Bolt group loose state optical fiber identification method based on strain response self-correlation characteristics

Abstract

The invention provides a bolt group loosening state optical fiber identification method based on strain response self-correlation characteristics. The bolt group loosening state optical fiber identification method comprises the steps of 1, designing a quasi-distributed fiber grating sensor layout scheme oriented to large-scale high-density bolt group loosening characteristic identification; 2, establishing a bolt mechanical transfer model; 3, establishing a dynamic model of the fastening structure; 4, constructing an optical fiber actual measurement strain response self-correlation characteristic function, and representing corresponding strain time sequence value correlation before and after bolt loosening; 5, proposing a strain response self-correlation change rate index, and judging whether the bolt is loosened or not and the loosening time based on the strain response self-correlation change rate index; and step 6, proposing a bolt looseness degree discrimination index based on the relative change rate of the pre-tightening force, and realizing bolt looseness degree discrimination. The method can be well suitable for monitoring the health state of the major equipment bolt group, the operation process is simple, and the method has good engineering practicability.

Description

Technical Field

[0001] The invention belongs to the field of structural health monitoring, and in particular relates to an optical fiber identification method for the loose state of a bolt group based on strain response autocorrelation characteristics. Background Art

[0002] Bolts are an important means of connecting key components of aircraft, and their tightening degree is of vital importance to ensuring flight safety. If the bolt fastening structure becomes loose, the overall structural strength will be reduced. The concealment and unpredictability of bolt loosening make it difficult for traditional manual detection and maintenance methods to meet the requirements of efficient monitoring of the loose state of aircraft bolt groups. Therefore, it is urgent to develop a new method for bolt loosening state identification with distributed synchronous perception capabilities.

[0003] In response to the demand for service health monitoring of bolt groups, traditional piezoelectric sensors have shortcomings such as difficult wiring, complex monitoring systems, and susceptibility to electromagnetic interference. Fiber Bragg grating sensors are widely used in the field of bolt fastening structure monitoring due to their advantages such as light weight, thin fiber diameter, resistance to electromagnetic interference, and large-scale network monitoring. The monitoring scheme that uses fiber Bragg grating sensors embedded in the bolt structure is not only cumbersome in process and inconvenient for subsequent sensor maintenance and repair, but also causes a significant decrease in the strength of the bolt structure itself. At present, researchers have proposed a bolt monitoring method based on the combination of fiber Bragg grating sensors and pattern recognition algorithms. In order to improve the recognition accuracy, this method requires the construction of a large-scale bolt loosening sample library, which has limitations such as time-consuming and labor-intensive, structural susceptibility to damage, and limited applicability. Summary of the invention

[0004] Purpose of the invention: The technical problem to be solved by the present invention is to provide a method for optical fiber identification of the loose state of a bolt group based on the autocorrelation characteristics of strain response in view of the shortcomings of the prior art. This method does not require the application of external loads, but only relies on the vibration response of the structure under test during service to achieve bolt event identification, bolt position identification, and bolt looseness degree identification.

[0005] The method of the present invention comprises the following steps: Step 1: Design a quasi-distributed fiber Bragg grating sensor layout scheme for loosening feature identification of large-scale high-density bolt groups; Step 2, establishing a bolt mechanical transmission model, solving the strain response and distribution characteristics of the pre-tightening force used for bolt tightening at the fiber Bragg grating sensor deployment position; Step 3, establish a fastening structure dynamics model, and calculate the strain response and distribution characteristics at the measuring point before and after the bolt is loosened; Step 4, constructing an autocorrelation characteristic function of the optical fiber measured strain response to characterize the numerical correlation of the strain time series before and after the bolt loosening; Step 5, proposing a strain response autocorrelation change rate index, and judging whether the bolt is loose and the time of loosening based on the strain response autocorrelation change rate index; Step 6, propose a bolt looseness degree judgment index based on the relative change rate of the preload force to realize the bolt looseness degree judgment.

[0006] Step 1 includes: in order to meet the real-time monitoring requirements of the loosening status of large-scale high-density bolt groups, fiber grating sensors are arranged along the tangent direction of each bolt hole to be measured, and the distance between the center of each sensor and the center of the bolt hole monitored by the sensor is At the same time, the fiber grating sensors are integrated with the help of wavelength division multiplexing technology and space division multiplexing technology to form a distributed fiber optic monitoring system.

[0007] Step 2 includes: Step 2-1, the relationship between the preload force and the torsional moment of the bolt is as follows: (1), Where F is the preload force on the bolt, T is the applied torsional moment, K is the friction coefficient, and d is the nominal diameter of the bolt; By recording the torsional moment, the preload force on the bolt can be calculated; Step 2-2, the bolt transfers the preload force to the surface of the connected structure through the gasket. The resulting effect is equivalent to applying uniform compressive stress to the connected structure. The stress intensity calculation formula is: (2), Where q is the stress intensity, S is the contact area between the gasket and the connected structure; Step 2-3, due to factors such as motor movement, external loads, and structural balance destruction, the connection structure will inevitably vibrate during service. Therefore, the strain response data measured by the fiber Bragg grating sensor arranged around the bolt mainly consists of two parts: one is the compressive strain caused by the bolt preload, and the other is the time-varying strain caused by structural vibration.

[0008] The compressive strain caused by the bolt preload is obtained through the circular hole strain distribution law. The calculation formula is: (3), Where r is the bolt hole radius.

[0009] Step 3 includes: Step 3-1, the fastening structure is subjected to vibration load during service, so the dynamic equation at the unit where the fiber Bragg grating sensor is located is expressed as: (4), Where m is the unit mass, c is the unit damping, k is the unit stiffness, is the vibration load; represents the unit displacement at time t, represents the unit speed at time t, represents the unit acceleration at time t; Through Laplace transform, the unit displacement expression is solved as follows: (5), in is the damped mode frequency, is the damping ratio, is the time t is the natural frequency of the variable; e is a natural constant, Represents a variable Perform integration; The unit strain expression caused by structural vibration is obtained from formula (5): (6), (7), in is the unit strain caused by structural vibration at time t, b is the unit geometric coefficient, is the time t is a function of a variable; Step 3-2, when the bolt is not loose, the strain measured by the fiber grating sensor is expressed as: (8), in is the strain measured by the fiber Bragg grating sensor of the bolt in healthy condition at time t; Step 3-3, during the service life of the fastened structure, when the bolts become loose, the strain measured by the fiber Bragg grating sensor is expressed as: (9), in represents the strain measured by the fiber Bragg grating sensor under the loosening condition of the bolt at time t, The strain measured by the fiber grating sensor under the loosening condition of the bolt is is the strain decay function.

[0010] In step 3-3, the strain change effect caused by the reduction of preload is calculated through strain conversion processing. Equivalent to the strain decay caused by damage to the structural measuring point .

[0011] Step 4 includes: Step 4-1, if the bolt is not loose, the autocorrelation function of the bolt strain response at any time t0 is It is expressed as: (10); Where E represents the autocorrelation operation; Step 4-2: If the bolt loosens at t=t0, the bolt strain response autocorrelation function is It is expressed as: (11) in is the autocorrelation variation of the strain response before and after the bolt loosening; Step 4-3, the mathematical expression of the autocorrelation change of the strain response ΔR(t0) before and after the bolt loosening is:

[0012] (12).

[0013] Step 5 includes: the mathematical expression of the strain response autocorrelation change rate index is: (13) According to formula (13), if the bolt is loosened at time t = t0, the strain response autocorrelation change rate R(t0) is not equal to 0; at the same time, when the strain response autocorrelation change rate R(t0) is not equal to 0 at time t = t0, it is judged that the bolt is loosened at time t = t0.

[0014] Step 6 includes: Step 6-1: According to formula (13), the strain decay caused by the looseness of the structural measuring point The expression is: (14), Step 6-2, according to formula (9), the effect of strain change caused by reducing the preload force Equivalent to the strain decay caused by damage to the structural measuring point : (15), The mathematical expression of the relative change of preload force ΔF obtained by simplifying formula (9) is: (16), Step 6-3, the bolt looseness determination index based on the relative change rate of the preload force is: (17), in It is the relative change rate of preload force. By calculating this index, the degree of preload force loss, that is, the degree of bolt loosening, can be determined.

[0015] In order to realize maintenance decision-making according to the situation, a calculation method of bolt residual strength based on the relative change rate of residual preload is proposed: (18) where F rem (t0) is the relative change rate of the residual preload, through which the residual strength of the bolt can be calculated.

[0016] The present invention also provides an electronic device, comprising a processor and a memory, wherein the memory stores program code, and when the program code is executed by the processor, the processor executes the steps of the described method.

[0017] The present invention also provides a storage medium storing a computer program or instruction. When the computer program or instruction is run on a computer, the steps of the method described are executed.

[0018] The present invention establishes a bolt mechanical transmission model to solve the effect of the pre-tightening force used for bolt tightening on the strain at the location where the fiber grating sensor is arranged. A fastening structure dynamic response model is established to calculate the strain response change characteristics at the measuring point before and after the bolt is loosened. A strain response autocorrelation characteristic function is constructed to evaluate the correlation of the time series values ​​of the strain response corresponding to the bolt before and after the bolt is loosened. A strain response autocorrelation change rate index is proposed, and then based on this index, it is judged whether the bolt is loose and the time when the loosening occurs. A bolt looseness degree judgment index based on the relative change rate of the pre-tightening force is proposed to facilitate the implementation of "condition-based maintenance" decision-making.

[0019] The method of the present invention has the following beneficial effects: (1) It can be well applied to the health status monitoring of bolt groups of major equipment, has a simple operation process, and has good engineering practicability.

[0020] (2) During the service life of the equipment, the loosening state of the bolts can be identified based on the vibration response characteristics of the structure without applying external loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0022] Figure 1 It is a flow chart of the method of the present invention.

[0023] Figure 2 It is a typical bolt fastening structure model.

[0024] Figure 3 It is the effect of identifying the loose positions of multiple bolts.

[0025] Figure 4 It is the effect of identifying the degree of bolt looseness. DETAILED DESCRIPTION

[0026] The embodiment of the present invention provides a method for optical fiber identification of the loose state of a bolt group based on the autocorrelation characteristics of strain response. This method does not require the application of external loads, and can achieve bolt event identification, bolt position identification, and bolt looseness degree identification by relying solely on the vibration response of the structure under test during service. Figure 1 As shown, the method of this embodiment includes the following steps: Step 1: Design of a quasi-distributed fiber Bragg grating sensor layout for loosening feature identification of large-scale high-density bolt groups; Construct a typical bolt-fastened structure model, such as Figure 2 The model includes an upper composite wall panel, a lower composite wall panel, and a hexagonal bolt of model M8. The upper composite wall panel has the same geometric dimensions as the lower composite wall panel, and its length, width, and thickness are 600 mm, 220 mm, and 2.4 mm, respectively.

[0027] Name the 20 bolts b1~b 20 , and the spacing between adjacent bolts along the X direction is 40 mm, and the spacing between adjacent bolts along the Y direction is 40 mm. Strain extraction points are set on the surface of the upper composite wall panel for the identification of loosening characteristics of bolt structures. Bolts b1~b 20 The corresponding strain extraction points are 1#~20#, such as Figure 2 shown.

[0028] In order to meet the real-time monitoring requirements of loosening status of large-scale high-density bolt groups, fiber Bragg grating sensors are arranged along the tangent direction of each bolt hole to be measured, and the distance between the center of each sensor and the center of the bolt hole it monitors is At the same time, the fiber grating sensor is highly integrated with the help of wavelength division multiplexing technology and space division multiplexing technology to form a distributed fiber optic monitoring system that is lightweight, highly reliable, and does not require a large number of transmission cables.

[0029] Step 2, establishing a bolt mechanical transmission model, solving the strain response and distribution characteristics of the pre-tightening force used for bolt tightening at the fiber Bragg grating sensor deployment position; (1) During the structural assembly process, in order to ensure the successful installation of the bolts, it is necessary to apply a torsional torque to them using tools such as torque wrenches. The relationship between the preload force and torsional torque on the bolts is as follows: (1) Where F is the preload force on the bolt, T is the applied torsional moment, K is the friction coefficient, and d is the nominal diameter of the bolt.

[0030] From formula (1), it can be seen that by recording the torsional moment, the preload force on the bolt can be calculated.

[0031] (2) The bolt transfers the preload force to the surface of the connected structure through the gasket. The resulting effect is equivalent to applying uniform compressive stress to the connected structure. The stress intensity calculation formula is as follows: (2) Where q is the stress intensity and S is the contact area between the gasket and the connected structure.

[0032] (3) Due to factors such as motor movement, external loads, and structural balance destruction, the connection structure will inevitably vibrate during service. Therefore, the strain response data measured by the fiber Bragg grating sensor arranged around the bolt mainly consists of two parts: one is the compressive strain caused by the bolt preload, and the other is the time-varying strain caused by structural vibration.

[0033] According to the circular hole strain distribution law, the calculation formula of the compressive strain caused by the bolt preload is derived as follows: (3) In the formula, is the compressive strain caused by the bolt preload, r is the radius of the bolt hole, and a is the distance between the center of the fiber Bragg grating sensor and the center of the bolt hole it monitors.

[0034] Step 3, establish a fastening structure dynamics model, and calculate the strain response and distribution characteristics at the measuring point before and after the bolt is loosened; (1) The fastening structure is subjected to vibration load during service, so the dynamic equation of the unit where the fiber Bragg grating sensor is located can be expressed as: (4) In the formula, m is the unit mass, c is the unit damping, k is the unit stiffness, is the vibration load; represents the unit displacement at time t, represents the unit speed at time t, represents the unit acceleration at time t.

[0035] Through Laplace transform, the unit displacement expression is solved as follows: (5) In the formula, is the damped mode frequency, is the damping ratio, is the time t is the natural frequency of the variable, Represents a variable Perform integration.

[0036] From formula (5), the unit strain expression caused by structural vibration is: (6) (7) In the formula, is the unit strain caused by structural vibration, b is the unit geometric coefficient, is time t, is a function of a variable.

[0037] (2) When the bolts are not loose, the strain measured by the fiber grating sensor can be expressed as: (8) in is the measured strain of the bolt under healthy working condition at time t by the fiber Bragg grating sensor; From formula (8), we can know that the measured strain of the fiber Bragg grating sensor is Compressive strain caused by bolt preload The element strain caused by structural vibration composition.

[0038] (3) During the service life of the fastened structure, when the bolts become loose, the strain measured by the fiber Bragg grating sensor can be expressed as: (9) in represents the strain measured by the fiber Bragg grating sensor under the loosening condition of the bolt at time t, The strain measured by the fiber grating sensor under the loosening condition of the bolt is is the strain decay function.

[0039] It can be seen from formula (9) that when the bolt loosens, the strain measured by the sensor will change. This rule is mainly caused by the reduction of preload.

[0040] Through strain conversion calculation, the strain change effect caused by the reduction of preload can be Equivalent to the strain decay caused by damage to the structural measuring point .

[0041] Step 4, constructing an autocorrelation characteristic function of the optical fiber measured strain response to characterize the numerical correlation of the strain time series before and after the bolt loosening; (1) If the bolt does not loosen, the autocorrelation function of its strain response at any time t0 can be expressed as: (10) In the formula, is the autocorrelation function of the strain response of the bolt in a healthy state, and E represents the autocorrelation operation.

[0042] (2) If the bolt becomes loose at time t=t0, its strain response autocorrelation function can be expressed as: (11) In the formula, is the autocorrelation function of the strain response under the loosened bolt state, It is the autocorrelation change of the strain response before and after the bolt loosening.

[0043] (3) Autocorrelation change of strain response before and after bolt loosening The mathematical expression is: (12) From formula (12), we can see that the strain autocorrelation change ΔR(t0) is related to the strain decay function There is a connection.

[0044] Step 5, a strain response autocorrelation change rate index is proposed, and then based on this index, whether the bolt is loose and when it is loosened are judged; The mathematical expression of the strain response autocorrelation change rate index is: (13) It can be seen from formula (13) that if the bolt is loose at time t = t0, the strain response autocorrelation change rate R(t0) is not equal to 0. At the same time, if the strain response autocorrelation change rate R(t0) is not equal to 0 at time t = t0, it can be determined that the bolt is loose at this moment.

[0045] b7, b 10 The autocorrelation change rate of strain response corresponding to multiple bolt loosening conditions, such as Figure 3 As shown. Figure 3 It can be seen that when two bolts are loosened, the strain vibration mode shows two mutation phenomena at positions 7# and 10#. It can be seen that when multiple bolts are loosened, the autocorrelation change rate of the strain response corresponding to each loose bolt will mutate.

[0046] Step 6, proposing a bolt looseness determination index based on the relative change rate of the preload force to achieve bolt looseness determination; (1) From formula (13), we can get the strain decay caused by the looseness of the structural measuring point: The expression is: (14) (2) From equation (9) in step 3, we can see that the effect of strain change caused by the reduction of preload is Equivalent to the strain decay caused by damage to the structural measuring point : (15) By simplifying formula (9), the mathematical expression of the relative change of preload force ΔF is: (16) (3) The bolt looseness determination index based on the relative change rate of preload force is: (17) In the formula, It is the relative change rate of preload. By calculating this index, the degree of preload loss, that is, the degree of bolt loosening, can be determined.

[0047] b7 and b are calculated by formula (17). 10 The looseness degree identification effect corresponding to multiple bolt loosening conditions, such as Figure 4 As shown. Figure 4 It can be seen that there is a good consistency between the bolt loosening degree calculated based on the relative change rate of the preload force and the actual loosening degree.

[0048] A calculation method for the residual strength of bolts based on the relative change rate of the residual preload is proposed: (18) In the formula, is the relative change rate of the residual preload. The residual strength of the bolt can be calculated through this index.

[0049] The present invention provides a method for optical fiber identification of the loose state of a bolt group based on the autocorrelation characteristics of strain response. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A method for optical fiber identification of loose bolt group status based on strain response autocorrelation characteristics, characterized in that: The following steps are involved: Step 1: Design a quasi-distributed fiber Bragg grating sensor layout scheme for loosening feature identification of large-scale high-density bolt groups; Step 2, establishing a bolt mechanical transmission model, solving the strain response and distribution characteristics of the pre-tightening force used for bolt tightening at the fiber Bragg grating sensor deployment position; Step 3, establish a fastening structure dynamics model, and calculate the strain response and distribution characteristics at the measuring point before and after the bolt is loosened; Step 4, constructing an autocorrelation characteristic function of the optical fiber measured strain response to characterize the numerical correlation of the strain time series before and after the bolt loosening; Step 5, proposing a strain response autocorrelation change rate index, and judging whether the bolt is loose and the time of loosening based on the strain response autocorrelation change rate index; Step 6, propose a bolt looseness degree judgment index based on the relative change rate of the preload force to realize the bolt looseness degree judgment.

2. The method according to claim 1, characterized in that: Step 1 includes: in order to meet the real-time monitoring requirements of the loosening status of large-scale high-density bolt groups, fiber grating sensors are arranged along the tangent direction of each bolt hole to be measured, and the distance between the center of each sensor and the center of the bolt hole monitored by the sensor is At the same time, the fiber grating sensors are integrated with the help of wavelength division multiplexing technology and space division multiplexing technology to form a distributed fiber optic monitoring system.

3. The method according to claim 2, characterized in that Step 2 includes: Step 2-1, the relationship between the preload force and the torsional moment of the bolt is as follows: （1）， Where F is the preload force on the bolt, T is the applied torsional moment, K is the friction coefficient, and d is the nominal diameter of the bolt; By recording the torsional moment, the preload force on the bolt can be calculated; Step 2-2, the bolt transfers the preload force to the surface of the connected structure through the gasket. The resulting effect is equivalent to applying uniform compressive stress to the connected structure. The stress intensity calculation formula is: （2）， Where q is the stress intensity, S is the contact area between the gasket and the connected structure; Step 2-3: Obtain the compressive strain caused by the bolt preload through the circular hole strain distribution law The calculation formula is: （3）， Where r is the bolt hole radius.

4. The method according to claim 3, characterized in that Step 3 includes: Step 3-1, the fastening structure is subjected to vibration load during service, so the dynamic equation at the unit where the fiber Bragg grating sensor is located is expressed as: （4）， Where m is the unit mass, c is the unit damping, k is the unit stiffness, is the vibration load; represents the unit displacement at time t, represents the unit speed at time t, represents the unit acceleration at time t; Through Laplace transform, the unit displacement expression is solved as follows: （5）， in is the damped mode frequency, is the damping ratio, is the time t is the natural frequency of the variable; e is a natural constant, Represents a variable Perform integration; The unit strain expression caused by structural vibration is obtained from formula (5): （6）， （7）， in is the unit strain caused by structural vibration at time t, b is the unit geometric coefficient, is the time t is a function of a variable; Step 3-2, when the bolt is not loose, the strain measured by the fiber grating sensor is expressed as: （8）， in is the strain measured by the fiber Bragg grating sensor of the bolt in healthy condition at time t; Step 3-3, during the service life of the fastened structure, when the bolts become loose, the strain measured by the fiber Bragg grating sensor is expressed as: （9）， in represents the strain measured by the fiber Bragg grating sensor under the loosening condition of the bolt at time t, The strain measured by the fiber grating sensor under the loosening condition of the bolt is is the strain decay function.

5. The method according to claim 4, characterized in that In step 3-3, the strain change effect caused by the reduction of preload is calculated through strain conversion processing. Equivalent to the strain decay caused by damage to the structural measuring point .

6. The method according to claim 5, characterized in that Step 4 includes: Step 4-1, if the bolt is not loose, the autocorrelation function of the bolt strain response at any time t0 is It is expressed as: (10)； Where E represents the autocorrelation operation; Step 4-2: If the bolt loosens at t=t0, the bolt strain response autocorrelation function is It is expressed as: (11)， Where ΔR(t0) is the autocorrelation change of the strain response before and after the bolt loosening; Step 4-3, the mathematical expression of the autocorrelation change of the strain response ΔR(t0) before and after the bolt loosening is: (12)。 7. The method according to claim 6, characterized in that Step 5 includes: the mathematical expression of the strain response autocorrelation change rate index is: (13)， According to formula (13), if the bolt is loosened at time t = t0, the strain response autocorrelation change rate R(t0) is not equal to 0; at the same time, when the strain response autocorrelation change rate R(t0) is not equal to 0 at time t = t0, it is judged that the bolt is loosened at time t = t0.

8. The method according to claim 7, characterized in that Step 6 includes: Step 6-1: According to formula (13), the strain decay caused by the looseness of the structural measuring point The expression is: （14）， Step 6-2, according to formula (9), the effect of strain change caused by reducing the preload force Equivalent to the strain decay caused by damage to the structural measuring point : （15）， The mathematical expression of the relative change of preload force ΔF obtained by simplifying formula (9) is: （16）， Step 6-3, the bolt looseness determination index based on the relative change rate of the preload force is: （17）， where F bolt (t0) is the relative rate of change of preload force; In order to realize maintenance decision-making according to the situation, a calculation method of bolt residual strength based on the relative change rate of residual preload is proposed: (18)， in is the relative change rate of the residual preload.

9. An electronic device, characterized in that: The method comprises a processor and a memory, wherein the memory stores program codes, and when the program codes are executed by the processor, the processor executes the steps of the method according to any one of claims 1 to 8.

10. A storage medium, characterized in that: A computer program or instruction is stored, and when the computer program or instruction is run on a computer, the steps of the method according to any one of claims 1 to 8 are executed.

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