Defect imaging method based on multi-modal guided waves

Abstract

The disclosure provides a defect imaging method based on multi-modal guided waves, comprising: setting an acoustic source and a signal receiving device on the upper surface of a structure to be measured to obtain wave field data in several different paths; calculating the arrival time difference; obtaining the average length and depth of the defects on each path; analyzing the distribution of the defect information measured by each path in the spatial position to locate the defect area; substituting the defect depth of each path into the thickness distribution calculation formula to obtain the defect thickness distribution, and combining the results of the defect area positioning to realize defect imaging. The defect imaging method based on multi-modal guided waves provided by the disclosure only needs a set of acoustic source and signal receiving device in the testing process, which can realize the identification of defect depth and length, reduces the testing cost, and improves the testing efficiency; the imaging method can be applied to various guided waves and has universal applicability; and the influence of the modal shift caused by the defects on the defect identification is effectively reduced.

Description

TECHNICAL FIELD

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

[0002] When a structure is affected by chemical corrosion or natural degradation during long-term use, structural damage such as local thinning of a wall or a board may occur. Guided waves have been widely used in non-destructive testing and structural health monitoring due to their long-distance propagation ability and sensitivity to damage. In a damaged structure, the presence of defects changes the propagation speed of guided waves and their reflection, transmission and mode conversion characteristics.

[0003] Current detection techniques can usually only give the approximate location and relative corrosion degree of the defect. Accurate description of the size of the defect is time-consuming and labor-intensive, and often requires a multi-sensor cluster or point-by-point scanning method to obtain the range and depth of the defect, which is difficult to calculate, time-consuming and costly. SUMMARY

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

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

[0006] Setting an acoustic source and a signal receiving device on the surface of a non-destructive structure to draw a dispersion curve of the non-destructive structure;

[0007] Setting an acoustic source and a signal receiving device on the surface of the structure to be tested, simulating the time-domain full-wave wave train of the guided wave field, and obtaining wave field data in several different paths;

[0008] Reading the arrival time of the multi-modal wave packet in each path, and calculating the wave packet arrival time difference corresponding to the modal in the dispersion curve of the non-destructive structure;

[0009] Substituting the arrival time difference into a defect information calculation formula to obtain the average length and depth of the defect on each path, the defect information calculation formula being derived from guided wave flight time theory;

[0010] Analyzing the distribution of the defect information measured by each path in the spatial position to locate the defect area;

[0011] Substituting the defect depth of each path into a thickness distribution calculation formula to obtain the defect thickness distribution, and combining the results of defect area positioning to realize defect imaging.

[0012] In some embodiments, setting an acoustic source and a signal receiving device on the surface of a non-destructive structure to draw a dispersion curve of the non-destructive structure comprises:

[0013] 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] In the formula, f is the central frequency of the sound source, and 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] On the surface of the lossless structure, a sound source is arranged to excite guided waves, and the lower surface of the lossless structure is a free condition. For the guided waves propagating in the lossless structure, according to the polarization mode, the guided waves are divided into symmetric modes and asymmetric modes. The dispersion equation of the symmetric mode is as follows:

[0019]

[0020] The dispersion equation of the asymmetric mode is as follows:

[0021]

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

[0023] Meanwhile, p and q satisfy the following formulas respectively:

[0024]

[0025] In the formula, c L and c T respectively represent the longitudinal wave velocity and the transverse wave velocity in the lossless structure, and thus 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 surface of the structure to be measured, the time-domain full-wave wave train of the guided wave field is simulated, and the wave field data in several different paths are obtained, including:

[0027] A rectangular detection area is selected on the structure to be measured, a group of sound sources and signal receiving devices are used, the sound sources are arranged on the edges of the rectangle, the signal receiving devices are arranged at positions corresponding to the edges opposite to the edges where the sound sources are arranged, the line connecting the sound source and the signal receiving device is perpendicular to the edge where the sound source is arranged, the sound sources excite multi-mode guided waves, the signals received by the signal receiving devices are recorded, and 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 a preset interval, the connection line between the sound source and the signal receiving device is perpendicular to the edge where the sound source is located, the sound source excites multi-modal guided waves, and the signals received by the signal receiving device are recorded,

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

[0030] In some embodiments, the arrival time of the multi-modal wave packet in each path is read by using a Hilbert transform.

[0031] In some embodiments, the arrival time of the multi-modal wave packet in each path is read, and the calculation of the time difference of the wave packet arrival corresponding to the modal in the dispersion curve of the lossless structure includes:

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

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

[0034] Supposing that the structure to be measured is a metal plate of an isotropic medium, the thickness is d, there is a defect with a length of r under the lower surface of the structure to be measured, the cross-sectional depth distribution of the defect is random, 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 denoted as d k The position of the defect changes the group velocity c g , and the expression of the influence of the defect on the flight time of the guided wave of any modal is as follows:

[0035]

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

[0037] The average value d' of the defect depth is used as the overall depth of the defect, and the expression of the influence of the defect on the flight time of the guided wave of any modal is approximately expressed as:

[0038]

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

[0040]

[0041] In the formula, L represents the interval between the sound source and the signal receiving device in the current path; and cg ' = c g (fd');

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

[0043]

[0044] In the formula, the function c of the defect depth g ' and K are unknown, the above expression is applicable to any modal guided wave, and the time difference of at least two modal guided waves is substituted to obtain the equation as follows:

[0045]

[0046] Under the premise of known material parameters, the function c of the defect depth g ' and K are solved by solving the above equation, and the defect length r is calculated according to K.

[0047] In some embodiments, the distribution of the defect information measured by each path in the spatial position is analyzed, and the positioning of the defect region comprises:

[0048] It is judged whether the wave packet arrival time difference calculated by each path is 0, if the wave packet arrival time difference is not 0, it is judged that the path is a defect path,

[0049] All defect paths are obtained, and the range of the region where the defect is located is obtained according to the intersection position of the defect path,

[0050] According to the range of the region where the defect is located, the accurate position of the defect region is obtained in combination with the calculated defect length.

[0051] In some embodiments, 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 in combination with the positioning result of the defect region, comprising:

[0052] The defect depth calculated on each path is extracted and assigned to the region with a width of 1 / 2 of the sound source spacing on both sides of the corresponding path,

[0053] The maximum peak value of the wave packet received by the signal receiving device of different paths is extracted as a thickness distribution weight factor, and the defect thickness distribution is obtained,

[0054] According to the positioning result of the defect region and the defect thickness distribution, imaging is performed to realize defect imaging.

[0055] Compared with the prior art, the technical scheme provided by the present disclosure has the following beneficial effects: the test process only needs a set of sound sources and signal receiving devices, and the defect depth and length can be identified, thereby reducing the test cost, simplifying the calculation process, and improving the test efficiency; the imaging method can be applied to various guided waves and various types of defects, and has universal applicability; the influence of the modal shift caused by the defect on the defect identification is effectively reduced, the influence of the error on the detection result is reduced, and the detection efficiency and accuracy are improved.

[0056] In addition, in the technical scheme of the present disclosure, any unexplained part can be realized by using conventional means in the art. BRIEF DESCRIPTION OF DRAWINGS

[0057] In order to more clearly illustrate the technical scheme of the embodiments of the present disclosure, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present disclosure, and other drawings can also be obtained by those skilled in the art without creative labor.

[0058] Figure 1 The flow chart of the defect imaging method based on multi-modal guided waves provided by an embodiment of the present disclosure.

[0059] Figure 2 The 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 by an embodiment of the present disclosure.

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

[0061] Figure 4 The signal comparison diagram of the signal received by the signal receiving device in the defect-free path and the defect path in S13 in the defect imaging method based on multi-modal guided waves provided by an embodiment of the present disclosure.

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

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

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

[0065] To make the objects, technical solutions and advantages of the embodiments of the present disclosure clearer, the following will be combined with the drawings in the embodiments of the present disclosure to describe the technical solutions in the embodiments of the present disclosure clearly and completely. Obviously, the described embodiments are only some, but not all of the embodiments of the present disclosure. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present disclosure.

[0066] Embodiment:

[0067] With reference to the drawings attached Figure 1 in the description, a multi-modal guided wave based defect imaging method provided by an embodiment of the present disclosure is shown, which includes the following steps:

[0068] S11: setting a sound source and a signal receiving device on the surface of a lossless structure, and drawing a dispersion curve of the lossless structure;

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

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

[0071] S14: substituting the arrival time difference into a defect information calculation formula to obtain the average length and depth of the defect on each path, the defect information calculation formula being obtained by deduction from guided wave time of flight theory;

[0072] S15: analyzing the distribution of the defect information measured by each path in the spatial position, and positioning the defect region;

[0073] S16: substituting the defect depth of each path into a thickness distribution calculation formula to obtain the defect thickness distribution, and realizing defect imaging in combination with the positioning result of the defect region.

[0074] In the embodiment, an aluminum plate with a length and width of 300 mm and a thickness d of 4 mm is selected as a structure to be measured to illustrate the multi-modal guided wave based defect imaging method provided by the present application. The aluminum plate can be regarded as an isotropic homogeneous medium, with a density p of 2710 kg / m 3 , a Young's modulus E of 69 GPa, and a Poisson's ratio v of 0.33. 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 measured.

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

[0076] The lossless structure adopts an aluminum plate with the same size and material as the structure to be measured. A sound source is arranged on the upper surface of the lossless structure to excite multi-modal guided waves, and a signal receiving device is arranged on the upper surface of the lossless structure to receive the propagation signal. The signal receiving device can adopt a sound wave signal receiving sensor. In the embodiment, the sound source excites multi-modal Lamb waves. The waveform graph of the time-domain waveform of the selected sound source pulse in the embodiment can be referred to the attached drawings of the specification Figure 2 The central frequency f0 of the sound source is 300 kHz.

[0077] In the 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] In the formula, f is the central frequency of the sound source, and 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 nearest two sides of the aluminum plate can be set to 50 mm. The multi-modal Lamb wave field time-domain full-wave data in the lossless structure is obtained by simulating the time-domain transient results in the lossless structure.

[0083] The sound source is arranged on the upper surface of the aluminum plate to excite Lamb guided waves. The lower surface of the aluminum plate is free. For the Lamb guided waves propagating in the aluminum plate, the Lamb guided waves are divided into symmetric modes and asymmetric modes according to the polarization mode. The dispersion equation of the symmetric mode is as follows:

[0084]

[0085] The dispersion equation of the asymmetric mode is as follows:

[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 Let represent the longitudinal wave velocity and the transverse wave velocity in the lossless structure, respectively. Based on the dispersion equation, the dispersion curve of the lossless structure can be plotted.

[0091] In S12, a sound source and a signal receiving device are set on the upper surface of the structure under test to simulate the full-wave train of the guided wave field in the time domain and obtain wave field data in several different paths. This may include the following steps:

[0092] A rectangular detection area is selected on the structure under test. A set of sound sources and signal receiving devices is used. The sound source is placed on one side of the detection area, and the signal receiving device is placed on the opposite side of the sound source, with the line connecting the sound source and the signal receiving device perpendicular to the side containing the sound source. The sound source excites a multimode 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 signal receiving device are changed according to a 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. This causes the sound source to excite a multimode guided wave, and the signal received by the signal receiving device is recorded.

[0094] Based on the propagation signals of the guided waves received by the signal receiving devices in each path, the time-domain full-wave train of the guided wave field is simulated using the finite element method.

[0095] In this embodiment, please refer to the appendix to the specification. Figure 3 As shown, a square area with a side length of L (200mm) was selected as the detection area, and the square detection area is located in the middle of the aluminum plate, meaning the center of the square coincides with the center of the aluminum plate. The spacing between each sound source placement position can be determined according to the required detection accuracy. 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, and the spacing can be set to 4mm. 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] Starting from a vertex of the square detection area, a sound source location point is set every 4mm along the two sides adjacent to that vertex. In this embodiment, a total of 102 sound source location points are set along the two adjacent sides of the detection area. The sound source location points can be denoted as horizontal A1-horizontal A51 and vertical A1-vertical A51, where horizontal A1 and vertical A1 are the same point. Horizontal A1, horizontal A51, and vertical A51 are the three vertices of the detection area. The signal receiving device corresponding to horizontal A1-horizontal A51 is denoted as horizontal R1-horizontal R51, and the signal receiving device corresponding to vertical A1-vertical A51 is denoted as vertical R1-vertical R51. Thus, including the four sides of the detection area, there are a total of 102 detection paths.

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

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

[0099] The Hilbert transform can be used in signal processing to extract the envelope of a signal. It converts the signal into an analytic signal containing the amplitude and phase information of the original signal, from which the envelope signal is extracted. The Hilbert transform can phase-shift the positive and negative frequencies by π / 2, achieving single-sidebanding, simplifying analysis and saving computational resources. The mathematical representation of Hilbert's transform 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. This is beneficial for the analysis of narrowband frequency-modulated and amplitude-modulated waves, and can reduce sampling frequency requirements and improve signal processing efficiency.

[0102] To compare the time difference results generated after the wave packet passes through a defect, a control group was set up, comparing the signals received by the signal receiving device with and without the defect. (Refer to the appendix of the instruction manual.) Figure 4The healthy path in the figure is the signal received by the signal receiving device when there is no damage, and the damaged path is the signal received by the signal receiving device when there is a defect, S0 represents a symmetric mode, and A0 represents an asymmetric mode.

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

[0104] Suppose that the structure to be measured is an isotropic aluminum plate with a thickness of d, and there is a defect with a length of r and a random cross-sectional depth distribution under the surface of the structure to be measured, the defect is divided into k parts along the length direction, and the defect length of each part is Δr, and the defect depth of each part is denoted as d k , the position of the defect changes the group velocity c g , and the influence of the defect on the time-of-flight of the guided wave of any mode is expressed as follows:

[0105]

[0106] In the formula, ΔTOF represents the time difference of the guided wave packet of a certain mode, f is the main frequency of the excitation sound source, the generation of the defect is generally not abrupt, and therefore the defect depth d k of each part is not much different, and therefore the average value d' of the defect depth can be used as the overall depth of the defect, and the influence of the defect on the time-of-flight of the guided wave of any mode is approximately expressed as:

[0107]

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

[0109]

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

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

[0112]

[0113] In the formula, the function c g ′ of the defect depth and K are unknown numbers, and the above expression is applicable to any guided wave, and the time difference of at least two guided waves is substituted to obtain the following equation:

[0114]

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

[0116] In this embodiment, the calculation results of the average length and depth of the defect are shown in the attached drawings of the specification Figure 5 wherein D represents the thickness of the structure, and β represents the time difference ratio of the arrival of the Lamb wave S0 mode and the A0 mode. wherein c represents the group velocity of the A0 mode corresponding to the remaining thickness.

[0117] In S15, the distribution of the defect information measured by each path in the spatial position is analyzed, and the positioning of the defect region can include the following steps:

[0118] It is judged whether the wave packet arrival time difference calculated by each path is 0, if the wave packet arrival time difference is not 0, it is judged that the path is a defect path,

[0119] All defect paths are obtained, and the range of the defect region is obtained according to the intersection position of the defect paths,

[0120] According to the range of the defect region, the accurate position of the defect region is obtained in combination with the calculated defect length.

[0121] In the attached drawings of the specification Figure 6 , in this embodiment, the detection region is divided into square grids, the side length L of the detection region is 200mm, and the side length L of the square grid is 2mm, therefore, the spacing between the sound sources should be no more than 2*L min In this embodiment, the spacing between the sound sources is 4mm, and there are a total of 100*100 grids in the detection region. min In this embodiment, the spacing between the sound sources is 4mm, and there are a total of 100*100 grids in the detection region.

[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 sources are 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, the time difference occurs, then the six paths of horizontal A25-horizontal R25, horizontal A26-horizontal R26, horizontal A27-horizontal R27, vertical A25-vertical R25, vertical A26-vertical R26 and vertical A27-vertical R27 are all defect paths, the intersection region of the defect paths is obtained, at this time, the intersection region of the defect paths can be preliminarily determined as the defect region.

[0123] After the range of the defect area is preliminarily determined, the defect area is further judged in combination with the defect length calculated in S14. For example, in the embodiment, the defect length on the path A26-R26 in the transverse direction is calculated to be 22.18 mm, so the range of 20 mm obtained from the intersection area of the defect paths should be extended to 22 mm from the center position of 20 mm, and the calculation results of other paths are similarly extended, and finally the accurate position of the defect area is 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 the defect imaging in combination with the results of the defect area positioning can include the following steps:

[0125] The defect depth calculated on each path is extracted and assigned to the area with a width of 1 / 2 of the sound source spacing on both sides of the corresponding path,

[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] The defect imaging is realized according to the results of the defect area positioning and the defect thickness distribution.

[0128] Specifically, the defect depth calculated on each path of the defect area is extracted and assigned to the area with a width of 1 / 2 of the sound source spacing on both sides of the corresponding path. In the embodiment, the spacing of the sound sources is 4 mm, so the defect depth calculated on each path is extracted and assigned to the grid with 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 intersection path, the thickness value of each grid should be the average of the grid thicknesses of the two intersection paths. The grid thickness of the corresponding area of the defect path is denoted as d(i,j), and the expression of the grid thickness is d(i,j) = (d i +d j ) / 2, where i can represent the sequence number of the defect path in the x direction, and j can represent the sequence number of the defect path in the y direction. In the embodiment, i represents the sequence number of the defect path in the transverse direction, and j represents the sequence number of the defect path in the longitudinal direction.

[0130] The maximum peak value of the wave packet received by 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 d(i,j) = (I j d i +I i d j ) / 2;

[0131] According to the results of the defect area positioning and the thickness distribution of each grid in the defect area, imaging is performed to realize imaging of the defect.

[0132] In this example, the results of the defect imaging are referenced in the description accompanying Figure 7

[0133] The technical solution provided by the present disclosure has the following beneficial effects compared with the prior art: only one set of sound sources and signal receiving devices are needed in the testing process, which can realize identification of the defect depth and length, reduces the testing cost, simplifies the calculation process, and improves the testing efficiency; the imaging method can be applied to various guided waves and various types of defects, and has universal applicability; the influence of the modal shift caused by the defect on the defect identification is effectively reduced, the influence of the error on the detection result is reduced, and the detection efficiency and accuracy are improved.

[0134] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present disclosure, and not to limit them; although the present disclosure has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the embodiments of the present disclosure.​

Claims

1. A method of defect imaging based on multi-modal guided waves, characterized in that, The method comprises the following steps: setting a sound source and a signal receiving device on the surface of the intact structure, and drawing a dispersion curve of the intact structure; setting a sound source and a signal receiving device on the surface of the structure to be measured, simulating a time-domain full-wave wave train of a guided wave field, and obtaining wave field data in a plurality of different paths; reading the arrival time of a multi-modal wave packet in each path, and calculating the wave packet arrival time difference corresponding to the mode in the dispersion curve of the intact structure; substituting the arrival time difference into a defect information calculation formula to obtain the average length and depth of the defect in each path, wherein the defect information calculation formula is derived from guided wave time-of-flight theory; analyzing the distribution of the defect information measured in each path in the spatial position to locate the defect region; substituting the defect depth of each path into a thickness distribution calculation formula to obtain the defect thickness distribution, and combining the results of the defect region positioning to realize defect imaging; wherein the step of setting a sound source and a signal receiving device on the surface of the structure to be measured, and simulating a time-domain full-wave wave train of a guided wave field to obtain wave field data in a plurality of different paths comprises: selecting a rectangular detection region on the structure to be measured, using a group of sound sources and signal receiving devices, setting the sound sources on the edges of the rectangle, and setting the signal receiving devices on the positions corresponding to the opposite edges of the sound sources, so that the connecting line of the sound source and the signal receiving device is perpendicular to the edge where the sound source is located, the sound source excites multi-modal guided waves, the signal receiving device receives signals, and the connecting line of the sound source and the signal receiving device is recorded as a detection path, changing the positions of the sound sources and the signal receiving devices according to a predetermined interval, so that the connecting line of the sound source and the signal receiving device is perpendicular to the edge where the sound source is located, the sound source excites multi-modal guided waves, and the signal receiving device records the received signals, simulating a time-domain full-wave wave train of a guided wave field according to the propagation signals of the guided waves received by the signal receiving device in each path by a finite element simulation method; the step of obtaining the defect information calculation formula comprises: The to-be-tested structure is a metal plate of isotropic medium, the thickness is d, there is a defect with a length of r and a random cross-section depth distribution under the lower surface of the to-be-tested structure, the defect is divided into k parts along the length direction, and the defect length of each part is The defect depth of each part is denoted as d k The position of the defect changes the group velocity c g , and the influence of the defect on the time of flight of any mode of guided wave is expressed as follows: wherein ΔTOF represents the flight time difference of a guided wave packet of a certain mode, f is the main frequency of the excitation sound source, using the average value d' of the defect depth as the overall depth of the defect, and then the expression of the influence of the defect on the time of flight of any modal guided wave is approximately expressed as: the expression of the average group velocity is as follows: In the formula, L represents the distance between the sound source and the signal receiving device in the current path. ; assuming that K = r / L, then K ∈ [0, 1], and the expression of the average group velocity is simplified as: where the function of defect depth and K is an unknown, the above expression applies to guided waves of any mode, and the time difference for at least two modes of guided waves is substituted to obtain the equation as follows: Solving the above equations with known material parameters gives the function of defect depth and K, from which the defect length r is calculated. the step of substituting the defect depth of each path into the thickness distribution calculation formula to obtain the defect thickness distribution, and combining the results of the defect region positioning to realize defect imaging comprises: extracting the calculated defect depth on each path and assigning it to the regions on both sides of the corresponding path with a width of 1 / 2 of the sound source interval, extracting the maximum peak value of the wave packet received by the signal receiving device in different paths as a thickness distribution weight factor to obtain the defect thickness distribution, performing imaging according to the results of the defect region positioning and the defect thickness distribution to realize defect imaging.

2. The defect imaging method based on multi-modal guided waves according to claim 1, wherein the step of setting a sound source and a signal receiving device on the surface of the intact structure, and drawing a dispersion curve of the intact structure comprises: the expression of the sound source function is as follows: wherein f is the acoustic source center frequency, T = 1 / f is the acoustic source pulse width; the expression of the frequency domain function corresponding to the sound source function is as follows: The guided wave is excited by a sound source on the surface of the lossless structure, and the lower surface of the lossless structure is free, so the guided wave propagating in the lossless structure is divided into symmetric mode and asymmetric mode according to the polarization mode, wherein the dispersion equation of the symmetric mode is: The dispersion equation of the asymmetric mode is: wherein is the wave number, is the circular frequency, denotes the phase velocity of the guided wave; At the same time, p and q respectively satisfy the following formulae: wherein c L and c T denote the longitudinal and transverse wave velocities in the lossless structure, respectively, whereby the dispersion curves of the lossless structure are plotted from the dispersion equation.

3. The multi-modal guided wave-based defect imaging method of claim 1, wherein, The arrival time of the multi-modal wave packet in each path is read by using Hilbert transform.

4. The multi-modal guided wave based defect imaging method according to claim 3, wherein the reading of the arrival time of the multi-modal wave packet in each path comprises: The wave packet time domain information and amplitude of each path signal are extracted respectively, the wave packet envelope line is drawn by using Hilbert transform, the peak value arrival time and amplitude of each modal wave packet are read, the wave packet arrival time of each path is subtracted from the wave packet arrival time of the corresponding mode in the lossless structure, and the wave packet arrival time difference is obtained.

5. The multi-modal guided wave based defect imaging method according to claim 1, wherein the analysis of the distribution of the defect information measured by each path in the spatial position comprises: It is judged whether the wave packet arrival time difference calculated by each path is 0, if the wave packet arrival time difference is not 0, it is judged that the path is a defect path, All defect paths are obtained, the range of the defect area is obtained according to the intersection position of the defect paths, The accurate position of the defect area is obtained according to the range of the defect area and the calculated defect length. ​ ​

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