A solid plate defect detection method based on Lamb wave end face echo

Abstract

The present application relates to the technical field of solid plate defect detection, in particular to a solid plate defect detection method based on Lamb wave end face echo, which comprises the following steps: arranging excitation and receiving wedge piezoelectric transducers on the same side of the solid plate, exciting and receiving single mode Lamb wave signals, obtaining multi-path signals containing direct wave, side echo and defect reflection echo, enhancing the identification ability of boundary and local defects, extracting defect reflection signals by analyzing the difference between direct wave and side echo, combining the time difference between the defect reflection signals and the side echo and the Lamb wave propagation speed, establishing a geometric model and accurately calculating the defect position. By adjusting the transducer position to make the time difference between the defect reflection wave and the direct wave constant, the defect echo amplitude is obtained under the condition of equal propagation, compared with the standard test block amplitude, the mapping relationship between amplitude and size is constructed, and the accurate quantification of defect size is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid plate defect detection, and in particular to a solid plate defect detection method based on Lamb wave end face echo. BACKGROUND

[0002] In the fields of aerospace, rail transportation, energy equipment and high-end manufacturing, solid plate structures as key load-bearing components, their integrity is directly related to the safety and reliability of the overall structure. However, in the actual service process, due to load fatigue, environmental corrosion or manufacturing defects, micro-cracks, holes or local debonding may occur in the internal or boundary area of the solid plate. If not detected and evaluated in time, it may cause catastrophic failure. Therefore, it is of great engineering significance to develop high-sensitivity and high-precision plate defect detection technology.

[0003] Currently, in the non-destructive testing of plate structures, Lamb waves have been widely studied and applied due to their long propagation distance, sensitivity to defects, and ability to achieve multi-modal excitation. The propagation behavior of Lamb waves in the plate will be disturbed by defects, forming identifiable reflection, scattering and mode conversion phenomena, thereby providing a basis for defect detection, positioning and quantification.

[0004] The existing defect detection method based on Lamb wave mainly relies on the overall change of signal amplitude or comparison threshold for defect recognition, which is less sensitive to defects in boundary areas or complex spatial locations, and it is difficult to effectively separate various echo components. Especially in the presence of multiple reflections and interference waves, it is easy to miss or misjudge. In addition, in terms of defect size quantification, most methods lack standardized mapping mechanisms and can only estimate the actual size by relative amplitude change, which cannot accurately invert the actual size. SUMMARY

[0005] To solve the above problems, the present application provides a solid plate defect detection method based on Lamb wave end face echo.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is:

[0007] A solid plate defect detection method based on Lamb wave end face echo, comprising:

[0008] S1, on one side of the solid plate, a single mode of Lamb wave is excited by a fixed angle wedge piezoelectric transducer, and another same angle wedge piezoelectric transducer arranged on the same side as the wedge piezoelectric transducer is used to receive the Lamb wave signal, to obtain the direct wave signal and the side echo signal;

[0009] S2, extracting defect reflection echo signals caused by defects from the received signals between the direct wave signals and the side echo signals, to obtain time information and amplitude information of the defect reflection echoes;

[0010] S3, calculating the defect position based on the time difference between the defect reflection echo time information and the side echo time information, and combining the pre-calibrated Lamb wave propagation speed;

[0011] S4, by adjusting the transducer position to keep the time difference between the defect reflection echo and the direct wave as a fixed value, synchronously acquiring the defect reflection echo amplitude, and comparing with the reference echo amplitude of the pre-set size defect comparison block to obtain the quantitative size information of the defect.

[0012] Further, the direct wave signal is obtained by the following steps:

[0013] Arranging excitation and receiving wedge piezoelectric transducers in the defect-free area of the solid plate, triggering the excitation transducer to emit single mode Lamb wave signals based on the set excitation frequency and excitation angle;

[0014] Receiving the Lamb wave signals directly propagating between the excitation point and the receiving point along the plate surface by the receiving transducer, and extracting the first arriving high amplitude pulse signal in the waveform from the Lamb wave receiving signal as the direct wave signal;

[0015] Time window constraint and amplitude filtering processing are performed on the direct wave signal to determine the direct wave time domain characteristics.

[0016] Further, the high amplitude pulse signal is obtained by the following steps:

[0017] Setting a time window in the Lamb wave receiving signal, scanning all data points in the time window, calculating the amplitude absolute value, and identifying the high amplitude pulse signal with local maximum amplitude.

[0018] Further, characterized in that the side echo signal is obtained by the following steps:

[0019] In the Lamb wave receiving signal, based on the identified direct wave signal, searching backward for the second level high amplitude pulse signal in the time window, and combining the expected propagation time interval corresponding to the distance from the plate end face to extract the reflection signal as the side echo signal;

[0020] Time window constraint and amplitude filtering processing are performed on the side echo signal to determine the side echo time information.

[0021] Further, the S2 includes the following steps:

[0022] Based on the direct wave time information and the side echo time information, a target time interval between the two is determined, and a Lamb wave receiving signal is intercepted in the time interval to obtain a candidate echo signal segment;

[0023] Based on the candidate echo signal segment, an amplitude peak extraction process is performed to identify local extreme points higher than a background noise threshold to obtain a plurality of suspected defect reflection echo points;

[0024] Based on the time position and amplitude variation characteristics of the suspected defect reflection echo points, envelope demodulation and time window filtering processing are performed to obtain preliminary characteristics of the defect reflection echo;

[0025] Based on the preliminary characteristics of the defect reflection echo, joint time-frequency domain analysis is performed to extract the effective time domain center value and amplitude envelope peak value of the echo signal as the final time information and amplitude information of the defect reflection echo.

[0026] Further, the joint time-frequency domain analysis includes:

[0027] Based on the preliminary characteristics of the defect reflection echo, a short-time Fourier transform process is performed to obtain the frequency energy distribution of the signal in the time domain to obtain a time-frequency spectrum matrix;

[0028] Based on the time-frequency spectrum matrix, the time center value and corresponding frequency component of the energy concentrated region are extracted to obtain a preliminary time-frequency feature vector;

[0029] The preliminary characteristics of the defect reflection echo are further subjected to wavelet packet decomposition processing to obtain the energy distribution of each frequency band at several scales to obtain a sub-band energy vector;

[0030] The time-frequency feature vector and the sub-band energy vector are spliced in a time alignment manner to generate a joint time-frequency feature vector;

[0031] Based on the concentration degree of the time component in the joint time-frequency feature vector, the final time information of the defect reflection echo is extracted; based on the frequency component with the maximum energy amplitude in the joint feature vector, the final amplitude information of the defect reflection echo is extracted.

[0032] Further, the S3 includes the following steps:

[0033] Based on the time information of the defect reflection echo and the time information of the side echo, the time difference between the defect echo and the side echo is calculated;

[0034] Based on the propagation speed of the Lamb wave in the solid plate pre-calibrated by the direct wave and the side echo in the defect-free area, combined with the time difference, the equivalent propagation distance from the defect to the transducer is calculated according to the propagation path relationship;

[0035] Based on the current position of the transducer on the surface of the solid plate, the known spatial relationship between the transducer and the plate boundary, and the equivalent propagation distance, a geometric solution model of the defect position is established, and the two-dimensional position coordinates of the defect in the solid plate coordinate system are solved.

[0036] Further, the step of keeping the time difference between the defect reflection echo and the direct wave as a fixed value by adjusting the position of the transducer comprises:

[0037] According to the defect reflection echo time information and the direct wave time information, an initial time difference value between the two is calculated as a fixed reference time difference;

[0038] Keeping the position of the excitation transducer unchanged, the receiving transducer is translated along the surface of the solid plate, the Lamb wave receiving signals at different positions are collected, and the defect reflection echo time and the direct wave time corresponding to each position are extracted respectively;

[0039] Based on the defect reflection echo time and the direct wave time corresponding to each position, the real-time time difference between the current defect reflection echo and the direct wave is calculated, and compared with the reference time difference, until the time difference between the defect reflection echo and the direct wave is equal to the reference time difference, and the spatial position of the receiving transducer is recorded synchronously.

[0040] Further, the quantitative size information of the defect is obtained by the following steps:

[0041] Based on the spatial position of the transducer, the Lamb wave receiving signal at this position is collected, and the defect reflection echo signal is extracted in the window corresponding to the fixed time difference;

[0042] The extracted defect reflection echo signal is subjected to amplitude envelope analysis, and the peak amplitude of the echo signal is obtained as the reflection intensity characteristic value of the current defect;

[0043] Under the conditions of equal distance propagation path and same excitation frequency, the echo signals of each known size reflector are collected, and the corresponding envelope peak amplitudes are extracted respectively, and a corresponding relationship model between reflection amplitude and defect size is constructed;

[0044] The amplitude characteristic value of the defect reflection echo is input into the corresponding relationship model, and the corresponding defect size is matched as the quantitative size information of the detected defect.

[0045] Further, the step of constructing the corresponding relationship model between reflection amplitude and defect size comprises the following steps:

[0046] A plurality of artificial reflectors of different sizes are preset on the standard comparison test block, respectively corresponding to different reflection target areas;

[0047] For each artificial reflector position, arrange a transducer excitation and reception configuration consistent with the actual detection path, excite the Lamb wave and collect the corresponding received signal;

[0048] Extract the reflection echo signal corresponding to each artificial reflector, calculate the amplitude envelope peak value, and establish a data pair containing the defect size and the corresponding echo amplitude;

[0049] Perform interpolation fitting processing on the data pair to construct a continuous function between the reflection echo amplitude and the defect size as the corresponding relationship model.

[0050] The beneficial effects of the present application are that the present application arranges excitation and reception wedge piezoelectric transducers on the same side of the solid plate, excites and receives single mode Lamb wave signals propagating in the plate, obtains multiple propagation path signals containing direct wave, side echo and defect reflection echo, significantly improves the recognition ability of edge area and local defects, and overcomes the limitation that the existing opposite arrangement method is easy to miss detection in the boundary area. Through the echo sequence difference between the direct wave and the side echo, the reflection signal caused by the defect is accurately extracted, and combined with the time difference between the defect echo and the side echo and the pre-calibrated Lamb wave propagation speed, a high-precision geometric model is constructed to realize quantitative calculation of the spatial position of the defect, effectively improve the positioning accuracy, and solve the problem of difficult separation and determination of the position of the echo in the multiple interference environment. Further, by adjusting the transducer position to keep the time difference between the defect reflection wave and the direct wave constant, the defect echo amplitude is ensured to be obtained under the condition of equal propagation, and the amplitude is mapped and compared with the reference amplitude of the standard test block to establish the function relationship between the amplitude and the size, realize the quantitative inversion of the actual size of the defect, and overcome the uncertainty problem of estimating the size only by the amplitude change in the traditional method. BRIEF DESCRIPTION OF DRAWINGS

[0051] Fig. 1 is a step flow chart of a solid plate defect detection method based on Lamb wave end face echo in the present application.

[0052] Fig. 2 is a step flow chart of the quantitative size information acquisition of the defect in the present application. DETAILED DESCRIPTION

[0053] Please refer to Figs. 1-2 The present application relates to a solid plate defect detection method based on Lamb wave end face echo, as shown in the figure, which comprises:

[0054] S1, on one side of the solid plate, excite single mode Lamb wave through fixed angle wedge piezoelectric transducer, and receive Lamb wave signal through another same angle wedge piezoelectric transducer arranged on the same side of the wedge piezoelectric transducer, get direct wave signal and side echo signal;

[0055] S2, extracting defect reflection echo signals caused by defects from the received signals between the direct wave signals and the side echo signals to obtain time information and amplitude information of the defect reflection echoes;

[0056] S3, calculating the defect position based on the time difference between the defect reflection echo time information and the side echo time information and in combination with the pre-calibrated Lamb wave propagation speed;

[0057] S4, synchronously acquiring the defect reflection echo amplitude by keeping the time difference between the defect reflection echo and the direct wave as a fixed value, and comparing the defect reflection echo amplitude with the reference echo amplitude of the pre-set size defect comparison block to obtain the quantitative size information of the defect.

[0058] It should be noted that the Lamb wave in the solid plate is an elastic wave propagating in the plate structure, which is formed by multiple reflections between the two free surfaces of the plate, has the characteristics of small energy attenuation and long propagation distance, and when the Lamb wave encounters defects or interfaces in the material, partial energy scattering and attenuation occur. By detecting the changes in the propagation speed and amplitude of the wave, the defect position, size and type in the material can be inferred. Modal conversion: Lamb waves have different propagation properties at different frequencies and modes, and different responses to defects in the material. By analyzing the waveforms and propagation characteristics under different modes, defect information in the material can be identified. Therefore, it has been widely used in the field of nondestructive testing. In some embodiments, the excitation transducer and the receiving transducer are arranged on the same side of the solid plate, and the two are coupled to the plate surface through a wedge structure, forming a fixed excitation angle. The excitation transducer excites a single mode Lamb wave signal, which propagates in the plate and is received by the receiving transducer, which includes direct waves, side echoes reflected by the plate end surface, and reflected signals that may be generated by defects in the plate. Through this same side arrangement, the echo signals corresponding to different propagation paths can be clearly separated in one receiving signal channel, significantly improving the detection capability of the edge region and local defects in the plate, and overcoming the problem of missing detection of the edge region caused by the existing opposite arrangement. The received signal is analyzed in the time domain, and the time positions of the direct wave and the side echo are extracted first, and then the intermediate echo signal reflected by the defect is identified in the time window between the two. The signal has characteristics such as amplitude mutation and energy concentration, and is confirmed as a defect echo by comparison with the background waveform. Further, by measuring the time difference between the defect echo and the side echo, and combining the propagation speed of the Lamb wave in the material, an equivalent propagation path model is established to deduce the spatial position of the defect in the plate coordinate system, realizing accurate positioning of the defect. On the basis of completing the defect positioning, in order to realize the quantitative evaluation of the defect size, the position of the receiving transducer is translated to adjust the time difference between the defect echo and the direct wave to a constant value, thereby ensuring that the defect reflection signal is collected under the same propagation path condition. The amplitude characteristics of the defect echo are extracted under the constant time difference condition, and compared with the echo amplitude corresponding to the known size reflector in the pre-set standard test block, and through the function mapping relationship between the amplitude and the size, the defect size is inverted and quantified. Compared with the prior art, the present embodiment has obvious innovations.First, the same side arrangement effectively improves the detectability of edge defects and enhances the distinguishability of multi-path propagation signals; second, by introducing side echoes as a stable reference point, a defect positioning method based on time difference is established, which improves the stability and accuracy of positioning; finally, through the amplitude acquisition under constant propagation conditions and the standard amplitude comparison mechanism, the standardization of defect size is realized, which overcomes the defects of traditional methods that are strongly dependent on the absolute value of amplitude and inaccurate size estimation, and embodies the comprehensive technical advantages of the application in defect detection integrity, spatial positioning ability and size quantification accuracy.

[0059] Further, the direct wave signal is obtained by the following steps:

[0060] Arranging excitation and receiving wedge piezoelectric transducers in the defect-free area of the solid plate, based on the set excitation frequency and excitation angle, triggering the excitation transducer to emit single-mode Lamb wave signals;

[0061] Receiving the Lamb wave signals directly propagating between the excitation point and the receiving point along the plate surface by the receiving transducer, and extracting the first arriving high amplitude pulse signal in the waveform from the Lamb wave receiving signal as the direct wave signal;

[0062] Time window constraint and amplitude filtering processing are performed on the direct wave signal to determine the time domain characteristics of the direct wave.

[0063] In some embodiments, a continuous plate surface is first selected as a signal reference acquisition area in a non-defect area of the solid plate, and an excitation and receiving wedge piezoelectric transducer is arranged. The excitation frequency and excitation angle of the transducer are set in advance according to the dispersion characteristics of the target Lamb wave mode and remain stable. After the excitation transducer emits a single-mode Lamb wave signal, the signal directly reaches the receiving transducer along the propagation path in the plate, which does not undergo any interface reflection or scattering process, so that the first group of waveforms received contains the purest direct wave information. In the analysis and processing of the received signal, the system performs amplitude absolute value conversion on the complete waveform data and identifies the position of the first high-amplitude pulse, i.e., the first energy concentration peak. This signal usually has a prominent peak feature and is located in the earliest effective wave band after the excitation trigger. Extracting it as a direct wave signal helps to establish a reference for subsequent echoes. In order to enhance the stability and repeatability of the direct wave signal, a limited time window is further set in its time neighborhood, and amplitude filtering processing is performed on the waveform in the window to suppress high-frequency noise and low-amplitude interference terms, extract the main wave packet structure, and form a direct wave reference template with clear time domain characteristics and clear waveform boundaries. Unlike existing technologies, it extracts the true propagation echo in the local standard path of the non-defect area, ensures that the extracted direct wave signal has structural symmetry and propagation consistency, and avoids the misjudgment problem caused by multi-path superposition and signal aliasing. At the same time, the signal quality is strengthened through the time window and filtering mechanism, so that it has accurate reference standards when comparing time and amplitude with defect echoes and side echoes, and improves the robustness and measurement accuracy of the overall defect detection process.

[0064] Further, the high-amplitude pulse signal is obtained by the following steps:

[0065] A time window is set in the Lamb wave receiving signal, all data points in the time window are scanned, the amplitude absolute value is calculated, and a high-amplitude pulse signal with a local maximum amplitude is identified.

[0066] It should be noted that, in order to extract effective high-amplitude pulse signals from the Lamb wave receiving signal, a time window with physical meaning is first set in the original receiving signal. The time window is usually determined according to prior propagation path information, so as to cover the expected effective echo area, and at the same time, to exclude the interference signals caused by excitation delay, initial coupling noise and far-end multiple reflections. Within the selected time window, the absolute value of the amplitude of each discrete sampling point of the receiving signal is calculated, and an envelope curve is formed to highlight the energy distribution characteristics of the waveform. Then, the system performs local extremum scanning on the amplitude curve, and judges which points in the continuous points satisfy the extremum condition, that is, the amplitude of the current sampling point is greater than its adjacent front and rear points, and at the same time, it is higher than the set noise threshold. The pulse signal satisfying the above condition is extracted as a high-amplitude pulse signal, which is regarded as a key signal unit that may contain defect information or reference information. This processing method is different from the traditional means of screening waveforms through overall amplitude change or fixed threshold, but adopts a dynamic analysis method based on the local extremum characteristics of the signal, which can accurately extract high-energy pulse signals reflecting the response characteristics of the structure in the complex waveform background. By limiting the time window and introducing the local extremum judgment mechanism, the extraction result is more stable and has stronger robustness, and a reliable signal basis is established for subsequent time sequence discrimination, defect identification and amplitude comparison. This method improves the defect identification accuracy while reducing the risk of false triggering caused by random noise.

[0067] Further, it is characterized in that the side echo signal is obtained by the following steps:

[0068] In the Lamb wave receiving signal, based on the identified direct wave signal, a secondary high-amplitude pulse signal in the time window is searched backward, and a reflection signal corresponding to the expected propagation time interval of the plate end face distance is extracted as a side echo signal;

[0069] The side echo signal is subjected to time window constraint and amplitude filtering processing to determine the side echo time information.

[0070] In this embodiment, in order to accurately extract the side echo signal from the Lamb wave receiving signal, first, the identified direct wave signal time position is taken as a reference, and a target time window is set to the rear thereof. The start and end range of the time window is calculated according to the physical distance between the receiving transducer and the plate end face and the pre-calibrated Lamb wave propagation speed, so as to ensure that the window covers the arrival time interval of the theoretical end face reflected wave, and at the same time, the interference section that may overlap with the defect reflected wave is eliminated. In the target time window, the amplitude curve of the receiving signal is subjected to local extreme value analysis, and a secondary high-amplitude pulse signal with prominent amplitude is searched. Since the Lamb wave is reflected at the plate end face and then returns along the original path in a specific mode, the reflected wave energy is concentrated and the position is stable, so the extracted secondary maximum value signal has obvious waveform characteristics. The pulse signal is taken as a side echo candidate signal, and the waveform in the neighborhood thereof is further subjected to time window interception and amplitude filtering processing, so as to weaken the background interference and retain the main waveform energy, and finally the waveform envelope peak time point is extracted, and the effective time information of the side echo is determined. The method effectively improves the extraction accuracy of the end face reflected signal by using the time reference system constructed based on the direct wave and the time prediction mechanism calculated based on the theoretical propagation path, and avoids the recognition error caused by the overlapping of the reflected wave and the unknown time sequence in the traditional method. Especially in the detection environment with complex boundary conditions or significant multiple reflection interference, the scheme can still realize high-robustness side echo extraction, and provides a stable reference time scale for the time positioning and difference analysis of the defect reflected wave.

[0071] Further, the S2 comprises the following steps:

[0072] Based on the direct wave time information and the side echo time information, a target time interval between the two is determined, and the Lamb wave receiving signal is intercepted in the time interval to obtain a candidate echo signal segment;

[0073] Based on the candidate echo signal segment, amplitude peak extraction processing is performed to identify local extreme points higher than a background noise threshold to obtain a plurality of suspected defect reflected echo points;

[0074] Based on the time position and amplitude variation characteristics of the suspected defect reflected echo points, envelope demodulation and time window filtering processing are performed to obtain defect reflected echo preliminary characteristics;

[0075] Based on the defect reflected echo preliminary characteristics, time-frequency domain joint analysis is performed to extract the effective time domain center value and amplitude envelope peak value of the echo signal as the final time information and amplitude information of the defect reflected echo.

[0076] In the embodiment, to accurately extract the reflection echo signal caused by structural defects, first, according to the time information of the direct wave and the side echo obtained by the previous identification, the target time interval between the two is determined. The time interval theoretically corresponds to the possible range of defect reflection wave, has the physical propagation path significance, and helps to separate the real and effective defect signal from the complex background. Then, the complete Lamb wave receiving signal is intercepted in the time interval to form a candidate echo signal segment, which provides an analysis basis for subsequent feature identification. In the candidate signal segment, the system performs amplitude peak extraction processing, scans all sampling points and identifies local extreme points with an amplitude higher than a set background noise threshold. This processing ensures that only signal structures with significant energy mutations are retained, and several possible defect-related reflection wave positions are preliminarily screened out. Then, for each suspected echo point, a local time window is constructed, the neighborhood waveform signal is extracted, and envelope demodulation processing is performed to extract the main peak feature from the amplitude envelope. To enhance the signal reliability and anti-interference ability, further combined with time window filtering technology, the secondary noise interference is suppressed, and the defect reflection echo preliminary feature with time domain focusing characteristics is output. On the basis of obtaining the preliminary feature, the system performs joint time-frequency analysis processing on the candidate signals. Specifically, first, the frequency energy distribution of the signal in different time periods is obtained through short-time Fourier transform; second, the frequency band energy change in different scales is analyzed through wavelet packet decomposition. Finally, the two kinds of features are aligned and fused on the time axis to extract the time center value and the corresponding envelope main peak amplitude with the most concentrated signal energy. This information is used as the final time information and amplitude information of the defect reflection echo, providing high-precision input for subsequent position inversion and size estimation. Compared with the traditional method of relying only on amplitude change or fixed threshold detection, the embodiment introduces time reference constraints, waveform structure analysis, frequency energy decomposition, and feature fusion mechanisms at multiple key processing nodes, making the defect echo recognition more stable and selective. Especially in the scene where multi-path interference is serious or the signal-to-noise ratio is low, the defect recognition rate can still be kept high.

[0077] Further, the joint time-frequency domain analysis includes:

[0078] Based on the preliminary defect reflection echo feature signal, short-time Fourier transform processing is performed to obtain the frequency energy distribution of the signal in the time domain, and a time-frequency spectrum matrix is obtained;

[0079] Based on the time-frequency spectrum matrix, the time center value and the corresponding frequency component of the energy concentration region are extracted to obtain a preliminary time-frequency feature vector;

[0080] The preliminary defect reflection echo feature signal is further subjected to wavelet packet decomposition processing to obtain the energy distribution of each frequency band at several scales, and a sub-band energy vector is obtained;

[0081] The time-frequency feature vector and the sub-band energy vector are spliced in time alignment manner to generate a joint time-frequency feature vector;

[0082] Based on the concentration degree of the time component in the joint time-frequency feature vector, the final time information of the defect reflection echo is extracted; based on the frequency component with the maximum energy amplitude in the joint feature vector, the final amplitude information of the defect reflection echo is extracted.

[0083] In the embodiment, to further improve the recognition accuracy and robustness of the defect reflection echo feature, a time-frequency domain joint analysis and processing procedure is introduced based on the extracted preliminary defect reflection echo feature signal. First, short-time Fourier transform is performed on the preliminary signal, the signal is analyzed locally in frequency by setting a suitable sliding window function and time step, and a complete time-frequency spectrum matrix is generated. The matrix reflects the frequency energy distribution of the signal in different time periods, and can directly display the two-dimensional changes of signal energy with time and frequency. After obtaining the time-frequency spectrum matrix, the region with the most concentrated energy is further extracted therefrom, the time center position and the corresponding main frequency component are analyzed, and a preliminary time-frequency feature vector reflecting the local frequency feature of the signal is constructed. The vector represents the time-frequency energy core of the defect reflection wave, which is helpful for subsequent feature fusion and enhancement of discrimination. To improve the ability to capture details of non-stationary or multi-scale signals, wavelet packet decomposition operation is performed on the same preliminary feature signal to extract its energy distribution in multiple frequency bands and scales. The sub-band energy vector of the signal is obtained by reconstructing the energy vector from the decomposition coefficients, reflecting the local variation characteristics in the frequency domain. Subsequently, the time-frequency feature vector obtained by short-time Fourier analysis and the sub-band energy vector obtained by wavelet packet decomposition are aligned in time axis and spliced to construct a unified length data structure with consistent time sequence, generating a joint time-frequency feature vector. The vector comprehensively retains the global frequency evolution information and local scale energy variation characteristics, improving the specificity and stability of defect wave recognition. In the joint feature vector, the concentration degree of the time component can obtain the final time information of the defect reflection wave, that is, the time center value of the signal energy concentration; at the same time, the position with the maximum energy amplitude is extracted from the frequency dimension to obtain the final amplitude information as the significant feature index of the defect reflection wave.

[0084] Further, the S3 comprises the following steps:

[0085] Based on the time information of the defect reflection echo and the time information of the side echo, the time difference between the defect echo and the side echo is calculated;

[0086] Based on the propagation speed of the Lamb wave in the solid plate calibrated by the direct wave and the side echo in the defect-free area in advance, combined with the time difference, the equivalent propagation distance from the defect to the transducer is calculated according to the propagation path relationship;

[0087] Based on the current position of the transducer on the surface of the solid plate, the known spatial relationship between the transducer and the plate boundary, and the equivalent propagation distance, a geometric solution model for the defect location is established, and the two-dimensional position coordinates of the defect in the solid plate coordinate system are obtained.

[0088] In this embodiment, to achieve spatial localization of the defect on the solid plate surface, the propagation time difference between the extracted defect reflection echo and side echo is first calculated based on their time information. This time difference reflects the difference between the path length from the defect location to the receiving transducer and the path length from the end face reflection point to the receiving transducer, and has direct geometric propagation significance. Subsequently, the Lamb wave propagation velocity, pre-calibrated in the defect-free region using direct wave and side echo, is called and multiplied with the aforementioned time difference to obtain the equivalent propagation distance corresponding to the defect echo. Further, based on the known spatial position of the transducer on the solid plate surface, combined with the position of the solid plate end face, the distance from the excitation transducer to the end face, and the angle between the receiving transducer and the possible path direction of the defect, a solution model including propagation path constraints and boundary geometric relationships is constructed. In this model, the defect location is represented as a unique solution in a set of points with a fixed distance from the receiving transducer and satisfying the propagation path symmetry constraint. By performing algebraic analysis on the aforementioned propagation distance, angle constraints, and boundary position parameters, the two-dimensional position coordinates of the defect in the solid plate coordinate system are finally calculated, outputting a point location result with practical spatial reference significance. Compared with existing methods that infer defect location based on overall waveform offset, this embodiment has a clearer physical basis and a quantifiable propagation model. It does not rely on empirical regression or inversion training models, avoiding excessive sensitivity to environmental noise, structural complexity, and other factors. Especially in scenarios with multipath interference or overlapping signal delays, this method can still establish a time difference framework through a stable side echo reference, effectively achieving high-precision calculation of the defect location.

[0089] Furthermore, the step of maintaining a fixed time difference between the defect-reflected echo and the direct wave by adjusting the transducer position includes:

[0090] Based on the defect reflection echo time information and the direct wave time information, the initial time difference between the two is calculated and used as a fixed reference time difference;

[0091] Keeping the position of the excitation transducer unchanged, the receiving transducer is translated along the surface of the solid plate to collect the Lamb wave received signals at different positions, and the defect reflection echo time and direct wave time corresponding to each position are extracted respectively.

[0092] Based on the defect reflection echo time and the direct wave time corresponding to each position, the real-time time difference between the defect reflection echo and the direct wave is calculated and compared with the reference time difference until the time difference between the defect reflection echo and the direct wave is stable and equal to the reference time difference, and the spatial position of the receiving transducer is recorded synchronously.

[0093] In the embodiment, first, the time difference between the extracted defect reflection echo time information and the direct wave time information is calculated as a fixed reference time difference value. The reference time difference represents the relative propagation time delay between the defect wave and the direct wave under a certain specific spatial layout, which is the target parameter for subsequent matching adjustment. Subsequently, under the premise of keeping the position of the excitation transducer unchanged, the receiving transducer is translated along the surface of the solid plate at a regular step distance. Each time the position is adjusted, the system collects the complete Lamb wave receiving signal under the current position, and reextracts the direct wave arrival time and the defect reflection wave arrival time under the current condition, and calculates the real-time time difference value between the two. By comparing the real-time time difference with the preset reference time difference, the system determines whether the current transducer position meets the time difference constant condition. When the real-time time difference and the reference value are stable and consistent within a tolerance range, it is considered that the receiving position meets the propagation path equivalence requirement. At this time, the position coordinates are recorded, and the defect echo signal corresponding to this position is taken as the amplitude sampling result under the standard condition, which is used for subsequent quantitative comparison with the standard defect amplitude model. This implementation process not only establishes an amplitude acquisition mechanism under the control of time difference, but also realizes the physical reconstruction of the propagation path equivalence condition through spatial displacement, avoiding the amplitude drift problem caused by inconsistent excitation path. Unlike the traditional method of directly sampling the waveform of the fixed point for amplitude comparison while ignoring the propagation geometry difference, the spatial adjustment strategy with time difference matching control is introduced to ensure that the defect reflection signal is acquired under highly consistent propagation conditions, thereby improving the accuracy and repeatability of quantitative evaluation of defect size.

[0094] Further, the quantitative size information of the defect is obtained by the following steps:

[0095] Based on the spatial position of the transducer, the Lamb wave receiving signal under the position is collected, and the defect reflection echo signal is extracted in the window corresponding to the fixed time difference;

[0096] The extracted defect reflection echo signal is subjected to amplitude envelope analysis, and the peak amplitude of the echo signal is obtained as the reflection intensity characteristic value of the current defect;

[0097] Under the conditions of equal distance propagation path and same excitation frequency, the echo signals of each known size reflector are collected on the standard test block with known size artificial reflectors, and the corresponding envelope peak amplitudes are extracted respectively to construct a corresponding relationship model between the reflection amplitude and the defect size;

[0098] The amplitude characteristic value of the defect reflection echo is input into the corresponding relation model, a corresponding defect size is matched, and quantitative size information of the detected defect is obtained.

[0099] In the embodiment, first, the Lamb wave receiving signal at the position is collected based on the receiving transducer spatial position recorded under the aforementioned time difference constant condition. The system intercepts the window interval positioned by the fixed time difference on the basis of the reference time corresponding to the direct wave, and extracts the defect reflection echo signal therefrom. In order to enhance the distinguishability of the echo in the amplitude, the extracted signal is subjected to amplitude envelope processing, the main envelope curve thereof is obtained, and the envelope peak value is taken as the intensity characteristic value of the current defect reflection, which is used for subsequent mapping analysis of the amplitude and the size. In order to establish the amplitude-size relation with physical reference significance, in the experimental preparation stage, the standard contrast test block is used for echo calibration. The standard test block is provided with a plurality of artificial reflectors of different sizes, simulating typical defect types, and under the transducer arrangement condition and excitation frequency consistent with the actual detection path, the echo of each reflector is collected. The envelope analysis is also performed on the collected standard echo signals of various types, the amplitude peak value corresponding to each known defect is extracted, a group of “known size-envelope amplitude” data pairs are constructed, and a continuous mapping model is formed by interpolation or regression algorithm fitting. The amplitude characteristic value of the defect reflection echo obtained in the actual detection is taken as the model input, the corresponding defect size is obtained by function matching, and the quantitative inversion of the geometric characteristics of the defect is realized. The method avoids the problem that the traditional amplitude comparison process relies on experience determination or a single threshold strategy, and instead is based on the physical equivalent propagation condition and the standard reference curve, thereby improving the accuracy and stability of the size identification. The embodiment realizes the normalized collection of the echo amplitude by keeping the time difference constant to ensure the consistency of the propagation path, and establishes an accurate reflection amplitude-size mapping model in combination with the standard test block, thereby breaking through the technical bottleneck that the defect amplitude is easily affected by the path change, unstable coupling and other factors in the traditional method, and significantly improving the engineering applicability and measurement reliability of the defect size evaluation capability based on the Lamb wave.

[0100] Further, the construction of the corresponding relation model between the reflection amplitude and the defect size comprises the following steps:

[0101] A plurality of artificial reflectors of different sizes are preset on the standard contrast test block, and correspond to different reflection target regions, respectively;

[0102] For each artificial reflector position, the transducer excitation and receiving configuration consistent with the actual detection path is arranged, the Lamb wave is excited, and the corresponding receiving signal is collected;

[0103] extracting the reflection echo signals corresponding to each artificial reflector and calculating the peak value of the amplitude envelope to establish a data pair containing the defect size and the echo amplitude corresponding to each other;

[0104] performing interpolation fitting processing on the data pair to construct a continuous function between the reflection echo amplitude and the defect size as a corresponding relationship model.

[0105] In the embodiment, to realize quantitative mapping between the defect echo amplitude and the actual geometric size, first, a standard contrast test block is taken as the modeling basis, the material of the test block is consistent with the measured solid plate, and a plurality of artificial reflectors with known sizes are preset at different positions, typical forms include blind holes or etched grooves with different diameters, which respectively correspond to the reflection target area simulating defects of different scales. Each artificial reflector constitutes a standard reflection unit for simulating the reflection behavior of Lamb waves under different physical scale conditions. Subsequently, for each artificial reflector, the transducer arrangement and excitation parameters consistent with the actual detection process are adopted to ensure that the excitation frequency, excitation angle and propagation path are consistent with the subsequent actual measurement process. Under the premise of keeping the propagation conditions stable, Lamb waves are excited and the reflection echo signals formed at each artificial reflector are collected. After the received echo signals are processed by amplitude envelope, the main envelope peak value is extracted as the representative amplitude feature of the reflector. Finally, a set of data pairs consisting of defect size and corresponding reflection echo amplitude is obtained, and a reflection response sample set under multi-scale samples is constructed. To establish a continuous function model with universality, interpolation or fitting processing is performed on the obtained discrete data pairs, including polynomial fitting, spline interpolation or exponential function fitting, to obtain the continuous mapping relationship between the reflection amplitude and the defect size. The function model can be used as the input basis for subsequent actual defect amplitude characteristic values to realize rapid inversion and quantitative output of the actual defect size. Unlike existing methods that rely on empirical estimation or relative amplitude change trend to identify defect size, the embodiment builds a physical quantitative mapping mechanism based on a standard test block, extracts the amplitude response under strictly uniform propagation path and excitation conditions, and ensures the accuracy, repeatability and physical interpretability of the model. Especially in the face of multi-size defect identification requirements in engineering scenarios, the model provides a highly consistent and verifiable echo-size correlation structure.

[0106] The above embodiments only describe the 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 ordinary engineering technicians in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for detecting defects in solid plates based on Lamb wave end-face echoes, characterized in that, include: S1. On one side of the solid plate, a single-mode Lamb wave is excited by a fixed-angle wedge piezoelectric transducer, and the Lamb wave signal is received by another wedge piezoelectric transducer with the same angle arranged on the same side as the wedge piezoelectric transducer, so as to obtain the direct wave signal and the side echo signal. S2. By using the signal received between the direct wave signal and the side echo signal, extract the defect reflection echo signal caused by the defect to obtain the time information and amplitude information of the defect reflection echo. S3. Based on the time difference between the defect reflection echo time information and the side echo time information, and combined with the pre-calibrated Lamb wave propagation velocity, calculate the defect location. S4. By adjusting the position of the transducer, the time difference between the defect reflected echo and the direct wave is kept at a fixed value. The amplitude of the defect reflected echo is acquired synchronously and compared with the reference echo amplitude of the test block of the preset size defect to obtain the quantitative size information of the defect. The direct wave signal is obtained through the following steps: An excitation and receiving wedge piezoelectric transducer is arranged in the defect-free area of ​​a solid plate. Based on the pre-set excitation frequency and excitation angle, the excitation transducer is triggered to emit a single-mode Lamb wave signal. The receiving transducer receives the Lamb wave signal that propagates directly along the plate surface between the excitation point and the receiving point, and extracts the high-amplitude pulse signal that arrives first in the waveform from the Lamb wave received signal as the direct wave signal; Time window constraints and amplitude filtering are applied to the direct wave signal to determine its time-domain characteristics. The side echo signal is obtained through the following steps: In the Lamb wave received signal, based on the identified direct wave signal, the next higher amplitude pulse signal within the search time window is used, and the reflected signal is extracted by combining it with the expected propagation time interval corresponding to the distance from the end face of the plate, as the side echo signal; The side echo signal is subjected to time window constraint and amplitude filtering to determine the side echo time information; S2 includes the following steps: Based on the direct wave time information and the side echo time information, the target time interval between the two is determined, and the Lamb wave received signal is intercepted within this time interval to obtain candidate echo signal segments; Based on the candidate echo signal segments, amplitude peak extraction processing is performed to identify local extreme points higher than the background noise threshold, thereby obtaining several suspected defect reflection echo points. Based on the temporal location and amplitude variation characteristics of the suspected defect reflection echo points, envelope demodulation and time window filtering are performed to obtain preliminary characteristics of the defect reflection echo. Based on the preliminary characteristics of the defect reflection echo, a joint time-frequency domain analysis is performed to extract the effective time-domain center value and amplitude envelope peak value of the echo signal, which are used as the final time and amplitude information of the defect reflection echo. The step of maintaining the time difference between the defect-reflected echo and the direct wave at a fixed value by adjusting the transducer position includes: Based on the defect reflection echo time information and the direct wave time information, the initial time difference between the two is calculated and used as a fixed reference time difference; Keeping the position of the excitation transducer unchanged, the receiving transducer is translated along the surface of the solid plate to collect the Lamb wave received signals at different positions, and the defect reflection echo time and direct wave time corresponding to each position are extracted respectively. Based on the defect reflection echo time and direct wave time corresponding to each location, the real-time time difference between the current defect reflection echo and direct wave is calculated and compared with the reference time difference until the time difference between the defect reflection echo and direct wave is stably equal to the reference time difference, and the spatial position of the receiving transducer is recorded synchronously.

2. The method for detecting defects in a solid plate based on Lamb wave end-face echoes according to claim 1, characterized in that, The high-amplitude pulse signal is obtained through the following steps: A time window is set in the Lamb wave received signal, all data points within the time window are scanned, the absolute value of their amplitude is calculated, and high amplitude pulse signals whose amplitude reaches a local maximum are identified.

3. The method for detecting defects in a solid plate based on Lamb wave end-face echoes according to claim 1, characterized in that, The joint time-frequency domain analysis includes: Based on the preliminary characteristic signal of the defect reflection echo, a short-time Fourier transform is performed to obtain the frequency energy distribution of the signal in the time domain, thus obtaining the time-spectrum matrix. Based on the time-frequency matrix, the time center value and corresponding frequency component of the energy concentration region are extracted to obtain a preliminary time-frequency feature vector; The initial characteristic signal of the defect reflection echo is further processed by wavelet packet decomposition to obtain the energy distribution of each frequency band at several scales and obtain the sub-band energy vector. The time-frequency feature vector and the sub-band energy vector are concatenated in a time-aligned manner to generate a joint time-frequency feature vector. Based on the concentration of the time component in the joint time-frequency feature vector, the final time information of the defect reflection echo is extracted; based on the frequency component with the largest energy amplitude in the joint feature vector, the final amplitude information of the defect reflection echo is extracted.

4. The method for detecting defects in a solid plate based on Lamb wave end-face echoes according to claim 3, characterized in that, S3 includes the following steps: Based on the time information of the defect reflection echo and the side echo, the time difference between the defect echo and the side echo is calculated. Based on the propagation speed of Lamb waves in the solid plate, which was pre-calibrated in a defect-free region by direct waves and side echoes, and combined with the time difference, the equivalent propagation distance from the defect to the transducer is calculated according to the propagation path relationship. Based on the current position of the transducer on the surface of the solid plate, the known spatial relationship between the transducer and the plate boundary, and the equivalent propagation distance, a geometric solution model for the defect location is established, and the two-dimensional position coordinates of the defect in the solid plate coordinate system are obtained.

5. The method for detecting defects in a solid plate based on Lamb wave end-face echoes according to claim 1, characterized in that, The quantitative size information of the defect is obtained through the following steps: Based on the spatial location of the transducer, the Lamb wave received signal at that location is acquired, and the defect reflected echo signal is extracted in the window corresponding to the fixed time difference. Amplitude envelope analysis is performed on the extracted defect reflection echo signal to obtain the peak amplitude of the echo signal, which is used as the reflection intensity characteristic value of the current defect. On a standard test block with artificial reflectors of known size, equidistant propagation paths and the same excitation frequency are set up. The echo signals of each reflector of known size are collected, and the corresponding envelope peak amplitude is extracted. A correspondence model between reflection amplitude and defect size is constructed. The amplitude characteristic value of the reflected echo from the defect is input into the correspondence model to match the corresponding defect size, which serves as the quantitative size information of the detected defect.

6. The method for detecting defects in a solid plate based on Lamb wave end-face echoes according to claim 5, characterized in that, The process of constructing the correspondence model between reflection amplitude and defect size includes the following steps: Several artificial reflectors of different sizes are pre-set on the standard comparison test block, each corresponding to a different reflection target area; For each artificial reflector position, a transducer excitation and receiving configuration consistent with the actual detection path is arranged to excite Lamb waves and collect the corresponding received signals. Extract the reflected echo signal corresponding to each artificial reflector, calculate its amplitude envelope peak value, and establish a data pair containing a one-to-one correspondence between defect size and echo amplitude; Interpolation fitting is performed on the data pairs to construct a continuous function between the reflected echo amplitude and the defect size, which serves as the corresponding relationship model.

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