Bolt loosening identification method for multi-bolt connection structure based on Lamb wave array signal

Abstract

The present invention provides a method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals, which includes the following steps: S1, installing a piezoelectric array and collecting Lamb wave signals propagating across the bolt joints in each path; S2, obtaining the dispersion curve of the Lamb wave in the aluminum plate based on the Rayleigh-Lamb equation, and further obtaining the group velocity of the Lamb wave; S3, constructing a Lamb wave propagation model across the bolt joint to obtain the theoretical direct arrival time of the S0 wave packet; S4, calculating the preload index of each bolt separately; S5, setting a detection threshold to achieve bolt status identification. The present invention establishes a Lamb wave propagation model across the bolt joint and extracts detection information based on the propagation model, which can realize the status identification of specific bolts in a multi-bolt connection structure and can timely find the specific location of loose bolts. The present invention has a certain ability to resist environmental interference and has the potential for real-time monitoring, and can realize the automation of non-destructive testing and status monitoring.

Description

Technical Field

[0001] The present invention relates to the technical fields of state health monitoring and non-destructive testing, and in particular to a method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals. Background Art

[0002] Bolted connection structures have the advantages of low cost, easy installation, and high integrity, and are therefore widely used in engineering fields such as civil engineering, machinery, electricity, transportation, aviation, and chemical engineering. Under the combined effects of adverse external factors such as vibration, corrosion, and impact, as well as human factors such as improper initial installation, bolted connections are prone to degradation, resulting in loss of bolt preload and loosening, which may affect the safe operation of the overall structure and equipment and create safety hazards. Therefore, in order to ensure the connection strength of the structure and prevent serious accidents, looseness detection and loosening location of bolted connection nodes have important engineering practical value. At present, the commonly used bolt detection methods in engineering include manual inspection methods such as visual inspection and tapping. However, this method relies heavily on the experience of workers and is inefficient. Therefore, it is necessary to develop efficient condition health monitoring and non-destructive testing methods to detect bolt loosening.

[0003] With the development of ultrasonic guided wave testing technology, Lamb wave-based detection methods are considered to be a highly promising condition health monitoring and nondestructive testing technology. Lamb wave ultrasonic testing excites Lamb waves on the surface of the test piece, collects the damaged Lamb wave signal through a sensor, and analyzes indicators such as frequency, energy, and amplitude to obtain status information such as the location and size of the damage. Lamb waves can propagate over long distances in plate-like structures and are sensitive to damage. Therefore, they have been widely used in the detection of simple thin-walled structures such as metal plates and have demonstrated excellent performance. The bolt connection structure involved in this patent is composed of two metal plates connected by multiple bolts. Lamb waves can propagate in such plate-like structures and have detection potential. Therefore, this patent uses Lamb waves to detect bolt loosening.

[0004] Due to the complexity of bolt connection structures, most of the current bolt loosening detection methods based on Lamb waves only analyze simple statistical indicators such as energy and amplitude, and do not consider the actual propagation process of Lamb waves in the structure. Therefore, the current bolt loosening detection methods based on Lamb waves have low detection capabilities and are mostly only capable of detecting the loosening of a single bolt connection structure. When applied to multi-bolt connection structures, they can often only provide a qualitative evaluation of the overall connection status and are unable to determine the specific location of the loose bolts. In view of this, this patent considers the propagation process of Lamb waves in bolt connection structures, constructs a Lamb wave cross-bolt propagation model, and on this basis uses Lamb wave array signals to realize the loosening detection and status identification of specific bolts in multi-bolt connection structures, thereby locating the loose bolts. Summary of the Invention

[0005] The present invention aims to provide a detection method capable of identifying the tightness state of bolts in a multi-bolt connection structure, which can realize the looseness judgment of specific bolts and find the specific position of the loose bolts.

[0006] The present invention proposes a method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals, which comprises the following steps:

[0007] S1, install the piezoelectric array and collect Lamb wave signals propagating across the bolt joints in each path;

[0008] S11, lap-joining the first aluminum plate and the second aluminum plate together by bolts, and attaching piezoelectric sheets to the first aluminum plate and the second aluminum plate on both sides of the bolt lap joint respectively;

[0009] S12, wherein the piezoelectric piece on the first aluminum plate is used as an excitation, the signal generator outputs an excitation signal s(t), which is amplified by the signal amplifier and then applied to the piezoelectric piece as the excitation; the excitation signal s(t) is selected with a center frequency of f c , Hanning window modulated sinusoidal signal:

[0010] s(t)=m(t)sin(ω c t) (1)

[0011] Among them, ω c =2πf c , represents the center angular frequency, t represents time, and m(t) represents the Hanning window;

[0012] S13, the piezoelectric piece on the second aluminum plate acts as a receiver, receives the signal in turn, and the oscilloscope completes the signal acquisition;

[0013] S2. Based on the Rayleigh-Lamb equation, the dispersion curve of the Lamb wave in the aluminum plate is obtained, and the group velocity of the Lamb wave is further obtained;

[0014] S3. Construct a Lamb wave propagation model across bolted joints to obtain the theoretical direct arrival time of the S0 wave packet;

[0015] S31. Construct a Lamb wave propagation model across bolted joints;

[0016] When the excitation end excites the Lamb wave, the Lamb wave will first propagate from the first aluminum plate to the bolt, then from the bolt to the second aluminum plate, and then from the bolt to the receiving end. Therefore, the propagation model of the Lamb wave across the bolt joint is established as:

[0017]

[0018] Where r(t) is the received signal collected by the receiving end; s(t) is the excitation signal applied by the excitation end; represents the amplitude spectrum, S(ω) is the result of Fourier transform of s(t), M is the number of bolts in the structure, N1 is the number of Lamb wave modes in the aluminum plate where the excitation end is located, N2 is the number of Lamb wave modes in the aluminum plate where the receiving end is located, and k x k represents the wave number of the xth mode in the aluminum plate where the excitation end is located, y represents the wave number of the yth mode in the aluminum plate where the receiving end is located, represents the distance between the excitation end and the i-th bolt, represents the distance between the receiving end and the i-th bolt, j is an imaginary unit;

[0019] S32, obtain the theoretical arrival time of the direct S0 wave packet in the measured signal of the piezoelectric piece, that is, only consider the S0 mode with the fastest group velocity, and simplify Equation (8) to:

[0020]

[0021] Among them, k S0 is the wave number of S0 mode;

[0022] Substituting formula (1) into formula (9), we get:

[0023]

[0024] Among them, c gS0 (ω c ) is the S0 mode at ω c The group velocity at is the phase offset, because the excitation signal is a narrowband signal, so the amplitude spectrum A i (ω,T) is simplified to A i (T), according to Equation (10), for an excitation-receiving pair, the theoretical arrival time t of the direct S0 Lamb wave packet passing through the i-th bolt is i for:

[0025]

[0026] S4. Calculate the preload index of each bolt separately;

[0027] Assume that there are M bolts and N excitation-receiving pairs in the bolt connection structure. For the i-th bolt, its preload index Q is i Defined as:

[0028]

[0029] in, is the theoretical arrival time of the direct wave packet of the signal collected by the zth pair of excitation-receiving pairs through the ith bolt, e z (t) is the envelope of the signal collected by the zth excitation-receiving pair;

[0030] S5. Setting a threshold to identify the status of the bolt;

[0031] Set the detection threshold V for the i-th bolt being detected i , detection threshold V i According to the percentage of the preload index measured when the bolt is tightened properly, when the preload index Q i Less than V i When , it is considered that the i-th bolt is loose. By comparing the preload indexes of all bolts, the tightness status of all bolts in the multi-bolt connection structure is obtained, and the loose bolts are located and identified.

[0032] Preferably, the Hanning window signal in step S12 is a 2-cycle sinusoidal signal, and the expression is:

[0033]

[0034] Among them, ω c =2πf c Indicates the center angular frequency, f c is the center frequency of the excitation signal s(t).

[0035] Preferably, the step S2 is to obtain the dispersion curve of the Lamb wave in the aluminum plate based on the Rayleigh-Lamb equation, and further obtain the group velocity of the Lamb wave; specifically:

[0036] S21. Based on the Rayleigh-Lamb equation, the relationship between the modal wave number k and the frequency f in the aluminum plate structure is used to obtain the wave number dispersion curve; the Rayleigh-Lamb equation is expressed as:

[0037] Symmetrical mode (3)

[0038] Antisymmetric modes (4)

[0039] Where 2h is the thickness of the aluminum plate, k is the number of waves transmitted in the horizontal direction, and the auxiliary variables p and q are defined as:

[0040]

[0041]

[0042] Where, ω=2πf, ω is the angular frequency; c L and c T are the velocities of longitudinal and shear waves in the aluminum plate, respectively;

[0043] By solving equations (3) and (4), we can obtain the relationship between the wave number k of each mode and the frequency f, that is, the wave number dispersion curve;

[0044] S22. Calculate the group velocity of the Lamb wave in the aluminum plate based on the wave number dispersion curve; use the following relationship:

[0045]

[0046] Calculate the group velocity c of the corresponding mode in the aluminum plate g .

[0047] Preferably, in step S5, the detection threshold V i It is set according to 60% of the preload force index measured when the bolts are tightened properly.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. The present invention considers the actual propagation path of Lamb waves in bolted connection structures and establishes a Lamb wave propagation model across bolted joints, which helps to achieve accurate information extraction.

[0050] 2. Based on the propagation model, the present invention can identify and distinguish the specific wave packets passing through a specific bolt, thereby extracting detection information, realizing the status identification of specific bolts in a multi-bolt connection structure, and timely discovering the specific location of loose bolts.

[0051] 3. The present invention comprehensively utilizes the multi-path Lamb wave information in the array signal, can more accurately identify the status information of specific bolts, and has a certain ability to resist environmental interference.

[0052] 4. This invention utilizes piezoelectric sheets to excite and receive Lamb waves, which is less expensive and faster than traditional methods such as ultrasonic C-scanning. Furthermore, piezoelectric sheets can be adhered to structural surfaces without affecting their functionality, offering the potential for real-time monitoring and enabling automated nondestructive testing and condition monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a flow chart of a method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals according to the present invention;

[0054] Figure 2a A top view of the sample of the present invention and the position of the piezoelectric piece;

[0055] Figure 2b A front view of the sample of the present invention and the position of the piezoelectric piece;

[0056] Figure 3a is the wave number dispersion curve of the aluminum plate of the present invention;

[0057] Figure 3b is the group velocity dispersion curve of the aluminum plate of the present invention;

[0058] Figure 4The multi-bolt connection structure and Lamb wave propagation process of the present invention;

[0059] Figure 5 is the theoretical arrival time of the measured signal T1-R1 and the direct wave packet passing through the two bolts of the present invention;

[0060] Figure 6 The pre-tightening index Q of bolt No. 1 under the four assembly conditions of the present invention is 1 With the No. 2 bolt preload indicator Q 2 . DETAILED DESCRIPTION

[0061] The present application will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the relevant invention and are not intended to limit the invention. It should also be noted that, for ease of description, only portions relevant to the relevant invention are shown in the accompanying drawings.

[0062] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0063] Figure 1 The present invention shows a method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals, which includes the following steps:

[0064] S1. Install the piezoelectric array and collect Lamb wave signals propagating across the bolt joints in each path.

[0065] S11, the first aluminum plate and the second aluminum plate are overlapped together by bolts, and piezoelectric sheets are respectively attached to the first aluminum plate and the second aluminum plate on both sides of the bolt overlap joint.

[0066] S12. The piezoelectric piece on the first aluminum plate is used as an excitation, and the signal generator outputs an excitation signal s(t), which is amplified by the signal amplifier and then applied to the piezoelectric piece used as the excitation in sequence.

[0067] The excitation signal s(t) is selected with a center frequency of f c , a 2-cycle sinusoidal signal modulated by a Hanning window:

[0068] s(t)=m(t)sin(ω c t) (1)

[0069] Among them, ω c =2πf c , represents the center angular frequency, t represents time, m(t) represents the Hanning window signal, and the expression is:

[0070]

[0071] S13. The piezoelectric piece on the second aluminum plate acts as a receiver to receive signals in sequence, and the oscilloscope completes signal acquisition.

[0072] In a specific embodiment, the sample size, bolt position, and piezoelectric piece position are as follows: Figure 2a and 2b As shown. The first and second aluminum plates are both 2mm thick and are assembled together using two M8 bolts. Five piezoelectric plates (T1-T5) are attached to the surface of the first aluminum plate as the excitation end; five piezoelectric plates (R1-R5) are attached to the surface of the second aluminum plate as the signal receiving end. The diameter of the piezoelectric plates is 7mm, and the center distance between adjacent piezoelectric plates is 50mm. The center frequency of the excitation signal is f c In this embodiment, 250 kHz is selected, and then each piezoelectric piece at the signal receiving end receives the excitation signal sent by the piezoelectric piece at the excitation end, and the signals of all 25 Lamb wave paths are collected.

[0073] S2. Based on the Rayleigh-Lamb equation, the dispersion curve of the Lamb wave in the aluminum plate is obtained, and the group velocity of the Lamb wave is further obtained.

[0074] S21. Based on the Rayleigh-Lamb equation, the relationship between the modal wave number k and the frequency f in the aluminum plate structure is obtained, and the wave number dispersion curve is obtained. The expression of the Rayleigh-Lamb equation is:

[0075] Symmetrical mode (3)

[0076] Antisymmetric modes (4)

[0077] Where 2h is the thickness of the aluminum plate, k is the number of waves transmitted in the horizontal direction, and the auxiliary variables p and q are defined as:

[0078]

[0079]

[0080] Where, ω=2πf, ω is the angular frequency; c L and c T are the velocities of longitudinal and shear waves in the aluminum plate, respectively.

[0081] By solving equations (3) and (4), we can obtain the relationship between the wave number k of each mode and the frequency f, that is, the wave number dispersion curve.

[0082] S22. The group velocity of the Lamb wave in the aluminum plate is obtained based on the wave number dispersion curve using the following relationship:

[0083]

[0084] Calculate the group velocity c of the corresponding mode in the aluminum plate g .

[0085] In a specific embodiment, the thickness of the aluminum plate used is 2 mm. Through the deduction of material parameters, its longitudinal wave velocity is 5533 m / s and the shear wave velocity is 2787 m / s. The curves of each mode wave number and group velocity are as follows: Figure 3a and 3b shown.

[0086] S3. Construct a Lamb wave propagation model across bolted joints to obtain the theoretical direct arrival time of the S0 wave packet.

[0087] S31. Construct a Lamb wave propagation model across bolted joints and analyze the direct path of the Lamb wave. Considering the stress concentration phenomenon near the bolts, that is, the actual contact area between the two plates is mainly concentrated around the bolts, the Lamb wave mainly propagates between the plates near the bolts. Figure 4 As shown in the figure, for a general multi-bolt lap joint structure, when the excitation end excites the Lamb wave, the Lamb wave will first propagate in the original aluminum plate to the bolt, then propagate from the bolt to the other aluminum plate, and then propagate from the bolt to the receiving end. Based on the above analysis of the direct path of the Lamb wave, a Lamb wave propagation model across the bolted joint can be further established. Assuming that the excitation signal applied by the excitation end is s(t), the received signal r(t) collected by the receiving end is:

[0088]

[0089] in, represents the amplitude spectrum, S(ω) is the result of Fourier transform of s(t), M is the number of bolts in the structure, N1 is the number of Lamb wave modes in the aluminum plate where the excitation end is located, N2 is the number of Lamb wave modes in the aluminum plate where the receiving end is located, and k x k represents the wave number of the xth mode in the aluminum plate where the excitation end is located, y represents the wave number of the yth mode in the aluminum plate where the receiving end is located, represents the distance between the excitation end and the i-th bolt, represents the distance between the receiving end and the i-th bolt, and j is an imaginary unit.

[0090] S32. Obtain the theoretical arrival time of the direct S0 wave packet in the measured signal of the piezoelectric piece. Equation (8) contains many modal components. To simplify the analysis, this patent only considers the S0 mode with the fastest group velocity. Therefore, Equation (8) can be simplified to:

[0091]

[0092] Among them, k S0 is the wave number of the S0 mode. Substituting formula (1) into formula (9), we can obtain:

[0093]

[0094] Among them, c gS0 (ω c ) is the S0 mode at ω c The group velocity at is the phase offset. Since the excitation signal is a narrowband signal, the amplitude spectrum A i (ω,T) can be simplified to A i (T).

[0095] According to the time relationship between the excitation signal and the received signal, the envelope of the excitation signal s(t) is m(t), and the envelope of the received signal r(t) is m(tt i ), the time difference between the two envelopes (i.e. wave packets) is t i , the arrival time can be considered as t i From formula (10), it can be seen that for this excitation-receiving pair, the theoretical arrival time t of the direct S0 Lamb wave packet passing through the i-th bolt is i for:

[0096]

[0097] In a specific embodiment, there are 25 excitation-receiving pairs, two bolts, and the center frequency of the excitation signal is 250 kHz. At this time, the group velocity of the S0 mode is approximately 4750 m / s. Taking the excitation-receiving pair T1-R1 as an example, the theoretical arrival time of the direct wave packet in the received signal through bolt No. 1 is:

[0098]

[0099] The theoretical arrival time of the direct wave packet of the received signal through bolt No. 2 is:

[0100]

[0101] When both bolts are tightened, the actual signal received by the stimulus-receiver pair T1-R1 is as follows: Figure 5 As shown in the figure, it can be seen that direct S0 wave packets from both bolts can be observed near the calculated theoretical arrival time, verifying the accuracy of the theoretical arrival time calculation. For the remaining excitation-receiving pairs, the theoretical arrival time of the direct wave packet passing through any bolt can also be calculated using the above method and formula, so it will not be repeated here.

[0102] S4. Calculate the preload index of each bolt separately.

[0103] Assume that there are M bolts and N excitation-receiving pairs in the bolt connection structure. For the i-th bolt, its preload index Q i Defined as:

[0104]

[0105] in, is the theoretical arrival time of the direct wave packet of the signal collected by the zth pair of excitation-receiving pairs through the i-th bolt, calculated by step S3. z (t) is the envelope of the signal collected by the zth stimulus-receiver pair, defined as:

[0106] e z (t)=abs(Hilbert(r z (t))) (15)

[0107] Among them, r z (t) is the signal collected by the zth pair of excitation-receiving pairs, Hilbert represents Hilbert transform, and abs() represents absolute value. Because the amplitude of the wave packet near the theoretical arrival time is expected to be positively correlated with the bolt preload, the preload index Q i It is expected to reflect the tightness of the i-th bolt.

[0108] In a specific embodiment, the detection object includes 2 bolts, so a total of 4 assembly conditions are considered: (1) Both bolts are in the "tight" state; (2) Bolt No. 1 is in the "loose" state, and bolt No. 2 is in the "tight" state; (3) Bolt No. 1 is in the "tight" state, and bolt No. 2 is in the "loose" state; (4) Both bolts are in the "loose" state. In the "tight" state, a torque of 16 Nm is applied to the bolt by a torque wrench, and in the "loose" state, it is only tightened by hand. In the four assembly conditions, the preload index Q of bolt No. 1 is 1 With the No. 2 bolt preload index Q 2 like Figure 6 shown.

[0109] S5. Set a threshold to identify the status of the bolt.

[0110] For the i-th bolt, set the detection threshold V i If its preload index Q i Less than V i , then the i-th bolt is considered loose, and vice versa. By comparing the preload indexes of all bolts, the tightness of all bolts in a multi-bolt connection structure can be obtained, and the loose bolts can be located and identified.

[0111] In this embodiment, the detection threshold V 1 and V 2 It can be set to 0.1. Figure 6As shown in the figure, the pre-tightening index of loose bolts is less than the detection threshold, while the pre-tightening index of bolts with acceptable tightness is greater than the threshold, thus achieving the location and identification of loose bolts. i The specific size should be set flexibly according to the actual situation. Generally, it can be set to about 60% of the preload index measured when the bolts are tightened properly.

[0112] Finally, it should be noted that the embodiments described above are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals, characterized by: It includes the following steps: S1, install the piezoelectric array and collect Lamb wave signals propagating across the bolt joints in each path; S11, lap-joining the first aluminum plate and the second aluminum plate together by bolts, and attaching piezoelectric sheets to the first aluminum plate and the second aluminum plate on both sides of the bolt lap joint respectively; S12, wherein the piezoelectric piece on the first aluminum plate is used as an excitation, the signal generator outputs an excitation signal s(t), which is amplified by the signal amplifier and then applied to the piezoelectric piece as the excitation; the excitation signal s(t) is selected with a center frequency of f c , Hanning window modulated sinusoidal signal: s(t)=m(t)sin(ω c t) (1) Among them, ω c =2πf c , represents the center angular frequency, t represents time, and m(t) represents the Hanning window; S13, the piezoelectric piece on the second aluminum plate acts as a receiver, receives the signal in turn, and the oscilloscope completes the signal acquisition; S2. Based on the Rayleigh-Lamb equation, the dispersion curve of the Lamb wave in the aluminum plate is obtained, and the group velocity of the Lamb wave is further obtained; S3. Construct a Lamb wave propagation model across bolted joints to obtain the theoretical direct arrival time of the S0 wave packet; S31. Construct a Lamb wave propagation model across bolted joints; When the excitation end excites the Lamb wave, the Lamb wave will first propagate from the first aluminum plate to the bolt, then from the bolt to the second aluminum plate, and then from the bolt to the receiving end. Therefore, the propagation model of the Lamb wave across the bolt joint is established as: Where r(t) is the received signal collected by the receiving end; s(t) is the excitation signal applied by the excitation end; represents the amplitude spectrum, S(ω) is the result of Fourier transform of s(t), M is the number of bolts in the structure, N1 is the number of Lamb wave modes in the aluminum plate where the excitation end is located, N2 is the number of Lamb wave modes in the aluminum plate where the receiving end is located, and k x k represents the wave number of the xth mode in the aluminum plate where the excitation end is located, y represents the wave number of the yth mode in the aluminum plate where the receiving end is located, represents the distance between the excitation end and the i-th bolt, represents the distance between the receiving end and the i-th bolt, j is an imaginary unit; S32, obtain the theoretical arrival time of the direct S0 wave packet in the measured signal of the piezoelectric piece, that is, only consider the S0 mode with the fastest group velocity, and simplify Equation (8) to: Among them, k S0 is the wave number of S0 mode; Substituting formula (1) into formula (9), we get: Among them, c gS0 (ω c ) is the S0 mode at ω c The group velocity at is the phase offset, because the excitation signal is a narrowband signal, so the amplitude spectrum A i (ω,T) is simplified to A i (T), according to Equation (10), for an excitation-receiving pair, the theoretical arrival time t of the direct S0 Lamb wave packet passing through the i-th bolt is i for: S4. Calculate the preload index of each bolt separately; Assume that there are M bolts and N excitation-receiving pairs in the bolt connection structure. For the i-th bolt, its preload index Q is i Defined as: in, is the theoretical arrival time of the direct wave packet of the signal collected by the zth pair of excitation-receiving pairs through the ith bolt, e z (t) is the envelope of the signal collected by the zth excitation-receiving pair; S5. Setting a threshold to identify the status of the bolt; Set the detection threshold V for the i-th bolt being detected i , detection threshold V i According to the percentage of the preload index measured when the bolt is tightened properly, when the preload index Q i Less than V i When , it is considered that the i-th bolt is loose. By comparing the preload indexes of all bolts, the tightness status of all bolts in the multi-bolt connection structure is obtained, and the loose bolts are located and identified.

2. The method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals according to claim 1, characterized in that: The Hanning window signal in step S12 is a 2-cycle sinusoidal signal, and the expression is: Among them, ω c =2πf c Indicates the center angular frequency, f c is the center frequency of the excitation signal s(t).

3. The method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals according to claim 1, characterized in that: The step S2 is to obtain the dispersion curve of the Lamb wave in the aluminum plate based on the Rayleigh-Lamb equation, and further obtain the group velocity of the Lamb wave; specifically: S21. Based on the Rayleigh-Lamb equation, the relationship between the modal wave number k and the frequency f in the aluminum plate structure is used to obtain the wave number dispersion curve; the Rayleigh-Lamb equation is expressed as: Where 2h is the thickness of the aluminum plate, k is the number of waves transmitted in the horizontal direction, and the auxiliary variables p and q are defined as: Where, ω=2πf, ω is the angular frequency; c L and c T are the velocities of longitudinal and shear waves in the aluminum plate, respectively; By solving equations (3) and (4), we can obtain the relationship between the wave number k of each mode and the frequency f, that is, the wave number dispersion curve; S22. Calculate the group velocity of the Lamb wave in the aluminum plate based on the wave number dispersion curve; use the following relationship: Calculate the group velocity c of the corresponding mode in the aluminum plate g .

4. The method for identifying loose bolts in a multi-bolt connection structure based on Lamb wave array signals according to claim 1, characterized in that: In step S5, the detection threshold V i It is set according to 60% of the preload force index measured when the bolts are tightened properly.

Images