Optical Fiber Identification Method for Loosening State of Bolt Group Based on Strain Response Autocorrelation Features

Abstract

The present invention provides a fiber optic identification method for the loosening state of a bolt group based on strain response autocorrelation features, including: Step 1, designing a quasi-distributed fiber optic grating sensor layout scheme for the identification of loosening features of a large-scale high-density bolt group; Step 2, establishing a bolt mechanical transmission model; Step 3, establishing a dynamic model of the fastening structure; Step 4, constructing a fiber optic measured strain response autocorrelation feature function to characterize the numerical correlation of the strain time series corresponding before and after the bolt loosening; Step 5, proposing a strain response autocorrelation change rate index, and based on the strain response autocorrelation change rate index, determining whether the bolt is loosened and the time of loosening; Step 6, proposing a bolt loosening degree discrimination index based on the relative change rate of the pre-tightening force to realize the discrimination of the bolt loosening degree. The present invention can be well applied to the health state monitoring of bolt groups of major equipment, has a simple operation process, and has good engineering practicability.

Description

Technical Field

[0001] The present invention belongs to the field of structural health monitoring, and particularly relates to a fiber optic identification method for the loosening state of bolt groups based on strain response autocorrelation characteristics. Background Art

[0002] As an important means of connecting key components of aircraft, the tightening degree of bolts is of crucial significance for ensuring flight safety. If the bolt fastening structure loosens, 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 efficient monitoring of the loosening state of aircraft bolt groups. Therefore, there is an urgent need to develop a new method for identifying bolt loosening states with distributed synchronous sensing capabilities.

[0003] For the requirements of in-service health monitoring of bolt groups, traditional piezoelectric sensors have deficiencies such as difficult wiring, complex monitoring systems, and susceptibility to electromagnetic interference. Due to the advantages of fiber Bragg grating sensors such as light weight, thin fiber diameter, electromagnetic interference resistance, and large-scale networking monitoring, they are widely used in the field of bolt fastening structure monitoring. The monitoring scheme of embedding fiber Bragg grating sensors in bolt structures is not only cumbersome in process, inconvenient for subsequent sensor maintenance and repair, but also causes a significant decrease in the strength of the bolt structure itself. Currently, researchers have proposed bolt monitoring methods based on the combination of fiber Bragg grating sensors and pattern recognition algorithms. Such methods require constructing a large-scale bolt loosening sample library to improve the identification accuracy, and have limitations such as time-consuming, laborious, easy damage to the structure, and limited applicability. Summary of the Invention

[0004] Object of the Invention: The technical problem to be solved by the present invention is to provide a fiber optic identification method for the loosening state of bolt groups based on strain response autocorrelation characteristics in view of the deficiencies of the prior art. This method can achieve bolt event discrimination, bolt position identification, and bolt loosening degree discrimination without applying external loads, only by relying on the vibration response of the measured structure during service.

[0005] The method of the present invention includes the following steps:

[0006] Step 1, design a quasi-distributed fiber Bragg grating sensor layout scheme for identifying the loosening characteristics of large-scale high-density bolt groups;

[0007] Step 2, establish a bolt mechanical transfer model, and solve for the strain response and distribution characteristics of the pre-tightening force for bolt fastening at the positions where the fiber Bragg grating sensors are arranged;

[0008] Step 3, establish a dynamic model of the fastening structure, and calculate the strain response and distribution characteristics at the measuring point positions before and after bolt loosening;

[0009] Step 4: Construct the autocorrelation characteristic function of the measured strain response of the optical fiber to characterize the numerical correlation of the strain time series corresponding to before and after the bolt loosening;

[0010] Step 5: Propose an autocorrelation change rate index of the strain response, and based on the autocorrelation change rate index of the strain response, determine whether the bolt is loosened and the time of loosening;

[0011] Step 6: Propose a bolt loosening degree discrimination index based on the relative change rate of the pre-tightening force to realize the discrimination of the bolt loosening degree.

[0012] Step 1 includes: For the real-time monitoring requirement of the loosening state of a large-scale high-density bolt group, fiber Bragg grating sensors are arranged along the tangent direction of each measured bolt hole, 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 Bragg grating sensors are integrated by means of wavelength division multiplexing technology and space division multiplexing technology to form a distributed optical fiber monitoring system.

[0013] Step 2 includes:

[0014] Step 2-1: The following relationship exists between the pre-tightening force and the torsional moment of the bolt:

[0015] (1),

[0016] where F is the pre-tightening force of the bolt, T is the applied torsional moment, K is the friction coefficient, and d is the nominal diameter of the bolt;

[0017] By recording the torsional moment, the pre-tightening force of the bolt is calculated;

[0018] Step 2-2: The bolt transfers the pre-tightening force to the surface of the connection structure through the gasket, and the resulting effect is equivalent to applying a uniform compressive stress to the connected structure. The stress intensity calculation formula is:

[0019] (2),

[0020] where q is the stress intensity and S is the contact area between the gasket and the connected structure;

[0021] Step 2-3: Due to factors such as motor movement, external load action, and structural balance destruction, the connection structure inevitably has vibration phenomena during service. Therefore, the measured strain response data of the fiber Bragg grating sensors arranged around the bolt mainly consists of two parts. One is the compressive strain caused by the bolt pre-tightening force, and the other is the time-varying strain caused by the structural vibration.

[0022] Through the circular hole strain distribution law, the compressive strain caused by the bolt pre-tightening force The calculation formula is:

[0023] (3),

[0024] where r is the radius of the bolt hole.

[0025] Step 3 includes:

[0026] Step 3-1, during the service process of the fastening structure, it is subjected to vibration loads. Therefore, the dynamic equation at the unit where the fiber Bragg grating sensor is located is expressed as:

[0027] (4),

[0028] where m is the unit mass, c is the unit damping, and k is the unit stiffness, is the vibration load; represents the unit displacement at time t, represents the unit velocity at time t, represents the unit acceleration at time t;

[0029] By Laplace transform, the expression for the unit displacement obtained by solving is:

[0030] (5),

[0031] where is the damped modal frequency, is the damping ratio, is the natural vibration frequency at time t with as the variable; e is the natural constant, represents the integration with respect to the variable ;

[0032] The expression for the unit strain caused by the structural vibration obtained from Equation (5) is:

[0033] (6),

[0034] (7),

[0035] where is the unit strain caused by the structural vibration at time t, b is the unit geometric coefficient, is a function at time t with as the variable;

[0036] Step 3-2, when the bolt does not loosen, the measured strain of the fiber Bragg grating sensor is expressed as:

[0037] (8),

[0038] where is the measured strain of the fiber Bragg grating sensor of the bolt under healthy conditions at time t;

[0039] Step 3-3, during the service of the fastening structure, when the bolt loosens, the measured strain of the fiber Bragg grating sensor is expressed as:

[0040] (9),

[0041] where represents the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition at time t, is the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition, is the strain decay function.

[0042] In Step 3-3, through strain conversion processing calculation, the strain change effect caused by the reduction of the pre-tightening force is equivalent to the strain decay caused by the damage of the structural measurement point .

[0043] Step 4 includes:

[0044] Step 4-1, if the bolt does not loosen, the autocorrelation function of the strain response of the bolt at any time t0 is expressed as:

[0045] (10);

[0046] where E represents the autocorrelation operation;

[0047] Step 4-2, if the bolt loosens at t = t0, the autocorrelation function of the bolt strain response is expressed as:

[0048] (11),

[0049] where is the autocorrelation change amount of the strain response corresponding before and after the bolt loosens;

[0050] Step 4-3, the mathematical expression of the autocorrelation change amount ΔR(t0) of the strain response corresponding before and after the bolt loosens is:

[0051] (12).

[0052] Step 5 includes: The mathematical expression of the autocorrelation change rate index of the strain response is:

[0053] (13),

[0054] According to Equation (13), if the bolt loosens at time t = t0, the autocorrelation change rate R(t0) of the strain response is not equal to 0; meanwhile, when the autocorrelation change rate R(t0) of the strain response is not equal to 0 at time t = t0, it is determined that the bolt loosens at time t = t0.

[0055] Step 6 includes:

[0056] Step 6-1, according to Equation (13), the strain decay caused by the loosening of the structural measurement point The expression is:

[0057] (14),

[0058] Step 6-2, according to Equation (9), the strain change effect caused by the reduction of the pre-tightening force is equivalent to the strain decay caused by the damage of the structural measurement point :

[0059] (15),

[0060] The mathematical expression of the relative change amount ΔF of the pre-tightening force simplified from Equation (9) is:

[0061] (16),

[0062] The bolt loosening degree discrimination index based on the relative change rate of the pre-tightening force is:

[0063] (17),

[0064] where is the relative change rate of the pre-tightening force. By calculating this index, the degree of loss of the pre-tightening force, that is, the bolt loosening degree, can be discriminated.

[0065] To realize condition-based maintenance decision-making, a calculation method for the remaining strength of the bolt based on the relative change rate of the remaining pre-tightening force is proposed:

[0066] (18),

[0067] where F rem (t0) is the relative change rate of the remaining pre-tightening force. Through this index, the remaining strength of the bolt can be calculated.

[0068] The present invention also provides an electronic device, including a processor and a memory. The memory stores program code. When the program code is executed by the processor, the processor executes the steps of the method.

[0069] The present invention also provides a storage medium storing a computer program or instructions, which, when running on a computer, execute the steps of the above method.

[0070] By establishing a bolt mechanical transfer model, the present invention solves for the effect of the pre-tightening force for bolt fastening on the strain at the fiber Bragg grating sensor layout position. A dynamic response model of the fastening structure is established to calculate the change characteristics of the strain response at the measuring point position before and after bolt loosening. A strain response autocorrelation characteristic function is constructed to evaluate the numerical correlation of the strain response time series corresponding to before and after bolt loosening. A strain response autocorrelation change rate index is proposed, and based on this index, it is further used to determine whether the bolt is loose and the time of loosening. A bolt loosening degree discrimination index based on the relative change rate of the pre-tightening force is proposed to facilitate the implementation of the "condition-based maintenance" decision.

[0071] The method of the present invention has the following beneficial effects: (1) It can be well applied to the health state monitoring of bolt groups of major equipment, the operation process is simple, and it has good engineering practicability.

[0072] (2) During the service period of the equipment, according to the vibration response characteristics of the structure, the bolt loosening state can be identified without applying external loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The following further specifically describes the present invention in conjunction with the drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0074] Figure 1 is the flowchart of the method of the present invention.

[0075] Figure 2 is a typical bolt fastening structure model.

[0076] Figure 3 is the discrimination effect of multiple bolt loosening positions.

[0077] Figure 4 is the identification effect of bolt loosening degree. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0078] The embodiment of the present invention provides a fiber optic identification method for bolt group loosening state based on strain response autocorrelation characteristics. This method can realize bolt event discrimination, bolt position identification, and bolt loosening degree discrimination without applying external loads, only by relying on the vibration response of the measured structure during service. As Figure 1 shown, the method of this embodiment includes the following steps:

[0079] Step 1, design a quasi-distributed fiber Bragg grating sensor layout scheme for identifying the loosening characteristics of a large-scale high-density bolt group;

[0080] Build a typical bolt fastening structure model, such as Figure 2 shown. The model includes an upper composite wall panel, a lower composite wall panel, and a hexagonal bolt of model M8. The upper and lower composite wall panels have the same geometric dimensions, with lengths, widths, and thicknesses of 600 mm, 220 mm, and 2.4 mm respectively.

[0081] Name 20 bolts in sequence as 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 study of bolt structure loosening feature identification. The strain extraction points corresponding to bolts b1~b 20 are 1#~20# respectively, as Figure 2 shown.

[0082] For the real-time monitoring requirement of the loosening state of a large-scale high-density bolt group, fiber Bragg grating sensors are arranged along the tangent direction of each measured bolt hole, and the distance between the center of each sensor and the center of the bolt hole it monitors is mm. At the same time, with the help of wavelength division multiplexing technology and space division multiplexing technology, the fiber Bragg grating sensors are highly integrated to form a distributed fiber optic monitoring system with light weight, high reliability advantages, and without a large number of transmission cables.

[0083] Step 2: Establish a bolt mechanical transfer model, and solve to obtain the strain response and distribution characteristics of the pre-tightening force for bolt fastening at the fiber Bragg grating sensor layout position;

[0084] (1) During the structure assembly process, to ensure the successful installation of the bolt, a torsional moment needs to be applied to it through tools such as a torque wrench. The relationship between the pre-tightening force and the torsional moment of the bolt is as follows:

[0085] (1),

[0086] In the formula, F is the pre-tightening force of the bolt, T is the applied torsional moment, K is the friction coefficient, and d is the nominal diameter of the bolt.

[0087] It can be seen from formula (1) that by recording the torsional moment, the pre-tightening force of the bolt can be calculated.

[0088] (2) The bolt transfers the pre-tightening force to the surface of the connection structure through the gasket, and the resulting effect is equivalent to applying a uniform compressive stress to the connected structure. The stress intensity calculation formula is as follows:

[0089] (2),

[0090] Wherein, q is the stress intensity, and S is the contact area between the gasket and the connected structure.

[0091] (3) Due to factors such as motor movement, external load action, and structural balance damage, vibration inevitably exists in the connected structure during service. Therefore, the strain response data measured by the fiber Bragg grating sensors arranged around the bolts mainly consists of two parts. One is the compressive strain caused by the bolt pre-tightening force, and the other is the time-varying strain caused by structural vibration.

[0092] Through the strain distribution law of the circular hole, the calculation formula for the compressive strain caused by the bolt pre-tightening force is derived as follows:

[0093] (3),

[0094] Wherein, is the compressive strain caused by the bolt pre-tightening force, 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.

[0095] Step 3: Establish a dynamic model of the fastening structure, and calculate the strain response and distribution characteristics at the measuring point position before and after bolt loosening;

[0096] (1) During service, the fastening structure is subjected to vibration loads. Therefore, the dynamic equation of the unit where the fiber Bragg grating sensor is located can be expressed as:

[0097] (4),

[0098] Wherein, 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 velocity at time t, represents the unit acceleration at time t.

[0099] By Laplace transform, the unit displacement expression is solved as:

[0100] (5),

[0101] Wherein, is the damped modal frequency, is the damping ratio, is at time t with as the variable natural vibration frequency, represents the variable for integration.

[0102] From equation (5), the unit strain expression caused by structural vibration is obtained as:

[0103] (6),

[0104] (7),

[0105] In the formula, is the element strain caused by structural vibration, b is the element geometric coefficient, at time t, taking as a function of the variable.

[0106] (2) When the bolt does not loosen, the measured strain of the fiber Bragg grating sensor can be expressed as:

[0107] (8),

[0108] where is the measured strain of the fiber Bragg grating sensor of the bolt under healthy conditions at time t; It can be seen from formula (8) that the measured strain of the fiber Bragg grating sensor is composed of the compressive strain caused by the bolt pre-tightening force and the element strain caused by structural vibration .

[0109] (3) During the service of the fastening structure, when the bolt loosens, the measured strain of the fiber Bragg grating sensor can be expressed as:

[0110] (9),

[0111] where represents the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition at time t, is the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition, is the strain decay function.

[0112] It can be seen from formula (9) that when the bolt loosens, the measured strain of the sensor will change, and this law is mainly caused by the reduction of the pre-tightening force.

[0113] Through strain conversion processing and calculation, the strain change effect caused by the reduction of the pre-tightening force can be equivalently regarded as the strain decay caused by the damage of the structural measurement point .

[0114] Step 4, construct the autocorrelation characteristic function of the measured strain response of the optical fiber to characterize the numerical correlation of the strain time series corresponding to before and after the bolt loosening;

[0115] (1) If the bolt does not loosen, the autocorrelation function of its strain response at any time t0 can be expressed as:

[0116] (10),

[0117] In the formula, is the autocorrelation function of the strain response in the healthy state of the bolt, and E represents the autocorrelation operation.

[0118] (2) If the bolt becomes loose at the moment t = t0, the autocorrelation function of its strain response can be expressed as:

[0119] (11),

[0120] In the formula, is the autocorrelation function of the strain response in the loose state of the bolt, is the change amount of the autocorrelation of the strain response corresponding before and after the bolt becomes loose.

[0121] (3) Before and after the bolt becomes loose, the change amount of the autocorrelation of the strain response has the following mathematical expression:

[0122] (12),

[0123] It can be seen from formula (12) that the change amount ΔR(t0) of the strain autocorrelation is related to the strain decay function There is a connection.

[0124] Step 5, propose an index of the change rate of the autocorrelation of the strain response, and then based on this index, determine whether the bolt becomes loose and when it becomes loose;

[0125] The mathematical expression of the index of the change rate of the autocorrelation of the strain response is:

[0126] (13),

[0127] It can be seen from formula (13) that if the bolt becomes loose at the moment t = t0, the change rate R(t0) of the autocorrelation of the strain response is not equal to 0. At the same time, if the change rate R(t0) of the autocorrelation of the strain response is not equal to 0 at the moment t = t0, it can be determined that the bolt becomes loose at this moment.

[0128] b7, b 10 The change rates of the autocorrelation of the strain response corresponding to multiple bolt loosening conditions are as Figure 3 shown. From this Figure 3 it can be seen that when two bolts become loose, there are two mutation phenomena in the strain vibration mode at the 7# and 10# positions. Thus, it can be seen that when multiple bolts become loose, the change rates of the autocorrelation of the strain response corresponding to each loose bolt all undergo mutations.

[0129] Step 6, propose an index for judging the degree of bolt loosening based on the relative change rate of the pre-tightening force to achieve the judgment of the degree of bolt loosening;

[0130] (1) It can be obtained from Equation (13) that the strain decay caused by the loosening of the structural measurement point The expression is:

[0131] (14),

[0132] (2) As can be seen from Equation (9) in Step 3, the strain change effect caused by the reduction of the pre-tightening force is equivalent to the strain decay caused by the damage of the structural measurement point :

[0133] (15),

[0134] By simplifying Equation (9), the mathematical expression of the relative change amount ΔF of the pre-tightening force is:

[0135] (16),

[0136] (3) The discrimination index of the bolt loosening degree based on the relative change rate of the pre-tightening force is:

[0137] (17),

[0138] In the formula, is the relative change rate of the pre-tightening force. By calculating this index, the degree of loss of the pre-tightening force, that is, the bolt loosening degree, can be discriminated.

[0139] The loosening degree identification effects corresponding to multiple bolt loosening conditions are calculated through Equation (17), such as 10 shown. It can be seen from this Figure 4 that there is a good consistency between the bolt loosening degree calculated based on the relative change rate of the pre-tightening force and the actual loosening degree. Figure 4 A calculation method for the remaining strength of bolts based on the relative change rate of the remaining pre-tightening force is proposed:

[0140] (18),

[0141] (18),

[0142] In the formula, is the relative change rate of the remaining pre-tightening force. Through this index, the remaining strength of the bolt can be calculated.

[0143] The present invention provides a method for identifying the loosening state of a bolt group by using fiber optic based on strain response autocorrelation features. There are many methods and ways to specifically implement this technical solution. The above description is only a preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be implemented by using existing technologies.

Claims

1. A fiber optic identification method for the loosening state of a bolt group based on strain response autocorrelation features, characterized in that It includes the following steps: Step 1: Design a quasi-distributed fiber Bragg grating sensor layout scheme for identifying the loosening characteristics of a large-scale high-density bolt group; Step 2: Establish a bolt mechanical transfer model, and solve to obtain the strain response and distribution characteristics of the pre-tightening force for bolt fastening at the fiber Bragg grating sensor layout position; Step 3: Establish a dynamic model of the fastening structure, and calculate the strain response and distribution characteristics at the measuring point position before and after bolt loosening; Step 4: Construct an autocorrelation characteristic function of the measured fiber strain response to characterize the numerical correlation of the strain time series corresponding to before and after bolt loosening; Step 5: Propose an autocorrelation change rate index of the strain response, and based on the autocorrelation change rate index of the strain response, determine whether the bolt is loosened and the time of loosening; Step 6: Propose a bolt loosening degree discrimination index based on the relative change rate of the pre-tightening force to realize the discrimination of the bolt loosening degree; Step 1 includes: For the real-time monitoring requirement of the loosening state of a large-scale high-density bolt group, fiber Bragg grating sensors are arranged along the tangent direction of each measured bolt hole, and the distance between the center of each sensor and the center of the bolt hole monitored by the sensor is a; At the same time, the fiber Bragg grating sensors are integrated by means of wavelength division multiplexing technology and space division multiplexing technology to form a distributed optical fiber monitoring system; Step 2 includes: Step 2-1: The following relationship exists between the pre-tightening force and the torsional moment of the bolt: F = T / Kd (1), where F is the pre-tightening force of 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 pre-tightening force of the bolt is calculated; Step 2-2: The bolt transfers the pre-tightening force to the surface of the connecting structure through the gasket, and the resulting effect is equivalent to applying a uniform compressive stress to the connected structure. The stress intensity calculation formula is: q = F / S (2), where q is the stress intensity and S is the contact area between the gasket and the connected structure; Step 2-3: Obtain the compressive strain ε caused by the bolt pre-tightening force according to the strain distribution law of the circular hole. bolt The calculation formula is as follows: ε bolt = F / S(1 - r 2 / a 2 ) (3), where r is the radius of the bolt hole; Step 3 includes: Step 3-1: During the service process of the fastening structure, it is affected by vibration loads. Therefore, the dynamic equation of the unit where the fiber Bragg grating sensor is located is expressed as: where m is the element mass, c is the element damping, k is the element stiffness, and f(t) is the vibration load; x(t) represents the element displacement at time t, represents the element velocity at time t, represents the element acceleration at time t; By Laplace transform, the unit displacement expression is solved as: where ω d is the damping modal frequency, ξ is the damping ratio, ω n (t - τ) is the natural vibration frequency with τ as the variable at time t; e is the natural constant, and dτ represents the integration with respect to the variable τ; The unit strain expression caused by structural vibration is obtained from Equation (5) as: where ε vib (t) is the element strain caused by the structural vibration at time t, b is the element geometric coefficient, and g(t - τ) is a function with τ as the variable at time t; Step 3-2: When the bolt does not loosen, the measured strain of the fiber Bragg grating sensor is expressed as: where ε0(t) is the measured strain of the fiber Bragg grating sensor of the bolt under healthy conditions at time t; Step 3-3: During the service process of the fastening structure, when the bolt is loosened, the measured strain of the fiber Bragg grating sensor is expressed as: where ε1(t) represents the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition at time t, and Δε bolt is the measured strain of the fiber Bragg grating sensor of the bolt under the loosening condition, and g0(t - τ) is the strain decay function.

2. The method according to claim 1, characterized in that, In step 3-3, through strain conversion processing and calculation, the strain change effect ΔF / S(1-r 2 / a 2 ) caused by the reduction of the pre-tightening force is equivalent to the strain decay caused by the damage of the structural measurement point 3. The method according to claim 2, wherein Step 4 includes: Step 4-1, if the bolt does not loosen, the autocorrelation function of the strain response of the bolt at any time t0 is expressed as: where E represents the autocorrelation operation; Step 4-2, if the bolt becomes loose at time t = t0, the autocorrelation function of the bolt strain response is expressed as: where ΔR(t0) is the autocorrelation change amount of the strain response corresponding to before and after bolt loosening; Step 4-3: The mathematical expression of the autocorrelation change amount ΔR(t0) of the strain response corresponding to before and after bolt loosening is:

4. The method according to claim 3, wherein Step 5 includes: The mathematical expression of the autocorrelation change rate index of the strain response is: According to Equation (13), if the bolt loosens at t = t0, the autocorrelation change rate R(t0) of the strain response is not equal to 0; at the same time, when the autocorrelation change rate R(t0) of the strain response is not equal to 0 at t = t0, it is determined that the bolt loosens at t = t0.

5. The method according to claim 4, wherein Step 6 includes: Step 6-1, according to Equation (13), the strain decay caused by the loosening of the structural measurement point The expression is: Step 6-2. According to Equation (9), the strain change effect ΔF / S(1 - r 2 / a 2 ) caused by the reduction of the pre-tightening force is equivalent to the strain decay caused by the damage of the structural measurement point The mathematical expression of the relative change in pre-tightening force ΔF obtained by simplifying Equation (9) is: In Step 6-3, the bolt loosening degree discrimination index based on the relative change rate of pre-tightening force is: Among which F bolt (t0) is the relative change rate of the pre-tightening force; To achieve condition-based maintenance decision-making, a method for calculating the remaining strength of bolts based on the relative change rate of the remaining pre-tightening force is proposed: Where F rem (t0) is the relative change rate of the remaining preload force.

6. An electronic device, characterized in that, It includes a processor and a memory. The memory stores program code. When the program code is executed by the processor, the processor is caused to execute the steps of the method according to any one of claims 1 to 5.

7. A storage medium, characterized in that, It stores a computer program or instruction. When the computer program or instruction runs on a computer, it executes the steps of the method according to any one of claims 1 to 5.

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