Defect imaging method based on multi-modal guided waves

Abstract

The invention provides a defect imaging method based on multi-modal guided waves, which comprises the following steps: arranging a sound source and a signal receiving device on the upper surface of a to-be-detected structure to obtain wave field data in a plurality of different paths; calculating an arrival time difference; obtaining the average length and depth of the defects on each path; analyzing the distribution of defect information measured by each path in a spatial position, and positioning a defect area; and substituting the defect depth of each path into a thickness distribution calculation formula to obtain defect thickness distribution, and realizing defect imaging in combination with a defect area positioning result. According to the defect imaging method based on the multi-modal guided waves, identification of the depth and length of the defect can be achieved only through one group of sound sources and signal receiving devices in the testing process, the testing cost is reduced, and the testing efficiency is improved; the imaging method can be suitable for various guided waves and has general applicability; and the influence of modal deviation caused by the defects on defect identification is effectively reduced.

Description

Technical Field

[0001] The present disclosure relates to the technical field of ultrasonic nondestructive testing, and in particular to a defect imaging method based on multi-modal guided waves. Background Art

[0002] Structural damage, such as local thinning of walls or panels, occurs when structures are affected by chemical corrosion or natural degradation over long periods of use. Guided waves have been widely used for nondestructive testing and structural health monitoring due to their long-distance propagation capabilities and sensitivity to damage. In damaged structures, the presence of defects can change the propagation speed of guided waves and their reflection, transmission, and mode conversion characteristics.

[0003] Current detection technologies can usually only give the approximate location of defects and the relative degree of corrosion. Accurately describing the size of defects is time-consuming and labor-intensive, and most require multi-sensor clusters or point-by-point scanning methods to obtain the range and depth of defects, which is difficult to calculate, time-consuming, and costly. Summary of the invention

[0004] The purpose of the present disclosure is to provide a defect imaging method based on multi-modal guided wave, which can solve one or more of the above-mentioned problems in the prior art.

[0005] According to one aspect of the present disclosure, a defect imaging method based on multi-modal guided wave is provided, comprising the following steps:

[0006] A sound source and a signal receiving device are arranged on the upper surface of the lossless structure to draw a dispersion curve of the lossless structure;

[0007] A sound source and a signal receiving device are set on the upper surface of the structure to be tested, and the full wave train of the guided wave field in the time domain is simulated to obtain wave field data in several different paths;

[0008] Read the arrival time of the multimodal wave packet in each path and calculate the arrival time difference of the wave packet of the corresponding mode in the dispersion curve of the lossless structure;

[0009] Substitute the arrival time difference into the defect information calculation formula to obtain the average length and depth of the defect on each path. The defect information calculation formula is derived from the guided wave flight time theory.

[0010] Analyze the spatial distribution of defect information measured on each path and locate the defect area;

[0011] The defect depth of each path is substituted into the thickness distribution calculation formula to obtain the defect thickness distribution, and the defect imaging is realized by combining the results of defect area positioning.

[0012] In some embodiments, a sound source and a signal receiving device are arranged on the upper surface of the lossless structure, and drawing a dispersion curve of the lossless structure includes:

[0013] Assume that the expression of the sound source function is as follows:

[0014] f(t)=exp(-((t-4e -6 ) / (T / 0.8)) 2 )*sin(2*π*f*t)

[0015] Where f is the center frequency of the sound source, T = 1 / f is the pulse width of the sound source;

[0016] The expression of the frequency domain function corresponding to the sound source function is as follows:

[0017]

[0018] An acoustic source is set on the upper surface of the lossless structure to excite the guided wave, and the lower surface of the lossless structure is a free condition. Then, the guided wave propagating in the lossless structure is divided into symmetric mode and asymmetric mode according to the polarization mode, where the dispersion equation of the symmetric mode is:

[0019]

[0020] The dispersion equation for the asymmetric mode is:

[0021]

[0022] In the formula, is the wave number, ω=2πf is the circular frequency, c p represents the phase velocity of the guided wave;

[0023] At the same time, p and q satisfy the following formulas respectively:

[0024]

[0025] In the formula, c L and c T They represent the longitudinal wave velocity and shear wave velocity in the lossless structure respectively, and the dispersion curve of the lossless structure is drawn according to the dispersion equation.

[0026] In some embodiments, a sound source and a signal receiving device are arranged on the upper surface of the structure to be tested, and the full-wave wave train of the guided wave field in the time domain is simulated to obtain wave field data in several different paths, including:

[0027] A rectangular detection area is selected on the structure to be tested, and a set of sound sources and signal receiving devices are used. The sound source is set on the side of the rectangle, and the signal receiving device is set at a position corresponding to the side opposite to the side where the sound source is located. The line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded. The line connecting the sound source and the signal receiving device is recorded as a detection path.

[0028] The positions of the sound source and the signal receiving device are changed according to the preset interval, so that the line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded.

[0029] According to the propagation signal of the guided wave received by the signal receiving device in each path, the full-wave wave train of the guided wave field in the time domain is simulated by the finite element simulation method.

[0030] In some embodiments, the Hilbert transform is used to read the arrival time of the multimodal wave packets in each path.

[0031] In some embodiments, reading the arrival time of the multimodal wave packet in each path and calculating the arrival time difference of the wave packet of the corresponding mode in the dispersion curve of the lossless structure includes:

[0032] The time domain information and amplitude of the wave packet of each path signal are extracted respectively, and the wave packet envelope is drawn using Hilbert transform. The peak arrival time and amplitude of each modal wave packet are read, and the wave packet arrival time of each path is subtracted from the wave packet arrival time of the corresponding mode in the lossless structure to obtain the wave packet arrival time difference.

[0033] In some implementations, the step of obtaining the defect information calculation formula includes:

[0034] Assume that the structure to be tested is a metal plate of isotropic medium with a thickness of d. There is a defect with a length of r and a random cross-sectional depth distribution on the lower surface of the structure to be tested. The defect is divided into k parts along the length direction. The defect length of each part is Δr, and the defect depth of each part is recorded as d. k The position of the defect changes due to the change in the frequency-thickness product, which causes the group velocity c g If the defect changes, the expression of the effect of the defect on the flight time of any mode guided wave is as follows:

[0035]

[0036] Where ΔTOF represents the flight time difference of a certain mode guided wave packet, f is the main frequency of the excitation sound source,

[0037] Taking the average value of the defect depth d' as the overall depth of the defect, the expression for the effect of the defect on the flight time of any mode guided wave is approximately expressed as:

[0038]

[0039] The expression for the average group velocity is obtained as follows:

[0040]

[0041] Where L represents the distance between the sound source and the signal receiving device in the current path; cg ′=c g (fd');

[0042] Assume K = r / L, then K∈[0,1], and the expression of the average group velocity is simplified to:

[0043]

[0044] In the formula, the function of defect depth c g ′ and K are unknowns. The above expression is applicable to any mode waveguide. Substituting the time difference of at least two mode waveguides into the equation, we get the following equation:

[0045]

[0046] Under the premise of known material parameters, solving the above equations to obtain the function c of the defect depth g ′ and K, and the defect length r is calculated based on K.

[0047] In some implementations, analyzing the distribution of the defect information measured on each path in spatial position and locating the defect area includes:

[0048] Determine whether the wave packet arrival time difference calculated for each path is 0. If the wave packet arrival time difference is not 0, the path is determined to be a defective path.

[0049] Get all defect paths, and obtain the area where the defects are located according to the intersection of the defect paths.

[0050] According to the area where the defect is located and the calculated defect length, the precise location of the defect area is obtained.

[0051] In some embodiments, substituting the defect depth of each path into the thickness distribution calculation formula to obtain the defect thickness distribution, and implementing defect imaging in combination with the result of defect area positioning includes:

[0052] Extract the defect depth calculated on each path and assign it to the area on both sides of the corresponding path with a width of 1 / 2 of the sound source spacing.

[0053] The maximum peak value of the wave packet received by the signal receiving device of different paths is extracted as the thickness distribution weight factor to obtain the defect thickness distribution.

[0054] Imaging is performed based on the results of defect area positioning and defect thickness distribution to achieve defect imaging.

[0055] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects: the test process only requires a set of sound sources and signal receiving devices to realize the identification of defect depth and length, which reduces the test cost, simplifies the calculation process, and improves the test efficiency; the imaging method can be applied to various waveguides and various types of defects, and has universal applicability; it effectively reduces the influence of the modal offset caused by the defect on the defect identification, reduces the influence of the error on the detection result, and improves the detection efficiency and accuracy.

[0056] In addition, in the technical solution disclosed in the present invention, unless otherwise specified, the technical solution can be implemented by adopting conventional means in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0058] Figure 1 A flowchart of a defect imaging method based on multi-modal guided wave provided in one embodiment of the present disclosure.

[0059] Figure 2 A waveform diagram of the time domain waveform of the sound source pulse used in S11 in the defect imaging method based on multi-modal guided waves provided in one embodiment of the present disclosure.

[0060] Figure 3 A schematic diagram of the setting of the detection area and the defect area in S12 in the defect imaging method based on multi-modal guided wave provided in one embodiment of the present disclosure.

[0061] Figure 4 A comparison diagram of signals received by a signal receiving device in a defect-free path and a defective path in S13 in a defect imaging method based on multi-modal guided wave provided in one embodiment of the present disclosure.

[0062] Figure 5 The calculation results of the average length and depth of the defect in S14 in the defect imaging method based on multi-modal guided wave provided in one embodiment of the present disclosure.

[0063] Figure 6 A schematic diagram of defect area positioning in S15 in a defect imaging method based on multi-modal guided wave provided in an embodiment of the present disclosure.

[0064] Figure 7 The defect imaging result in S16 of the defect imaging method based on multi-modal guided wave provided in one embodiment of the present disclosure. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solution and advantages of the embodiments of the present disclosure clearer, the technical solution in the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present disclosure.

[0066] Example:

[0067] Reference Manual Attached Figure 1 , showing a defect imaging method based on multi-modal guided wave provided by an embodiment of the present disclosure, comprising the following steps:

[0068] S11: a sound source and a signal receiving device are arranged on the upper surface of the lossless structure, and a dispersion curve of the lossless structure is drawn;

[0069] S12: setting a sound source and a signal receiving device on the upper surface of the structure to be tested, simulating the full-wave wave train in the time domain of the guided wave field, and obtaining wave field data in several different paths;

[0070] S13: Read the arrival time of the multi-modal wave packet in each path, and calculate the arrival time difference of the wave packet of the corresponding mode in the dispersion curve of the lossless structure;

[0071] S14: Substitute the arrival time difference into the defect information calculation formula to obtain the average length and depth of the defect on each path. The defect information calculation formula is derived from the guided wave flight time theory;

[0072] S15: Analyze the spatial distribution of defect information measured on each path and locate the defect area;

[0073] S16: Substituting the defect depth of each path into the thickness distribution calculation formula to obtain the defect thickness distribution, and combining the result of defect area positioning to realize defect imaging.

[0074] In this embodiment, an aluminum plate with a length and width of 300 mm and a thickness of 4 mm is selected as the structure to be tested to illustrate the defect imaging method based on multi-modal guided wave provided by this application. The aluminum plate can be regarded as an isotropic homogeneous medium with a density of 2710 kg / m 3 , Young's modulus E is 69 GPa, Poisson's ratio ν is 0.33, and a defect with a length and width of 20 mm and a thickness d' of 1.5 mm is constructed in the center of the three-dimensional interface of the structure to be tested.

[0075] In S11, a sound source and a signal receiving device are arranged on the upper surface of the lossless structure, and drawing a dispersion curve of the lossless structure may include the following steps:

[0076] The lossless structure uses an aluminum plate of the same size and material as the structure to be tested. A sound source is set on the upper surface of the lossless structure to excite a multi-mode waveguide, and a signal receiving device is set on the upper surface of the lossless structure to receive the propagation signal. The signal receiving device can use an acoustic wave signal receiving sensor. In this embodiment, the sound source excites a multi-mode Lamb wave. The waveform diagram of the time domain waveform of the sound source pulse selected in this embodiment can refer to the attached specification. Figure 2 As shown, the center frequency f0 of the sound source is 300kHz.

[0077] In this embodiment, a cosine envelope pulse is used as the sound source function. At this time, the expression of the sound source function is as follows:

[0078] f(t)=exp(-((t-4e -6 ) / (T / 0.8)) 2 )*sin(2*π*f*t)

[0079] Where f is the center frequency of the sound source, T = 1 / f is the pulse width of the sound source;

[0080] The expression of the frequency domain function corresponding to the sound source function is as follows:

[0081]

[0082] In S11, the distance between the sound source and the signal receiving device can be set to 200 mm, and the distance between the sound source and the two closest sides of the aluminum plate can be set to 50 mm. The time domain transient results are simulated in the lossless structure to obtain the time domain full-wave data of the multi-modal Lamb wave field in the lossless structure.

[0083] An acoustic source is set on the upper surface of the aluminum plate to excite the Lamb waveguide, and the lower surface of the aluminum plate is free. The Lamb waveguide propagating in the aluminum plate is divided into symmetric mode and asymmetric mode according to the polarization mode. The dispersion equation of the symmetric mode is:

[0084]

[0085] The dispersion equation for the asymmetric mode is:

[0086]

[0087] In the formula, is the wave number, ω=2πf is the circular frequency, c p represents the phase velocity of the guided wave, and h represents the thickness of the structure;

[0088] At the same time, p and q satisfy the following formulas respectively:

[0089]

[0090] In the formula, c L and c T They represent the longitudinal wave velocity and shear wave velocity in the lossless structure respectively. Therefore, the dispersion curve of the lossless structure can be drawn according to the dispersion equation.

[0091] In S12, setting a sound source and a signal receiving device on the upper surface of the structure to be tested, simulating the full-wave wave train in the time domain of the guided wave field, and obtaining wave field data in several different paths may include the following steps:

[0092] A rectangular detection area is selected on the structure to be tested, and a set of sound sources and signal receiving devices are used. The sound source is set on one side of the detection area, and the signal receiving device is set at a position corresponding to the side opposite to the side where the sound source is located. The line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded. The line connecting the sound source and the signal receiving device is recorded as a detection path.

[0093] The positions of the sound source and the signal receiving device are changed according to the preset interval, so that the line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded.

[0094] According to the propagation signal of the guided wave received by the signal receiving device in each path, the full-wave wave train of the guided wave field in the time domain is simulated by the finite element simulation method.

[0095] In this embodiment, refer to the attached manual Figure 3 As shown, a square area with a side length of L is selected as the detection area, the side length L of the square is 200mm, and the square detection area is located in the middle of the aluminum plate, that is, the center of the square coincides with the center of the aluminum plate. The spacing between the positions where the sound source is placed each time can be determined according to the accuracy requirements of the detection. For example, in this embodiment, when the accuracy A is 2mm, the detection area can be divided into a square grid with a side length of 2mm. At this time, the spacing can be set to 4mm. In order to ensure the accuracy and precision of defect imaging, the side length of the detection area can be an integer multiple of the preset spacing.

[0096] Taking a vertex of the square detection area as the starting point, every 4 mm on the two sides adjacent to the vertex is used as a sound source position point. In the present embodiment, there are 102 sound source position points, which are arranged along two adjacent sides of the detection area. The sound source position points can be recorded as horizontal A1-horizontal A51 and vertical A1-vertical A51, wherein the horizontal A1 and the vertical A1 are the same point, the horizontal A1, the horizontal A51 and the vertical A51 are the three vertices of the detection area, the signal receiving device corresponding to the horizontal A1-horizontal A51 is recorded as horizontal R1-horizontal R51, and the signal receiving device corresponding to the vertical A1-vertical A51 is recorded as vertical R1-longitudinal R51. Therefore, there are a total of 102 detection paths including the four sides of the detection area.

[0097] The sound source is respectively set at the corresponding position point, and the signal receiving device is set on the side opposite to the position point, and the connection line between the sound source and the signal receiving device is perpendicular to the side where the sound source is located, until the connection line between the sound source and the signal receiving device coincides with two sides that are not adjacent to the starting point. Therefore, each connection line between the sound source and the signal receiving device can be used as a detection path. According to the Lamb signals of multiple paths received by the signal receiving device, the Lamb wave field time domain full wave train is simulated by the finite element simulation method, thereby obtaining the wave field data in different detection paths.

[0098] In S13, the Hilbert transform can be used to read the arrival time of the multimodal wave packet in each path. Specifically, the time domain information and amplitude of the wave packet of each path signal are extracted respectively, the wave packet envelope is drawn using the Hilbert transform, the peak arrival time and amplitude of each modal wave packet are read, and the wave packet arrival time of each path is subtracted from the wave packet arrival time of the corresponding mode in the lossless structure to obtain the wave packet arrival time difference.

[0099] Hilbert transform can be used for signal processing to extract the envelope of the signal. The signal is converted into an analytical signal containing the amplitude and phase information of the original signal, from which the envelope signal is extracted. Hilbert transform can phase shift the positive and negative frequencies by π / 2 respectively, realize single-sideband, simplify analysis and save computing resources. Hilbert's mathematical representation is as follows:

[0100]

[0101] When processing real signals, the Hilbert transform converts them into complex signals and obtains amplitude and phase information through demodulation, which is beneficial to the analysis of narrowband FM and AM waves, can reduce sampling frequency requirements and improve signal processing efficiency.

[0102] In order to compare the time difference of the wave packet after passing through the defect, the signals received by the signal receiving device with defect and the signal receiving device without loss are set as the control group. Figure 4In the figure, the healthy path is the signal received by the signal receiving device when it is intact, and the damaged path is the signal received by the signal receiving device when it is defective. S0 represents the symmetric mode and A0 represents the asymmetric mode.

[0103] In S14, a calculation formula for defect information can be obtained according to the guided wave flight time theory. The defect information includes the average length and depth of the defect. The specific steps of obtaining the calculation formula for defect information include:

[0104] Assume that the structure to be tested is an aluminum plate of isotropic medium with a thickness of d. There is a defect with a length of r and a random cross-sectional depth distribution on the lower surface of the structure to be tested. The defect is divided into k parts along the length direction. The defect length of each part is Δr, and the defect depth of each part is recorded as d. k The position of the defect changes due to the change in the frequency-thickness product, which causes the group velocity c g If the defect changes, the expression of the effect of the defect on the flight time of any mode guided wave is as follows:

[0105]

[0106] Where ΔTOF represents the flight time difference of a certain mode guided wave packet, f is the main frequency of the excitation sound source, and the generation of defects is generally not sudden, so the defect depth d of each part k There is little difference between them, so the average value of the defect depth d' can be used as the overall depth of the defect. The expression for the influence of the defect on the flight time of any mode guided wave is approximately expressed as:

[0107]

[0108] The expression for the average group velocity is obtained as follows:

[0109]

[0110] Where L represents the distance between the sound source and the signal receiving device in the current path; c g ′=c g (fd');

[0111] Assume K = r / L, then K∈[0,1], and the expression of the average group velocity is simplified to:

[0112]

[0113] In the formula, the function of defect depth c g ′ and K are unknowns. The above expression is applicable to any mode waveguide. Substituting the time difference of at least two mode waveguides into the equation, we get the following equation:

[0114]

[0115] Under the premise of knowing the density, Young's modulus and thickness of the material parameters of the structure to be tested, the above equation is solved to obtain the function c of the defect depth g ′ and K, and the defect length r is calculated based on K.

[0116] In this embodiment, the calculation results of the average length and depth of the defects refer to the appendix of the specification. Figure 5 As shown, D represents the thickness of the structure, and β represents the arrival time difference ratio between the Lamb wave S0 mode and the A0 mode; represents the group velocity of the A0 mode corresponding to the residual thickness.

[0117] In S15, analyzing the distribution of the defect information measured on each path in spatial position, locating the defect area may include the following steps:

[0118] Determine whether the wave packet arrival time difference calculated for each path is 0. If the wave packet arrival time difference is not 0, the path is determined to be a defective path.

[0119] Get all defect paths, and obtain the area where the defects are located according to the intersection of the defect paths.

[0120] According to the area where the defect is located and the calculated defect length, the precise location of the defect area is obtained.

[0121] Reference Manual Attached Figure 6 In this embodiment, the detection area is divided into square grids, the side length L of the detection area is 200 mm, and the side length L of the square grid is min is 2mm, so the distance between sound sources should not be greater than 2*L min In this embodiment, the distance between the sound sources is 4 mm, and there are a total of 100*100 grids in the detection area.

[0122] When the time difference is not equal to 0, it means that there is a loss in the path. In this embodiment, when the sound source is located at A25-A27 in the horizontal and vertical directions and the signal receiving device is located at R25-R27 in the horizontal and vertical directions, time difference occurs. Then, the six paths of horizontal A25-horizontal R25, horizontal A26-horizontal R26, horizontal A27-horizontal R27, longitudinal A25-longitudinal R25, longitudinal A26-longitudinal R26 and longitudinal A27-longitudinal R27 are all defective paths. The intersection area of ​​the defective paths is obtained. At this time, the intersection area of ​​the defective paths can be preliminarily determined as a defective area.

[0123] After the range of the defect area is preliminarily determined, the defect area is further judged in combination with the defect length calculated by S14. For example, in this embodiment, the defect length on the horizontal path A26-R26 is calculated to be 22.18 mm. The 20 mm range originally obtained by the intersection area of ​​the defect path should be extended from the center position of 20 mm to 22 mm. The calculation results of other paths are similar, and the precise position of the defect area is finally obtained.

[0124] In S16, the defect depth of each path is substituted into the thickness distribution calculation formula to obtain the defect thickness distribution, and defect imaging can be realized by combining the result of defect area positioning, which may include the following steps:

[0125] Extract the defect depth calculated on each path and assign it to the area on both sides of the corresponding path with a width of 1 / 2 of the sound source spacing.

[0126] The maximum peak value of the wave packet received by the signal receiving device of different paths is extracted as the thickness distribution weight factor to obtain the defect thickness distribution.

[0127] Imaging is performed based on the results of defect area positioning and defect thickness distribution to achieve defect imaging.

[0128] Specifically, the defect depth calculated on each path of the defect area is extracted and assigned to the area on both sides of the corresponding path with a width of 1 / 2 of the sound source spacing. In this embodiment, the sound source spacing is 4 mm, and the defect depth calculated on each path is extracted and assigned to the grids within a range of 2 mm on both sides of the path, so that each grid in the defect area has a thickness value.

[0129] Due to the existence of the cross path, the thickness of each grid should be the average of the grid thickness of the two cross paths. The grid thickness of the area corresponding to the defect path is recorded as d(i, j), and the expression of the grid thickness is d(i, j) = (d i +d j ) / 2, where i may represent the defect path sequence number in the x direction, and j may represent the defect path sequence number in the y direction. In this embodiment, i represents the defect path sequence number in the horizontal direction, and j represents the defect path sequence number in the vertical direction;

[0130] The maximum peak value of the wave packet of the signal receiving device of different defect paths is extracted as the thickness distribution weight factor I, and the final expression of the thickness of each grid in the defect area is obtained as d(i, j) = (I j d i +I i d j ) / 2;

[0131] Imaging is performed based on the results of defect area positioning and the thickness distribution of each grid in the defect area to achieve defect imaging.

[0132] In this example, the defect imaging results refer to the attached manual. Figure 7 shown.

[0133] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects: the test process only requires a set of sound sources and signal receiving devices to realize the identification of defect depth and length, which reduces the test cost, simplifies the calculation process, and improves the test efficiency; the imaging method can be applied to various waveguides and various types of defects, and has universal applicability; it effectively reduces the influence of the modal offset caused by the defect on the defect identification, reduces the influence of the error on the detection result, and improves the detection efficiency and accuracy.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present disclosure, rather than to limit them. Although the present disclosure has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present disclosure.

Claims

1. A defect imaging method based on multi-modal guided wave, characterized in that: The following steps are involved: A sound source and a signal receiving device are arranged on the upper surface of the lossless structure to draw a dispersion curve of the lossless structure; A sound source and a signal receiving device are set on the upper surface of the structure to be tested, and the full wave train of the guided wave field in the time domain is simulated to obtain wave field data in several different paths; Read the arrival time of the multimodal wave packet in each path and calculate the arrival time difference of the wave packet of the corresponding mode in the dispersion curve of the lossless structure; Substitute the arrival time difference into the defect information calculation formula to obtain the average length and depth of the defect on each path. The defect information calculation formula is derived from the guided wave flight time theory; Analyze the spatial distribution of defect information measured on each path and locate the defect area; The defect depth of each path is substituted into the thickness distribution calculation formula to obtain the defect thickness distribution, and the defect imaging is realized by combining the results of defect area positioning.

2. The defect imaging method based on multi-modal guided wave according to claim 1, characterized in that: The sound source and the signal receiving device are arranged on the upper surface of the lossless structure, and the dispersion curve of the lossless structure is drawn, comprising: Assume that the expression of the sound source function is as follows: Where f is the center frequency of the sound source, T = 1 / f is the pulse width of the sound source; The expression of the frequency domain function corresponding to the sound source function is as follows: An acoustic source is set on the upper surface of the lossless structure to excite the guided wave, and the lower surface of the lossless structure is a free condition. Then, the guided wave propagating in the lossless structure is divided into symmetric mode and asymmetric mode according to the polarization mode, where the dispersion equation of the symmetric mode is: The dispersion equation for the asymmetric mode is: In the formula, is the wave number, ω=2πf is the circular frequency, c p represents the phase velocity of the guided wave; At the same time, p and q satisfy the following formulas respectively: In the formula, c L and c T They represent the longitudinal wave velocity and shear wave velocity in the lossless structure respectively, and the dispersion curve of the lossless structure is drawn according to the dispersion equation.

3. The defect imaging method based on multi-modal guided wave according to claim 1, characterized in that: The sound source and signal receiving device are set on the upper surface of the structure to be tested, and the full wave train of the guided wave field in the time domain is simulated to obtain the wave field data in several different paths including: A rectangular detection area is selected on the structure to be tested, and a set of sound sources and signal receiving devices are used. The sound source is set on the side of the rectangle, and the signal receiving device is set at a position corresponding to the side opposite to the side where the sound source is located. The line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded. The line connecting the sound source and the signal receiving device is recorded as a detection path. The positions of the sound source and the signal receiving device are changed according to the preset interval, so that the line connecting the sound source and the signal receiving device is perpendicular to the side where the sound source is located, so that the sound source excites the multi-mode guided wave, and the signal received by the signal receiving device is recorded. According to the propagation signal of the guided wave received by the signal receiving device in each path, the full-wave wave train of the guided wave field in the time domain is simulated by the finite element simulation method.

4. The defect imaging method based on multi-modal guided wave according to claim 1, characterized in that: The Hilbert transform is used to read the arrival time of the multimodal wave packets in each path.

5. The defect imaging method based on multi-modal guided wave according to claim 4 is characterized in that: Reading the arrival time of the multimodal wave packet in each path and calculating the arrival time difference of the wave packet of the corresponding mode in the dispersion curve of the lossless structure include: The time domain information and amplitude of the wave packet of each path signal are extracted respectively, and the wave packet envelope is drawn using Hilbert transform. The peak arrival time and amplitude of each modal wave packet are read, and the wave packet arrival time of each path is subtracted from the wave packet arrival time of the corresponding mode in the lossless structure to obtain the wave packet arrival time difference.

6. The defect imaging method based on multi-modal guided wave according to claim 1, characterized in that: The steps of obtaining the defect information calculation formula include: Assume that the structure to be tested is a metal plate of isotropic medium with a thickness of d. There is a defect with a length of r and a random cross-sectional depth distribution on the lower surface of the structure to be tested. The defect is divided into k parts along the length direction. The defect length of each part is Δr, and the defect depth of each part is recorded as d. k The position of the defect changes due to the change in the frequency-thickness product, which causes the group velocity c g If the defect changes, the expression of the effect of the defect on the flight time of any mode guided wave is as follows: Where ΔTOF represents the flight time difference of a certain mode guided wave packet, f is the main frequency of the excitation sound source, Taking the average value of the defect depth d' as the overall depth of the defect, the expression for the effect of the defect on the flight time of any mode guided wave is approximately expressed as: The expression for the average group velocity is obtained as follows: Where L represents the distance between the sound source and the signal receiving device in the current path; c g ′=c g (fd'); Assume K = r / L, then K∈[0,1], and the expression of the average group velocity is simplified to: In the formula, the function of defect depth c g ′ and K are unknowns. The above expression is applicable to any mode waveguide. Substituting the time difference of at least two mode waveguides into the equation, we get the following equation: Under the premise of known material parameters, solving the above equations to obtain the function c of the defect depth g ′ and K, and the defect length r is calculated based on K.

7. The defect imaging method based on multi-modal guided wave according to claim 1, characterized in that: The analyzing the distribution of the defect information measured on each path in the spatial position and locating the defect area comprises: Determine whether the wave packet arrival time difference calculated for each path is 0. If the wave packet arrival time difference is not 0, the path is determined to be a defective path. Get all defect paths, and obtain the area where the defects are located according to the intersection of the defect paths. According to the area where the defect is located and the calculated defect length, the precise location of the defect area is obtained.

8. The defect imaging method based on multi-modal guided wave according to claim 3, characterized in that: Substituting the defect depth of each path into the thickness distribution calculation formula to obtain the defect thickness distribution, and combining the defect area positioning result to realize defect imaging includes: Extract the defect depth calculated on each path and assign it to the area on both sides of the corresponding path with a width of 1 / 2 of the sound source spacing. The maximum peak value of the wave packet received by the signal receiving device of different paths is extracted as the thickness distribution weight factor to obtain the defect thickness distribution. Imaging is performed based on the results of defect area positioning and defect thickness distribution to achieve defect imaging.

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