A fiber optic grating bolt stress sensor

Abstract

This utility model discloses a fiber Bragg grating bolt stress sensor, comprising a cylindrical load-bearing cylinder with a centrally located through-hole for the bolt to pass through. A protective sleeve surrounds the cylindrical load-bearing cylinder, and several strain-sensitive components are evenly distributed on its outer periphery. These strain-sensitive components are located between the protective sleeve and the cylindrical load-bearing cylinder. Each strain-sensitive component includes an elastic body and a strain fiber Bragg grating. The elastic body employs an S-shaped sensitizing micro-displacement amplification structure, and a fiber optic mounting groove is provided on the surface of the elastic body, within which the strain fiber Bragg grating is fixed. By employing an S-shaped elastic structure to enhance the sensitizing strain-sensitive component and utilizing the micro-displacement amplification structure to amplify the deformation of the cylindrical load-bearing cylinder under axial load, the strain sensitivity is improved, thereby enhancing the sensor's detection accuracy.

Description

Technical Field

[0001] This utility model relates to bolt stress detection, specifically to a fiber optic grating bolt stress sensor. Background Technology

[0002] Bolts are a primary means of structural connection in fields such as water conservancy and hydropower, bridge construction, and power equipment. Therefore, monitoring the condition of bolts has become a major issue for structural safety. By measuring the stress of bolts, it is possible to determine whether the bolts are within the appropriate preload range, thereby avoiding structural connection failures caused by excessive looseness or tightness. It can also detect problems such as bolt loosening or fatigue fracture in a timely manner, thus preventing potential equipment failures or accidents.

[0003] Traditional bolt stress testing methods primarily rely on torque wrenches to control bolt preload and theoretical calculations of stress conditions. However, these methods suffer from several significant problems: first, fluctuations in the friction coefficient between threaded pairs lead to large deviations in preload; second, the nonlinear conversion between torque and preload easily introduces secondary errors; and third, they lack online monitoring capabilities, making it impossible to monitor bolt stress decay in real time during service life. In bolt stress monitoring applications, the main sensing technologies include piezoelectric ceramics, ultrasonic sensors, and resistance strain gauges. Besides traditional sensing technologies, fiber optic grating technology offers advantages such as simple structure, high stability, high precision, wide dynamic range, and resistance to electromagnetic interference. It can also utilize wavelength division multiplexing (WDM) technology to form fiber optic sensor networks. After connecting to a fiber optic grating demodulation device, it can perform non-destructive testing of bolts over a wide range and long distances, but the detection accuracy of existing fiber optic grating sensors remains insufficient. Utility Model Content

[0004] Purpose of the utility model: To address the above-mentioned shortcomings, this utility model provides a fiber optic grating bolt stress sensor that improves detection accuracy.

[0005] Technical Solution: To solve the above problems, this utility model adopts a fiber optic grating bolt stress sensor, including a cylindrical load-bearing cylinder with a through hole in the center for the bolt to pass through. A protective sleeve is provided around the cylindrical load-bearing cylinder, and several strain-sensitive components are evenly distributed on the outer periphery of the cylindrical load-bearing cylinder. The strain-sensitive components are located between the protective sleeve and the cylindrical load-bearing cylinder. The strain-sensitive components include an elastic body and a strain fiber optic grating. The elastic body adopts an S-shaped sensitized micro-displacement amplification structure, and an optical fiber mounting groove is provided on the surface of the elastic body. The strain fiber optic grating is fixed in the optical fiber mounting groove.

[0006] Furthermore, the elastomer is installed axially along the cylindrical load-bearing cylinder, and the fiber mounting groove passes through the bends of all adjacent S-bends on the same side of the elastomer to amplify the strain deformation of the load-bearing cylinder structure, thereby improving the strain sensitivity of the sensitive component. The strain fiber grating is placed in the fiber mounting groove by adhesive bonding on both sides. The strain fiber grating is pre-stretched to an initial value before adhesive bonding and is in a taut state.

[0007] Furthermore, it also includes a temperature-sensitive component, which includes a fixed base and a temperature-compensated fiber optic grating. The fixed base is fixedly installed on the outer periphery of the cylindrical load-bearing cylinder and located between the protective cylinder and the cylindrical load-bearing cylinder. The fixed base is provided with a fixing groove, and the temperature-compensated fiber optic grating is fixed in the fixing groove. The strain fiber optic grating and the temperature-compensated fiber optic grating are connected in series.

[0008] Furthermore, the temperature-sensitive component is fixed at the middle section of the outer surface of the cylindrical load-bearing cylinder, and can be used for temperature monitoring and stress temperature compensation. The fixing groove of the fixing base extends along the axis perpendicular to the cylindrical load-bearing cylinder. The temperature-compensated fiber optic grating is placed in the fixing groove by adhesive bonding on both sides. The temperature-compensated fiber optic grating is in a relaxed state before adhesive bonding and encapsulation.

[0009] Furthermore, the cylindrical load-bearing cylinder is an annular cylindrical structure. Its outer surface is provided with at least three circumferentially uniformly arranged strain-sensitive component fixing areas and one temperature-sensitive component fixing area. The strain-sensitive component is fixed vertically to the strain-sensitive component fixing area by two combined screws, and the temperature-sensitive component is also horizontally fixed to the temperature-sensitive component fixing area by two combined screws, respectively used for monitoring strain and temperature physical quantities. The strain-sensitive components are evenly arranged circumferentially along the axial direction of the outer surface of the cylindrical load-bearing cylinder, with a quantity of at least three. Ultimately, the overall stress change of the fiber optic grating bolt stress sensor is reflected by the average wavelength change of the arranged strain-sensitive components.

[0010] Furthermore, the cylindrical load-bearing cylinder has a bilaterally symmetrical I-shaped cross-section along the axial direction. The cylindrical load-bearing cylinder includes an upper convex portion, a middle concave portion, and a lower convex portion arranged along the axial direction. The outer surfaces of the upper and lower convex portions of the cylindrical load-bearing cylinder contact the inner surface of the protective cylinder, and grooves for O-ring fixing are provided on the outer surfaces of the upper and lower convex portions. A waterproof seal is achieved between the cylindrical load-bearing cylinder and the protective cylinder through O-rings provided on the upper and lower convex portions. The strain-sensitive component is located on the outer surface of the middle concave portion. The inner and outer diameters of the load-bearing cylinder can be determined according to the size and load-bearing capacity of the bolt to be monitored. The way the inner diameter of the load-bearing cylinder fits into the bolt allows for long-term monitoring of bolt stress.

[0011] Furthermore, the casing is provided with two through holes that connect the inner and outer sides. Optical cable connectors are fixedly installed in the through holes. Fiber optic protection components are threadedly connected to the outer side of the optical cable connectors. A series-connected strain fiber grating and temperature-compensated fiber grating form a sensing link. The optical fibers at both ends of the sensing link pass through the optical cable connectors fixed on the upper and lower sides of the casing and exit from the fiber optic protection components, forming two optical fiber outputs.

[0012] Furthermore, both the strain fiber grating and the temperature-compensated fiber grating use bend-insensitive fibers with a bending radius of up to 5 mm, ensuring minimal light intensity loss within a confined bending space. In the sensing link formed by the strain fiber grating and the temperature-compensated fiber grating, the wavelength difference between any two gratings is greater than 2 nm to prevent overlap of multiple wavelengths.

[0013] Multiple fiber optic strain sensors evenly distributed along the circumference of the outer surface of the load-bearing cylinder are used for monitoring bolt stress. A fiber optic temperature sensor fixed in the neutral layer is also built in for temperature monitoring and stress-temperature compensation. When the load-bearing cylinder is deformed by an axial load, the strain sensors deform synchronously, causing a change in the output wavelength of the strain grating. By connecting to a fiber optic demodulator to measure the strain wavelength, the strain that causes the deformation of the load-bearing cylinder can be calculated. Substituting the calibration coefficients, the axial load force on the bolt stress gauge can be obtained.

[0014] Beneficial effects: Compared with the prior art, the advantages of this invention are that by adopting an S-shaped elastic structure to enhance the strain sensitivity of the strain-sensitive component, and by utilizing a micro-displacement amplification structure to amplify the deformation of the cylindrical load-bearing cylinder under axial load, the strain sensitivity is improved, thereby enhancing the detection accuracy of the sensor. The built-in temperature-sensitive component enables temperature compensation, resulting in a sensor with high sensitivity, high accuracy, and long-term stable stress change response output. The overall sensor structure is simple and the size is small.

[0015] By using bend-insensitive optical fibers as the sensitive unit for strain and temperature sensing, the small bending radius of the fiber body ensures minimal light intensity loss within confined spaces, thus guaranteeing the output stability of the fiber Bragg grating sensor. The bolt-fitting installation method replaces the previous approach of embedding sensitive components within the bolt for stress monitoring, achieving non-destructive bolt monitoring. Furthermore, the sensor structure can be flexibly designed according to the bolt size and load-bearing capacity. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view of the fiber optic grating bolt stress sensor of this utility model.

[0017] Figure 2 This is a top view of the fiber optic grating bolt stress sensor of this utility model.

[0018] Figure 3This is a schematic diagram of the structure of the elastomer in this utility model.

[0019] Figure 4 This is a schematic diagram of the installation of strain and temperature sensitive components in this utility model. Detailed Implementation

[0020] like Figure 1 As shown, this embodiment of a fiber optic grating bolt stress sensor includes a cylindrical load-bearing cylinder 1, O-rings 2, a protective sleeve 3, a strain-sensitive component 4, a combination screw 5, a set screw 6, a temperature-sensitive component 7, an optical cable connector 8, and an optical fiber protection component 9. The cylindrical load-bearing cylinder has a through hole in the center for the bolt to pass through. The outer surface of the cylindrical load-bearing cylinder 1 is provided with a sensitive component fixing area. The strain-sensitive component 4 and the temperature-sensitive component 7 are fixed by the combination screw 5. The series optical fiber fused between the strain-sensitive component 4 and the temperature-sensitive component 7 passes through the optical cable connector 8 welded to the protective sleeve 3 and forms two optical fiber outputs through the optical fiber protection component 9. The cylindrical load-bearing cylinder 1 and the protective sleeve 3 are waterproofed and sealed by the upper and lower O-rings 2 and fixedly connected by the set screw 6.

[0021] The cylindrical load-bearing cylinder 1 is an annular cylindrical structure, combined with... Figure 4 The outer surface of the cylindrical load-bearing cylinder 1 is provided with at least three circumferentially distributed strain-sensitive component fixing areas Ⅰ and one temperature-sensitive component fixing area Ⅱ. The strain-sensitive component 4 is fixed vertically to the strain-sensitive component fixing area Ⅰ by two combination screws 5, and the temperature-sensitive component 7 is also fixed horizontally to the temperature-sensitive component fixing area Ⅱ by two combination screws, which are used for monitoring strain and temperature physical quantities respectively.

[0022] The cylindrical load-bearing cylinder 1 has a cross-sectional shape that is symmetrical on both sides along the axial direction. It includes an upper convex part, a middle concave part, and a lower convex part arranged along the axial direction. The outer surfaces of the upper and lower convex parts of the cylindrical load-bearing cylinder 1 are in contact with the inner surface of the protective cylinder 3. The outer surfaces of the upper and lower convex parts are provided with grooves for fixing the O-rings 2. The cylindrical load-bearing cylinder 1 and the protective cylinder 3 are waterproofed and sealed by the O-rings 2 provided on the upper and lower convex parts. The strain-sensitive component 4 is set on the outer surface of the middle concave part. The inner and outer diameters of the load-bearing cylinder can be determined according to the size and load-bearing capacity of the bolt to be monitored. The way the inner diameter of the cylindrical load-bearing cylinder 1 is fitted with the bolt can realize long-term monitoring of bolt stress.

[0023] Combination Figure 3The strain-sensitive component 4 includes an elastic body 41 and a strain fiber grating 42. The elastic body 41 adopts an S-shaped sensitizing micro-displacement amplification structure. The elastic body is installed along the axial direction of the cylindrical load-bearing cylinder 1. An optical fiber mounting groove is provided on the surface of the elastic body. The optical fiber mounting groove passes through the bends of all adjacent S-bends on the same side of the elastic body. It is used to amplify the strain deformation of the cylindrical load-bearing cylinder 1 structure, thereby improving the strain sensitivity of the sensitive component. The fiber grating of the strain-sensitive component 4 is placed in the fixing groove by adhesive bonding on both sides. Before adhesive bonding and encapsulation, the strain fiber grating needs to be pre-stretched to a certain initial value and is in a taut state.

[0024] The strain-sensitive components 4 are evenly arranged around the outer surface of the cylindrical load-bearing cylinder 1 along the axial direction, with a number of no less than 3. Finally, the overall stress change of the fiber optic grating bolt stress sensor is reflected by the average wavelength change of the arranged strain-sensitive components 4.

[0025] Considering the cross-sensitivity of fiber Bragg gratings to strain and temperature during measurement, special design is needed to eliminate or reduce the interference of temperature on the measurement results. In this embodiment, the fiber Bragg grating bolt stress sensor incorporates a temperature-sensitive component 7, which is horizontally fixed to the middle section of the outer surface of the cylindrical load-bearing cylinder 1. The temperature-sensitive component 7 includes a fixed base and a temperature-compensated fiber Bragg grating. The fixed base is fixedly installed on the outer periphery of the cylindrical load-bearing cylinder 1, located between the protective sleeve and the cylindrical load-bearing cylinder 1. The fixed base has a fixing groove, and the temperature-compensated fiber Bragg grating is fixed within the fixing groove. It can be used for temperature monitoring and stress temperature compensation. The fiber Bragg grating is placed in the fixing groove by adhesive bonding on both sides, and the fiber Bragg grating needs to be in a relaxed state before adhesive bonding.

[0026] Both the strain-sensitive component 4 and the temperature-sensitive component 7 require the use of bend-insensitive optical fibers with a bending radius of up to 5mm. Minimize light intensity loss in a confined bending space. The optical fibers between the strain-sensitive component 4 and the temperature-sensitive component 7 are connected in series by fusion splicing to form a sensing link. The optical fibers on both sides of the sensing link pass through the upper and lower optical cable connectors 8 welded to the upper and lower sides of the protective sleeve 3, and finally exit through the optical fiber protection component 9, which is fixedly connected to the optical cable connector 8 by threads, to form two optical fiber outputs.

[0027] The wavelength difference between the two gratings in the sensing link composed of strain-sensitive component 4 and temperature-sensitive component 7 must be greater than 2nm in order to prevent multiple wavelengths from overlapping and causing wavelength value confusion.

[0028] The cylindrical load-bearing cylinder 1 and the protective cylinder 3 are sealed with an O-ring 2 for waterproofing and are connected and fixed by a set screw 6. Specifically, the O-ring 2 is placed in the groove on the upper and lower sides of the cylindrical load-bearing cylinder 1. To facilitate the assembly of the cylindrical load-bearing cylinder 1 and the protective cylinder 3, an appropriate amount of silicone grease is applied to the O-ring 2 and the inner side of the cylindrical load-bearing cylinder 1 for lubrication. The optical fibers on both sides of the sensing link composed of the strain-sensitive component 4 and the temperature-sensitive component 7 pass through the optical cable connector 8 from the inside of the cylindrical load-bearing cylinder 1 to the outside of the protective cylinder 3. Finally, the upper and lower surfaces of the cylindrical load-bearing cylinder 1 and the protective cylinder 3 are flattened by tapping with a rubber hammer. Finally, the cylindrical load-bearing cylinder 1 and the protective cylinder 3 are connected and fixed by the set screw 6.

[0029] After assembling the utility model's self-compensated fiber optic grating bolt stress gauge, temperature coefficient calibration of the entire instrument is required to reduce the interference of temperature on stress measurement results. The main calibration method is to place the finished fiber optic grating bolt stress gauge in an environmental test chamber and place a standard platinum resistance thermometer near the instrument. Select a test point near each of five temperature points: -20℃, 0℃, +20℃, +40℃, and +60℃. After reaching and stabilizing at each temperature, maintain the temperature for 3 hours. Measure the temperature wavelength and strain wavelength at the stable temperature of each test point (the strain wavelength is the average of the measured values ​​of each strain-sensitive element). Calculate the instrument's temperature correction coefficient using the following formula:

[0030]

[0031] In the formula, b is the temperature correction coefficient; λ T1 λ represents the wavelength value at the lowest temperature point. T2 λ1 represents the wavelength value at the highest temperature point, λ2 represents the average measured strain wavelength at the highest temperature, and λ1 represents the average measured strain wavelength at the lowest temperature.

[0032] To verify the beneficial effects of the fiber Bragg grating bolt stress sensor in this embodiment, an experimental study was conducted on the test environment of the fiber Bragg grating bolt stress gauge with built-in temperature compensation in this embodiment. The experimental conditions are as follows: The experimental instruments used are: universal testing machine and dynamic fiber Bragg grating demodulator.

[0033] The assembled fiber Bragg grating bolt stress gauge was placed on a universal testing machine. The instrument was divided into six ranges according to its full-scale load capacity. The universal testing machine provided the pressure for each range. The sensor was connected to a fiber Bragg grating demodulator to measure the changing wavelength values. The average wavelength change value of the multiple built-in strain-sensitive components was recorded. The main performance parameters of the sensor, including nonlinearity error, hysteresis, and repeatability, were tested. Taking a fiber Bragg grating bolt stress gauge with a load capacity of 900 kN as an example, the test results are shown in the table below:

[0034]

[0035] The full-scale output, nonlinearity, hysteresis, and repeatability errors of the fiber Bragg grating bolt stress gauge were calculated based on the experimental data in the table above. It can be found that the full-scale output of the tested fiber Bragg grating bolt stress gauge is 1.3 nm; the nonlinearity is 0.64%, less than 1%; the hysteresis is 0.76%, less than 1%; and the non-repeatability is 0.11%, less than 1%. It can be seen that the developed fiber Bragg grating bolt stress gauge has high measurement accuracy, stable output performance, and high reliability.

[0036] In summary, the technical solution in this embodiment uses an S-shaped enhanced strain-sensitive component and a temperature-sensitive component connected in series to form a sensing link for measuring the stress and temperature of bolts. This reduces the interference of temperature on strain measurement. The entire sensor uses a bend-insensitive optical fiber as the measurement sensing element to minimize light intensity loss in a confined space. Multiple sensors can be connected in series to form a quasi-distributed bolt stress monitoring system through wavelength division multiplexing technology. The bolt stress sensor can achieve non-destructive monitoring by using an installation method that fits into the bolt. This bolt stress sensor has a simple structure, small size, and high measurement accuracy. The structure of the sensor can be flexibly designed according to the bolt size and load-bearing range.

Claims

1. A fiber optic grating bolt stress sensor, characterized in that, The system includes a cylindrical load-bearing cylinder (1), which has a through hole in the center for bolts to pass through. The cylindrical load-bearing cylinder (1) is covered with a protective sleeve (3). Several strain-sensitive components (4) are evenly distributed on the outer periphery of the cylindrical load-bearing cylinder (1). The strain-sensitive components (4) are located between the protective sleeve and the cylindrical load-bearing cylinder (1). The strain-sensitive components (4) include an elastic body (41) and a strain fiber grating (42). The elastic body adopts an S-shaped sensitized micro-displacement amplification structure. An optical fiber mounting groove is provided on the surface of the elastic body, and the strain fiber grating is fixed in the optical fiber mounting groove.

2. The fiber optic grating bolt stress sensor according to claim 1, characterized in that, The elastomer is installed along the axial direction of the cylindrical load-bearing cylinder (1). The fiber mounting groove passes through the bends of all adjacent S-bends on the same side of the elastomer. The strain fiber grating is placed in the fiber mounting groove by adhesive bonding on both sides. The strain fiber grating is pre-stretched to an initial value before adhesive bonding and is in a taut state.

3. The fiber optic grating bolt stress sensor according to claim 2, characterized in that, It also includes a temperature-sensitive component (7), which includes a fixed base and a temperature-compensated fiber optic grating. The fixed base is fixedly installed on the outer periphery of the cylindrical load-bearing cylinder (1) and located between the protective cylinder and the cylindrical load-bearing cylinder (1). The fixed base is provided with a fixing groove, and the temperature-compensated fiber optic grating is fixed in the fixing groove. The strain fiber optic grating and the temperature-compensated fiber optic grating are connected in series.

4. The fiber optic grating bolt stress sensor according to claim 3, characterized in that, The temperature-sensitive component (7) is fixed at the middle section of the outer surface of the cylindrical load-bearing cylinder (1). The fixing groove of the fixing base extends along the axial direction perpendicular to the cylindrical load-bearing cylinder (1). The temperature-compensated fiber grating is placed in the fixing groove by adhesive bonding on both sides. The temperature-compensated fiber grating is in a relaxed state before adhesive bonding and encapsulation.

5. The fiber optic grating bolt stress sensor according to claim 3, characterized in that, The cylindrical load-bearing cylinder (1) is an annular cylindrical structure. The outer surface of the cylindrical load-bearing cylinder (1) is provided with no less than three uniformly arranged strain-sensitive component fixing areas and one temperature-sensitive component fixing area along the circumference. The strain-sensitive component (4) is fixed to the strain-sensitive component fixing area vertically by two combined screws, and the temperature-sensitive component (7) is also fixed horizontally to the temperature-sensitive component fixing area by two combined screws.

6. The fiber optic grating bolt stress sensor according to claim 1, characterized in that, The cylindrical load-bearing cylinder (1) has an axially symmetrical I-shaped cross-section. The cylindrical load-bearing cylinder (1) includes an upper convex part, a middle concave part and a lower convex part arranged along the axial direction. The outer surfaces of the upper convex part and the lower convex part of the cylindrical load-bearing cylinder (1) are in contact with the inner surface of the protective cylinder (3). The outer surfaces of the upper convex part and the lower convex part are provided with grooves for fixing O-rings (2). The cylindrical load-bearing cylinder (1) and the protective cylinder (3) are waterproofed and sealed by O-rings (2) provided on the upper convex part and the lower convex part. The strain-sensitive component (4) is provided on the outer surface of the middle concave part.

7. The fiber optic grating bolt stress sensor according to claim 5, characterized in that, The casing is provided with two through holes that connect the inner and outer sides. Optical cable connectors (8) are fixedly installed in the through holes. Optical fiber protection components (9) are threadedly connected to the outer side of the optical cable connectors (8). The series-connected strain fiber grating and temperature-compensated fiber grating form a sensing link. The optical fibers at both ends of the sensing link pass through the optical cable connectors (8) fixed on the upper and lower sides of the casing (3) and exit from the optical fiber protection components (9) to form two optical fiber outputs.

8. The fiber optic grating bolt stress sensor according to claim 7, characterized in that, Both the strained fiber grating and the temperature-compensated fiber grating use bend-insensitive fiber.

9. The fiber optic grating bolt stress sensor according to claim 7, characterized in that, The wavelength difference between any two gratings in the sensing link formed by the strained fiber grating and the temperature-compensated fiber grating is greater than 2 nm.

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