Long-gauge fiber grating strain sensor and damage monitoring method

Abstract

The invention discloses a long-scale-distance fiber grating strain sensor and a damage monitoring method, and the sensor comprises a plurality of long-scale-distance strain sensing units which are connected in series through rigid anchoring ends. A sensing fiber core of the long-scale-distance strain sensing unit is divided into a plurality of small-scale-distance subunits by a plurality of flexible anchoring areas which are arranged at intervals, and each small-scale-distance subunit comprises a fiber grating sensing element; the flexible anchoring area generates radial anchoring force on the sensing fiber core, the sensing fiber core can generate relative slippage when the flexible anchoring area overcomes the radial anchoring force, and the radial anchoring force generated by the flexible anchoring area enables local deformation of the sensor outer sheath to be transmitted to the sensing fiber core in a desensitization mode. The strain sensor provided by the invention can accurately identify and measure the local damage event of the structure in the long gauge length unit, and provides high-precision deformation information perception and fine identification and accurate measurement of the local damage state in the long gauge length for health monitoring of various structures.

Description

Technical Field

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

[0002] In the field of fiber Bragg grating sensing, there are two main types of fiber Bragg grating strain sensors: point fiber Bragg grating strain sensors and long gauge length fiber Bragg grating strain sensors. Point-type fiber Bragg grating strain sensors are fiber Bragg grating sensing elements that are fully pasted or packaged with a shorter gauge length. Compared with the huge infrastructure structures, they are point-type configurations in engineering monitoring applications. Long-term engineering practice shows that this type of sensor can only be arranged in different parts of the structure in a discrete form and can only be used to measure the local strain of the structure. It is difficult to achieve effective coverage of the key areas of the structure and obtain effective data that can reflect the overall deformation and damage state of the structure. Long-gauge fiber Bragg grating strain sensors determine the effective sensing characteristic scale according to the characteristics of the monitoring object and application scenario. By packaging the fiber Bragg grating element with a long gauge length (the gauge length is generally between tens of centimeters and several meters), that is, the fiber Bragg grating sensing element and the outer sheath of the sensor are deformed and coupled by a fixed-point anchoring method with a certain gauge length. The measured deformation is the average strain between the long gauge lengths, so that the sensing system can effectively cover the key areas of the measured structure with a small number of sensing units. Further combined with relevant mechanical theories, it can achieve comprehensive and direct mapping of macro and micro characteristics such as structural deformation angle, load, internal force and dynamic parameters, as well as effective identification of early structural damage.

[0003] Although the existing long-scale packaged fiber Bragg grating strain sensors can provide effective data for macro-micro dynamic and static status monitoring of various structures, since the structural damage of general infrastructure occurs at the material scale, the existing long-scale sensing system can only obtain the overall representation of structural damage within a certain spatial range, while it is still difficult to obtain more in-depth and refined damage information within the long scale, such as the specific location, quantity and quantitative measurement of the damage. The acquisition of this information is of great significance for improving structural durability and refined maintenance evaluation.

[0004] In view of the above problems, in the field of health monitoring of various civil engineering infrastructure structures, strain sensors with simple structure, easy large-scale manufacturing, and capable of simultaneously realizing accurate monitoring of long-scale strain information and refined structural damage information are needed, thereby laying an information foundation for realizing a low-cost, high-precision, dynamic and static, multi-level precise structural health monitoring system, as well as a refined predictive maintenance and intelligent management system for infrastructure. Summary of the invention

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

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: The present invention first provides a long-gauge fiber Bragg grating strain sensor, comprising a plurality of long-gauge strain sensing units connected in series by a rigid anchoring end. In the long-gauge strain sensing unit, a plurality of flexible anchoring areas arranged at intervals divide the sensing fiber core of the long-gauge strain sensing unit into a plurality of small-gauge sub-units, each of which 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 generate relative slippage under the condition that the radial anchoring force is overcome in the flexible anchoring area. The radial anchoring force generated by the flexible anchoring area desensitizes the deformation of the local outer sheath of the sensor and transmits it to the sensing fiber core.

[0007] The flexible anchoring area in the long-gauge strain sensing unit is composed of a flexible anchoring clamp 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 clamp 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.

[0008] 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.

[0009] As a further preferred embodiment of the present invention, the sensing fiber core has two or more fiber grating regions within the long gauge length between the rigid anchoring ends, forming a fiber grating string with evenly spaced grating regions within the long gauge length, and the grating string sensing element is an ultra-weak grating or a strong grating or a combination thereof; As a further preferred embodiment of the present invention, the grating in the sensor core is processed by multi-point dense femtosecond grating technology, and the integrated grating is recorded synchronously with the drawing process during the fiber core drawing tower drawing forming stage, so that the grating area has superior characteristics such as dense spatial arrangement and a large range; As a further preferred embodiment of the present invention, the fiber grating element in the sensing core may have a grating reflectivity in the range of strong grating or ultra-weak grating reflectivity according to different principles of demodulation equipment; As a further preferred embodiment of the present invention, the sensing fiber core is processed by different high-performance coating processes or cabling processes according to specific application scenarios, and the sensing fiber core is in the form of a fiber grating string bare fiber or a fiber grating string tight sleeve fiber. The coating material used in the above processing technology has excellent elasticity, harsh environment tolerance and long-term stability, and forms a tight wrapping for the internal optical fiber core, effectively avoiding interlayer slippage between different materials, so that the deformation of the anchoring area can be accurately and reliably transmitted to the optical fiber core; As a further preferred embodiment of the present invention, the sensing fiber core adopts a fiber grating string tight-jacketed fiber as the sensing fiber core, and the sensing fiber core is composed of a fiber grating string bare fiber and an outer fiber sheath sheathed thereon, and the outer fiber sheath is tightly coupled with the bare fiber, which can effectively transfer external deformation to the fiber core; As a further preferred embodiment of the present invention, the rigid anchoring end of the long gauge fiber Bragg grating strain sensor is composited with a high-toughness and high-elasticity anchoring glue and a high-performance fiber, which can 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; As a further preferred embodiment of the present invention, the rigid anchoring end of the long gauge fiber Bragg grating strain sensor is composited with a special resin having high toughness, high elasticity and high durability and a high-performance basalt fiber braided material, and the anchoring resin also has strong permeability to the polymer layer; during the anchoring molding process, the resin penetrates the optical fiber coating layer directly to the optical fiber glass core and forms a direct composite with it through a strict temperature control process, and the additional stress on the optical fiber core in the anchoring area during the anchoring molding process is mild and controllable, and the solidified anchoring area has excellent rigidity and flexibility, thereby realizing the anti-slip anchoring of the optical fiber and stress control of the anchoring area; As a further preferred embodiment 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, and 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; As a further preferred embodiment 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 friction between the inner sheath and the sensing fiber core is very small when the sensor is in normal size; and when subjected to the external annular pressure, the inner sheath is easy to produce radial contraction, thereby generating a uniform annular pressure on the internal sensing fiber core; As a further preferred embodiment of the present invention, the inner sheath material of the sensor has good impermeability and glue-insulating properties, which can ensure that the gap between the inner sheath and the sensing fiber core is not penetrated by colloid during the processing and molding of the outer sheath; and effectively isolate the adverse effects of environmental water vapor, corrosive elements, etc. on the sensing element during the long-term use of the sensor; As a further preferred embodiment of the present invention, the sensor is provided with a flexible anchoring fixture on the outside of the inner sheath. 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 with the outside of the sensing fiber core, thereby generating a certain friction force at the interface of the anchoring region, so that the deformation of the outer sheath of the sensor can be fully or partially transmitted to the sensing fiber core in the small gauge length subunit through the action of the friction force; As a further preferred embodiment of the present invention, the flexible anchoring fixture is formed by bonding an inner elastic layer and an outer rigid layer. Under the action of external force extrusion or specific physical self-excitation, the outer rigid layer can undergo precise and controllable shrinkage deformation, driving the inner elastic layer of the fixture to squeeze the inner sheath of the sensor, so that the inner sheath produces uniform radial shrinkage; As a further preferred embodiment of the present invention, the flexible anchoring clamp, if adopting an external extrusion strategy to make the clamp shrink and deform, the outer rigid layer of the flexible anchoring clamp is made of a material with certain rigidity, 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 size; As a further preferred embodiment of the present invention, the flexible anchoring fixture, such as adopting a specific physical self-excitation strategy to make the fixture shrink and deform, the outer rigid layer of the flexible anchoring fixture adopts a shape memory material with a certain rigidity and good corrosion resistance, and is excited by specific physical conditions such as temperature changes, so that the anchoring fixture undergoes precise shrinkage and deformation according to a pre-designed size; As a further preferred embodiment of the present invention, the inner elastic layer of the flexible anchoring clamp is made of a high-elasticity, high-durability polymer material, so that when it contracts, it can generate uniform extrusion stress within the clamp range and can maintain stable mechanical properties in a long-term harsh environment; As a further preferred embodiment of the present invention, the long gauge fiber Bragg grating strain sensor is provided with a sensor outer sheath outside the inner sheath, the rigid anchoring area and the flexible anchoring area, and the outer sheath is tightly combined with the above structure to form an integral structure of deformation coupling; As a further preferred embodiment of the present invention, the sensing fiber core of the long gauge length fiber Bragg grating strain sensor has a uniform prestrain formed by being pulled by the rigid anchoring area in the long gauge length, that is, the sensing fiber core at each flexible subunit has a prestrain of the same magnitude in a free state, and the prestrain is formed during the sensor processing, so that the sensor can measure not only tensile strain but also compressive strain; As a further preferred embodiment of the present invention, the outer sheath of the sensor is a flexible rod with a certain rigidity, has a certain structural strength and rigidity, can provide a stable and reliable structural support for the pre-strain of the sensor core, and effectively protect the internal structure of the sensor; As a further preferred embodiment of the present invention, the outer sheath of the sensor adopts different materials and laminated structures according to specific application scenarios, so that the sensor has good flexibility, construction performance, durability and tolerance to harsh environments, thereby ensuring the long-term stability of the sensor performance while cooperating with the structure to deform; As a further preferred embodiment 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 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 layout in different application scenarios, and at the same time has superior characteristics such as light weight and high strength, flexibility and durability, and green and low carbon.

[0010] The present invention also provides a damage monitoring method based on a long gauge fiber Bragg grating strain sensor, which detects the damage of the fiber Bragg grating by comparing the strain difference between the small gauge sub-units and the strain difference between the small gauge sub-units. By comparing the damage, the generation and evolution of damage can be judged and quantitatively monitored. is the critical anchor slip strain.

[0011] The present invention has the following beneficial effects: The present invention adds a flexible anchoring area within the long gauge length between the rigid anchoring ends of the long gauge length sensor, and divides the sensing fiber core into a number of small gauge length sub-units, so that the long gauge length sensor can have the ability to finely identify and measure damage inside the long gauge length, thereby solving the problem that the existing long gauge length strain sensor is difficult to distinguish fine damage information within the gauge length.

[0012] The flexible anchoring design of the sensor of the present invention makes it difficult for local damage and deformation of the structure to cause large local deformation of the optical fiber, resulting in breakage and failure of the sensing system; the concentrated damage and deformation will be jointly borne by several sub-units, realizing the desensitization of local optical fiber sensing (that is, the actual deformation borne by the optical fiber at the local damage is smaller than the local deformation increment caused by the damage, and the sensing strain is smaller than that of rigid anchoring). Under the premise of realizing refined identification and precise measurement of damage characteristics, the safety and stability of the sensing system can be greatly improved.

[0013] The flexible anchoring area design of the sensor of the present invention makes the grating sensing strain of each subunit the average strain of the subunit, which can be used to perform accurate strain-displacement conversion and long-scale strain conversion, thereby achieving high-precision measurement of the fine damage deformation of the structure, greatly improving the efficiency of the long-scale strain sensing system in fine-grained and accurate monitoring and evaluation of structures.

[0014] Increased economical explanation: The rigid anchoring end of the sensor is difficult to process and has high cost, while the flexible anchoring area is easy to process and has low cost. The combination of the two not only improves the functionality of the sensing system, but also has good economical efficiency.

[0015] The sensor design of the present invention has a simple structure, is easy to process, is low in cost, strong and stable, and is convenient for large-scale sensor production and industrial application. It is of great significance for the identification and measurement of local fine damage of various structures and for the accurate assessment of the macroscopic and microscopic dynamic and static states of the structures.

[0016] In summary, the sensor proposed in the present invention is in the shape of a flexible rod, which is slender and compact, and is convenient for achieving comprehensive coverage of key areas of the structure and accurate perception of multi-level deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a conceptual and structural schematic diagram of a long gauge fiber Bragg grating strain sensor with a built-in flexible subunit according to the present invention; Figure 2 yes Figure 1 Schematic diagram of the structure of the long gauge length strain sensing unit; Figure 3 yes Figure 1 Structural cross-section of the long gauge length strain sensing unit in; Figure 4 yes Figure 3 AA screenshots; Figure 5 It is a schematic diagram of different packaging forms of a sensing fiber core of a long gauge fiber Bragg grating strain sensor with a built-in flexible subunit according to the present invention; Figure 6 It is a structural schematic diagram of a flexible anchoring fixture of a long gauge length fiber Bragg grating strain sensor with a built-in flexible subunit according to the present invention; Figure 7 It is a schematic diagram of a forming mode of a flexible anchoring fixture of a long gauge length fiber Bragg grating strain sensor with a built-in flexible subunit according to the present invention; Figure 8 It is a schematic diagram of the layout of a long gauge fiber Bragg grating strain sensor with a built-in flexible subunit in the present invention for use in an engineering structure; Fig. 9 It is a schematic diagram of different forms of a long gauge fiber Bragg grating strain sensing intelligent composite material with built-in flexible subunits of the present invention; Fig.10 The present invention is a schematic diagram of the principle and application of a long gauge fiber Bragg grating strain sensor with a built-in flexible subunit in structural deformation and crack monitoring.

[0018] In the figure: 1. Rigid anchoring end; 2. Sensing fiber core; 3. Fiber Bragg grating; 4. Sensor inner sheath; 5. Sensor outer sheath; 6. Flexible anchoring fixture; 7. Flexible anchoring area; 8. Fiber Bragg grating glass core; 9. Coating layer; 10. Fiber outer 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. DETAILED DESCRIPTION

[0019] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

[0020] In the description of the present invention, it should be understood that the terms "left side", "right side", "upper part", "lower part" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second" and the like do not indicate the importance of the components, and therefore cannot be understood as limiting the present invention. The specific dimensions used in this embodiment are only for illustrating the technical solution, and do not limit the scope of protection of the present invention.

[0021] like Figure 1 As shown, a long-gauge fiber Bragg grating strain sensor with built-in flexible subunits is composed of a plurality of long-gauge sensing units with built-in subunits connected in series via a rigid anchoring end 1.

[0022] The structure of the long gauge sensing unit is as follows: Figure 2 As shown, it includes a sensing fiber core 2, a sensor inner sheath 4, a flexible anchoring area 7 and a sensor outer sheath 5; wherein the sensing fiber core 2 is an arrayed fiber grating string; the sensor rigid anchoring end 1 firmly and durably combines the sensing fiber core 2 and the sensor outer sheath 5 to form a long gauge length strain sensing unit; within the long gauge length between the rigid anchoring ends, a plurality of flexible anchoring areas 7 divide the sensing fiber core 2 into a plurality of small gauge length sub-units, each of which contains a fiber grating sensing element, and the flexible anchoring area 7 can desensitize the significant deformation locally occurring in the sensor outer sheath 5 and transmit it to the sensing fiber core 2.

[0023] like Figure 2 As shown, there are generally two or more fiber Bragg grating regions in the long gauge length between the rigid anchoring ends of the sensing fiber core 2, forming a fiber Bragg grating string with evenly spaced grating regions in the long gauge length, and the number of grating regions is determined according to the degree of refinement of the perception of structural damage information in the application scenario; the grating in the sensing fiber core is processed by multi-point dense femtosecond grating technology, and the integrated grating is burned synchronously with the drawing process during the fiber core drawing tower drawing forming stage, so that the grating region has superior characteristics such as dense spatial arrangement and a large range.

[0024] The fiber grating element in the sensing core may have a grating reflectivity in the range of strong grating or ultra-weak grating reflectivity depending on the principle of the demodulation device. The demodulation principle includes but is not limited to the wavelength division multiplexing principle and the time division multiplexing principle.

[0025] The sensing fiber core can choose different packaging structures according to specific application scenarios, and is processed by different high-performance coating processes or cabling processes according to the application environment requirements. The sensing fiber core can be in the form of a bare fiber with a fiber Bragg grating string or a tight-sleeved fiber with a fiber Bragg grating string. The coating material used in the processing technology has excellent elasticity, harsh environment tolerance and long-term stability, and forms a tight wrap around 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.

[0026] If the sensing fiber core uses a fiber Bragg grating string tight-buffered fiber as the sensing fiber core, see Figure 5 The sensing core 2 is composed of a bare fiber of a fiber grating string and an outer fiber sheath 10 sheathed thereon. The bare fiber of the fiber grating string is composed of a fiber grating glass core 8 and an outer coating 9. The outer fiber sheath 10 is tightly coupled with the bare fiber of the fiber grating string, and can effectively transfer external deformation to the core. For example, a tight-packed fiber grating string is preferably adopted. The fiber element of the grating string can adopt G652D single-mode optical fiber, and the glass core and cladding of the optical fiber are 9.5μm and 125μm in diameter, respectively. The coating layer and the tight-packed outer sheath are preferably made of acrylic and nylon, respectively.

[0027] The rigid anchoring end of the long gauge fiber Bragg grating strain sensor is composed of a high-toughness and high-elasticity anchoring glue and a high-performance fiber. The sensing fiber core and the outer sheath of the sensor can be firmly bonded, and the sensing fiber core and the outer sheath of the sensor in the rigid anchoring area can be deformed and coupled under long-term large strain and harsh environmental conditions, effectively preventing interface slip and creep. For the rigid anchoring end, we prefer to use 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 molding process, the resin penetrates the optical fiber coating layer directly to the optical fiber glass core through a strict temperature control procedure and forms a direct composite with it. The additional stress of the optical fiber core in the anchoring area during the anchoring molding process is mild and controllable. The solidified anchoring area has excellent rigidity and flexibility, thereby realizing the anti-slip anchoring of the optical fiber and stress control of the anchoring area. The length of the rigid anchoring area needs to be determined comprehensively based on the resin characteristics and the type of sensing fiber core, and is generally about 2-5cm long.

[0028] The inner diameter of the sensor inner sheath outside the sensing core should be slightly larger than the outer diameter of the selected sensing core. The sensor inner sheath is placed outside the sensing core so that there is sliding space between the sensing core and the sensor inner sheath. Preferably, the sensor inner sheath is made of a material with a low elastic modulus and self-lubricating properties, so that the friction between it and the sensing core is very small when it is in normal size. When subjected to the external annular pressure, it is easy to produce radial contraction, and produce uniform annular pressure on the internal sensing core. Preferably, the inner sheath material should also have good impermeability and glue isolation properties, so that the gap between the inner sheath and the sensing core can be ensured not to be alternately penetrated during the processing and molding of the outer sheath. It can effectively prevent environmental water vapor, corrosive elements, etc. from having adverse effects on the sensing element during the long-term use of the sensor.

[0029] The flexible anchoring fixture 6 is one of the core components of the long gauge fiber Bragg grating strain sensor, and is also one of the features that distinguish this application from the prior art methods. The sensor is provided with a flexible anchoring fixture 6 on the outside of the inner sheath. During the sensor preparation process, the flexible anchoring fixture 6 can shrink 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, thereby generating a certain friction force at the interface of the anchoring area, so that the deformation of the outer sheath of the sensor can be fully or partially transmitted to the sensing fiber core in the small gauge subunit through the action of friction. The length of the flexible anchoring area needs to be determined comprehensively based on the fixture style, inner sheath characteristics, sensing fiber core type, etc., and is generally about 1-5 cm long.

[0030] like Figure 6 and Figure 7 As shown, the flexible anchoring clamp can adopt a variety of different design styles, but is generally made of an inner elastic layer 11 and an outer rigid layer 12 bonded together. The outer rigid layer can undergo precisely controllable shrinkage deformation under the action of external force or specific physical self-excitation, driving the inner elastic layer of the clamp to squeeze the inner sheath of the sensor, causing the inner sheath to produce uniform radial shrinkage. Figure 6 and Figure 7 Two different styles of flexible anchoring clamps are preferably shown.

[0031] For flexible anchoring clamps, if an external extrusion strategy is used to shrink and deform the clamp, the outer rigid layer is made of a material with a certain rigidity, good plastic deformation ability and corrosion resistance, and a special precision extrusion device is used to shrink and deform it strictly according to the designed size; if a specific physical self-excitation strategy is used to shrink and deform the clamp, the outer rigid layer of the flexible anchoring clamp is made of a shape memory material with a certain rigidity and good corrosion resistance, such as a memory metal or a memory polymer, and is stimulated by specific physical conditions such as temperature changes, so that the anchoring clamp shrinks and deforms precisely according to the pre-designed size. Preferably, the inner elastic layer of the flexible anchoring clamp is made of a highly elastic and durable polymer material, so that it can generate uniform extrusion stress within the clamp range when it shrinks, and maintain stable mechanical properties in a long-term harsh environment.

[0032] like Figure 1 As shown, the long gauge fiber Bragg grating strain sensor of the present invention has an outer sensor sheath on the outer side of the inner sensor sheath, the rigid anchoring area and the flexible anchoring area, and the outer sensor sheath is tightly combined with the above structure to form an integral structure of deformation coupling. The sensing fiber core of the long gauge fiber Bragg grating strain sensor of the present invention has a uniform pre-strain formed by the pulling of the rigid anchoring area in the long gauge, that is, the sensing fiber core at each flexible subunit has a pre-strain of the same size in a free state, and the pre-strain is formed during the processing of the sensor, 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-stretched to about 3000με.

[0033] Preferably, the outer sheath of the sensor of the present invention is a flexible rod with a certain rigidity, has a certain structural strength and rigidity, can provide stable and reliable structural support for the pre-strain of the sensor core, and effectively protect the internal structure of the sensor to prevent potential mechanical impact. The outer sheath of the sensor of the present invention can adopt different materials and laminated structures according to specific application scenarios, so that the sensor has good flexibility, construction, durability and tolerance to harsh environments, thereby ensuring the long-term stability of the sensor performance while cooperating with the structure to deform.

[0034] 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. Fig. 9 The schematic diagram shows different forms of smart composites formed by combining sensors with high-performance composite materials.

[0035] Fig.10This 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.

[0036] Fig.10 The examples show the structural strain information sensed by the sensor of the present invention at different times. It should be noted that Fig.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:

[0037] in, is the long gauge length strain of the i-th long gauge length unit, is the strain of the jth subunit of the ith long gauge length unit, and n is the number of subunits. Through this formula, the long gauge length strain can be calculated based on the sensor of the present invention, and combined with relevant mechanical analysis theory to achieve various structural macro-micro, dynamic and static monitoring and evaluation goals. It is worth pointing out that the long gauge length strain and the relative displacement between long gauge lengths have the following relationship:

[0038] Where GL is the long gauge length of the sensor, is the relative displacement between the long gauge lengths of the ith unit.

[0039] To illustrate the role of the sensor of the present invention in damage perception and quantification within a long gauge length, the sensor test of this embodiment shows that at time T2, a structural crack appears inside the subunit U3, and the crack width gradually expands over time. At time T1, the structure has not yet cracked, and it can be noted that the difference between the strains of each subunit is small, reflecting the approximate distribution of the structural strain at that time. Between time T1 and time T2, the strain within the U3 range increases suddenly, but the strains of the adjacent U2 and U4 remain relatively stable, which can reflect that concentrated damage deformation has occurred inside U3, indicating the specific location of the structural damage.

[0040] like Fig.10 As shown, due to the flexible anchoring design between the sensor subunits of the present invention, when the differential strain of adjacent subunits reaches a certain level, the key subunit sensing fiber core with a large strain will produce relative slip with it at the flexible anchoring area, so that the differential strain between the key subunit and the adjacent subunit has an upper limit value - that is, Fig.10 The critical anchor slip strain shown in . The specific size of is related to many factors such as the specific material and size of each part of the sensor, the design of the flexible anchoring area, etc. In general, the stronger the anchoring effect of the flexible anchoring area on the sensor core, the It is worth noting that, since the present invention adopts a precisely controllable flexible anchoring region forming method, the anchoring effect of different flexible anchoring regions on the sensing fiber core can be roughly the same, that is, the anchoring effect of different anchoring regions on the sensing fiber core is roughly the same. The values ​​are also roughly the same; therefore, the strain differences between the subunits can be By comparing the damage patterns, the generation and evolution of damage can be determined and quantitatively monitored.

[0041] In this embodiment, since cracks occur in the range of U3, U3 is the key subunit. At time T2, the gap between U3 and the adjacent U2 reaches , and then reached 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-unit will drive its adjacent units to deform together. As the degree of damage increases, more adjacent units will participate in the joint deformation and share the strain increment caused by the damage deformation. This avoids the disadvantage that general local strain sensors often suffer from large strains that lead to sensor fracture and failure when facing discontinuous deformation of structural damage. Through the flexible anchoring design, the strain of the key sub-unit of this sensor can be effectively controlled, significantly reducing the risk of sensor failure. On the other hand, as the damage progresses, the strain sequence sensed by each sub-unit shows a distinct pattern in terms of temporal and spatial distribution, that is, it presents an equidistant step-like arrangement centered on the key sub-unit, and the step range gradually expands as the damage progresses; therefore, the damage can be located and measured by observing this phenomenon and calculating the relevant information based on the strain of each sub-unit. Based on the above observations, by comparing Moment and The strain distribution of the subunit at the time can give a measurement formula for the damage deformation in the key subunit: 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, and can generally be selected based on experience. The selection should be significantly smaller than , but it must have a certain magnitude to avoid misjudgment of active subunits due to monitoring noise.

[0042] The preferred embodiments of the present invention are 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 number of flexible anchoring areas arranged at intervals 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 produce relative slippage under the condition that the radial anchoring force is overcome in the flexible anchoring area, and the radial anchoring force generated by the flexible anchoring area desensitizes the local deformation of the sensor outer sheath and transmits it to the sensing fiber core.

2. The long gauge fiber Bragg grating strain sensor according to claim 1, characterized in that: The flexible anchoring area in the long-gauge strain sensing unit is composed of a flexible anchoring clamp 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 clamp 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.

3. The long gauge fiber Bragg grating strain sensor according to claim 2, characterized in that: 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.

4. The long gauge fiber Bragg grating strain sensor according to claim 3, 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.

5. The long gauge fiber Bragg grating strain sensor according to claim 2, 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.

6. The long gauge fiber Bragg grating strain sensor according to any one of claims 1 to 5, 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.

7. The long gauge fiber Bragg grating strain sensor according to claim 6, 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.

8. The long gauge fiber Bragg grating strain sensor according to claim 7, 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.

9. A damage monitoring method for a long gauge length fiber Bragg grating strain sensor according to any one of claims 1 to 8, 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.

10. The damage monitoring method according to claim 9, 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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