Bolt loosening detection method, system and device based on passive flow-induced random guided waves

Abstract

The application provides a bolt loosening detection method, system and device based on passive flow-induced random guided waves, which has the following steps: a plurality of sensors are pasted on two plates connected by bolts, and random guided wave signals propagated in the plates caused by environmental noise are collected; the collected random guided wave signals are processed by using cross-correlation and band-pass filtering methods; an imaging equation is constructed by using the obtained cross-correlation functions under different filtering frequencies; a to-be-imaged area containing all bolts is divided into discrete grids, each node of the grid is assumed to be a scattering point of guided wave propagation across the plate, the pixel value required for imaging is calculated by using the extracted bolt tightness features, and imaging analysis of the tightness of the multi-bolt structure is realized. The application proposes a guided wave propagation model in the multi-bolt structure, extracts the propagation path and energy information of the passive random guided wave, obtains a multi-bolt tightness imaging method based on delay stacking, and realizes online detection of the tightness of the multi-bolt structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of bolt loosening detection, in particular to a bolt loosening detection method, system and device based on passive flow-induced random guided waves. BACKGROUND

[0002] Bolt connection is widely used in various industrial fields. Due to mechanical stress and corrosion in the use environment, bolt connection structure often appears loosening. Bolt loosening will cause the parts that should be tightly connected to be separated, which brings serious safety hazards to the whole mechanical structure, and even causes safety accidents. In order to ensure the stable operation of the mechanical structure, it is very important to monitor the tightness of the bolt in real time and timely locate the loosening bolt.

[0003] The artificial detection method is still the most commonly used bolt loosening detection method at present. This method is time-consuming and laborious, depends on the experience of the detection personnel, and is difficult to apply to online detection. Therefore, it is necessary to develop a structural health monitoring method to replace manual work and realize online detection of the tightness of the bolt structure. SUMMARY

[0004] In view of the problems existing in the prior art, the present application provides a bolt loosening detection method, system and device based on passive flow-induced random guided waves. A plurality of piezoelectric ceramic sensors are first pasted on two plates connected by bolts, and random guided wave signals propagating in the plates caused by environmental noise are collected. The collected random guided wave signals are processed by using cross-correlation and band-pass filtering. The cross-correlation functions obtained at different filtering frequencies are used for guided wave path identification and feature extraction. The imaging area containing all the bolts is divided into discrete grids, each node of the grid is assumed to be a scattering point of the guided wave across the plate, and the pixel value required for imaging is calculated by using the extracted bolt tightness features, so as to realize online detection of the tightness of the multi-bolt structure.

[0005] The present application provides a bolt loosening detection method based on passive flow-induced random guided waves, which specifically comprises the following steps:

[0006] S1, collecting random guided wave signals: using sensors to collect a plurality of groups of random guided wave signals propagating between the flat plate structures connected by bolts;

[0007] S2, random guided wave signal processing: performing cross-correlation calculation on the plurality of groups of random guided wave signals collected in step S1 to obtain cross-correlation signals and performing band-pass filtering to obtain the cross-correlation signals at different filtering frequencies after filtering;

[0008] S3, constructing a bolt tightness imaging equation: using the cross-correlation signals at different filtering frequencies obtained in step S2 to identify the guided wave path and construct a bolt tightness imaging equation, which specifically comprises the following sub-steps:

[0009] S31, divide the entire detection area into discrete grids, each node of the grid is defined as a scattering point of the guided wave propagating across the plate, assuming that a set of sensor pairs of r and respectively located at r1' and r2' constitute a theoretical propagation path r1'→r→r2', calculate the theoretical length of each guided wave path at the filter frequency f, the corresponding guided wave path theoretical length calculation formula of node r is as follows:

[0010] d(r)=||r1'-r||+||r-r2'|

[0011] Wherein, r is the node, r1' represents the position of the sensor on the first plate structure, r2' represents the position of the sensor on the second plate structure;

[0012] S32, calculate the theoretical flight time of the guided wave path: for each node r1=(x,y), the positions of the two sensors of the i-th sensor pair are r i1 '=(x i1 ,y i1 ) and r i2 '=(x i2 ,y i2 ), i=(1,2,…,m), m represents the number of sensor pairs;

[0013] The theoretical flight time τ ij (x,y) of the guided wave path is calculated as follows:

[0014]

[0015] Wherein, c g (f) represents the group velocity of the guided wave at frequency f; f j j=(1,2,…,n);

[0016] S33, construct the imaging equation of the sensor pair by the theoretical flight time of the guided wave path calculated in step S32:

[0017]

[0018] Wherein, represents the cross-correlation envelope amplitude at the theoretical flight time of the cross-correlation signal of the i-th sensor pair after the band-pass filter with the center frequency f j ;

[0019] S34, superimpose the calculation results of m sensor pairs and n filter frequencies to obtain the final bolt tightness imaging equation:

[0020]

[0021] S4, evaluating the tightness of the bolt: all the grid node coordinates are brought into the final bolt tightness imaging equation P(x, y) for calculation, the imaging amplitude at each grid node is calculated using the imaging equation in step S3, a two-dimensional image is drawn based on multiple amplitudes, and the tightness of each bolt is evaluated.

[0022] Preferably, step S1 specifically comprises the following sub-steps:

[0023] S11, several sensors are attached to the surface of the plate structure connected by the bolt for measuring random guided wave signals, and any two sensors distributed on both sides of the bolt form a sensor pair;

[0024] S12, using a digital sampling device to receive random guided wave signals within 10-30s.

[0025] Preferably, the sensor arrangement and bolt spacing in step S11 satisfy the following relationship:

[0026] |d(r1)-d(r2)|≥5c g (f) / f

[0027] Where f represents the center frequency of the band-pass filter, and in the cross-correlation function after band-pass filtering with a center frequency of f, the width of the wave packet is 5 / f, which includes five peaks, c g (f) represents the guided wave group velocity at the center frequency f, and for a pair of sensors located at r1' and r2' respectively and two bolts located at r1 and r2 respectively, two guided wave paths through the two bolts are formed, respectively r1'→r1→r2' and r1'→r2→r2', and the lengths are calculated by d1(r1) = ||r1'-r1||+||r1-r2'|| and d2(r2) = ||r1'-r2||+||r2-r2'||.

[0028] Preferably, the different filter center frequencies used in the cross-correlation band-pass filtering in step S2 are denoted as f1, f2, …, f n , and m groups of sensor pairs are arranged on both sides of the bolt joint.

[0029] Preferably, step S2 specifically comprises the following sub-steps:

[0030] S21, calculating the cross-correlation of different sensor pairs, taking two sensors distributed on different plane structures and a bolt connecting the two plane structures as a guided wave path, and the time-domain random guided wave signals collected by the two sensors are x1(t) and x2(t) respectively, and the cross-correlation function expression is:

[0031]

[0032] Wherein, x1(t) represents the incident wave signal received by the bolt first side sensor, x2(t) represents the transmitted wave signal received by the bolt second side sensor, τ represents the time delay, and τ∈(-t0, t0), t0 represents the time domain signal length, and the wave packet in the cross-correlation function represents the time of flight of the random guided wave propagating through the path between the two sensors, that is, a guided wave path; the cross-correlation signal obtained by band-pass filtering the cross-correlation function is denoted as C 12 (τ; f), wherein f represents the center frequency of the band-pass filtering;

[0033] S22, since the guided wave propagation has a dispersion effect, the cross-correlation signal is filtered using band-pass filtering, and the cross-correlation signal filtered by the band-pass filtering of the center frequency f is denoted as C 12 (τ; f);

[0034] S23, the upper envelope value of the cross-correlation signal C 12 (τ; f) is calculated, denoted as

[0035] S24, the cross-correlation signal is normalized, and the normalized cross-correlation signal The calculation formula is as follows:

[0036]

[0037] Wherein, C 11 (0) represents the zero point value of the autocorrelation of the incident wave signal, which represents the energy level of the incident wave signal, and the specific calculation is as follows:

[0038]

[0039] Preferably, the sensor in step S11 is a piezoelectric ceramic sensor.

[0040] Preferably, the specific evaluation method in step S4 is: taking the plurality of amplitudes as the pixel values of the imaging heat map, so as to draw a two-dimensional image in the imaging area, and the pixel value size of the two-dimensional image represents the bolt tightness, and according to the two-dimensional image, the tightness of each bolt can be evaluated; in the imaging image, when the bolt is fastened, the pixel value of the area where the bolt is located is greater than that of other areas, and when the bolt is completely loosened, the pixel value of the area where the bolt is located is less than that of other areas.

[0041] The second aspect of the application provides a bolt loosening detection system based on flow-induced random guided waves, which comprises a random guided wave signal acquisition unit, a random guided wave signal processing unit, a bolt tightness imaging equation construction unit and a bolt tightness evaluation unit.

[0042] The random guided wave signal acquisition unit acquires a plurality of groups of random guided wave signals propagating between the plate structures connected by the bolt by using the sensor;

[0043] The random guided wave signal processing unit performs cross-correlation calculation on the multiple groups of random guided wave signals collected in step S1 to obtain cross-correlation signals and performs band-pass filtering to obtain multiple groups of cross-correlation signals at different filtered frequencies after filtering.

[0044] The bolt tightness imaging equation construction unit performs guided wave path recognition and feature extraction on the cross-correlation signals at different filtered frequencies obtained in step S2 and constructs a bolt tightness imaging equation.

[0045] The bolt tightness evaluation unit brings all the grid node coordinates into the final bolt tightness imaging equation P(x, y) for calculation, uses the amplitude of the imaging equation at each grid node to draw a two-dimensional image, and the pixel value size of the two-dimensional image represents the bolt tightness, and according to the two-dimensional image, the tightness of each bolt is evaluated.

[0046] In a third aspect, the application provides a bolt loosening detection device based on flow-induced random guided waves, which comprises a digital sampling device, a piezoelectric ceramic sensor, an air compressor and a jet gun, the digital sampling device is connected with the piezoelectric ceramic sensor, the jet gun is connected with the air compressor, and the air compressor and the jet gun are used to excite and simulate passive random guided waves under real working conditions on a planar structure.

[0047] Compared with the prior art, the application has the following advantages:

[0048] (1) The application proposes a guided wave propagation model in a multi-bolt structure, in a bolted structure, guided waves only propagate between plates through fastened bolts, and the transmitted guided wave energy is proportional to the bolt torque, further using the cross-correlation of random guided wave signals combined with band-pass filtering to extract the propagation path and energy information of random guided waves, and deducing a multi-bolt tightness imaging method based on delay superposition, realizing online detection of the tightness of a multi-bolt structure.

[0049] (2) The detection method based on passive random guided waves proposed by the application does not require complex and cumbersome guided wave excitation equipment, only needs to attach a number of piezoelectric ceramic sensors with small volume and light weight on the inner surface of the structure, which does not need to be powered, has almost no influence on the volume and weight of the structure and the aerodynamic shape, and can be applied to online detection of the loosening of the bolts of the aircraft wing skin and other various plates. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 Fig. 1 is a main flowchart of the bolt loosening detection method based on passive flow-induced random guided waves of the application;

[0051] Figure 2 Fig. 2 is a flowchart of bolt tightness imaging in the bolt loosening detection method based on passive flow-induced random guided waves of the application;

[0052] Figure 3 Fig. 1 is a schematic diagram of a double bolt connection structure sample and a flow-induced random wave excitation device based on the bolt loosening detection method of the present application;

[0053] Figure 4 Fig. 2 is a structural schematic diagram of a bolt loosening detection device for a flat plate structure based on the flow-induced random wave of the present application;

[0054] Figure 5 Fig. 3 is a schematic diagram of experimental samples, sensor arrangement and blowing excitation positions under different experimental conditions of the present application;

[0055] Figures 6a-6b Fig. 4 is a cross-correlation band-pass filter image under different experimental conditions of the present application;

[0056] Figures 7a-7d Fig. 5 is a two-dimensional imaging result under different tightness conditions of the present application;

[0057] Figure 8 Fig. 6 is a schematic block diagram of the bolt loosening detection system of the present application. DETAILED DESCRIPTION

[0058] To make the technical contents, purposes and effects of the present application clear, the following will be described in detail in combination with the drawings of the specification.

[0059] The present application provides a bolt loosening detection method based on passive flow-induced random wave, as shown in Figure 1 and Figure 2 specifically comprising the following steps:

[0060] S1, collecting random wave signals: using sensors to collect multiple groups of random wave signals propagating between the flat plate structures connected by bolts.

[0061] This step specifically comprises the following sub-steps:

[0062] S11, attaching a plurality of sensors on the surface of the flat plate structure connected by bolts for measuring random wave signals, and any two sensors distributed on both sides of the bolt form a sensor pair. In a specific embodiment, the sensor is a piezoelectric ceramic sensor.

[0063] The sensor arrangement and the bolt spacing satisfy the following relationship:

[0064] |d(r1)-d(r2)|≥5c g (f) / f

[0065] wherein f represents the center frequency of the band-pass filter, and in the cross-correlation function after the band-pass filter with the center frequency f, the width of the wave packet is 5 / f, which includes five peak values, c g(f) denotes the group velocity of the guided wave at the center frequency f, and for a pair of sensors located at r1' and r2' respectively and two bolts located at r1 and r2 respectively, two guided wave paths are formed, which are r1'→r1→r2' and r1'→r2→r2' respectively, and the lengths of the two paths are calculated by d1(r1) = ||r1'-r1||+||r1-r2'|| and d2(r2) = ||r1'-r2||+||r2-r2'|| respectively.

[0066] S12, receiving random guided wave signals within 10-30 seconds using a digital sampling device.

[0067] S2, random guided wave signal processing: calculating the cross-correlation signals of the multiple groups of random guided wave signals collected in step S1 and performing band-pass filtering to obtain multiple groups of cross-correlation signals at different filtered frequencies.

[0068] Preferably, the different filtered frequencies used in the cross-correlation band-pass filtering in step S2 are denoted as f1, f2,..., f n , and m groups of sensor pairs are arranged on both sides of the bolt joint.

[0069] Preferably, step S2 specifically includes the following sub-steps:

[0070] S21, calculating the cross-correlation of different sensor pairs, taking two sensors distributed on different planar structures and a bolt connecting the two planar structures as a guided wave path, and the time-domain random guided wave signals collected by the two sensors are x1(t) and x2(t) respectively, and the cross-correlation function expression is:

[0071]

[0072] wherein x1(t) represents the incident wave signal received by the first side sensor of the bolt, x2(t) represents the transmitted wave signal received by the second side sensor of the bolt, τ represents the time delay, and τ ∈ (-t0, t0), t0 represents the length of the time-domain signal, and the wave packet in the cross-correlation function represents the flight time of the random guided wave propagating through the path between the two sensors, i.e. a guided wave path; the cross-correlation signal obtained after the cross-correlation function is band-pass filtered is denoted as C 12 (τ; f), wherein f represents the center frequency of the band-pass filtering.

[0073] S22, since the guided wave propagation has a dispersion effect, the cross-correlation is filtered using band-pass filtering, and the cross-correlation signal after band-pass filtering at the center frequency f is denoted as C 12 (τ; f).

[0074] S23, calculating the upper envelope value of the cross-correlation signal C 12 (τ; f), denoted as

[0075] S24, normalizing the cross-correlation signal, the normalized cross-correlation signal The calculation formula is as follows:

[0076]

[0077] Wherein, C 11 (0) represents the zero point value of the incident wave signal autocorrelation, which represents the energy level of the incident wave signal, and the specific calculation is as follows:

[0078]

[0079] S3, construct bolt tightness imaging equation: use the cross-correlation signal at different filter frequencies obtained in step S2 to identify guided wave path and construct bolt tightness imaging equation, which includes the following sub-steps:

[0080] S31, divide the entire detection area into discrete grids, and each node of the grid is defined as a scattering point of guided wave cross-plate propagation. It is assumed that a group of sensor pairs of each node r and located at r1′ and r2′ constitute a theoretical propagation path r1′→r→r2′. At the filter frequency f, the theoretical length of each guided wave path is calculated. The calculation formula of the theoretical length of the guided wave path corresponding to the node r is as follows:

[0081] d(r) = ||r1′-r||+||r-r2′||

[0082] Wherein, r is the node, r1′ represents the position of the sensor on the first plate structure, and r2′ represents the position of the sensor on the second plate structure.

[0083] S32, calculate the theoretical flight time of the guided wave path: for each node r1=(x,y), the positions of the two sensors of the i-th sensor pair are r i1 ′=(x i1 ,y i1 ) and r i2 ′=(x i2 ,y i2 ), i=(1,2,...,m), and m represents the number of sensor pairs.

[0084] The calculation formula of the theoretical flight time τ ij (x,y) of the guided wave path is as follows:

[0085]

[0086] Wherein, c g (f) represents the group velocity of the guided wave at frequency f; f jdenotes the jth filtered frequency, j = (1, 2, …, n).

[0087] S33, the imaging equation of the sensor pair is constructed by the theoretical flight time of the guided wave path calculated in step S32:

[0088]

[0089] wherein, denotes the correlation envelope amplitude of the cross-correlation signal of the ith group of sensor pairs at the theoretical flight time after the filtered center frequency f j of the band-pass filter.

[0090] S34, the imaging results of the m groups of sensors at the n filtered frequencies are calculated according to the imaging equation in step S33, and the calculation results of the m groups of sensors and the n filtered frequencies are superimposed to obtain the final bolt tightness imaging equation:

[0091]

[0092] S4, evaluating the tightness of the bolt: all grid node coordinates are brought into the final bolt tightness imaging equation P(x, y) for calculation, a two-dimensional image is drawn using the amplitude of the imaging equation at each grid node, and the pixel value size of the two-dimensional image represents the bolt tightness. Then, according to the two-dimensional image, the tightness of each bolt is evaluated. The evaluation method is: multiple amplitudes are used as pixel values of an imaging heat map, so that a two-dimensional image is drawn in the imaging area, the pixel value size of the two-dimensional image represents the bolt tightness, and according to the two-dimensional image, the tightness of each bolt can be evaluated. In the imaging image, when the bolt is tightened, the pixel value of the area where the bolt is located is greater than that of other areas, and when the bolt is completely loosened, the pixel value of the area where the bolt is located is less than that of other areas.

[0093] The second aspect of the application provides a bolt loosening detection system based on the bolt loosening detection method of passive flow-induced random guided waves, as shown in Figure 8 which comprises a random guided wave signal acquisition unit 1, a random guided wave signal processing unit 2, a bolt tightness imaging equation construction unit 3, and a bolt tightness evaluation unit 4.

[0094] The random guided wave signal acquisition unit 1 acquires multiple groups of random guided wave signals propagating between the plate structures connected by the bolt by using sensors.

[0095] The random guided wave signal processing unit 2 performs cross-correlation calculation on the multiple groups of random guided wave signals acquired in step S1 to obtain cross-correlation signals and performs band-pass filtering to obtain multiple groups of cross-correlation signals after filtering at different filtered frequencies.

[0096] The bolt tightness imaging equation construction unit 3 uses the cross-correlation signals at different filtering frequencies obtained in step S2 to perform waveguide path identification and feature extraction and construct the bolt tightness imaging equation.

[0097] The bolt tightness evaluation unit 4 brings all grid node coordinates into the final bolt tightness imaging equation P(x, y) for calculation, and uses the amplitude of the imaging equation at each grid node to draw a two-dimensional image. The pixel value size of the two-dimensional image represents the bolt tightness. Based on the two-dimensional image, the tightness of each bolt is evaluated.

[0098] The third aspect of the present invention provides a bolt loosening detection device based on a bolt loosening detection method using passive flow-induced random guided waves, such as Figure 4 As shown, the detection device includes a digital sampling device 101, a piezoelectric ceramic sensor 102, an air compressor, and an air gun. This detection device is used to provide passive random guided waves and collect signals. The digital sampling device 101 is connected to the piezoelectric ceramic sensor 102, and the air gun is connected to the air compressor. The air compressor and air gun are used to excite and simulate passive random guided waves under real working conditions on a planar structure 103. The two planar structures 103 are connected by bolts 104. Specific embodiments

[0100] The following is a further description of a bolt loosening detection method based on passive flow-induced random guided waves proposed by the present invention in conjunction with an embodiment:

[0101] In a specific embodiment of the present invention, two 6061 aluminum plates connected by two M8 bolts are used. The elastic modulus E of the aluminum plate is 71 GPa, the Poisson's ratio v is 0.33, and the density ρ is 2750 kg / m 3 The specifications of a single aluminum plate are 400mm×400mm×1mm, and the distance between the two bolts is 100mm. The experiment uses an air compressor and a spray gun to generate compressed gas and excite flow-induced vibration noise on the aluminum plate. The gas flow rate is about 30m / s. A total of eight piezoelectric ceramic sensors are arranged on the two aluminum plates to receive random guided wave signals. The experimental setup and passive random guided wave excitation device are as follows Figure 3 and Figure 4 The specific parameters of the experimental sample, sensor layout and blowing excitation position are shown in Figure 5 During the experiment, a torque wrench was used to change the bolt tightening conditions to: (i) T1 = 0 N·m, T2 = 0 N·m; (ii) T1 = 16 N·m, T2 = 0 N·m; (iii) T1 = 0 N·m, T2 = 16 N·m; and (iv) T1 = 16 N·m, T2 = 16 N·m. Four experimental conditions were used in total. After each adjustment, air blowing was performed on the left side of the sample.

[0102] Firstly, the time of flight of the guided wave path through different bolts is verified. Figure 6a The cross-correlation image of sensors 2 and 5 under experimental condition (ii), Figure 6b The cross-correlation image of sensors 3 and 8 under experimental condition (iii), Figure 6a and Figure 6b The band-pass filter frequency in each of the above figures is 20 kHz, 40 kHz, 60 kHz and 80 kHz respectively. The red cross represents the theoretical time of flight of the guided wave path through the first bolt, and the red circle represents the theoretical time of flight of the guided wave path through the second bolt. When the bolt is tightened, a wave packet appears at the theoretical time of flight of the bolt on the cross-correlation image. Conversely, when the bolt is loosened, there is no wave packet or a very small wave packet at the theoretical position on the cross-correlation image, which is consistent with the theoretical prediction result, ensuring that the tightening state of the bolt can be clearly distinguished from the imaging result.

[0103] Then, a 400 mm x 400 mm area containing two bolts to be detected is discretized, and then the tightening degree of the bolts is imaged for this area using the imaging equation in step S3. Four pairs of sensors are used for imaging, Figure 5 The sensor pairs in the above figures are 1&6, 2&5, 3&7 and 4&8, and 10 band-pass filter center frequencies are used, including 10 kHz, 20 kHz, …, 100 kHz. Figure 5 The midpoint of the two bolts is set as the coordinate origin, and the imaging results of the four tightening conditions are shown in FIG. 7. Figures 7a-7d The red solid hexagon represents the actual position of the completely tightened bolt, and the black hollow hexagon represents the actual position of the completely loosened bolt. From the imaging results in FIG. 7, it can be seen that when the bolt is completely tightened, the corresponding area will be highlighted in the imaging figure. Conversely, when the bolt is completely loosened, there will be no highlight around the corresponding area, or the imaging intensity is much smaller than when the bolt is completely tightened.

[0104] In this embodiment, by analyzing the two-dimensional imaging of the tightening degree of the double-bolt connection structure, it is proved that the proposed passive detection method can simultaneously identify the tightening state of multiple bolts. In addition, in specific practical applications, the size of the imaging function increases with the number of fastening bolts, because the greater the torque on the bolt, the higher the guided wave energy transmitted through it, and the amplitude of the imaging equation is related to the guided wave energy transmitted through the bolt, which can also help to judge the tightening degree of the bolt joint.

[0105] The above-described embodiments are only preferred embodiments of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A bolt loosening detection method based on passive flow-induced random guided waves, characterized in that: Specifically comprising the following steps: S1, collecting random guided wave signals: using sensors to collect multiple groups of random guided wave signals propagating between bolt-connected flat plate structures; S2, random guided wave signal processing: performing cross-correlation calculation on the multiple groups of random guided wave signals collected in step S1 to obtain cross-correlation signals and performing band-pass filtering to obtain multiple groups of cross-correlation signals at different filtered frequencies after filtering; S3, constructing a bolt tightness imaging equation: using the cross-correlation signals at different filtered frequencies obtained in step S2 to identify guided wave paths and construct a bolt tightness imaging equation, specifically comprising the following sub-steps: S31, dividing the entire detection area into discrete grids, each node of the grid is defined as a scattering point of guided wave cross-plate propagation, assuming that a group of sensors at r1' and r2' respectively located at each node r and constitute a theoretical propagation path r1'→r→r2', the theoretical length of each guided wave path is calculated at the filtered frequency f, the theoretical length calculation formula of the guided wave path corresponding to the node r is as follows: d(r)＝||r1′-r||+||r-r2′|| Wherein, r is the node, r1' represents the position of the sensor on the first flat plate structure, and r2' represents the position of the sensor on the second flat plate structure; S32, calculate the theoretical flight time of the waveguide path: for each node r1=(x, y), the positions of the two sensors of the i-th group of sensor pairs are r i1 ′=(x i1 ,y i1 ) and r i2 ′=(x i2 ,y i2 ), i=(1, 2, …, m), and m represents the number of groups of sensor pairs; Waveguide path theory time of flight τ ij The (x, y) is calculated as follows: where c g (f) denotes the group velocity of the guided wave at frequency f; f j denotes the jth filter frequency, j = (1, 2,..., n); S33, constructing an imaging equation of the sensor pair through the guided wave path theoretical flight time calculated in step S32: wherein, represents the cross-correlation envelope amplitude at the theoretical time of flight of the cross-correlation signal of the ith pair of sensors after band-pass filtering with a center frequency of f j i. S34, superimposing the calculation results of m sensor pairs and n filtered frequencies to obtain the final bolt tightness imaging equation: S4, evaluating the tightness of the bolt: bringing all grid node coordinates into the final bolt tightness imaging equation P(x,y) for calculation, using the imaging equation in step S3 to calculate the imaging amplitude at each grid node, drawing a two-dimensional image based on multiple amplitudes and evaluating the tightness of each bolt.

2. The passive flow-induced random guided wave based bolt loosening detection method according to claim 1, characterized in that: Step S1 specifically comprises the following sub-steps: S11, attaching a plurality of sensors to the surface of the flat plate structure connected by the bolt for measuring random guided wave signals, any two sensors distributed on both sides of the bolt constitute a sensor pair; S12, using a digital sampling device to receive random guided wave signals within 10-30s.

3. The passive flow-induced random guided wave based bolt loosening detection method according to claim 2, characterized in that: The sensor arrangement in step S11 and the bolt spacing satisfy the following relationship: | d(r1) - d(r2) | > 5c g (f) / f where f represents the center frequency of the band-pass filter, and the width of the wave packet is 5 / f in the cross-correlation function after the band-pass filter with the center frequency f, c g (f) represents the group velocity of the guided wave at the center frequency f, and for a pair of sensors located at r1' and r2' and two bolts located at r1 and r2, two guided wave paths are formed, r1'→r1→r2' and r1'→r2→r2', which propagate through the two bolts, and the lengths of the two paths are calculated by d1(r1) = ||r1'-r1||+||r1-r2'|| and d2(r2) = ||r1'-r2||+||r2-r2' ||, respectively.

4. The passive flow-induced random guided wave based bolt loosening detection method of claim 1, wherein: The center frequencies of the different filter frequencies used in the cross-correlation band-pass filtering in step S2 are denoted as f1, f2,..., f n And m groups of sensor pairs are arranged on both sides of the bolt joint.

5. The passive flow-induced random guided wave based bolt loosening detection method of claim 1, wherein: Step S2 specifically comprises the following sub-steps: S21, calculating the cross-correlation of different sensor pairs, taking two sensors distributed on different flat plate structures and a bolt connecting the two flat plate structures as a guided wave path, the time domain random guided wave signals collected by the two sensors are x1(t) and x2(t) respectively, and the cross-correlation function expression is: Wherein, x1(t) represents the incident wave signal received by the bolt first side sensor, x2(t) represents the transmitted wave signal received by the bolt second side sensor, τ represents the time delay, and τ ∈ (-t0, t0), t0 represents the length of the time domain signal, and the wave packet in the cross-correlation function represents the time of flight of the random guided wave propagating through the path between the two sensors, that is, a guided wave path; the cross-correlation signal obtained after the cross-correlation function is band-pass filtered is denoted as C 12 (τ; f), wherein f represents the center frequency of the band-pass filtering; S22, filter the cross-correlation signal using a band-pass filter, the cross-correlation signal after band-pass filtering with a center frequency f is denoted as C 12 (τ;f); S23, calculating the cross-correlation signal C 12 the upper envelope value of (τ;f), denoted by S24, normalizing the cross-correlation signal, normalized cross-correlation signal The calculation formula is as follows: where C 11 (0) represents the zero point value of the autocorrelation of the incident wave signal, which represents the energy level of the incident wave signal, and is calculated as follows:

6. The passive flow-induced random guided wave based bolt loosening detection method of claim 1, wherein: The sensor in step S11 is a piezoelectric ceramic sensor.

7. The passive flow-induced random guided wave based bolt loosening detection method of claim 1, wherein: The specific evaluation method in step S4 is: taking multiple amplitudes as pixel values of an imaging heat map, thereby drawing a two-dimensional image in the imaging area, the pixel value size of the two-dimensional image represents the bolt tightness, and according to the two-dimensional image, the tightness of each bolt can be evaluated; in the imaging image, when the bolt is tightened, the pixel value of the area where the bolt is located is greater than that of other areas, and when the bolt is completely loosened, the pixel value of the area where the bolt is located is less than that of other areas.

8. A bolt loosening detection system for use in the passive flow-induced random wave based bolt loosening detection method of claim 1, characterized by: It comprises a random guided wave signal acquisition unit, a random guided wave signal processing unit, a bolt tightness imaging equation construction unit, and a bolt tightness evaluation unit; The random guided wave signal acquisition unit acquires multiple groups of random guided wave signals propagating between the bolt-connected flat plate structures by using sensors; The random guided wave signal processing unit performs cross-correlation calculation on the multiple groups of random guided wave signals acquired in step S1 to obtain cross-correlation signals and performs band-pass filtering to obtain multiple groups of cross-correlation signals after filtering at different filtering frequencies; The bolt tightness imaging equation construction unit performs guided wave path recognition on the cross-correlation signals at different filtering frequencies obtained in step S2 and constructs a bolt tightness imaging equation; The bolt tightness evaluation unit brings all the grid node coordinates into the final bolt tightness imaging equation P(x, y) for calculation, uses the amplitude of the imaging equation at each grid node to draw a two-dimensional image, and uses the pixel value size of the two-dimensional image to represent the bolt tightness; according to the two-dimensional image, the tightness of each bolt is evaluated.

9. A bolt loosening detection device for use in the passive flow-induced random wave-based bolt loosening detection method of claim 1, characterized by: It includes a digital sampling device, a piezoelectric ceramic sensor, an air compressor and a jet gun, the digital sampling device is connected with the piezoelectric ceramic sensor, the jet gun is connected with the air compressor, and the air compressor and the jet gun are used for exciting and simulating passive random guided waves under real working conditions on a plane structure.

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