A method and system for detecting residual stress in thin plates based on thickness resonance mode

Abstract

The present invention discloses a method and system for detecting residual stress of thin plates based on thickness resonance mode, which relates to the technical field of stress detection. The method includes: obtaining the dispersion curve of the thin plate to be measured, and determining the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; conducting a stress coefficient calibration experiment on the thin plate to be measured to determine the experimental Lamb wave A-scan data under different stresses; determining the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signals of each experimental Lamb wave A-scan data; exciting a Lamb wave signal at a preset excitation point of the thin plate to be measured and determining the corresponding measured Lamb wave A-scan data; and determining the residual stress of the thin plate to be measured by using the stress-thickness resonance normalized amplitude curve based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data. By utilizing the characteristic that the amplitude increases with the stress value under the S1 thickness resonance mode to measure the residual stress of the thin plate, the detection efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of stress detection, and particularly to a method and system for detecting residual stress of thin plates based on thickness resonance mode. Background Art

[0002] Thin plate parts are widely used in multiple industries such as automobiles, mechanical equipment casings, aircraft skins, and sheet metal products. Under the action of external loads, such long-term service metal structures may cause stress concentration at key parts, which may further lead to problems such as crack formation, material failure, and even sudden fracture, posing a threat to life and property safety. Therefore, in order to ensure the quality of thin plate parts during the production and acceptance stages, and to accurately evaluate their structural life throughout the service cycle, it is necessary to detect the residual stress of thin plate parts.

[0003] The measurement techniques of residual stress can be divided into destructive testing methods and non-destructive testing methods. Among them, the destructive testing methods obtain residual stress by removing the sample material and based on the displacement or strain within the region, including drilling method, ring core method, and peeling method, etc. Although the destructive testing methods are relatively mature in theory and usually have high measurement accuracy, they will cause irreversible damage to the workpiece during the implementation process. The non-destructive testing methods (Non-Destructive Testing, NDT) can more accurately evaluate the microstructure and mechanical properties of materials without damaging the tested samples, and effectively identify and locate potential defects such as bubbles, cracks, and material non-uniformity. The non-destructive testing methods mainly include diffraction method (X-ray Diffraction, XRD), ultrasonic method, magnetic method, Raman spectroscopy method, and nanoindentation method. Among them, the diffraction method has perfect detection standards but the instrument price is expensive, the magnetic method has high detection sensitivity but is only applicable to ferromagnetic workpieces, the Raman spectroscopy method can measure the microstructure but the accuracy is extremely susceptible to external conditions, the nanoindentation method is convenient and fast for detection but the theory is not perfect, and the ultrasonic method is widely recognized in industrial applications due to its advantages of high safety, no pollution, fast response speed, and the equipment is easy to carry.

[0004] Currently, the ultrasonic stress detection method based on Zero-Group-Velocity Lamb Waves (ZGV Lamb Waves) detects stress through the zero group velocity mode (ZGV). Its mechanism is that the frequency of the ZGV mode will change monotonically, that is, frequency shift, with the change of stress in the thin plate part. Since the thickness change of the thin plate part during the stretching process will affect the frequency shift, thickness compensation processing is required, and the detection process is relatively cumbersome, resulting in low detection efficiency of the residual stress of thin plates. Summary of the Invention

[0005] The present invention provides a method and system for detecting residual stress in thin plates based on thickness resonance mode, which solves the technical problem that the existing ultrasonic stress detection method needs to perform thickness compensation processing for stress detection through the zero group velocity mode, resulting in low efficiency of detecting residual stress in thin plates.

[0006] A method for detecting residual stress in thin plates based on thickness resonance mode provided by the first aspect of the present invention includes:

[0007] Obtain the dispersion curve of the thin plate to be measured, and determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve;

[0008] Conduct a stress coefficient calibration experiment on the thin plate to be measured to determine the experimental A-scan data of Lamb waves under different stresses;

[0009] According to the resonance wave time-domain signals of the respective experimental Lamb wave A-scan data, determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency;

[0010] Excite a Lamb wave signal at a preset excitation point of the thin plate to be measured, and determine the corresponding measured Lamb wave A-scan data;

[0011] Based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data, use the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured.

[0012] Optionally, the obtaining the dispersion curve of the thin plate to be measured and determining the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve includes:

[0013] Obtain the structural parameters of the thin plate to be measured, and determine the dispersion curve of the thin plate to be measured based on the structural parameters;

[0014] Determine the theoretical frequency-thickness product of the S1 thickness resonance mode according to the dispersion curve;

[0015] Extract the plate thickness from the structural parameters, perform a ratio operation on the theoretical frequency-thickness product and the plate thickness, and output the theoretical thickness resonance frequency.

[0016] Optionally, the determining the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signals of the respective experimental Lamb wave A-scan data includes:

[0017] Extract the corresponding resonance wave time-domain signals from the respective experimental Lamb wave A-scan data according to a preset time range;

[0018] After windowing each of the resonance wave time-domain signals through a Hanning window function, perform a fast Fourier transform respectively, and perform amplitude normalization processing with the amplitude of the S1 zero-group velocity Lamb wave mode frequency as an index, and output multiple experimental frequency-domain diagrams;

[0019] Take the peak amplitude closest to the theoretical thickness resonance frequency in each experimental frequency-domain diagram as the corresponding theoretical thickness resonance normalized amplitude;

[0020] Use the theoretical thickness resonance normalized amplitudes and the associated stresses for curve fitting to generate a stress-thickness resonance normalized amplitude curve.

[0021] Optionally, for the measured thickness resonance normalized amplitude determined based on the measured A-scan data of Lamb waves, using the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured includes:

[0022] Perform windowed fast Fourier transform on the resonance wave time-domain signals within a preset time range in the measured A-scan data, and output a measured frequency-domain diagram;

[0023] According to the theoretical thickness resonance frequency, determine the measured thickness resonance normalized amplitude of the S1 thickness resonance mode from the measured frequency-domain diagram;

[0024] Input the measured thickness resonance normalized amplitude into the stress-thickness resonance normalized amplitude curve for solution, and output the residual stress of the thin plate to be measured.

[0025] Optionally, there are multiple excitation points; it further includes:

[0026] Perform mechanical scanning on the thin plate to be measured to determine the residual stresses at multiple excitation points, and use each of the residual stresses to construct the residual stress field of the thin plate to be measured.

[0027] A thin plate residual stress detection system provided in the second aspect of the present invention includes a waveform generator, an amplifier, an incident probe, a receiving probe, an incident wedge, a receiving wedge, an oscilloscope, a processor, and a universal testing machine;

[0028] The waveform generator is connected to the amplifier, and the amplifier is connected to the incident probe;

[0029] The processor is connected to the oscilloscope, and the oscilloscope is connected to the receiving probe;

[0030] The incident probe is connected to the thin plate to be measured through the incident wedge at a preset incident angle, the receiving probe is connected to the thin plate to be measured through the receiving wedge, and the universal testing machine is connected to the thin plate to be measured;

[0031] The universal testing machine is used for performing stress coefficient calibration experiments with different stresses on the thin plate to be measured;

[0032] The waveform generator is used to generate an excitation signal, which is amplified by an amplifier and then transmitted to the incident probe to generate an incident ultrasonic signal;

[0033] The incident wedge is used to convert the ultrasonic signal into a Lamb wave signal and excite it towards the thin plate to be measured;

[0034] The receiving wedge is used to transmit the Lamb wave signal returned by the thin plate to be measured to the receiving probe to generate a received ultrasonic signal;

[0035] The oscilloscope is used to generate experimental Lamb wave A-scan data or measured Lamb wave A-scan data with different stresses according to the received ultrasonic signal;

[0036] The processor is used to obtain the dispersion curve of the thin plate to be measured, determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signal of each experimental Lamb wave A-scan data; and determine the residual stress of the thin plate to be measured by using the stress-thickness resonance normalized amplitude curve based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data.

[0037] Optionally, when determining the measured Lamb wave A-scan data, both the incident wedge and the receiving wedge can also be air or water.

[0038] Optionally, the determination process of the incident angle includes:

[0039] Determine the phase velocity of the S1 zero group velocity Lamb wave mode based on the dispersion curve;

[0040] Perform an arcsine operation on the ratio of the longitudinal wave velocity in the incident wedge or water or air to the phase velocity to determine the incident angle.

[0041] A computer device provided in the third aspect of the present invention includes a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor executes the steps of the method for detecting the residual stress of a thin plate based on the thickness resonance mode as described in any one of the above.

[0042] A computer-readable storage medium provided in the fourth aspect of the present invention stores a computer program, and when the computer program is executed, it implements the method for detecting the residual stress of a thin plate based on the thickness resonance mode as described in any one of the above.

[0043] A computer program product provided by the fifth aspect of the present invention includes a computer program / instructions, and when the computer program / instructions are executed by a processor, the method for detecting residual stress of a thin plate based on thickness resonance mode as described in any one of the above is implemented.

[0044] As can be seen from the above technical solutions, the present invention has the following advantages:

[0045] The above solution of the present invention provides a method for detecting residual stress of a thin plate based on thickness resonance mode, including: obtaining the dispersion curve of the thin plate to be measured, and determining the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; conducting a stress coefficient calibration experiment on the thin plate to be measured to determine the experimental Lamb wave A-scan data of different stresses; determining the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signals of each experimental Lamb wave A-scan data; exciting a Lamb wave signal at a preset excitation point of the thin plate to be measured, and determining the corresponding measured Lamb wave A-scan data; and determining the residual stress of the thin plate to be measured by using the stress-thickness resonance normalized amplitude curve based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data. The above solution can accurately measure the residual stress of the thin plate by utilizing the characteristic that the amplitude increases with the change of the stress value in the S1 thickness resonance mode, without considering the thickness change of the thin plate, thereby improving the detection efficiency of the residual stress of the thin plate. Description of the Drawings

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention, and those of ordinary skill in the art can obtain other drawings without creative efforts based on these drawings.

[0047] Figure 1 It is a step flowchart of a method for detecting residual stress of a thin plate based on thickness resonance mode provided by an embodiment of the present invention;

[0048] Figure 2 It is a schematic diagram of the frequency-thickness product-phase velocity dispersion curve provided by an embodiment of the present invention;

[0049] Figure 3 It is a technical flowchart of a method for detecting residual stress of a thin plate based on thickness resonance mode provided by an embodiment of the present invention;

[0050] Figure 4 It is a schematic diagram of the structure of a two-dimensional finite element model excited by a point source;

[0051] Figure 5 It is a dispersion curve diagram of 6061 aluminum alloy;

[0052] Figure 6 Schematic diagram of time-domain signals at 0 MPa and 200 MPa;

[0053] Figure 7 Frequency-domain comparison diagram under the stress of 0 MPa and 200 MPa;

[0054] Figure 8 Schematic diagram of the change of the resonance mode frequency of the S1 thickness with stress;

[0055] Figure 9 Schematic diagram of the change of the resonance mode amplitude of the S1 thickness with stress;

[0056] Figure 10 Stress nephogram of a uniform specimen under a tensile force of 10.5 kN;

[0057] Figure 11 Stress distribution diagram of a uniform specimen under a tensile force of 10.5 kN;

[0058] Figure 12 Typical frequency-domain diagram under a stress of 100 MPa;

[0059] Figure 13 Stress field reconstruction diagram of a uniform plate;

[0060] Figure 14 Schematic structural diagram of a thin plate residual stress detection system based on the thickness resonance mode provided by an embodiment of the present invention;

[0061] Figure 15 Schematic structural diagram of a thin plate residual stress detection system based on the thickness resonance mode provided by an embodiment of the present invention using different propagation media. Detailed implementation manners

[0062] An embodiment of the present invention provides a thin plate residual stress detection method and system based on the thickness resonance mode, which is used to solve the technical problem that the existing ultrasonic stress detection method needs to perform thickness compensation processing for stress detection through the zero group velocity mode, resulting in low efficiency of thin plate residual stress detection.

[0063] In order to make the invention purpose, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0064] Please refer to Figure 1 , Figure 1The flowchart of the steps of a method for detecting residual stress in thin plates based on thickness resonance mode provided by an embodiment of the present invention.

[0065] A method for detecting residual stress in thin plates based on thickness resonance mode provided by the present invention includes:

[0066] Step 101: Obtain the dispersion curve of the thin plate to be measured, and determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve.

[0067] Step 101 includes the following sub-steps:

[0068] Obtain the structural parameters of the thin plate to be measured, and determine the dispersion curve of the thin plate to be measured based on the structural parameters;

[0069] Determine the theoretical frequency-thickness product of the S1 thickness resonance mode according to the dispersion curve;

[0070] Extract the plate thickness from the structural parameters, perform a ratio operation on the theoretical frequency-thickness product and the plate thickness, and output the theoretical thickness resonance frequency.

[0071] It should be noted that there are various ways to obtain the dispersion curve of the thin plate to be measured. In one implementation, the COMSOL Multiphysics finite element software can be used. By inputting the structural parameters such as the plate thickness, geometric shape, elastic modulus, density, and Poisson's ratio of the thin plate to be measured, which reflect the geometric and material properties of the template to be measured, the Rayleigh-Lamb equation is solved to determine the dispersion curve of the thin plate to be measured; the Rayleigh-Lamb equation includes:

[0072] For symmetric modes (S):

[0073] ;

[0074] For antisymmetric modes (A):

[0075] ;

[0076] In the formula, is half of the plate thickness, is the first propagation constant, is the second propagation constant, is the wave number; among them, the propagation constant satisfies the following formula:

[0077] ;

[0078] ;

[0079] In the formula, is the angular frequency, is the longitudinal wave velocity in the thin plate, is the shear wave velocity in the thin plate;

[0080] According to the calculation results of the above equations, with the help of commercial MATLAB software, data processing is carried out on the calculation results to plot the required dispersion curves, and then the dispersion characteristics are simulated and analyzed; it can be understood that the dispersion curves can have various forms. For example, with the frequency-thickness product as the abscissa and the phase velocity as the ordinate (as shown in Figure 2 ), with the frequency-thickness product as the abscissa and the group velocity as the ordinate, with the wave number as the abscissa and the frequency as the ordinate;

[0081] In this embodiment, the focus is on the cut-off frequency of the S1 thickness resonance mode, that is, the S1 mode. Based on the dispersion curve, the frequency-thickness product of the S1 thickness resonance mode can be determined as the theoretical frequency-thickness product. Combining with the plate thickness of the thin plate to be measured, the theoretical thickness resonance frequency can be calculated using the following formula:

[0082] ;

[0083] In the formula, is the theoretical thickness resonance frequency, is the theoretical frequency-thickness product, is the plate thickness.

[0084] Step 102: Conduct a stress coefficient calibration experiment on the thin plate to be measured to determine the experimental Lamb wave A-scan data under different stresses.

[0085] Step 102 includes the following sub-steps:

[0086] It should be noted that a stress coefficient calibration experiment can be carried out on the thin plate to be measured under different set stress conditions through a universal testing machine. It can be understood that the stress coefficient calibration experiment includes a tensile experiment and a compression experiment. A Lamb wave signal is emitted through instruments such as a waveform generator, a probe, and a wedge to excite the S1 thickness resonance mode of the thin plate to be measured, and the corresponding A-scan data can be determined through an oscilloscope as the experimental Lamb wave A-scan data.

[0087] Step 103: Determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signals of each experimental Lamb wave A-scan data.

[0088] Step 103 includes the following sub-steps:

[0089] Extract the corresponding resonance wave time-domain signals from each experimental Lamb wave A-scan data according to a preset time range;

[0090] After windowing each resonance wave time-domain signal through a Hanning window function, perform a fast Fourier transform respectively, and perform amplitude normalization processing with the amplitude of the S1 zero group velocity Lamb wave mode frequency as an index, and output multiple experimental frequency domain diagrams;

[0091] Take the peak amplitude closest to the theoretical thickness resonance frequency in each experimental frequency-domain graph as the corresponding normalized amplitude of the theoretical thickness resonance;

[0092] Use the normalized amplitudes of each theoretical thickness resonance and the associated stress for curve fitting to generate a stress-normalized amplitude of thickness resonance curve.

[0093] It should be noted that, select the resonance wave time-domain signals within the preset time range from each experimental Lamb wave A-scan data, apply a Hanning window function to each resonance wave time-domain signal for windowing processing and perform a fast Fourier transform. Then, use the amplitude of the S1 zero group velocity Lamb wave mode (S1-ZGV) frequency as an index for amplitude normalization processing, so as to obtain multiple experimental frequency-domain graphs. In the experimental frequency-domain graph, with frequency as the abscissa and the normalized amplitude of thickness resonance as the ordinate, the normalized amplitude of thickness resonance can be understood as the ratio of the amplitude of the thickness resonance frequency to the amplitude of the S1-ZGV frequency. By using a Hanning window, it is possible to prevent spectral leakage during the Fourier transform process and improve the frequency accuracy after transformation. By performing amplitude normalization processing with the amplitude of the S1-ZGV frequency, it is possible to avoid the influence of the voltage change of the excitation signal on the amplitude of the thickness resonance mode, and further affect the numerical value of the fitting curve, thereby improving the stability of the curve fitting; extract the peak amplitude closest to the theoretical thickness resonance frequency of the S1 thickness resonance mode from the experimental frequency-domain graph as the corresponding normalized amplitude of the theoretical thickness resonance, and perform stress-amplitude curve fitting on the normalized amplitudes of the theoretical thickness resonance under different stresses, so as to establish the relationship between the stress and the amplitude of the S1 thickness resonance mode and obtain the stress-normalized amplitude of thickness resonance curve. Thus, the calibration process of the stress is completed.

[0094] Step 104: Excite a Lamb wave signal at a preset excitation point on the thin plate to be measured, and determine the corresponding measured Lamb wave A-scan data.

[0095] It should be noted that after determining the stress calibration index, a Lamb wave signal should be excited at the preset excitation point set on the thin plate to be measured, and the measured Lamb wave A-scan data at the position of this excitation point can be collected.

[0096] Step 105: Based on the measured normalized amplitude of thickness resonance determined from the measured Lamb wave A-scan data, use the stress-normalized amplitude of thickness resonance curve to determine the residual stress of the thin plate to be measured.

[0097] Step 105 includes the following sub-steps:

[0098] Perform windowed fast Fourier transform on the resonance wave time-domain signals within the preset time range in the measured A-scan data, and output the measured frequency-domain graph;

[0099] Determine the measured thickness resonance normalized amplitude of the S1 thickness resonance mode from the measured frequency domain diagram according to the theoretical thickness resonance frequency;

[0100] Input the measured thickness resonance normalized amplitude into the stress-thickness resonance normalized amplitude curve for solution, and output the residual stress of the thin plate to be measured.

[0101] It should be noted that, for the resonance wave time domain signal of the measured A-scan data within the preset time range, after windowing processing using a window function and then performing fast Fourier transform to generate the measured frequency domain diagram, the amplitude in the S1 thickness resonance mode is obtained from the frequency domain diagram according to the theoretical thickness resonance frequency as the measured thickness resonance normalized amplitude, and then the residual stress in the thin plate to be measured is calculated using the stress-thickness resonance normalized amplitude curve obtained by stress calibration.

[0102] Preferably, there are multiple excitation points; the method further includes:

[0103] Perform mechanical scanning on the thin plate to be measured to determine the residual stress of multiple excitation points, and construct the residual stress field of the thin plate to be measured using each residual stress.

[0104] It should be noted that the residual stress of a single excitation point only reflects the stress state of the thin plate to be measured within a local range. In order to obtain the stress distribution of the entire thin plate to be measured, multiple excitation points can be set on the thin plate to be measured, and the residual stress of each excitation point can be obtained according to step 105, and then the residual stress distribution within the global range can be obtained.

[0105] For better illustration, Figure 3 The schematic diagram of the technical process of the thin plate residual stress detection method based on the thickness resonance mode provided by the embodiment of the present invention is shown; it should be noted that this technical process is only briefly described, and the specific implementation process of each step can be understood by referring to the relevant content in the foregoing embodiments:

[0106] 1. Excite the S1 thickness resonance mode: In this process, first solve the dispersion curve of the thin plate to be measured, select a probe with a suitable center frequency according to the theoretical thickness resonance frequency determined by the dispersion curve, and select the best incident angle of the probe to ensure that the S1 thickness resonance mode of the thin plate to be measured can be excited;

[0107] 2. Establish the relationship between stress and the amplitude of the thickness resonance mode: In this process, a tensile test is carried out on the thin plate to be measured under different stresses. An excitation signal is sent to the thin plate to be measured through a probe, and the signal returned from the thin plate to be measured is transmitted to an oscilloscope, so as to obtain the A-scan data under different stresses. The time-domain signals of the resonance waves in each A-scan data are processed by windowed fast Fourier transform to obtain the corresponding frequency-domain diagrams. The theoretical thickness resonance normalized amplitudes are respectively extracted from each frequency-domain diagram and curve-fitted with the corresponding stresses, and then the stress-amplitude relationship formula is obtained;

[0108] 3. Solve the local stress: In this process, a certain excitation point set on the thin plate to be measured is detected by moving the probe. After obtaining the corresponding measured thickness resonance normalized amplitude, the stress-amplitude relationship formula is used to solve for the local residual stress of the thin plate to be measured;

[0109] 4. Scan the stress over the entire range: In this process, the entire surface of the thin plate to be measured is scanned by automatically or manually moving the probe. Multiple excitation points set on the surface of the thin plate to be measured are processed according to the steps of solving the local stress, so as to obtain the residual stress values in the global range of the thin plate to be measured.

[0110] To clearly illustrate the effect achieved by the method for detecting the residual stress of a thin plate based on the thickness resonance mode provided by the embodiments of the present invention, a numerical simulation experiment is carried out through finite element modeling:

[0111] First, a thin plate workpiece is numerically simulated by the commercial software COMSOL Multiphysics to construct a two-dimensional finite element model of point source excitation as shown in Figure 4 The geometric shape of the model is a thin plate with a thickness of 3 mm. The material is selected as 6061 aluminum alloy, and increasing damping absorption layers are set on the left and right sides of the thin plate in the model to effectively eliminate the influence of acoustic reflection from the boundary; at the same time, in order to obtain the stress values of the thin plate workpiece at multiple positions, 80 equally spaced (interval of 2 mm) normal displacement signals are collected on the surface of the model (including the excitation points). The signals of multiple excitation points can be received in a linear array manner. In addition, tensile stresses are applied in the range of 0 - 200 MPa to the thin plate workpiece at a step of 20 MPa to simulate the different stresses suffered in the stress coefficient calibration experiment;

[0112] Second, according to the known structural parameters of the thin plate workpiece, the Rayleigh-Lamb equation is solved by the COMSOL Multiphysics finite element software, and the calculation results are processed by the MATLAB commercial software to draw as shown in Figure 5The dispersion curve shown; in the dispersion curve graph, it can be seen that when the wave number k = 0, it represents the cut-off frequency, that is, the thickness resonance mode. From the marked box in the figure, the theoretical thickness resonance frequency of the S1 thickness resonance mode is 1.0567 MHz. On this basis, the center frequency of the excitation signal can be selected near the theoretical thickness resonance frequency. In the simulation, the excitation signal selected is a 3-cycle sine signal modulated by a Hanning window with a center frequency of 0.9 MHz. This excitation signal is a broadband signal, and its frequency domain also contains the frequency components of the thickness resonance mode;

[0113] Then, tests are carried out under the loading conditions of 0 MPa and 200 Mpa tensile stress, and the received signal at the first excitation point of the model, that is, the A-scan data, is extracted; According to Figure 6 (a), it can be observed that there are resonance waves lasting for a period of time in the A-scan data at 0 MPa. The resonance wave time-domain signal from 4 - 50 μs is intercepted from this A-scan data, as Figure 6 shown in (b). Its resonance wave shows the characteristic of uniform amplitude decay. The analysis results show that this signal contains the S1-ZGV mode and an extremely weak S1 thickness resonance mode; Under the action of 200 Mpa tensile stress, in Figure 6 (c), it can be seen that there are also resonance waves lasting for a period of time in the A-scan data. The resonance wave time-domain signal from 4 - 50 μs is intercepted as Figure 6 shown in (d). In comparison, the resonance wave time-domain signal under the action of tensile stress shows obvious periodic changes in amplitude. This is due to the superposition of the S1-ZGV mode and other modes with similar frequencies under the action of stress, that is, it shows that the S1 thickness resonance mode may exist in the resonance wave;

[0114] Next, in order to further determine whether the S1 thickness resonance mode exists in the resonance wave, the windowed fast Fourier transform is performed on the two intercepted resonance wave time-domain signals to obtain the frequency domain comparison under the action of 0 MPa and 200 MPa stress as Figure 7 shown; The resonance peak with a larger amplitude in the figure is the S1-ZGV mode. However, after applying a tensile stress of 200 MPa in the simulation, a new resonance peak appears at the position of 1.0497 MHz, which is quite close to the theoretical thickness resonance frequency of the S1 thickness resonance mode, 1.0567 MHz. In addition, the frequency of the S1 thickness resonance mode is higher than that of the S1-ZGV mode, which can be clearly distinguished in the frequency domain graph; According to existing research, the change of excitation conditions will affect the coupling efficiency of the S1-ZGV mode, which may lead to the emergence of new modes near the cut-off frequency of Lamb waves. In this study, the application of stress destroys the optimal coupling condition of the S1-ZGV mode. Under the induction of stress, the S1-ZGV mode will undergo mode conversion, thus triggering the generation of the S1 thickness resonance mode;

[0115] Furthermore, under the action of tensile stress in the range of 0 - 200 MPa (step size: 20 MPa), a windowed fast Fourier transform is performed on the time-domain signal of the resonance wave within the range of 110 - 250 μs. It can be understood that selecting different time ranges has little impact on the results, but it is necessary to ensure that the intercepted time range is long enough. If the intercepted time range is too short, it will result in a wider frequency spectrum, thus masking the resonance mode of the S1 thickness; the frequencies and amplitudes of the resonance peaks of the S1 thickness resonance mode under different stresses are extracted, and the trend changes are constructed as shown in Figure 8 and Figure 9 shown. According to Figure 8 , it can be seen that the change in the frequency of the S1 thickness resonance mode under the action of 0 - 200 MPa is small, while as shown in Figure 9 , the amplitude change of the S1 thickness resonance mode shows a relatively smooth increasing trend, and its change amount is also quite sensitive to the change of the stress value. Therefore, it can be seen that the amplitude change under mode conversion is an effective index for detecting stress, and compared with the frequency shift of the Hz / MPa level without thickness compensation in the existing S1-ZGV frequency domain method, the amplitude change index has more advantages; by performing a cubic curve fitting on the simulation data, the stress-thickness resonance normalized amplitude curve of the relationship between stress (X) and amplitude (Y) can be obtained as: ;

[0116] Finally, for a uniform 6061 aluminum plate with a material yield strength of 275 MPa, a thickness of 3 mm, and a width of 35 mm in the middle part, a tensile force of 10.5 kN is applied to the workpiece in the COMSOL software. The stress cloud diagram and stress distribution diagram are as shown in Figure 10 and Figure 11 shown; under the action of a 10.5 kN tensile force, according to the formula , where is the stress, is the applied force, and is the cross-sectional area, the expected theoretical stress received at the middle position of the aluminum plate is 100 MPa, which is in good agreement with the static analysis result; at this time, an external stress of 100 MPa is applied in the point-source excitation two-dimensional finite element model, the A-scan data of the receiving point (excitation point) is extracted, and a windowed fast Fourier transform is performed on the time-domain signal of the resonance wave within the range of 110 - 250 μs. The obtained frequency domain diagram is as shown in Figure 12 shown. It can be seen that the amplitude of the S1 thickness resonance mode under the action of 100 MPa is 0.1557. According to the fitted stress-thickness resonance normalized amplitude curve, the stress σ≈93.702 MPa can be solved, and this result is very close to the expected stress; by performing the same operation on multiple equally spaced position points collected, the stress field reconstruction diagram of the uniform thin plate can be obtained as shown in Figure 13As shown, the stress field has been detected with good results. Referring to the standard deviation formula, the error of the stress measurement method based on the thickness resonance mode is defined as: , where is the measured stress, is the theoretical stress calculated by the formula, is the th stress. From this, the standard deviation of the detected stress is calculated to be 8.7457 MPa. The error of this stress detection method is less than 10 MPa, which proves that this method has high accuracy.

[0117] In the embodiment of the present invention, the residual stress of the thin plate is accurately measured by using the characteristic that the amplitude of the thickness resonance mode increases with the change of the stress value. Thus, the stress field inside the thin plate can be reconstructed by using the Lamb wave technology of the thickness resonance mode. This method does not need to consider the thickness change of the thin plate, so there is no need to perform thickness compensation processing. Relatively speaking, it is more convenient and thus improves the detection efficiency of the residual stress of the thin plate. At the same time, it has wider applicability and stronger practical value in local and global stress measurements, provides a new method and theoretical basis for evaluating the residual stress in workpieces, and shows potential application prospects in the field of non-destructive testing.

[0118] Please refer to Figure 14 , Figure 14 which is a schematic structural diagram of a thin plate residual stress detection system based on the thickness resonance mode provided by the embodiment of the present invention.

[0119] A thin plate residual stress detection system based on the thickness resonance mode provided by the present invention includes:

[0120] A waveform generator, an amplifier, an incident probe, a receiving probe, an incident wedge, a receiving wedge, an oscilloscope, a processor, and a universal testing machine;

[0121] The waveform generator is connected to the amplifier, and the amplifier is connected to the incident probe;

[0122] The processor is connected to the oscilloscope, and the oscilloscope is connected to the receiving probe;

[0123] The incident probe is connected to the thin plate to be measured through the incident wedge at a preset incident angle, the receiving probe is connected to the thin plate to be measured through the receiving wedge, and the universal testing machine is connected to the thin plate to be measured;

[0124] The universal testing machine is used to perform stress coefficient calibration experiments on the thin plate to be measured with different stresses;

[0125] The waveform generator is used to generate an excitation signal, which is amplified by the amplifier and then transmitted to the incident probe to generate an incident ultrasonic signal;

[0126] The incident wedge is used to convert ultrasonic signals into Lamb wave signals and excite them to the thin plate to be measured;

[0127] The receiving wedge is used to transmit the Lamb wave signals returned from the thin plate to be measured to the receiving probe to generate received ultrasonic signals;

[0128] The oscilloscope is used to generate experimental Lamb wave A-scan data or measured Lamb wave A-scan data of different stresses based on the received ultrasonic signals;

[0129] The processor is used to obtain the dispersion curve of the thin plate to be measured, and determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; according to the resonance wave time-domain signals of each experimental Lamb wave A-scan data, determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency; based on the measured thickness resonance normalization amplitude determined from the measured Lamb wave A-scan data, use the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured.

[0130] It should be noted that first, the processor determines the theoretical thickness resonance frequency of the S1 thickness resonance mode based on the dispersion curve of the thin plate (workpiece to be measured), then further selects a probe with a suitable frequency for the experiment, and according to the calculation formula of Snell's law for the longitudinal wave velocity in the propagation medium and the phase velocity of the S1 zero group velocity Lamb wave mode perform an arcsine operation on the ratio to determine the incident angle of the incident probe

[0131] After selecting an appropriate probe and incident angle, the detection operation can be carried out. In the experimental system, when the propagation medium is in the form of a wedge, the probe and the wedge, as well as the wedge and the thin plate to be measured, are in contact coupling using an ultrasonic couplant to improve the ultrasonic propagation efficiency and the signal-to-noise ratio of the detection result. A waveform generator and an amplifier are used as the excitation module, and an oscilloscope and a processor are used as the receiving module. When performing signal detection, the waveform generator generates an excitation signal, which is amplified by the amplifier and then transmitted to the incident probe. The incident probe emits an incident ultrasonic signal into the thin plate to be measured at a certain incident angle. The ultrasonic signal passes through the incident wedge and enters the thin plate to be measured, and is converted into a Lamb wave signal in the thin plate. The Lamb wave propagates in the thin plate and returns to the receiving wedge, where it is received by the receiving probe to generate a received ultrasonic signal. The receiving probe transmits the received ultrasonic signal into the oscilloscope to generate A-scan data. The receiving angle of the receiving probe can be set according to the incident angle. According to the above signal detection process, when the universal testing machine performs stress coefficient calibration experiments on the thin plate to be measured with different stresses, the corresponding experimental Lamb wave A-scan data is detected. The processor determines the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency based on the resonance wave time-domain signal of each experimental Lamb wave A-scan data, and then determines the measured thickness resonance normalized amplitude based on the measured Lamb wave A-scan data detected at the preset excitation point position. Furthermore, the residual stress of the thin plate to be measured is solved through the stress-thickness resonance normalized amplitude curve.

[0132] Optionally, the processor can be a computer.

[0133] Optionally, when determining the measured Lamb wave A-scan data, the incident wedge and the receiving wedge can both be air or water.

[0134] It should be noted that when performing the stress coefficient calibration experiment, the propagation medium can be in the form of a wedge. During the actual measurement after stress calibration, as Figure 15 shown, in addition to the wedge, the propagation medium can also be water or air to perform oblique incidence to excite the mode of interest.

[0135] Optionally, obtaining the dispersion curve of the thin plate to be measured and determining the theoretical thickness resonance frequency of the S1 thickness resonance mode based on the dispersion curve includes:

[0136] Obtaining the structural parameters of the thin plate to be measured and determining the dispersion curve of the thin plate to be measured based on the structural parameters;

[0137] Determining the theoretical frequency-thickness product of the S1 thickness resonance mode based on the dispersion curve;

[0138] Extracting the plate thickness from the structural parameters, performing a ratio operation on the theoretical frequency-thickness product and the plate thickness, and outputting the theoretical thickness resonance frequency.

[0139] Optionally, according to the resonance wave time-domain signals of each experimental Lamb wave A-scan data, determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency, including:

[0140] Extract the corresponding resonance wave time-domain signals from each experimental Lamb wave A-scan data according to a preset time range;

[0141] After windowing each resonance wave time-domain signal through a Hanning window function, perform fast Fourier transform respectively, and perform amplitude normalization processing with the amplitude of the S1 zero-group velocity Lamb wave mode frequency as an index, and output multiple experimental frequency-domain diagrams;

[0142] Take the peak amplitude closest to the theoretical thickness resonance frequency in each experimental frequency-domain diagram as the corresponding theoretical thickness resonance normalized amplitude;

[0143] Use curve fitting of each theoretical thickness resonance normalized amplitude and the associated stress to generate a stress-thickness resonance normalized amplitude curve.

[0144] Optionally, based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data, use the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured, including:

[0145] Perform windowed fast Fourier transform on the resonance wave time-domain signals in the measured A-scan data within a preset time range, and output the measured frequency-domain diagram;

[0146] According to the theoretical thickness resonance frequency, determine the measured thickness resonance normalized amplitude of the S1 thickness resonance mode from the measured frequency-domain diagram;

[0147] Input the measured thickness resonance normalized amplitude into the stress-thickness resonance normalized amplitude curve for solution, and output the residual stress of the thin plate to be measured.

[0148] Optionally, there are multiple excitation points; the processor is further configured to:

[0149] Perform mechanical scanning on the thin plate to be measured to determine the residual stress of multiple excitation points, and use each residual stress to construct the residual stress field of the thin plate to be measured.

[0150] It should be noted that by automatically or manually moving the probe to scan the entire surface of the thin plate to be measured, the residual stress of multiple excitation points set on the surface of the thin plate to be measured can be obtained, and then the residual stress value in the global range of the thin plate to be measured can be determined.

[0151] An embodiment of the present invention further provides a computer device, including a memory and a processor, and a computer program is stored in the memory; when the computer program is executed by the processor, the processor executes the steps of the method for detecting the residual stress of a thin plate based on the thickness resonance mode in any of the above embodiments.

[0152] An embodiment of the present invention also provides a computer-readable storage medium, on which a computer program / instructions are stored. When the computer program / instructions are executed by a processor, the steps of the method for detecting residual stress of a thin plate based on the thickness resonance mode in any of the above embodiments are implemented.

[0153] An embodiment of the present invention also provides a computer program product, including a computer program / instructions. When the computer program / instructions are executed by a processor, the steps of the method for detecting residual stress of a thin plate based on the thickness resonance mode in any of the above embodiments are implemented.

[0154] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes of the above-described systems and modules can refer to the corresponding processes in the foregoing method embodiments, and will not be elaborated herein.

[0155] In several embodiments provided by the present application, it should be understood that the disclosed systems and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed coupling or direct coupling or communication connection between each other can be through some interfaces. The indirect coupling or communication connection of the devices or units can be in electrical, mechanical or other forms.

[0156] The units described as separate components may or may not be physically separated. The components displayed as units may or may not be physical units, that is, they can be located in one place, or can be distributed to multiple network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0157] In addition, in each embodiment of the present invention, the functional units can be integrated in a processing unit, or each unit can exist physically alone, or two or more units can be integrated in one unit. The above integrated units can be implemented in the form of hardware or in the form of software functional units.

[0158] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to enable a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as USB flash drives, mobile hard disks, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs.

[0159] As described above, the above embodiments are only used to illustrate the technical solution of the present invention and are not intended to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of various embodiments of the present invention.

Claims

1. A method for detecting residual stress in thin plates based on thickness resonance mode, characterized in that Including: Obtain the dispersion curve of the thin plate to be measured, and determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; Conduct a stress coefficient calibration experiment on the thin plate to be measured to determine the experimental Lamb wave A-scan data under different stresses; According to the resonance wave time-domain signals of the respective experimental Lamb wave A-scan data, determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency; Excite a Lamb wave signal at a preset excitation point of the thin plate to be measured, and determine the corresponding measured Lamb wave A-scan data; Based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data, use the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured; The step of using the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured based on the measured thickness resonance normalized amplitude determined from the measured Lamb wave A-scan data includes: Perform windowed fast Fourier transform on the resonance wave time-domain signals within a preset time range in the measured Lamb wave A-scan data, and output a measured frequency-domain diagram; According to the theoretical thickness resonance frequency, determine the measured thickness resonance normalized amplitude of the S1 thickness resonance mode from the measured frequency-domain diagram; Input the measured thickness resonance normalized amplitude into the stress-thickness resonance normalized amplitude curve for solution, and output the residual stress of the thin plate to be measured.

2. The method for detecting residual stress of thin plates based on thickness resonance mode according to claim 1, characterized in that The step of obtaining the dispersion curve of the thin plate to be measured and determining the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve includes: Obtain the structural parameters of the thin plate to be measured, and determine the dispersion curve of the thin plate to be measured based on the structural parameters; Determine the theoretical frequency-thickness product of the S1 thickness resonance mode according to the dispersion curve; Extract the plate thickness from the structural parameters, perform a ratio operation on the theoretical frequency-thickness product and the plate thickness, and output the theoretical thickness resonance frequency.

3. The method for detecting residual stress of a thin plate based on the thickness resonance mode according to claim 1, wherein The step of determining the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency according to the resonance wave time-domain signals of the respective experimental Lamb wave A-scan data includes: Extract the corresponding resonance wave time-domain signals from each of the experimental Lamb wave A-scan data according to a preset time range; After performing windowing processing on each of the resonance wave time-domain signals through a Hann window function, perform fast Fourier transform respectively, and perform amplitude normalization processing with the amplitude of the S1 zero-group velocity Lamb wave mode frequency as an index, and output multiple experimental frequency-domain diagrams; Use the peak amplitude closest to the theoretical thickness resonance frequency in each experimental frequency-domain diagram as the corresponding theoretical thickness resonance normalized amplitude; Perform curve fitting on each of the theoretical thickness resonance normalized amplitudes and the associated stresses to generate a stress-thickness resonance normalized amplitude curve.

4. The method for detecting residual stress of thin plates based on thickness resonance mode according to claim 1, wherein, There are multiple excitation points; it further includes: Perform mechanical scanning on the thin plate to be measured to determine the residual stresses at multiple excitation points, and construct the residual stress field of the thin plate to be measured using each of the residual stresses.

5. A thin plate residual stress detection system based on thickness resonance mode, characterized in that Including a waveform generator, an amplifier, an incident probe, a receiving probe, an incident wedge, a receiving wedge, an oscilloscope, a processor, and a universal testing machine; The waveform generator is connected to the amplifier, and the amplifier is connected to the incident probe; The processor is connected to the oscilloscope, and the oscilloscope is connected to the receiving probe; The incident probe is connected to the thin plate to be measured through the incident wedge at a preset incident angle, the receiving probe is connected to the thin plate to be measured through the receiving wedge, and the universal testing machine is connected to the thin plate to be measured; The universal testing machine is used to perform stress coefficient calibration experiments with different stresses on the thin plate to be measured; The waveform generator is used to generate an excitation signal, which is amplified by an amplifier and then transmitted to the incident probe to generate an incident ultrasonic signal; The incident wedge is used to convert the ultrasonic signal into a Lamb wave signal and excite it to the thin plate to be measured; The receiving wedge is used to transmit the Lamb wave signal returned by the thin plate to be measured to the receiving probe to generate a received ultrasonic signal; The oscilloscope is used to generate experimental Lamb wave A-scan data or measured Lamb wave A-scan data with different stresses according to the received ultrasonic signal; The processor is used to obtain the dispersion curve of the thin plate to be measured, and determine the theoretical thickness resonance frequency of the S1 thickness resonance mode according to the dispersion curve; according to the resonance wave time-domain signal of each experimental Lamb wave A-scan data, determine the stress-thickness resonance normalized amplitude curve associated with the theoretical thickness resonance frequency; based on the measured thickness resonance normalization amplitude determined from the measured Lamb wave A-scan data, use the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured; The step of using the stress-thickness resonance normalized amplitude curve to determine the residual stress of the thin plate to be measured based on the measured thickness resonance normalization amplitude determined from the measured Lamb wave A-scan data includes: Perform windowed fast Fourier transform on the resonance wave time-domain signal within a preset time range in the measured Lamb wave A-scan data, and output a measured frequency-domain diagram; According to the theoretical thickness resonance frequency, determine the measured thickness resonance normalization amplitude of the S1 thickness resonance mode from the measured frequency-domain diagram; Input the measured thickness resonance normalization amplitude into the stress-thickness resonance normalized amplitude curve for solution, and output the residual stress of the thin plate to be measured.

6. The thin plate residual stress detection system based on the thickness resonance mode according to claim 5, characterized in that, When determining the measured Lamb wave A-scan data, both the incident wedge and the receiving wedge can also be air or water.

7. The thin plate residual stress detection system based on the thickness resonance mode according to claim 5 or 6, characterized in that The determination process of the incident angle includes: Determine the phase velocity of the S1 zero group velocity Lamb wave mode based on the dispersion curve; Perform an arcsine operation on the ratio of the longitudinal wave velocity in the incident wedge or water or air to the phase velocity to determine the incident angle.

8. A computer device, characterized in that, It includes a memory and a processor. A computer program is stored in the memory. When the computer program is executed by the processor, the processor executes the steps of the method for detecting residual stress of a thin plate based on thickness resonance mode according to any one of claims 1-4.

9. A computer-readable storage medium having computer programs / instructions stored thereon, characterized in that, When the computer program / instructions are executed by the processor, the steps of the method for detecting residual stress of a thin plate based on thickness resonance mode according to any one of claims 1-4 are implemented.

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