Cavity-containing bolt pre-tightening force and temperature composite sensing optical fiber sensor and method

Abstract

The application discloses a kind of cavity-containing bolt pre-tightening force and temperature compound sensing optical fiber sensor and method, including bolt, glue and fiber grating, the cavity, longitudinal hole and transverse through hole are equipped in bolt, two transverse through holes are arranged in the cavity both sides, longitudinal hole sequentially passes through transverse through hole and internal cavity along bolt length direction, longitudinal hole is used to lay optical fiber, one half grating area of fiber grating is suspended and arranged at the cavity, the other half grating area surface is attached with metal plating layer and then is pasted in longitudinal hole by glue, since the cavity and longitudinal hole are different in bolt cross-sectional area, when bolt is subjected to axial stretching, two sections grating area will have different axial strain;Simultaneously after metal plating layer, there will be no chirp phenomenon, and metal plating layer can change the response coefficient of grating area to temperature.The application realizes the simultaneous monitoring of bolt temperature and force signal, and obtains accurate bolt temperature and force signal according to the different sensing coefficients of two parts of fiber grating to temperature and force.

Description

Technical Field

[0001] Specifically, this invention relates to a fiber optic sensor and method for sensing bolt preload and temperature in a cavity. Background Technology

[0002] Bolts are commonly used to connect and fasten two or more components. They offer high reliability and detachability, and are widely used in technical fields such as railway engineering, civil engineering, and the automotive industry. However, during long-term use, factors such as vibration, temperature, and alternating loads can cause bolted connections to slip, loosen, or even separate, reducing the reliability and stability of the structure and creating safety hazards. Therefore, accurate measurement of bolt preload is crucial for the safety and reliability of bolted connections.

[0003] In recent decades, various techniques for measuring bolt stress states have been reported, primarily including structural dynamic parameter detection, acoustoelastic effect methods, piezoresistive impedance methods, and FBG sensor-based methods. Currently, each of these monitoring methods has its own drawbacks. Structural dynamic parameter detection methods can only detect the required data after the structure has loosened or damaged. Acoustic and piezoresistive methods are costly and susceptible to noise and electromagnetic interference. In contrast, fiber optic sensors offer significant advantages, such as immunity to noise and electromagnetic interference, small fiber optic grating size for easy installation inside the bolt, and ease of multi-signal measurement. However, most current FBG-based bolt force sensors do not consider the influence of temperature on the sensing signal while monitoring bolt preload. Some researchers have attempted to simultaneously measure temperature and force by embedding multiple FBGs inside the bolt, but this method causes significant damage to the bolt structure.

[0004] In summary, a temperature-compensated bolt force sensor that is simple in structure, resistant to noise and electromagnetic interference, and causes minimal damage to the bolt structure is needed to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a cavity-based fiber optic sensor and method for combined sensing of bolt preload and temperature, enabling simultaneous monitoring of bolt temperature and force signals. Based on the different sensing coefficients of the two fiber optic gratings for temperature and force, accurate temperature and force signals are obtained through decoupling. Compared to other fiber optic grating bolt force sensors, this invention causes less damage to the bolt structure, achieving simultaneous sensing of bolt temperature and force signals using only a single FBG. Accurate bolt temperature and force signals are obtained through the decoupling matrix between temperature-force and wavelength offset.

[0006] The technical solution adopted in this invention is: A cavity-based composite fiber optic sensor for sensing bolt preload and temperature includes a bolt, adhesive, and a fiber optic grating. The bolt has a cavity, a longitudinal hole, and a transverse through hole inside. Two transverse through holes are arranged on both sides of the internal cavity. The longitudinal hole passes through the transverse through hole and the internal cavity in sequence along the length of the bolt. The longitudinal hole is used to lay optical fibers. The internal cavity is used to suspend and arrange the fiber optic grating. The transverse through hole is used for injecting adhesive to bond the optical fibers inside the bolt. Half of the fiber Bragg grating is suspended in the cavity inside the bolt, while the other half has a metal coating on its surface and is glued to the longitudinal hole of the bolt. The presence of the cavity can improve the sensor sensitivity by 25%.

[0007] Preferably, the bolts are made using titanium alloy 3D printing technology. Preferably, the adhesive used is a UV-curable adhesive. Preferably, the fiber grating metal coating is a nickel coating.

[0008] Preferably, the cavity diameter is half the bolt diameter.

[0009] Preferably, the metal coating is a nickel metal coating applied to the grating surface by a metal coating process, and the coating thickness is approximately 350 μm; Metal coating process refers to applying a metallic nickel coating to half of the fiber grating region by sequentially performing a sensitization and activation fiber grating process, a chemical nickel plating process, and an electroplating nickel process.

[0010] Preferably, in the sensitized and activated fiber Bragg grating process, the optical fiber is immersed in the sensitization solution and the activation solution for 15 minutes in sequence; in the electroless nickel plating process, the sensitized and activated fiber Bragg grating is immersed in an 85°C electroless nickel plating solution for 2 hours; in the electroless nickel plating process, the electroless nickel-plated fiber Bragg grating forms a closed-loop circuit through a DC power supply, a nickel plate, a copper wire, and an electroless nickel plating solution, and is electroplated in an environment of 40°C electroless nickel plating solution, wherein the current is set to 1.5mA and the electroplating time is 3 hours.

[0011] A monitoring method using the above-described cavity-containing bolt preload and temperature composite sensing fiber optic sensor includes the following steps: when a bolt is connected to a mechanical device, its preload will cause the bolt to undergo axial strain. When this strain is transmitted to the fiber optic grating, the two fiber optic gratings will produce different axial strains because the cross-sectional areas of the two fiber optic gratings installed inside the bolt are different. When the external temperature changes, both fiber Bragg gratings are affected by the temperature. Due to the presence of the metal coating, the center wavelengths of the two fiber Bragg gratings have different sensitivities to temperature. Under the same temperature, the center wavelengths of the two fiber Bragg gratings will have different offsets. By analyzing the difference in the sensitivity of the two grating regions to temperature and force response, and combining the sensor's mechanical model, a decoupling matrix between the center wavelength of the fiber Bragg grating and temperature and force is established. Through decoupling, the changes in temperature and force parameters monitored by the final sensor are obtained.

[0012] Preferably, the center wavelength offset of the two fiber grating segments after being subjected to axial force load and temperature load is... and They can be represented as:

[0013]

[0014] in, This refers to the change in the center wavelength of the uncoated fiber grating. The initial center wavelength of the uncoated fiber grating segment. This refers to the change in the center wavelength of the fiber grating in the coated section. The initial center wavelength of the coated fiber grating is given. The effective elastic-optic coefficient of the optical fiber is (~0.22). The coefficient of thermal expansion of the uncoated fiber grating is... The coefficient of thermal expansion of the fiber grating in the coated section is... The thermo-optic coefficient of the uncoated fiber grating. The thermo-optic coefficient of the fiber grating in the coated section. This represents the axial strain of the uncoated fiber grating section after the bolt is subjected to axial tensile force. This represents the axial strain of the coated fiber grating section after the bolt is subjected to axial tensile force. This refers to the change in ambient temperature.

[0015] Center wavelength offset and force of uncoated and coated fiber gratings and / or temperature The relation is:

[0016]

[0017] in, The force sensitivity coefficient of the uncoated fiber grating. This represents the force sensitivity coefficient of the fiber grating in the coated section. The temperature sensitivity coefficient of the uncoated fiber grating section. This represents the temperature sensitivity coefficient of the fiber grating in the coated section. This represents the change in the axial tensile force acting on the bolt.

[0018]

[0019] Calculations show that u=0 and v=L3=L4, from which the deformation at both ends of the cavity can be obtained. Where u is the transverse displacement component at the center of both ends of the cavity, the transverse direction being the bolt width direction (i.e., the x-axis direction), and x refers to the coordinate on the x-axis. v is the longitudinal displacement component at the center of both ends of the cavity, the longitudinal direction being the bolt axis (i.e., the y-axis direction), and y refers to the coordinate on the y-axis. Let be the Poisson's ratio of the bolt material, q be the external load, h be the cavity diameter, and E = Eb be the bolt's elastic modulus. Where is the cavity radius.

[0020] The beneficial effects of this invention are: This invention uses only a single fiber Bragg grating. One half of the grating area is suspended in the cavity inside the bolt, while the other half, with a metal coating, is glued to the longitudinal hole. Because the bolt cross-sectional areas at the cavity and the longitudinal hole are different, the two grating areas will experience different axial strains when the bolt is subjected to axial tension. The cavity further improves the sensor sensitivity. Simultaneously, the metal coating prevents chirping in the reflection spectrum of the grating area glued to the longitudinal hole, and it also alters the grating area's temperature response coefficient. This invention enables simultaneous monitoring of bolt temperature and force signals. Based on the different temperature and force sensing coefficients of the two fiber Bragg gratings, accurate bolt temperature and force signals are obtained. Furthermore, using only a single fiber Bragg grating results in a simple manufacturing process, small size, and suitability, especially for bolts as small as M3. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a temperature self-compensating bolt force sensor based on a metallized fiber optic grating in an embodiment of the present invention.

[0022] Figure 2 This is a cross-sectional view of a temperature self-compensating bolt force sensor based on a metallized fiber Bragg grating according to an embodiment of the present invention.

[0023] Figure 3 yes Figure 2 A partial K-section view.

[0024] Figure 4 This is a schematic diagram of a temperature self-compensating bolt force sensor based on a metallized fiber optic grating under axial tensile force, according to an embodiment of the present invention.

[0025] Figure 5 yes Figure 4 A schematic diagram showing the change of local part M before and after being subjected to tensile force; Figure 6 This is a schematic diagram of the deformation of the internal cavity of the bolt under axial tensile force according to an embodiment of the present invention, and a schematic diagram of the deformation model transformation. Figure 7 yes Figure 6 A schematic diagram of the local stress deformation of N and a schematic diagram of the deformation model transformation; Figure 8 This is a schematic diagram of the embedded fiber grating of a temperature self-compensating bolt force sensor based on a metallized fiber grating according to an embodiment of the present invention. Figure 9 This is a force analysis diagram of the embedded fiber grating of the temperature self-compensating bolt force sensor based on metallized fiber grating according to an embodiment of the present invention; In the diagram: 1—bolt, 2—fiber optic protective sleeve, 3—UV adhesive, 4—fiber grating, 101—longitudinal hole, 102—transverse through hole, 103—cavity, 401—metal coating, 402—coated fiber grating section, 403—uncoated fiber grating section. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] A fiber optic sensor for combined sensing of bolt preload and temperature, containing a cavity, such as... Figures 1-7 As shown, it includes a bolt 1, an optical fiber protective sleeve 2, glue 3, and a fiber optic grating 4. The bolt has a cavity 103, a longitudinal hole 101, and a transverse through hole 102 inside. The two transverse through holes are arranged on both sides of the internal cavity. The longitudinal hole passes through the transverse through hole and the internal cavity in sequence along the length of the bolt. The longitudinal hole is used to lay optical fibers, and the internal cavity is used to suspend and arrange the fiber optic grating. The optical fibers are glued to the inside of the bolt by injecting glue through the transverse through hole. A fiber optic grating is a section of grating mounted on an optical fiber. One half of the grating area is suspended in the cavity inside the bolt, while the other half is coated with a metal layer through a metal plating process, forming two parts of the fiber optic grating: the uncoated section and the coated section. These parts are then attached to the longitudinal hole of the bolt using UV adhesive. By utilizing the difference in the response of the two fiber optic grating sections to temperature and force signals, the sensor achieves decoupled temperature-force measurement. The presence of the cavity can improve the sensor sensitivity by 25%.

[0028] Furthermore, the temperature-force decoupling measurement is achieved by creating an internal cavity within the bolt to decouple the bolt temperature-force parameters under a single fiber optic grating.

[0029] In this embodiment, preferably, the bolt is made of titanium alloy metal 3D printing technology; it has high strength, a tensile strength of up to 900MPa, and a wide range of material applications. In this embodiment, preferably, the adhesive used is a UV-curable adhesive, which is cured by irradiation with an ultraviolet lamp. In this embodiment, preferably, the adhesive is a UV-curable adhesive with high fluidity, which can be easily filled into the bolt opening. The adhesive can only be cured by UV light irradiation, the curing time is easy to control, and the temperature range is -30 to 80°C.

[0030] In this embodiment, preferably, the thickness of the metal coating is approximately .

[0031] Preferably, the cavity diameter is half the bolt diameter. In this embodiment, preferably, the metal coating is a nickel metal coating attached to the grating surface by a metal coating process; Metal coating process refers to the process of sequentially applying a metallic nickel coating to half of the fiber grating region through a sensitization activation fiber grating process, a chemical nickel plating process, and an electroplating nickel process to form a coated fiber grating segment.

[0032] In this embodiment, preferably, in the sensitization and activation fiber Bragg grating process, the optical fiber is immersed in a sensitizing solution and an activation solution in sequence; The sensitization solution is prepared by fully dissolving 10 ml of hydrochloric acid and 2 g of stannous chloride, then adding deionized water to a final volume of 200 ml. The activation solution is prepared by fully dissolving 0.6 ml of hydrochloric acid and 0.06 g of palladium chloride, then adding deionized water to a final volume of 200 ml.

[0033] In this embodiment, preferably, in the electroless nickel plating process, the sensitized and activated fiber grating is immersed in an 85°C electroless nickel plating solution. The method for preparing the chemical nickel plating solution is as follows: place 4g of nickel sulfate, 3g of sodium hypophosphite, 3g of boric acid and 3ml of propionic acid in a beaker, add deionized water to 200ml and stir to dissolve.

[0034] In this embodiment, preferably, in the electroplating nickel process, the electroless nickel-plated fiber Bragg grating is connected to a DC power supply, a nickel plate, a copper wire, and an electroplating nickel solution to form a closed-loop circuit (specifically, the fiber Bragg grating and the nickel plate are connected to the cathode and anode of the DC power supply, respectively, and the area of ​​the fiber Bragg grating to be plated and the nickel plate are immersed in the electroplating nickel solution). Electroplating is carried out for 3 hours at an electroplating nickel solution temperature of 40°C and a current density of 1.5 mA. The method for preparing the nickel plating solution is as follows: place 70g of nickel sulfate, 12.5g of nickel chloride, 0.25g of sodium dodecyl sulfate and 8.75g of boric acid in a beaker, add deionized water to 250ml and stir to dissolve.

[0035] In this embodiment, preferably, the metallized fiber grating has a simple coating process and low coating cost. Half of the fiber grating is nickel-plated, and the other half is not nickel-plated. During the nickel plating process, the adhesion of metallic nickel in the half-grating region of the metallized fiber grating generates additional uniform stress on the fiber grating, resulting in two reflection peaks in the reflection spectrum of the fiber grating.

[0036] A monitoring method using the above-described cavity-containing bolt preload and temperature composite sensing fiber optic sensor includes the following steps: when a bolt is connected to a mechanical device, its preload will cause the bolt to undergo axial strain. When this strain is transmitted to the fiber optic grating, the two fiber optic gratings will produce different axial strains because the cross-sectional areas of the two fiber optic gratings installed inside the bolt are different. Furthermore, when the external temperature changes, both fiber Bragg gratings are simultaneously affected by the temperature. Due to the presence of the coating, the two fiber Bragg gratings have different temperature response coefficients. By analyzing the differences in the temperature and force responses of the two grating regions, and combining this with the sensor's mechanical model, a matrix relating the fiber Bragg grating center wavelength to temperature and force is established. Through decoupling, the final temperature and force parameter changes monitored by the sensor are obtained.

[0037] When the external temperature changes, both fiber Bragg gratings are affected by the temperature. Due to the presence of the metal coating, the center wavelengths of the two fiber Bragg gratings have different sensitivities to temperature. Under the same temperature, the center wavelengths of the two fiber Bragg gratings will have different offsets. By analyzing the difference in the sensitivity of the two grating regions to temperature and force response, and combining the sensor's mechanical model, a decoupling matrix between the center wavelength of the fiber Bragg grating and temperature and force is established. Through decoupling, the changes in temperature and force parameters monitored by the final sensor are obtained.

[0038] In this embodiment, preferably, the center wavelength offset of the two fiber Bragg gratings after being subjected to axial force load and temperature load is... and They can be represented as:

[0039]

[0040] in, This refers to the change in the center wavelength of the uncoated fiber grating. The initial center wavelength of the uncoated fiber grating segment. This refers to the change in the center wavelength of the fiber grating in the coated section. The initial center wavelength of the coated fiber grating is given. The effective elastic-optic coefficient of the optical fiber is (~0.22). The coefficient of thermal expansion of the uncoated fiber grating is... The coefficient of thermal expansion of the fiber grating in the coated section is... The thermo-optic coefficient of the uncoated fiber grating. The thermo-optic coefficient of the fiber grating in the coated section. This represents the axial strain of the uncoated fiber grating section after the bolt is subjected to axial tensile force. This represents the axial strain of the coated fiber grating section after the bolt is subjected to axial tensile force. This refers to the change in ambient temperature.

[0041] Center wavelength offset and force of uncoated and coated fiber gratings and / or temperature The relation is:

[0042]

[0043] in, The force sensitivity coefficient of the uncoated fiber grating. This represents the force sensitivity coefficient of the fiber grating in the coated section. The temperature sensitivity coefficient of the uncoated fiber grating section. This is the temperature sensitivity coefficient of the fiber grating in the coated section.

[0044]

[0045] Calculations show that u=0 and v=L3=L4, from which the deformation at both ends of the cavity can be obtained. Where u is the transverse displacement component at the center of both ends of the cavity, the transverse direction being the bolt width direction (i.e., the x-axis direction), and x refers to the coordinate on the x-axis. v is the longitudinal displacement component at the center of both ends of the cavity, the longitudinal direction being the bolt axis (i.e., the y-axis direction), and y refers to the coordinate on the y-axis. Let be the Poisson's ratio of the bolt material, q be the external load, h be the cavity diameter, and E = Eb be the bolt's elastic modulus. Where is the cavity radius.

[0046] Working principle of the invention: See Figures 1-3 A composite fiber optic sensor for sensing bolt preload and temperature with a cavity is disclosed. The sensor comprises: a titanium alloy 3D-printed bolt 1, an optical fiber protective sleeve 2, UV adhesive 3, and a fiber optic grating 4. The titanium alloy 3D-printed bolt 1 serves as the sensor body, with a longitudinal hole 101, a transverse through hole 102, and a cavity 103 internally. An uncoated section of the fiber optic grating 403 is suspended within the cavity 103. The longitudinal hole to the left of this cavity is connected and fixed to the coated section of the fiber optic grating 402 via UV adhesive 3. The remaining longitudinal holes are filled with UV adhesive 3 to fix the optical fiber. The transverse through hole 102 serves as an injection port for the UV adhesive 3.

[0047] The fiber optic protective sleeve 2 and UV adhesive 3 are used for sensor encapsulation. At the same time, the fiber optic protective sleeve 2 can protect the fiber optic tail from the influence of shaking on the fiber optic grating sensing signal, and protect the sensor's pigtail from breakage due to external interference.

[0048] A fiber optic grating refers to a section of fiber optic grating 4 disposed on an optical fiber, including a coated section fiber optic grating 402 and an uncoated section fiber optic grating 403. The coated section fiber optic grating 402 is suspended in the cavity 103 of the titanium alloy 3D printed bolt 1, and the coated section fiber optic grating 403 is attached to the longitudinal hole inside the bolt by UV glue 3.

[0049] A method for implementing the metallized coating of a temperature self-compensating bolt force sensor based on a metallized fiber Bragg grating, as described above: In the sensitized and activated fiber Bragg grating process, the sensitization solution is prepared by mixing 10 ml of hydrochloric acid and 2 g of stannous chloride, stirring thoroughly, and then adding deionized water to a final volume of 200 ml. The fiber sensitization method involves immersing the fiber in the sensitization solution for 15 minutes and then cleaning it in an ultrasonic cleaner for 30 seconds. The activation solution is prepared by mixing 0.6 ml of hydrochloric acid and 0.06 g of palladium chloride, stirring thoroughly, and then adding deionized water to a final volume of 200 ml. The fiber activation method involves immersing the fiber in the activation solution for 15 minutes and then cleaning it in an ultrasonic cleaner for 30 seconds. In the electroless nickel plating process, the electroless nickel plating solution is prepared by dissolving 4 g of nickel sulfate, 3 g of sodium hypophosphite, 3 g of boric acid, and 3 ml of propionic acid in 200 ml of deionized water. The electroless nickel plating method involves immersing the sensitized and activated fiber Bragg grating in an 85°C electroless nickel plating solution for 2 hours. In the nickel electroplating process, the nickel electroplating solution is prepared by dissolving 70g of nickel sulfate, 12.5g of nickel chloride, 0.25g of sodium dodecyl sulfate and 8.75g of boric acid in 250ml of deionized water. The nickel electroplating method involves forming a closed-loop circuit between the chemically nickel-plated fiber grating and a DC power supply, a nickel plate, a copper wire and the nickel electroplating solution, and electroplating for 3 hours at a solution temperature of 40℃ and a current density of 1.5mA.

[0050] This embodiment of the disclosure metallizes the fiber Bragg grating, thus dividing the entire fiber Bragg grating into two segments: a non-metallized area and a generally metallized area. Because the coated area exerts additional stress on the fiber Bragg grating, and the two areas are mounted at bolts with different cross-sectional areas, the fiber Bragg grating will exhibit different axial strains after being subjected to axial tension and temperature loads. The center wavelength shift of the two fiber Bragg grating segments after being subjected to axial force and temperature loads is... and They can be represented as:

[0051]

[0052] in, The initial center wavelength of the uncoated fiber grating 402 is given. This is the initial center wavelength of the coated fiber grating 403. The effective elastic-optic coefficient of the optical fiber is (~0.22). and These are the thermal expansion coefficients of the two fiber gratings. and These are the thermo-optic coefficients of the two fiber gratings, and These represent the axial strains of the two fiber Bragg grating segments after the bolts are subjected to axial tensile force. This represents the change in ambient temperature.

[0053] Figure 4 and Figure 5 This is a stress-deformation model of the bolts and optical fibers after the sensor is subjected to axial tensile force. From... Figure 4 and Figure 5 From this, we can obtain that the axial strains of the two fiber Bragg grating segments after the bolt is subjected to axial tension are as follows:

[0054] in and These represent the lengths of the bolts before force is applied to the coated and uncoated fiber grating sections, respectively. and These represent the lengths of the bolt cross-sections after stress is applied to the coated and uncoated fiber grating sections, respectively. and The dimensions of the indentation deformation at both ends of the cavity after the bolt is subjected to axial tensile force. is the strain transfer coefficient of the fiber grating in the coated section.

[0055] in:

[0056]

[0057] in This refers to the axial tensile force acting on the bolt. The elastic modulus of the bolt. The elastic modulus of the adhesive. The cross-sectional area of ​​the bolt at the uncoated section of the fiber grating. This represents the cross-sectional area of ​​the bolt at the fiber grating section of the coated segment. This represents the cross-sectional area of ​​the adhesive at the fiber grating section of the coating.

[0058] Figure 6 and Figure 7This diagram illustrates the stress and deformation of the internal cavity of a bolt under axial tensile force, along with the transformation of the deformation model. When the bolt is subjected to axial tensile force, the two ends of its internal cavity deform inwards. Through analysis and calculation, the stress of the original structure can be considered as Figure 6 (a) After conversion of the intermediate model Figure 6 (b) and Figure 6 (c) Superposition of two load forms. Figure 6 In (b), a load of the same magnitude but opposite direction to the external load is applied inside the cavity. Figure 6 In (c), a load of the same magnitude and direction as the external load is applied to the bolt. Figure 6 In (b), the stress distribution within the cuboid is as follows:

[0059]

[0060] in, , and Let x, y, and z represent the stresses of the cuboid in the x, y, and z directions, respectively, where y is the axial direction of the bolt, and the x and z directions are perpendicular to the y direction. Therefore, only the stresses need to be calculated. Figure 6 The stress distribution at each point in (c) can be obtained. Figure 6 The mechanical model in (c) can be equivalent to applying a uniformly distributed axial load to a long cylindrical column with fixed supports around it. According to Saint-Venant's principle, different types of forces acting on the ends of a rod only affect its deformation over a length less than or equal to the diameter of the rod's end. Therefore, Figure 7 The model in (d) can be equivalent to: Figure 7 (e) shows a short cylinder (diameter equal to its height) with fixed supports around it, bearing a uniformly distributed axial load. The uniform surface load at the end of the cylinder can be divided into countless uniform line loads, each line load affecting... A The effect at point 0 is the same. Its mechanical model can be equivalent to a short beam fixed at both ends and subjected to a uniformly distributed load on its surface. For example... Figure 7 As shown in (f), when a statically indeterminate rectangular beam fixed at both ends is subjected to a uniformly distributed load, the normal stress boundary conditions at its upper and lower boundaries are:

[0061]

[0062] because:

[0063] Therefore, stress function for:

[0064]

[0065] By using stress function Compatibility equation: The form of the undetermined function , , The following can be obtained:

[0066]

[0067] The load is symmetrically distributed, and the stress distribution is also symmetrically distributed. With both the upper and lower boundary conditions and the shear stress boundary condition at zero, the undetermined constants can be obtained as follows:

[0068] Therefore, the stress components are:

[0069] The stress components contain two undetermined constants. and This can be determined by displacement boundary conditions. Therefore, consider the physical equations:

[0070]

[0071] in Let be the Poisson's ratio of the bolt material. The geometric equations for the stress components are:

[0072]

[0073] Where u is the displacement component along the transverse direction at the center of both ends of the cavity (transverse direction refers to the x-direction), and v is the displacement component along the longitudinal direction at the center of both ends of the cavity (longitudinal direction refers to the y-direction). Substituting the stress components into the physical equation, we get:

[0074] Define the constant as ω ,but:

[0075]

[0076]

[0077] Calculations show that u=0 and v=L3=L4. This allows us to determine the deformation at both ends of the cavity.

[0078] Figure 8 A schematic diagram of the embedded fiber Bragg grating structure and Figure 9 This is a force analysis diagram of the embedded fiber Bragg grating. The length of the intermediate layer. The shear modulus of the intermediate layer. For the target layer thickness, The elastic modulus of the target layer, The thickness is the intermediate layer. In this strain transfer model, since the intermediate layer includes UV adhesive 3 and the metal plating 401, there is two layers of strain transfer. Therefore, the strain transfer coefficient in this model... The calculations are as follows:

[0079]

[0080]

[0081] Therefore, the relationship between the center wavelength offset of the two fiber gratings and force or temperature can be expressed as:

[0082] in, and These are the force sensitivity coefficients of the two fiber Bragg gratings. and These are the temperature sensitivity coefficients of the two fiber grating segments, respectively. This represents the change in the axial tensile force acting on the bolt. This represents the change in ambient temperature.

[0083] In actual measurement, by analyzing the difference in the response of the center wavelengths of the two fiber gratings to temperature and force signals, the sensor can achieve decoupled measurement of temperature and force signals, thus realizing temperature self-compensation of the bolt force sensor.

[0084] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A monitoring method using a cavity-containing fiber optic sensor for combined sensing of bolt preload and temperature, characterized in that: The cavity-containing bolt preload and temperature composite sensing fiber optic sensor includes a bolt, glue, and fiber optic grating. The bolt has a cavity, a longitudinal hole, and a transverse through hole inside. Two transverse through holes are arranged on both sides of the internal cavity. The longitudinal hole passes through the transverse through hole and the cavity in sequence along the length of the bolt. The longitudinal hole is used to lay the optical fiber. The internal cavity is used to suspend and arrange the fiber optic grating. The transverse through hole is used for glue injection to attach the optical fiber inside the bolt. Half of the fiber Bragg grating is suspended in the cavity, while the other half has a metal coating on its surface and is glued to the longitudinal bolt hole. The monitoring method includes the following steps: when a bolt is connected to a mechanical device, its preload will cause the bolt to undergo axial strain. When this strain is transmitted to the fiber Bragg grating, the two fiber Bragg gratings will produce different axial strains because the cross-sectional areas of the bolts at the two parts of the fiber Bragg grating are different. When the external temperature changes, both fiber Bragg gratings are affected by the temperature. Due to the presence of the metal coating, the center wavelengths of the two fiber Bragg gratings have different sensitivities to temperature. Under the same temperature, the center wavelengths of the two fiber Bragg gratings will have different offsets. By analyzing the difference in the sensitivity of the two grating regions to temperature and force response, and combining the sensor's mechanical model, a decoupling matrix between the center wavelength of the fiber Bragg grating and temperature and force is established. Through decoupling, the changes in temperature and force parameters monitored by the final sensor are obtained respectively. The center wavelength shift of two fiber Bragg grating segments after being subjected to axial force and temperature loads and They can be represented as: in, The initial center wavelength of the uncoated fiber grating segment. The initial center wavelength of the coated fiber grating is given. The effective elastic-optical coefficient of the optical fiber. The coefficient of thermal expansion of the uncoated fiber grating is... The coefficient of thermal expansion of the fiber grating in the coated section is... The thermo-optic coefficient of the uncoated fiber grating. The thermo-optic coefficient of the fiber grating in the coated section. This represents the axial strain of the uncoated fiber grating section after the bolt is subjected to axial tensile force. This represents the axial strain of the coated fiber grating section after the bolt is subjected to axial tensile force. This refers to the change in ambient temperature. After the bolts are subjected to axial tension, the axial strains of the two fiber Bragg grating segments are as follows: in and These represent the lengths of the bolts before force is applied to the coated and uncoated fiber grating sections, respectively. and These represent the lengths of the bolt cross-sections after stress is applied to the coated and uncoated fiber grating sections, respectively. and The dimensions of the indentation deformation at both ends of the cavity after the bolt is subjected to axial tensile force. The strain transfer coefficient of the fiber grating in the coated section; in: in The axial tensile force acting on the bolt. The elastic modulus of the bolt. This refers to the elastic modulus of the adhesive. This represents the cross-sectional area of ​​the bolt at the uncoated section of the fiber grating. This represents the cross-sectional area of ​​the bolt at the fiber grating section of the coated segment. The cross-sectional area of ​​the adhesive at the fiber grating section of the coating segment; The relationship between the center wavelength shift of the uncoated and coated fiber gratings and force and / or temperature is as follows: in, The force sensitivity coefficient of the uncoated fiber grating. This represents the force sensitivity coefficient of the fiber grating in the coated section. The temperature sensitivity coefficient of the uncoated fiber grating section. This represents the temperature sensitivity coefficient of the fiber grating in the coated section. This represents the change in the axial tensile force acting on the bolt.

2. The monitoring method as described in claim 1, characterized in that: The bolts are made using titanium alloy 3D printing technology.

3. The monitoring method as described in claim 1, characterized in that: The adhesive used is a UV-curable adhesive.

4. The monitoring method as described in claim 1, characterized in that: The fiber grating's metal coating is a nickel plating.

5. The monitoring method as described in claim 1, characterized in that: The cavity diameter is half the bolt diameter.

6. The monitoring method as described in claim 4, characterized in that: The nickel plating is applied to the grating surface using a metal plating process, with a plating thickness of 350 μm. Metal coating process refers to applying a metallic nickel coating to half of the fiber grating region by sequentially performing a sensitization and activation fiber grating process, a chemical nickel plating process, and an electroplating nickel process.

7. The monitoring method as described in claim 6, characterized in that: In the sensitized and activated fiber Bragg grating process, the optical fiber is immersed in the sensitization solution and the activation solution for 15 minutes in sequence; in the electroless nickel plating process, the sensitized and activated fiber Bragg grating is immersed in an 85°C electroless nickel plating solution for 2 hours; in the electroless nickel plating process, the electroless nickel-plated fiber Bragg grating is connected to a closed-loop circuit via a DC power supply, a nickel plate, a copper wire, and an electroless nickel plating solution, and electroplated in an environment of 40°C electroless nickel plating solution, wherein the current is set to 1.5mA and the electroplating time is 3 hours.

8. The monitoring method as described in claim 1, characterized in that: Calculations show that u=0 and v=L3=L4, from which the deformation at both ends of the cavity can be obtained. Where u is the transverse displacement component at the center of both ends of the cavity, x is the coordinate on the x-axis, v is the longitudinal displacement component at the center of both ends of the cavity, and y is the coordinate on the y-axis. Let be the Poisson's ratio of the bolt material, q be the external load, h be the cavity diameter, and E = Eb be the bolt's elastic modulus. Where is the cavity radius.

Images