge length strain sensing unit, a plurality of spaced flexible anchoring areas divide the sensing fiber core of the long gauge length strain sensing unit into a plurality of small gauge length sub-units, each of which contains a fiber grating sensing element; the flexible anchoring area generates a radial anchoring force on the sensing fiber core, and the sensing fiber core can generate relative slippage under the condition that the radial anchoring force is overcome by the flexible anchoring area, and the radial anchoring force generated by the flexible anchoring area desensitizes the deformation of the sensor outer sheath locally and transmits it to the sensing fiber core; The flexible anchoring area in the long gauge length strain sensing unit is composed of a flexible anchoring fixture arranged outside the inner sheath of the sensor; the inner diameter of the inner sheath of the sensor is larger than the outer diameter of the sensing fiber core, and the inner sheath of the sensor is arranged outside the sensing fiber core, so that there is a sliding space between the sensing fiber core and the inner sheath of the sensor; the flexible anchoring fixture squeezes the inner sheath of the sensor, so that the diameter of the inner sheath of the sensor shrinks to contact the outside of the sensing fiber core, thereby generating friction at the interface of the flexible anchoring area, and the deformation of the outer sheath of the sensor is fully or partially transmitted to the sensing fiber core in the small gauge length subunit through the action of friction; The flexible anchoring clamp is an elastic member or an elastic-rigid composite member formed by bonding an inner elastic layer and an outer rigid layer; the elastic member squeezes the inner sheath of the sensor, causing the inner sheath of the sensor to shrink radially; the outer rigid layer of the elastic-rigid composite member shrinks and deforms under the action of external force or physical self-excitation, driving the inner elastic layer to squeeze the inner sheath of the sensor, causing the inner sheath of the sensor to shrink radially.

2. The long gauge fiber Bragg grating strain sensor according to claim 1, characterized in that: The outer rigid layer of the flexible anchoring clamp is made of shape memory material, and through physical conditions stimulation, the anchoring clamp undergoes precise shrinkage and deformation according to a pre-designed size.

3. The long gauge fiber Bragg grating strain sensor according to claim 1, characterized in that: The inner sheath of the sensor is made of a material with self-lubricating properties; when subjected to external annular pressure, it is easy to produce radial contraction, generating uniform annular pressure on the internal sensing fiber core; The rigid anchoring ends at both ends of the long gauge strain sensing unit are composited with high-toughness and high-elasticity anchoring glue and high-performance fiber, which firmly composite the sensing fiber core and the outer sheath of the sensor, so that the sensing fiber core and the outer sheath of the sensor in the rigid anchoring area are deformed and coupled under long-term large strain and harsh environmental conditions, effectively preventing interface slip and creep.

4. The long gauge fiber Bragg grating strain sensor according to any one of claims 1 to 3, characterized in that: Two adjacent long gauge length strain sensing units are connected in series using rigid anchoring ends; a sensor outer sheath is arranged outside the series-connected long gauge length strain sensing units, and the outer sheath is tightly combined with the long gauge length strain sensing units to form an integral structure of deformation coupling.

5. The long gauge fiber Bragg grating strain sensor according to claim 4, characterized in that: The fiber grating sensing element in the sensing core is an ultra-weak grating or a strong grating and a combination thereof, and the grating reflectivity is within the range of the strong grating or the ultra-weak grating reflectivity; the sensing core is an arrayed fiber grating string; the rigid anchoring end of the sensor firmly and durably compounds the sensing core and the outer sheath of the sensor to form a long gauge length strain sensing unit.

6. The long gauge fiber Bragg grating strain sensor according to claim 5, characterized in that: The fiber grating sensing element in the sensing core is processed by a multi-point femtosecond dense grating method, and integrated grating engraving is performed synchronously with the drawing process during the fiber core drawing tower drawing and forming stage, so that the grating area has a dense spatial arrangement and a large range characteristic.

7. A damage monitoring method for a long gauge length fiber Bragg grating strain sensor according to any one of claims 1 to 6, characterized in that: The strain difference between each small-scale sub-unit is By comparing the damage, the generation and evolution of damage can be judged and quantitatively monitored. is the critical anchor slip strain; the long gauge length strain of the long gauge length strain sensing unit is the average strain of each small gauge length sub-unit in the long gauge length strain sensing unit.

8. The damage monitoring method according to claim 7, characterized in that: According to the generation and evolution process of damage, the key subunits in each small-gauge-length subunit in the long-gauge-length sensing unit are determined, and the damage deformation in the key subunit is: in, for Moment critical unit The deformation of the structure damage, is the subunit gauge length, n is the number of subunits in the long gauge length, for Moment Subunit The strain at for Moment Subunit The active subunit determination function is defined as: in, It is a constant related to the deformation mode of the monitored structure, the accuracy of the sensing system, and the monitoring frequency.

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