A method and system for detecting damage of a thin plate structure based on a chain piezoelectric array

Abstract

The application provides a kind of thin plate structure damage detection method based on chain piezoelectric array, including arranging a linear uniform multiple-input multiple-output piezoelectric ultrasonic array in the form of chain in the center of thin plate structure, and multiple focus points are preset in advance;Obtain the phase delay and frequency offset of the transmission array focus control conversion;With the abscissa of each focus point as the symmetry axis, determine the transmission array element and the receiving array element activated by each focus point, and when carrying out global damage scanning on the thin plate structure based on the phase delay and frequency offset, obtain the response signal generated by the receiving array element activated by each focus point, and combined with the preset threshold, the effective damage scattering signal is obtained;Characteristic extraction is carried out on the effective damage scattering signal and input into the following imaging algorithm, and the damage positioning imaging result is obtained. By implementing the present application, the problem of traditional ultrasonic Lamb detection sound attenuation is solved, and compared with the ultrasonic phased array technology, the angle distance dependent beam and higher imaging resolution are obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thin plate structure health monitoring, and particularly relates to a thin plate structure damage detection method and system based on a chain piezoelectric array. BACKGROUND

[0002] Thin plate structures have the characteristics of bearing large loads with small mass and less material, and are widely used in the fields of aerospace, petrochemical industry, shipbuilding, etc. However, thin plate structures are prone to damage due to impact during service because of their small thickness, which affects the structural integrity and safety in use, and therefore it is necessary to monitor the health of thin plate structures.

[0003] In recent years, there are numerous detection methods for thin plate structures. Among them, the Lamb wave detection method can achieve the characteristics of large-area propagation with one excitation, and is more efficient and fast, and is more widely used in the detection of large thin plate structures. However, there is a serious problem of acoustic attenuation in large thin plate structures, and as the propagation distance increases, the energy of the Lamb wave gradually attenuates. When detecting some small damage, the energy of the scattered wave is small, which leads to the problem of missed detection of damage.

[0004] To solve the problem of Lamb wave energy attenuation, some scholars have proposed piezoelectric ultrasonic phased array technology for damage detection of thin plate structure. The piezoelectric ultrasonic phased array technology is an active structural health monitoring technology based on Lamb wave. The time delay of each piezoelectric element in the piezoelectric array is controlled by electronic method, the signal wave front with energy much larger than that of single piezoelectric element excitation is formed in the thin plate through signal interference, so that the damage scattering signal is enhanced. At the same time, the direction of the beam can be controlled to realize multi-directional scanning or scanning of a specific area of the structure. The ultrasonic phased array detection has many advantages: flexible beam deflection performance, which can scan each area of the detected object without moving the ultrasonic probe to realize detection of complex workpieces; good focusing performance, the depth and position of the focusing point of the ultrasonic phased array probe are flexible and variable, which can realize focusing of each point of the measured object; high imaging resolution, the phased array focuses and deflects the emission beam, so that the ultrasonic wave irradiates each area of the detected object, and then the echo signal is focused, variable aperture, variable trace, etc. Through signal image reconstruction, clear and uniform high-resolution imaging of the defect position can be obtained. However, the ultrasonic phased array still has some deficiencies in practical application: on the one hand, the ultrasonic phased array beam pointing is constant in distance in each scanning snapshot, that is, the beam pointing is independent of the distance. This means that in some applications such as distance-dependent interference or clutter suppression, it cannot meet the expectation that the array beam can point to different distances at the same angle in the same snapshot. On the other hand, the ultrasonic phased array in the form of uniform linear array (ULA) has a blind area in beam deflection, and the beam performance is good within the range of 0° to 60°, and the beam deflection performance decreases at a large angle of more than 60°.

[0005] To this end, some scholars have proposed two-dimensional ultrasonic phased array technology for damage detection of thin plate structure. For example, Xu and Giurgiutiu et al. proposed a rectangular array, Wilcox et al. proposed a circular array, and Malinowski et al. proposed an improved cross-shaped array form, which realizes beam deflection and focusing in the global range of the structure, effectively solving the problem of angle blind area. However, two-dimensional array means that more piezoelectric elements are needed, which will increase the difficulty of array arrangement before detection, and some nonlinear arrays also have array arrangement error problem, which will affect the damage positioning result.

[0006] Therefore, it is necessary to propose a new thin plate structure damage detection method which can effectively solve the problem of traditional ultrasonic Lamb detection sound attenuation. SUMMARY

[0007] The technical problem to be solved by the embodiments of the present application is to provide a thin plate structure damage detection method and system based on chain piezoelectric array, which can effectively solve the problem of traditional ultrasonic Lamb detection sound attenuation.

[0008] To solve the above technical problems, the embodiment of the present application provides a thin plate structure damage detection method based on a chain piezoelectric array, which comprises the following steps:

[0009] A linear uniform multiple-input multiple-output piezoelectric ultrasonic array in a chain shape is arranged in the center of the thin plate structure, and a plurality of focus points are preset around the piezoelectric ultrasonic array in advance; wherein the piezoelectric ultrasonic array is composed of a transmitting array and a receiving array, and both the transmitting array and the receiving array are composed of elements arranged in a linear and equidistant manner;

[0010] When point focusing is achieved by the method of frequency-phase-frequency offset focusing, the phase delay and frequency offset that should be applied to the excitation signal emitted by the transmitting array when focusing on each focus point are obtained;

[0011] With the abscissa of each focus point as the symmetry axis, the transmitting elements and receiving elements that should be activated corresponding to each focus point are determined, and the excitation signal of the transmitting elements corresponding to each focus point is continuously transformed according to the phase delay and frequency offset that should be applied to the excitation signal emitted by each transmitting array, so as to perform global damage scanning on the thin plate structure, to obtain the response signal generated by the receiving elements activated corresponding to each focus point, and further combined with a preset threshold, to screen effective damage scattering signals from the obtained response signal;

[0012] The effective damage scattering signals screened are subjected to feature extraction, and input into a preset follow-up imaging algorithm, to obtain damage positioning imaging results.

[0013] Among them, the receiving array has seven transmitting elements, and the central element is located at the center of the thin plate structure; the receiving array and the transmitting array are in a symmetrical relationship and are distributed on both sides of the focus point.

[0014] Among them, the phase delay Δ of the excitation signal emitted by the transmitting array when focusing on each focus point is calculated by the formula tn ; wherein Δf n is the frequency offset of the excitation signal emitted by the transmitting array when focusing on each focus point; n is the serial number of the excitation element, and n≤N; c is the propagation speed of the excitation signal emitted by the transmitting array in the thin plate structure, and the excitation signal is expressed as A is the amplitude, m is the number of lamb wave peaks, t is the propagation time, T n is the period of the excitation signal, f n is the excitation frequency of the nth element, and its expression is f n =Δf n+f0;r0 is the distance from each focus point P(r0, θ0) to the center element, and θ0 is the included angle formed between each focus point P(r0, θ0) and the transmitting array.

[0015] The method further comprises:

[0016] When the thin plate structure is globally scanned for damage, if damage is scanned, the acquired response signal is composed of three wave packets; the first wave packet is an unfocused direct wave signal, the second wave packet is a damage scattering signal that is bounced back after focusing on damage, and the third wave packet is a boundary emission signal; or

[0017] If no damage is scanned, the acquired response signal is composed of two wave packets; the first wave packet is an unfocused direct wave signal, and the second wave packet is a boundary emission signal.

[0018] The specific steps of performing feature extraction on the screened effective damage scattering signals and inputting the preset follow-up imaging algorithm to obtain damage positioning imaging results comprise:

[0019] The covariance matrix of each effective damage scattering signal data X(t) is determined as R=E{x(t)x H (t)};

[0020] The covariance matrix of each effective damage scattering signal data X(t) is subjected to eigenvalue decomposition, and there are where D s is a diagonal matrix composed of K largest eigenvalues, D n is a diagonal matrix composed of eigenvalues other than the K largest eigenvalues, E s is a signal subspace corresponding to the K largest eigenvalues, E n is a noise subspace corresponding to small eigenvalues other than the K largest eigenvalues;

[0021] Spectral estimation is performed by using a MUSIC algorithm ; wherein a(r, θ) and b(θ) represent transmitting and receiving steering vectors, respectively.

[0022] A follow-up imaging algorithm is used to obtain damage positioning imaging results corresponding to each effective damage scattering signal data X(t), until all effective damage scattering signal spectrum estimations are completed.

[0023] The embodiment of the application also provides a thin plate structure damage detection system based on a chain piezoelectric array, comprising:

[0024] A piezoelectric ultrasonic array arrangement unit is used to arrange a linear and uniform multiple-input and multiple-output piezoelectric ultrasonic array in a chain shape at the center of a thin plate structure, and a plurality of focus points are preset around the piezoelectric ultrasonic array; wherein the piezoelectric ultrasonic array is composed of a transmitting array and a receiving array, and both the transmitting array and the receiving array are composed of array elements arranged in a linear and equidistant manner;

[0025] A frequency-phase offset fixed-point focusing unit is used to obtain phase delay and frequency offset to be applied to the excitation signal transmitted by the transmitting array when focusing on each focus point when fixed-point focusing is achieved by a frequency-phase frequency offset focusing method;

[0026] A symmetric receiving and point-by-point scanning and screening unit is used to determine the transmitting array elements and receiving array elements to be activated corresponding to each focus point with the horizontal coordinate of each focus point as a symmetric axis, and to continuously transform the focus point position and the excitation signal of the transmitting array elements to be activated corresponding to the focus point to perform global damage scanning on the thin plate structure to obtain response signals generated by the receiving array elements to be activated corresponding to each focus point, and further combined with a preset threshold to screen effective damage scattering signals from the obtained response signals;

[0027] A damage positioning and imaging unit is used to perform feature extraction on the screened effective damage scattering signals, and input into a preset follow-up imaging algorithm to obtain damage positioning and imaging results.

[0028] The receiving array has seven transmitting array elements, and the central array element is located at the center of the thin plate structure; the receiving array and the transmitting array are in a symmetric relationship and are distributed on both sides of the focus point.

[0029] The phase delay Δt to be applied to the excitation signal transmitted by the transmitting array when focusing on each focus point is calculated by the formula n ; wherein Δf n is the frequency offset to be applied to the excitation signal transmitted by the transmitting array when focusing on each focus point; n is the serial number of the excitation array element, and n≤N; c is the propagation speed of the excitation signal transmitted by the transmitting array in the thin plate structure, and the excitation signal is represented as A is the amplitude, m is the number of lamb wave peaks, t is the propagation time, T n is the excitation signal period, f n is the excitation frequency of the nth array element, and its expression is f n = Δf n + f0; r0 is the distance from each focus point P(r0, θ0) to the central array element, and θ0 is the included angle between each focus point P(r0, θ0) and the transmitting array.

[0030] The embodiment of the present application has the following beneficial effects:

[0031] The present application can effectively solve the problem of acoustic attenuation in traditional ultrasonic Lamb detection, and has angle-distance dependent beam and higher imaging resolution compared with the ultrasonic phased array technology. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings obtained according to these drawings without creative labor are still within the scope of the present application.

[0033] Figure 1 A flowchart of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application;

[0034] Figure 2 A scene diagram of the piezoelectric ultrasonic array arranged on the thin plate structure in step S1 of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application;

[0035] Figure 3 A scene diagram of the frequency-phase offset focusing array in step S2 of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application;

[0036] Figure 4 A scene diagram of the symmetric receiving of the activated transmitting array element and receiving array element corresponding to a certain focusing point in step S3 of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application;

[0037] Figure 5 A scene diagram of the global damage scanning of the thin plate structure by the focusing wave in step S4 of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application;

[0038] Figure 6 A scene diagram of the global damage scanning of the thin plate structure by the focusing wave in step S4 of a thin plate structure damage detection method based on a chain piezoelectric array provided by the embodiment of the present application; Figure 5Comparison of the received array element response signals when using focused waves to perform global damage scanning on thin plate structures; where 6(a) is the waveform of the received array element response signal when the focused wave scan detects damage; 6(b) is the waveform of the received array element response signal when the focused wave does not detect damage.

[0039] Figure 7 A schematic diagram of an aluminum plate structure model and array arrangement in an application scenario of a thin plate structure damage detection method based on a chain piezoelectric array provided in an embodiment of the present invention.

[0040] Figure 8 A schematic diagram of frequency-controlled focused wave damage scattering signal within a damaged aluminum plate model in an application scenario of a thin plate structure damage detection method based on a chain piezoelectric array provided in an embodiment of the present invention.

[0041] Figure 9 This invention provides a schematic diagram of the propagation of frequency-controlled focused waves at various moments within a damaged aluminum plate model, and a damage localization result diagram of the damaged aluminum plate model, in an application scenario of a thin plate structure damage detection method based on a chain piezoelectric array.

[0042] Figure 10 This is a schematic diagram of a thin-plate structure damage detection system based on a chain piezoelectric array, provided in an embodiment of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0044] like Figure 1 As shown in the figure, a method for detecting damage to a thin plate structure based on a chain piezoelectric array is provided in an embodiment of the present invention. The method includes the following steps:

[0045] Step S1: Arrange a linear, uniform, multi-input multi-output piezoelectric ultrasonic array in a chain at the center of the thin plate structure, and pre-set multiple focal points around the piezoelectric ultrasonic array; wherein, the piezoelectric ultrasonic array consists of a transmitting array and a receiving array, and both the receiving array and the transmitting array are composed of array elements arranged linearly and equally spaced.

[0046] The specific process is as follows: Figure 2As shown, a chain-like multi-input multi-output piezoelectric ultrasonic array is arranged in a thin plate structure. The array consists of a transmitting array forming the transmitting end and a receiving array forming the receiving end. Both the transmitting and receiving ends are linear arrays with equal spacing, and the array spacing is d. The number of array elements depends on the size of the plate. The central array element is located at the exact center of the thin plate. The transmitting array consists of seven array elements extending to the left and right from the central array element (e.g., the middle circle shows the transmitting array element). The remaining array elements are the receiving array elements (e.g., the circles on both sides show the receiving array elements). That is, the receiving arrays are symmetrically distributed on both sides of the focal point. The receiving array elements are denoted as R1, R2...R... m …R M . Figure 2 In the diagram, the pentagrams represent the preset focal point positions, denoted as P1, P2…P… k …P K The transmitting array achieves global scanning by focusing on each focal point point by changing different frequency offsets and phase delays.

[0047] Step S2: When achieving fixed-point focusing by frequency phase and frequency offset focusing, obtain the phase delay and frequency offset that should be applied to the excitation signal emitted when the transmitting array focuses on each focal point respectively;

[0048] The specific process is as follows: the first element of the frequency-phase offset focusing array is the reference element, with an element frequency of f0. Each element has a different frequency, and a frequency offset much smaller than its carrier frequency is added relative to the reference element. The excitation frequency of the nth element can be expressed as: f n =Δf n +f0=(n-1)·Δf+f0,n=0,1,...,N-1.

[0049] During excitation, multiple transmitting elements transmit excitation signals within the same excitation cycle by controlling the excitation phase and carrier frequency. Each excitation signal is... A is the amplitude, m is the number of lambda crests (here m = 5), t is the propagation time, T n It is the excitation signal period.

[0050] Due to the applied frequency offset, the periods of the excitation signals for each array element are different, namely T0, T1, ..., T... N-1 Given the frequency offset Δf n Then, if focusing is performed on a focal point P(r0,θ0) in space, the formula is used... The phase delay that should be applied to the excitation signal emitted when the transmitting array is focused on the focal point is calculated. Δt nwherein n is the serial number of the excitation array element, and n≤N, here N=7; c is the propagation speed of the excitation signal emitted by the transmitting array in the thin plate structure, r0 is the distance from each focus point P(r0, θ0) to the center element, and θ0 is the angle formed between each focus point P(r0, θ0) and the transmitting array.

[0051] Step S3: Taking the horizontal coordinate of each focus point as the symmetry axis, the transmitting array element and the receiving array element corresponding to each focus point are determined, and the focus point position and the excitation signal of the transmitting array element corresponding to each focus point are continuously transformed according to the phase delay and frequency offset that should be applied to the excitation signal emitted by each transmitting array, so as to perform global damage scanning on the thin plate structure, to obtain the response signal generated by the receiving array element corresponding to each focus point, and further combined with the preset threshold, the effective damage scattering signal is screened from the obtained response signal.

[0052] The specific process is that, first, in the focusing scanning process, not all array elements need to be activated in the "chain", the number of receiving array elements and the number of excitation array elements are consistent, and the element spacing is the same. While focusing on a certain focus point, the elements in symmetrical relationship with the transmitting array elements are activated, which are recorded as the receiving array, as shown in Figure 4 .

[0053] Secondly, based on the phase delay and frequency offset that should be applied to the excitation signal emitted by the transmitting array in step S2, the focusing scheme of the transmitting array element corresponding to each focus point is determined, and the excitation signal of the transmitting array element corresponding to each focus point is adjusted to perform global damage scanning on the thin plate structure, as shown in Figure 5 .

[0054] When the focusing wave scans the damage, the focusing wave rebounds at the damage, forms a damage scattering echo opposite to the original propagation direction, and is received by the receiving end element. At this time, the response signal of the receiving element is as shown in Figure 6 (a), the response signal is composed of three parts, the first wave packet is the unfocused direct wave signal, the second wave packet is the damage scattering signal after focusing and rebounding, and the third wave packet is the boundary reflection signal.

[0055] When the focusing wave does not scan the damage, the focusing wave rebounds at the damage, forms a damage scattering echo opposite to the original propagation direction, and is received by the receiving end element. At this time, the response signal of the receiving element is as shown in Figure 6 (b), the response signal is composed of two parts, the first wave packet is the unfocused direct wave signal, and the second wave packet is the boundary reflection signal. There is no damage scattering signal or very small damage scattering signal at the position of the original second wave packet.

[0056] Finally, a threshold TH is set, the damage scattering signal interval can be obtained according to the known relationship between the focusing position and the receiving array position, and the signal in the interval is extracted as X(t) by windowing. The average amplitude of the Hilbert envelope of the signal is calculated and recorded as S. If S < TH, it indicates that the focused wave does not scan the damage position, the excitation array focuses on the next focusing point, and the above operation is repeated. If S >= TH, it indicates that the focused wave scans the damage position, and the signal X(t) is stored, so that the effective damage scattering signal is screened out for the next imaging processing.

[0057] Step S4, the effective damage scattering signal screened out is subjected to feature extraction, and is input into a preset follow-up imaging algorithm to obtain damage positioning imaging results.

[0058] The specific process is as follows: first, the covariance matrix of each effective damage scattering signal data X(t) is determined and expressed as R = E{x(t)x H (t)}.

[0059] Secondly, the eigenvalue decomposition is performed on the covariance matrix of each effective damage scattering signal data X(t), and there are where D s is a diagonal matrix composed of K largest eigenvalues, D n is a diagonal matrix composed of eigenvalues other than the K largest eigenvalues, E s is a signal subspace corresponding to the K largest eigenvalues, and E n is a noise subspace corresponding to the small eigenvalues other than the K largest eigenvalues.

[0060] Then, the MUSIC algorithm is used for spectrum estimation; wherein a(r, theta) and b(theta) represent the transmitting direction vector and the receiving direction vector respectively.

[0061] Finally, the follow-up imaging algorithm is used to obtain the damage positioning imaging results corresponding to each effective damage scattering signal data X(t), until the spectrum estimation of all damage scattering signals is completed.

[0062] As shown in Figures 7 to 9 , the application scenario of the thin plate structure damage detection method based on the chain piezoelectric array in the embodiment of the application is further described, and the specific process is as follows:

[0063] Taking an aluminum plate structure as an example, the size is 1000mm*1000mm*2mm, and the model is established in the ABAQUS software, and the material parameters are shown in Table 1:

[0064] Table 1

[0065]

[0066] The piezoelectric receiving array is arranged at the center of the model, and the damage position is set as (105°, 300 mm).

[0067] (1) Chain array arrangement: as shown in Figure 7 , a Cartesian coordinate system is established at the center of the aluminum plate, and 99 piezoelectric elements are arranged together, with a spacing of 10 mm between each element. There are 92 receiving elements, denoted as R 92 1,…,R 32 7, and the transmitting elements are denoted as T 33 1,…,T 34 7.

[0068] (2) Frequency-phase offset focusing: the reference element excitation excitation signal is set as a Lamb wave of 50 kHz, the sampling frequency is set as 10 MHz, the sampling time is set as 0.8 ms, the frequency offset is 1 kHz, and the frequencies of each element are 50 kHz, 51 kHz, 52 kHz, 53 kHz, 54 kHz, 55 kHz, and 56 kHz. The time delay of each element is 0 us, -2.6 us, -5.1 us, -7.5 us, -9.7 us, and -13.7 us, at which time the wave front is focused on the damage position.

[0069] (3) Symmetric receiving: the center coordinates of the activated receiving array are (-160, 0), and the coordinates of each element are (-190, 0), (-180, 0), (-170, 0), (-160, 0), (-150, 0), (-140, 0), (-130, 0), as shown in Figure 8 , at which time the activated receiving element numbers are R 32 1,…,R 33 7. 34 35 36 37 38

[0070] (4) Array signal processing: the signal is windowed and processed, and the damage scattering signal between 0.35 ms and 0.55 ms is extracted (as shown in Figure 9 ), the Hilbert transform is used to calculate the average amplitude S of the scattering signal, and the amplitude S is compared with the threshold TH to obtain S>TH, then the signal is an effective signal, denoted as X(t).

[0071] (5) Following imaging: the covariance matrix of X(t) is calculated, and the matrix is decomposed to determine the signal subspace and noise subspace, and the spectral peak search is performed to obtain the damage imaging result. As shown in Figure 9 , the marked position in the figure is the damage imaging result (302 mm, 105°), which is relatively close to the actual damage.

[0072] Figure 10As shown, in the embodiment of the present application, a thin plate structure damage detection system based on chain piezoelectric array is provided, which comprises:

[0073] The piezoelectric ultrasonic array arrangement unit 110 is used for arranging a linear uniform multiple-input multiple-output piezoelectric ultrasonic array in the form of a chain at the center of the thin plate structure, and a plurality of focus points are preset around the piezoelectric ultrasonic array; wherein the piezoelectric ultrasonic array is composed of a transmitting array and a receiving array, and both the transmitting array and the receiving array are composed of elements arranged in a linear and equidistant manner;

[0074] The frequency phase offset fixed-point focusing unit 120 is used for obtaining the phase delay and frequency offset that should be applied to the excitation signal emitted by the transmitting array when focusing on each focus point when fixed-point focusing is realized by the method of frequency phase frequency offset focusing;

[0075] The symmetrical receiving and point-by-point scanning and screening unit 130 is used for determining the transmitting elements and receiving elements that should be activated corresponding to each focus point with the horizontal coordinate of each focus point as the symmetrical axis, and constantly transforming the focus point position and the excitation signal of the transmitting elements activated corresponding to the focus point to perform global damage scanning on the thin plate structure according to the phase delay and frequency offset that should be applied to the excitation signal emitted by each transmitting array, so as to obtain the response signal generated by the receiving elements activated corresponding to each focus point, and further combining the preset threshold value to screen the effective damage scattering signal from the obtained response signal;

[0076] The damage positioning imaging unit 140 is used for performing feature extraction on the screened effective damage scattering signal, and inputting into the preset follow-up imaging algorithm to obtain the damage positioning imaging result.

[0077] Among them, the receiving array has seven transmitting elements, and the central element is located at the center of the thin plate structure; the receiving array and the transmitting array are in a symmetrical relationship and are distributed on both sides of the focus point.

[0078] Among them, the phase delay Δ that should be applied to the excitation signal emitted by the transmitting array when focusing on each focus point is calculated by the formula tn ; wherein Δf n is the frequency offset that should be applied to the excitation signal emitted by the transmitting array when focusing on each focus point; n is the serial number of the excitation element, and n≤N; c is the propagation speed of the excitation signal emitted by the transmitting array in the thin plate structure, and the excitation signal is represented as A is the amplitude, m is the number of lamb wave peaks, t is the propagation time, T n is the period of the excitation signal, f n is the excitation frequency of the nth element, and its expression is f n= Δf n + f0; r0 is the distance from each focus point P(r0, θ0) to the center element, and θ0 is the angle formed between each focus point P(r0, θ0) and the transmitting array.

[0079] The embodiments of the present application have the following beneficial effects:

[0080] On one hand, the present application realizes ultrasonic guided wave focusing by frequency offset and phase delay among each element in the transmitting array, and obtains larger signal gain and stronger damage scattering signal. On the other hand, the present application realizes large thin plate structure damage positioning imaging by arranging "chain" array, point-by-point scanning and threshold judgment method, and combining damage positioning algorithm, and reduces damage missed detection and false detection, thereby solving the problem that the existing two-dimensional ultrasonic phased array technology affects damage positioning result due to array arrangement error.

[0081] It is worth noting that the units included in the above system embodiments are only divided according to functional logic, but are not limited to the above division, as long as the corresponding functions can be realized. In addition, the specific names of the functional units are only for easy mutual distinction, and are not used to limit the protection scope of the present application.

[0082] Those skilled in the art can understand that all or part of the steps of the above-mentioned embodiment methods can be completed by programs instructing related hardware, and the programs can be stored in a computer readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc.

[0083] The above only describes the preferred embodiments of the present application, and of course cannot limit the protection scope of the present application, so equivalent changes made according to the claims of the present application are still within the scope of the present application.

Claims

1. A method for detecting damage in thin-plate structures based on a chain piezoelectric array, characterized in that, The method includes the following steps: A linear, uniform, multi-input, multi-output piezoelectric ultrasonic array is arranged in a chain at the center of the thin plate structure, and multiple focal points are pre-set around the piezoelectric ultrasonic array; wherein the piezoelectric ultrasonic array consists of a transmitting array and a receiving array, and both the receiving array and the transmitting array are composed of array elements arranged linearly and equally spaced. When achieving fixed-point focusing by frequency phase and frequency offset focusing, the phase delay and frequency offset to be applied to the excitation signal emitted when the transmitting array focuses on each focal point are obtained. Using the horizontal coordinate of each focal point as the axis of symmetry, the transmitting and receiving array elements that need to be activated for each focal point are determined. Based on the phase delay and frequency offset that should be applied to the excitation signal emitted by each transmitting array, the position of the focal point and the excitation signal of the corresponding activated transmitting array element are continuously changed to perform a global damage scan on the thin plate structure, so as to obtain the response signal generated by the receiving array element activated for each focal point. Furthermore, combined with a preset threshold, effective damage scattering signals are selected from the obtained response signals. Feature extraction is performed on the selected effective damage scattering signals, and the results are input into a preset follow-up imaging algorithm to obtain damage localization imaging results.

2. The method for detecting damage to thin-plate structures based on a chain piezoelectric array as described in claim 1, characterized in that, The receiving array has seven transmitting elements, with the central element located at the exact center of the thin plate structure; the receiving array and the transmitting array are symmetrically distributed on both sides of the focal point.

3. The method for detecting damage to thin-plate structures based on a chain piezoelectric array as described in claim 1, characterized in that, Through formula The phase delay Δt that should be applied to the excitation signal emitted when the transmitting array is focused at each focal point is calculated. n ;Δf n is the frequency offset that should be applied to the excitation signal emitted when the transmitting array focuses on each focal point; n is the index of the excitation array element, and n≤N; c is the propagation speed of the excitation signal emitted by the transmitting array in the thin plate structure, and the excitation signal is expressed as A is the amplitude, m is the number of lambda crests, t is the propagation time, and T is the propagation time. n It is the period of the excitation signal, f n The excitation frequency of the nth array element is expressed as f. n =Δf n +f0; r0 is the distance from each focal point P(r0,θ0) to the central array element, and θ0 is the angle formed between each focal point P(r0,θ0) and the transmission array.

4. The method for detecting damage to thin-plate structures based on a chain piezoelectric array as described in claim 1, characterized in that, The method further includes: When performing a global damage scan on the thin plate structure, if damage is detected, the acquired response signal consists of three wave packets; wherein, the first wave packet is an unfocused direct wave signal, the second wave packet is a damage scattering signal that bounces back after being focused upon encountering damage, and the third wave packet is a boundary emission signal. If no damage is detected, the acquired response signal consists of two wave packets; the first wave packet is an unfocused direct wave signal, and the second wave packet is a boundary emission signal.

5. The method for detecting damage to thin-plate structures based on a chain piezoelectric array as described in claim 1, characterized in that, The specific steps for extracting features from the selected effective damage scattering signals and inputting them into a preset follow-up imaging algorithm to obtain damage localization imaging results include: The covariance matrix of each effective damage scattering signal data X(t) is represented as R = E{x(t)x H (t)}; For each valid damage scattering signal data X(t), the covariance matrix is ​​decomposed into eigenvalues. Among them, D s It is a diagonal matrix consisting of K largest eigenvalues, D n It is a diagonal matrix consisting of eigenvalues ​​excluding the K largest eigenvalues, E s E is the signal subspace corresponding to the K largest eigenvalues. n It is the noise subspace corresponding to the small eigenvalues ​​other than the K largest eigenvalues; Using the MUSIC algorithm Perform spectral estimation; where a(r,θ) and b(θ) represent the transmit steering vector and the receive steering vector, respectively; The follow-up imaging algorithm is used to obtain the damage localization imaging result corresponding to each effective damage scattering signal data X(t) until the spectrum estimation of all effective damage scattering signals is completed.

6. A damage detection system for thin-plate structures based on a chain piezoelectric array, characterized in that, include: A piezoelectric ultrasonic array arrangement unit is used to arrange a linear, uniform, multi-input multi-output piezoelectric ultrasonic array in a chain at the center of a thin plate structure, and multiple focal points are pre-set around the piezoelectric ultrasonic array; wherein, the piezoelectric ultrasonic array consists of a transmitting array and a receiving array, and both the receiving array and the transmitting array are composed of array elements arranged linearly and equally spaced. The frequency phase offset fixed-point focusing unit is used to obtain the phase delay and frequency offset amount that should be applied to the excitation signal emitted when the transmitting array focuses on each focal point when fixed-point focusing is achieved by the frequency phase offset focusing method. The symmetrical receiving and point-by-point scanning and filtering unit is used to determine the transmitting and receiving array elements that need to be activated for each focal point, with the horizontal coordinate of each focal point as the axis of symmetry. Based on the phase delay and frequency offset to be applied to the excitation signal emitted by each transmitting array, the unit continuously changes the position of the focal point and the excitation signal of the corresponding activated transmitting array element to perform a global damage scan on the thin plate structure, so as to obtain the response signal generated by the receiving array element activated for each focal point. Furthermore, it combines a preset threshold to filter out the effective damage scattering signal from the obtained response signal. The damage localization imaging unit is used to extract features from the selected effective damage scattering signals and input them into a preset follow-up imaging algorithm to obtain damage localization imaging results.

7. The thin-plate structure damage detection system based on a chain piezoelectric array as described in claim 6, characterized in that, The receiving array has seven transmitting elements, with its central element located at the exact center of the thin plate structure; the receiving array is symmetrical to the receiving array and distributed on both sides of the receiving array.

8. The thin-plate structure damage detection system based on a chain piezoelectric array as described in claim 7, characterized in that, Through formula The phase delay Δt that should be applied to the excitation signal emitted when the transmitting array is focused at each focal point is calculated. n ;wherein, Δf n is the frequency offset that should be applied to the excitation signal emitted when the transmitting array focuses on each focal point; n is the index of the excitation array element, and n≤N; c is the propagation speed of the excitation signal emitted by the transmitting array in the thin plate structure, and the excitation signal is expressed as A is the amplitude, m is the number of lambda crests, t is the propagation time, and T is the propagation time. n It is the period of the excitation signal, f n The excitation frequency of the nth array element is expressed as f. n =Δf n +f0; r0 is the distance from each focal point P(r0,θ0) to the central array element, and θ0 is the angle formed between each focal point P(r0,θ0) and the transmission array.

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