Electromagnetic ultrasonic transducer, excitation system, excitation method and simulation method for exciting unidirectional single-mode SH1 guided waves

Abstract

The present invention discloses an electromagnetic ultrasonic transducer, an excitation system, an excitation method and a simulation method for exciting unidirectional single-mode SH1 guided waves; the transducer is composed of a set of alternating coil arrays and periodic permanent magnets for double-sided excitation, each coil is wound by enameled wire wound on a single rectangular permanent magnet, and the coil winding direction alternates between clockwise and counterclockwise; adjacent rectangular permanent magnets are 1 / 4 wavelength apart, and are arranged alternately according to periodic magnetic poles, and are installed in a 3D printed fixed slot. Each coil and permanent magnet constitutes an independent electromagnetic ultrasonic excitation unit, which utilizes the interference effect of waves to realize the excitation of unidirectional single-mode SH1 waves, and controls the excitation direction of the waves by changing the delay sequence of two excitation currents. The present invention can effectively suppress the SH0 symmetric mode, reduce the complexity of multi-mode, and reduce the difficulty of subsequent signal processing, and has important value for defect detection of small-sized plates.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic ultrasonic detection technology, and in particular to an electromagnetic ultrasonic transducer and method for exciting unidirectional single-mode SH1 guided waves. Background Art

[0002] At present, SH1 guided wave, as the first-order mode of horizontal shear wave, has the advantages of not being easy to attenuate and having low requirements for pretreatment of the inspected surface because the particle vibration direction is parallel to the surface of the test piece. Moreover, compared with the zero-order mode (SH0) guided wave, SH1 guided wave has higher frequency and more complex wave structure characteristics, which can provide more abundant defect information during ultrasonic testing and is very suitable for the detection of plate cracks, corrosion, and welding defects. However, according to the SH guided wave dispersion curve, when SH1 guided wave is excited, SH0 guided wave will be excited at the same time, and mode conversion will also occur when interacting with defects. In addition, the multi-directional excitation characteristics of the transducer and the influence of the end face reflection of the plate make the echo signal very complex, which is not conducive to further signal analysis. The development of SH1 guided wave electromagnetic ultrasonic transducer that can excite unidirectional single mode can not only effectively reduce the complexity of multi-mode, but also reduce the interference signal of end face reflection by unidirectional emission guided wave, and reduce the difficulty of signal processing, which is of great significance for the defect detection of small-sized plates.

[0003] In order to obtain a pure mode SH1 wave, Song XC et al. proposed a dual periodic permanent magnet electromagnetic ultrasonic transducer (PPM-EMAT) placed symmetrically on both sides of an aluminum plate. Experiments verified that the dual PPM-EMAT can suppress the SH0 mode and produce a purer SH1 mode (Qiu GZ, Song XC, Zhang X, Tu J, Chen T. Pure SH1guided-wave generation method with dual periodic-permanent-magnetelectromagnetic acoustic transducers for plates inspection. Sensors, 2019, 19, 3019). Khalili P. et al. proposed a method to use a single PPM-EMAT to excite single-mode SH1 guided waves with a frequency-thickness product of about 3MHz-mm in a plate (Khalili P, Cegla F. Excitation of single-mode shear-horizontal guided waves and evaluation of their sensitivity to very shallow crack-like defects. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2021, 68(3), 818-828). Kubrusly AC proposed a mode selection method for dual PPM-EMAT to excite pure SH guided waves, which can realize the mode conversion of SH0 and SH1 waves at the wall thinning point (Kubrusly AC, Freitas MA, von der Weid JP, Dixon S. Mode selectivity of SH guided waves by dual excitation and reception applied to mode conversion analysis. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. 2018, 65(7), 1239-1249). However, although the current PPM-EMAT method can generate single-mode SH1 waves, the excitation of unidirectional single-mode SH1 guided waves has not yet been achieved. Summary of the invention

[0004] The object of the present invention is to provide an electromagnetic ultrasonic transducer, an excitation system, an excitation method and a simulation method for exciting a unidirectional single-mode SH1 guided wave, which can realize the excitation of the unidirectional single-mode SH1 guided wave.

[0005] The technical solution adopted by the present invention is:

[0006] An electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves, comprising an aluminum plate and an EMAT-A structure and an EMAT-B structure which are placed on the upper and lower surfaces of the aluminum plate and have the same structure, wherein the EMAT-A structure comprises an electromagnetic ultrasonic excitation unit; the electromagnetic ultrasonic excitation unit is composed of four rectangular permanent magnet units with the same structure and a coil, wherein the four rectangular permanent magnet units are arranged in rows along the length direction of the aluminum plate at intervals in sequence, and each rectangular permanent magnet unit is arranged in vertical rows of two rectangular permanent magnets; the rectangular permanent magnet units between two adjacent rectangular permanent magnet units are spaced 1 / 4 wavelength apart;

[0007] The coil is an alternating coil array, that is, each coil is composed of an enameled wire wound on a single rectangular permanent magnet; one end of the coil in each rectangular permanent magnet unit is recorded as the head end, and the other end is recorded as the tail end, and the winding direction of the coils on the two rectangular permanent magnets in each rectangular permanent magnet unit is one clockwise and the other counterclockwise;

[0008] In each electromagnetic ultrasonic excitation unit, the tail ends of the coils of two rectangular permanent magnet units separated by a rectangular permanent magnet unit are connected to each other, and the first sections are respectively connected to the pulse excitation current, so as to realize the excitation of unidirectional single-mode SH1 waves through the interference of waves; the EMAT-B structure is the same as the EMAT-A structure.

[0009] The connection between the coil and the pulse excitation current in the electromagnetic ultrasonic excitation unit is specifically as follows: from left to right, when the coil head end of the first rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the first pulse excitation current 1, the coil head end of the third rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the first pulse excitation current 1; at the same time, the coil head end of the first rectangular permanent magnet unit in the EMAT-B structure is connected to the input end of the first pulse excitation current 1, and the coil head end of the third rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the first pulse excitation current 1; at this time, the coil head end of the second rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the second pulse excitation current 2, and the coil head end of the fourth rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the second pulse excitation current 2; at the same time, the coil head end of the second rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the second pulse excitation current 2, and the coil head end of the fourth electromagnetic ultrasonic excitation unit in the EMAT-B structure is connected to the input end of the second pulse excitation current 2.

[0010] There are multiple electromagnetic ultrasonic excitation units, which are arranged in a row, and the corresponding coil in each electromagnetic ultrasonic excitation unit has the same connection relationship with the corresponding pulse excitation current.

[0011] The invention also includes fixing grooves for fixing the relative positions of the rectangular permanent magnets to remain unchanged. The fixing grooves are arranged on the upper and lower surfaces of the aluminum plate corresponding to the rectangular permanent magnets.

[0012] The distance ΔD between adjacent slots in the x direction is λ / 4, the slot gap Δd is 0.5 mm, and the gap Δh in the y direction is 2 mm; the notch Δs on the side is 1 mm, which is used to adjust the winding direction of the coil.

[0013] The fixing groove adopts two 3D printed R4600 nylon frame structures.

[0014] The excitation frequency f is 0.3MHz, the phase velocity c p is 3621m / s, the group velocity is 2654m / s, then the wavelength λ=c p / f=12mm

[0015] The invention discloses an excitation system based on the electromagnetic ultrasonic transducer for exciting the unidirectional single-mode SH1 waveguide, comprising: a high-energy pulse test system, a PC, an oscilloscope, an impedance matching module, a preamplifier, a high-pass filter and a receiving probe; the first output end of the high-energy pulse test system is used to output a first pulse excitation current through an impedance matching circuit, and the second output end is used to output a second pulse excitation current through an impedance matching circuit, which are respectively used to be connected to the electromagnetic ultrasonic transducer for exciting the unidirectional single-mode SH1 waveguide; the receiving probe is arranged on an aluminum plate, and is linked to the receiving end of the high-energy pulse test system through a high-pass filter and a preamplifier in turn; the PC and the oscilloscope are respectively connected to the high-energy pulse test system.

[0016] The excitation method based on the electromagnetic ultrasonic transducer for exciting the unidirectional single-mode SH1 guided wave comprises the following steps:

[0017] A: Excite the single-mode SH1 wave: When a pulse excitation current is passed through the coil, the Lorentz force generated in the skin layer of the aluminum plate under the action of the alternating magnetic pole magnetic field, the Lorentz force and displacement at the symmetrical positions on the upper and lower sides of the aluminum plate are opposite, which can effectively suppress the generation of the symmetrical mode SH0, and at the same time, enhance the excitation of the asymmetric mode SH1 wave, that is, stimulate the single-mode SH1 wave;

[0018] B: Excited unidirectional single-mode SH1 wave: The second pulse excitation current i2 is adjusted to be delayed by 1 / 4 cycle compared with the first pulse excitation current i1, so as to generate SH1 waves in the coil along the positive and negative directions of the x-axis; f represents the excitation signal frequency, and c represents the wave phase velocity; the wave signal is enhanced on the right side due to interference, while the interference destructive effect on the left side makes the wave signal almost zero; that is, a single-mode SH1 wave that only propagates to the right is generated;

[0019] C: When it is necessary to excite a single-mode SH1 wave propagating to the left, the delay sequence of the first excitation current i1 and the second excitation current i2 of the coil is changed, that is, the first pulse excitation current i1 is delayed by 1 / 4 cycle than the second pulse excitation current i2, thereby obtaining a unidirectional single-mode SH1 wave propagating to the left.

[0020] The simulation method based on the electromagnetic ultrasonic transducer for exciting the unidirectional single-mode SH1 guided wave comprises the following steps:

[0021] S1: Establish a coordinate system and set the solver: establish a three-dimensional rectangular coordinate system with the row arrangement direction of the permanent magnets, i.e. the SH1 waveguide propagation direction, as the x-axis, the vertical arrangement direction as the y-axis, and the thickness direction of the aluminum plate as the z-axis; add a steady-state solver to solve steady-state variables, and add a transient solver to solve time-varying variables related to the pulse excitation current;

[0022] S2: Build geometric models and add physical fields and material properties: Build geometric models of the air solution domain, permanent magnet, coil, and aluminum plate, and set their respective material properties; use the AC / DC module in the finite element simulation software to build the static magnetic field of the permanent magnet and the dynamic magnetic field of the coil, and add the magnetic field module and solid mechanics module to the physical field of the aluminum plate;

[0023] S3: Set solution conditions and mesh generation: In order to eliminate the influence of reflected waves, except for the upper and lower end faces of the aluminum plate, the remaining four end faces are set as low reflection boundary conditions; in order to reduce the calculation time, the mesh generation of the air domain and the permanent magnet is appropriately coarsened, and the mesh generation of the coil and the surface of the aluminum plate close to the coil is appropriately refined;

[0024] S4: Solving the simulation model and post-processing the data: In order to ensure the convergence and accuracy of the transient solution, the pre-processor of the iterative processor is used to improve the convergence speed of the calculation. The transient calculation time step cannot be set too large; after the solution is completed, the relevant data is post-processed and analyzed.

[0025] The present invention uses a transducer composed of a set of alternating coil arrays and periodic permanent magnets with double-sided excitation, and uses each coil and permanent magnet to form an independent electromagnetic ultrasonic excitation unit, and uses the interference effect of the wave to achieve the excitation of a unidirectional single-mode SH1 wave, and controls the excitation direction of the wave by changing the delay sequence of the two pulse excitation currents. Each coil is wound by an enameled wire wound on a single rectangular permanent magnet, and the coil winding direction alternates between clockwise and counterclockwise; adjacent rectangular permanent magnets are 1 / 4 wavelength apart, and are arranged alternately according to periodic magnetic poles, and installed in a 3D printed fixed slot. This method can effectively suppress the SH0 symmetric mode, reduce the complexity of multi-modality, and reduce the difficulty of subsequent signal processing, which is of great significance for defect detection of small-sized plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 It is the SH1 waveguide dispersion curve diagram of the present invention;

[0028] Figure 2 This is a schematic diagram of the EMAT-excited unidirectional single-mode SH1 wave of the present invention;

[0029] Figure 3 This is a structural diagram of a 3D-printed fixing groove of the present invention;

[0030] Figure 4 It is a wave structure diagram of the SH waveguide of the present invention;

[0031] Figure 5 Schematic diagram of unidirectional excitation of the present invention;

[0032] Figure 6 It is the EMAT three-dimensional simulation model diagram of the present invention;

[0033] Figure 7 The surface particle displacement cloud diagram of the aluminum plate of the present invention;

[0034] Figure 8 It is the y-component diagram of the particle displacement on the y=0 section of the present invention;

[0035] Fig. 9 It is a simulation result diagram of EMAT excitation and reception signals of the present invention;

[0036] Fig.10 It is the EMAT experimental system diagram of the present invention;

[0037] Fig.11 It is an experimental waveform diagram of the received signal of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] like Figure 1 , 2 As shown in Figure 3, the present invention includes an electromagnetic ultrasonic transducer and method for exciting a unidirectional single-mode SH1 waveguide, characterized in that: it includes an aluminum plate and an EMAT-A structure and an EMAT-B structure which are placed on the upper and lower surfaces of the aluminum plate and have the same structure, the EMAT-A structure includes an electromagnetic ultrasonic excitation unit; the electromagnetic ultrasonic excitation unit is composed of four rectangular permanent magnet units and coils with the same structure, the four rectangular permanent magnet units are arranged in rows along the length direction of the aluminum plate at intervals in sequence, and each rectangular permanent magnet unit is arranged in vertical rows of two rectangular permanent magnets; the rectangular permanent magnet units between two adjacent rectangular permanent magnet units are 1 / 4 wavelength apart;

[0040] The coil is an alternating coil array, that is, each coil is composed of an enameled wire wound on a single rectangular permanent magnet; one end of the coil in each rectangular permanent magnet unit is recorded as the head end, and the other end is recorded as the tail end, and the winding direction of the coils on the two rectangular permanent magnets in each rectangular permanent magnet unit is one clockwise and the other counterclockwise;

[0041] In each electromagnetic ultrasonic excitation unit, the tail ends of the coils of two rectangular permanent magnet units separated by a rectangular permanent magnet unit are connected to each other, and the first sections are respectively connected to the pulse excitation current, so as to realize the excitation of unidirectional single-mode SH1 waves through the interference of waves; the EMAT-B structure is the same as the EMAT-A structure. The connection between the coil and the pulse excitation current in the electromagnetic ultrasonic excitation unit is as follows: from left to right, when the coil head end of the first rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the first pulse excitation current 1, the coil head end of the third rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the first pulse excitation current 1; at the same time, the coil head end of the first rectangular permanent magnet unit in the EMAT-B structure is connected to the input end of the first pulse excitation current 1, and the coil head end of the third rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the first pulse excitation current 1; at this time, the coil head end of the second rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the second pulse excitation current 2, and the coil head end of the fourth rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the second pulse excitation current 2; at the same time, the coil head end of the second rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the second pulse excitation current 2, and the coil head end of the fourth electromagnetic ultrasonic excitation unit in the EMAT-B structure is connected to the input end of the second pulse excitation current 2.

[0042] There are multiple electromagnetic ultrasonic excitation units, which are arranged in a row, and the corresponding coil in each electromagnetic ultrasonic excitation unit has the same connection relationship with the corresponding pulse excitation current. When multiple units are arranged, the same excitation current is used, and in theory, an infinite number of units can be achieved (that is, as long as the number of groups is an integer multiple of 4), but in actual use, factors such as current transmission attenuation make the best effect generally achieved when there are 4 groups.

[0043] It also includes a fixed groove, which is arranged on the upper and lower surfaces of the aluminum plate corresponding to the rectangular permanent magnet. The distance ΔD between adjacent grooves in the x direction is λ / 4, the groove gap Δd is 0.5mm, the gap Δh in the y direction is 2mm, and the groove depth Δr is 6mm; the side notch Δs is 1mm, which is used to adjust the winding direction of the coil; the fixed groove adopts two 3D printed R4600 nylon material frame structures; the size error range of the fixed groove is ±0.2mm. If it exceeds the size range, the permanent magnet and the coil may not be properly placed in the fixed groove.

[0044] The size of the single rectangular permanent magnet is 2mm×20mm×9mm, and the size error range of the permanent magnet is ±0.2mm. If the size range is exceeded, the permanent magnet and the coil may not be properly placed in the fixed slot. The coil is wound with copper enameled wire with a diameter of 0.25-0.5mm. If the wire diameter is too small, the resistance increases and the current decreases. If the wire diameter is too large, the number of turns that the coil can accommodate becomes smaller.

[0045] The excitation frequency f is 0.3MHz, the phase velocity c p is 3621m / s, the group velocity is 2654m / s, then the wavelength λ=c p / f=12mm.

[0046] The invention comprises: a high-energy pulse test system, a PC, an oscilloscope, an impedance matching module, a preamplifier, a high-pass filter and a receiving probe; the first output end of the high-energy pulse test system is used to output a first pulse excitation current through an impedance matching circuit, and the second output end is used to output a second pulse excitation current through an impedance matching circuit, which are respectively used to be connected to the transducer accordingly; the receiving probe is arranged on an aluminum plate, and is connected to the receiving end of the high-energy pulse test system through a high-pass filter and a preamplifier in turn; the PC and the oscilloscope are respectively connected to the high-energy pulse test system.

[0047] The following is a further explanation with a specific example. In actual use, the working point must be determined first: choosing a suitable working point can suppress multi-mode and reduce the complexity of signal analysis. l =6300m / s, shear wave velocity c t =3100m / s. In this paper, the thickness of the aluminum plate is selected as 10mm, and the dispersion curve is as follows Figure 1 (a) and (b). When the frequency is relatively low (less than 200kHz), the wavelength is relatively long and the volume of the transducer is relatively large, which is not conducive to the miniaturization of the transducer. When the frequency is greater than 320kHz (SH2 mode cutoff frequency), additional SH2 mode waves will appear, increasing the complexity of the signal. Taking the above factors into consideration, the excitation frequency range of the SH1 waveguide is 200kHz to 320kHz. The working point of the present invention is set to point M, that is, the excitation frequency f is 0.3MHz, and the phase velocity c is 0. p is 3621m / s, group velocity c g is 2654m / s, then the wavelength λ=c p / f=12mm. It should be noted that if the working point excitation frequency is set to other values ​​between 200kHz and 320kHz, the corresponding wavelength will also change according to the dispersion curve.

[0048] The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 waves proposed in the present invention comprises a set of double-sided excited alternating coil arrays, periodic rectangular permanent magnets and fixed slot devices symmetrically placed on the upper and lower surfaces of an aluminum plate, and its structure is as follows: Figure 2 As shown. In the figure, the upper and lower surfaces of the aluminum plate are EMAT-A and EMAT-B respectively. EMAT-A is composed of eight independent rectangular "permanent magnet + coil" electromagnetic ultrasonic excitation units 1# to 8#. The rectangular permanent magnets are arranged in an alternating magnetic pole arrangement according to the PPM periodic permanent magnet. The blue and white squares represent the permanent magnet poles, and the adjacent permanent magnets are 1 / 4 wavelength apart. The coil is not a conventional runway coil, but an alternating coil array, that is, each coil is composed of an enameled wire wound around a single rectangular permanent magnet, and the winding direction of each coil alternates between clockwise and counterclockwise.

[0049] EMAT-B has the same structure as EMAT-A, consisting of eight "permanent magnet + coil" units from 1'# to 8'#, but the magnetization direction is opposite to 1# to 8#, which can not only effectively enhance the vertical magnetic field between the upper and lower EMATs, but also the permanent magnets on the upper and lower surfaces have a certain attraction effect, which is conducive to the installation and fixation of the permanent magnets. Since the winding direction of the coil alternates between clockwise and counterclockwise, the Lorentz forces generated by the excitation units in the same column have the same direction. For example, the Lorentz forces generated in the first column (units 1# and 5#) are represented by vertical gray arrows, and the Lorentz forces generated in the second column (units 2# and 6#) are represented by blue vertical arrows. EMAT-B and EMAT-A share two excitation currents i1 and i2. Both i1 and i2 are tone-burst signals with a frequency of 300kHz, a peak value of 20A and a frequency of 8. The phase of i2 lags behind the phase of i1 by 90°, that is, i2 is delayed by 1 / 4 cycle compared with i1. The current direction of the coil near the aluminum plate is along the x-axis, so that the direction of the induced current in the skin layer of the aluminum plate is opposite to the x-axis, while the magnetization direction of the permanent magnet is in the z-axis direction or the opposite direction. The coils under adjacent magnets are independent of each other, and the induced eddy currents of each coil in the aluminum plate are not related when the coil is energized. Each "permanent magnet + coil" unit on both sides of the aluminum plate independently excites electromagnetic ultrasound, and the interference of waves is used to achieve the excitation of unidirectional single-mode SH1 waves.

[0050] In order to minimize the error caused by the position of the coils and magnets and prevent the permanent magnets from attracting each other, the double-sided EMAT permanent magnet and coil array are mounted on two 3D printed (R4600 nylon) frame structures with fixed slots, one of which has a geometry such as Figure 3As shown. The size of a single rectangular permanent magnet is 2mm×20mm×9mm, and the coil is wound with copper enameled wire with a diameter of 0.25mm. The permanent magnet and the wound coil are installed in the slots, the distance ΔD between adjacent slots in the x direction is 3mm (=λ / 4), the slot gap Δd is 0.5mm, the gap Δh in the y direction is 2mm, and the slot depth Δr is 6mm; the notch Δs on the side is 1mm, which is used to adjust the winding direction of the coil. The EMAT is in contact with the surface of the aluminum plate to minimize the lift-off distance.

[0051] A method for exciting an electromagnetic ultrasonic transducer for exciting a unidirectional single-mode SH1 guided wave comprises the following steps:

[0052] A: Excite the single-mode SH1 wave: A pulse excitation current is passed through the coil. Under the action of the alternating magnetic pole magnetic field, the Lorentz force generated in the skin layer of the aluminum plate. At this time, the Lorentz force and displacement at the symmetrical positions on the upper and lower sides of the aluminum plate are opposite, which can effectively suppress the generation of the symmetrical mode SH0. At the same time, the excitation of the asymmetric mode SH1 wave is enhanced, that is, the single-mode SH1 wave is excited;

[0053] B: Excited unidirectional single-mode SH1 wave: The second pulse excitation current i2 is adjusted to be delayed by 1 / 4 cycle compared with the first pulse excitation current i1, so as to generate SH1 waves in the coil along the positive and negative directions of the x-axis; f represents the excitation signal frequency, and c represents the wave phase velocity; the wave signal is enhanced on the right side due to interference, while the interference destructive effect on the left side makes the wave signal almost zero; that is, a single-mode SH1 wave that only propagates to the right is generated;

[0054] C: When it is necessary to excite a single-mode SH1 wave propagating to the left, the delay sequence of the first excitation current i1 and the second excitation current i2 of the coil is changed, that is, the first pulse excitation current i1 is delayed by 1 / 4 cycle than the second pulse excitation current i2, thereby obtaining a unidirectional single-mode SH1 wave propagating to the left.

[0055] The excitation mechanism of the unidirectional unimodal SH1 wave described in the present invention is specifically as follows:

[0056] In the system composed of the coil and the aluminum plate of the EMAT, there is no free charge; if the influence of the displacement current is ignored, the Lorentz force f on the aluminum plate particle is L , the particle motion equation and the displacement field of the excited SH wave can be expressed by equations (1) to (3):

[0057] f L =J e ×B0 (1)

[0058]

[0059] u y (x,z,t)=An U n (z)e j(kx-ωt) (3)

[0060] Where B0 is the magnetic induction intensity, J e is the eddy current density in the aluminum plate, G and κ are the Lame constants, ρ is the material density, k is the wave number, and n is the modal order. n is the modal amplitude, U n (z) is the displacement distribution of each mode along the z coordinate.

[0061] Based on the Lorentz force mechanism, this paper combines the new EMAT structure of "PPM permanent magnet and alternating coil" to excite the unidirectional single-mode SH1 guided wave: through the double-sided excitation method, the particle displacements of the symmetrical positions on the upper and lower sides of the aluminum plate are opposite, which effectively suppresses the generation of the symmetric mode SH0 and enhances the excitation of the asymmetric mode SH1 wave; the mutual interference of the generated waves by the PPM permanent magnet and the two sets of alternating coil arrays is used to realize the excitation of the unidirectional single-mode SH1 wave. The principles of its single-mode excitation and unidirectional excitation are explained as follows:

[0062] (1) Single mode excitation. Figure 4 The SH waveguide structure diagram shown shows the distribution of displacement along the plate thickness direction. Since SH1 is an antisymmetric mode and SH0 is a symmetric mode, the in-plane displacement u of the SH1 mode is y The SH0 mode is anti-symmetric about the center line of the plate along the thickness direction. It can be seen that when only EMAT-A is excited and EMAT-B is not excited, SH0 will be excited at the same time as SH1 mode. In order to eliminate SH0, the EMAT-A and EMAT-B double-sided symmetric excitation method is adopted. When the upper and lower coil arrays are connected as shown in Figure 2 When the current is i1 or i2, the Lorentz force generated in the skin layer of the aluminum plate under the action of the alternating magnetic field is and ⊙ represent the direction of the force perpendicular to the paper, inward and outward, respectively. Since the eddy currents of the coils near the upper and lower surfaces are in opposite directions, the relationship between the Lorentz force and the particle displacement at the top (A surface) and bottom (B surface) of the aluminum plate can be expressed by formula (4):

[0063] f Ly,A =-f Ly,B , u y,A =-u y,B (4)

[0064] That is, the Lorentz force and displacement at the symmetrical positions on the upper and lower sides of the aluminum plate are opposite. This can effectively suppress the generation of the symmetric mode SH0, and at the same time, enhance the excitation of the asymmetric mode SH1 wave, that is, stimulate the single-mode SH1 wave.

[0065] (2) Unidirectional single-mode excitation. Figure 5The principle of exciting unidirectional SH1 is given. In the middle figure, the white and gray rectangles represent the two permanent magnet coil units 1# and 2#, which are spaced λ / 4 apart in the x-direction. In addition, the pulse excitation current i2 of the 2# coil is delayed by 1 / 4 cycle compared with the pulse excitation current i1 of the 1# coil, generating SH1 waves in the positive and negative directions along the x-axis. f represents the excitation signal frequency, and c represents the wave phase velocity. The wave signal is enhanced on the right side due to interference, while the interference destructive effect on the left side makes the wave signal almost zero; that is, a single-mode SH1 wave that only propagates to the right is generated.

[0066] When it is necessary to excite a single-mode SH1 wave propagating to the left, it is only necessary to change the delay sequence of the coil excitation currents i1 and i2, that is, the pulse excitation current i1 of coil #1 is delayed by 1 / 4 cycle compared with the pulse excitation current i2 of coil #2, thereby obtaining a unidirectional single-mode SH1 wave propagating to the left.

[0067] A simulation method based on the electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves comprises the following steps:

[0068] S1: Establish a coordinate system and set up a solver. A three-dimensional rectangular coordinate system is established with the row arrangement direction of the permanent magnets (i.e., the SH1 waveguide propagation direction) as the x-axis, the vertical arrangement direction as the y-axis, and the thickness direction of the aluminum plate as the z-axis; a steady-state solver is added to solve steady-state variables, and a transient solver is added to solve time-varying variables related to the pulse excitation current.

[0069] S2: Build a geometric model and add physical fields and material properties. Build the geometric models of the air solution domain, permanent magnet, coil, and aluminum plate, and set their respective material properties; use the AC / DC module in the finite element simulation software to build the static magnetic field of the permanent magnet and the dynamic magnetic field of the coil, and add the magnetic field module and solid mechanics module to the physical field of the aluminum plate.

[0070] S3: Set the solution conditions and meshing. In order to eliminate the influence of reflected waves, the other four end faces are set as low reflection boundary conditions except for the upper and lower end faces of the aluminum plate. In order to reduce the calculation time, the meshing of the air domain and the permanent magnet is appropriately coarsened, and the meshing of the coil and the surface of the aluminum plate near the coil is appropriately refined.

[0071] S4: Solve the simulation model and post-process the data. In order to ensure the convergence and accuracy of the transient solution, use the pre-processor of the iterative processor to improve the convergence speed of the calculation. The transient calculation time step cannot be set too large. After the solution is completed, post-process the relevant data and analyze the simulation results.

[0072] This simulation method can simulate the dynamic change law and characteristics of the magnetic field, pulse eddy current field, and sound field of the unidirectional single-mode SH1 wave electromagnetic ultrasonic transducer during the transduction process by constructing an integrated three-dimensional simulation model including elements such as magnetic field / pulse eddy current field / ultrasonic field multi-field coupling, and can well reflect the actual working conditions of the unidirectional single-mode SH1 wave electromagnetic ultrasonic transducer. However, most of the existing SH waveguide simulation technologies use a simplified equivalent excitation simulation method, which applies an edge load on the aluminum plate instead of the Lorentz force to achieve the excitation of the waveguide, which is different from the actual situation of the electromagnetic ultrasonic transducer.

[0073] In order to study the excitation effect of the proposed unidirectional single-mode SH1 wave EMAT, a three-dimensional finite element model of static magnetic field / eddy current field / ultrasonic field multi-physics field coupling was established using the multi-physics field simulation software Comsol Multiphics. Figure 6 As shown. The model consists of a PPM permanent magnet, an alternating coil array, an aluminum plate, and an air domain. The permanent magnet uses NdFeB with a remanence of 1.2T, the number is 16, and the permanent magnet spacing is 3mm. For the convenience of calculation, the rectangular coil under the permanent magnet is used to approximate the equivalent of multiple turns of closely arranged enameled wire. The effective area of ​​the rectangular coil winding is equal to that of the enameled wire, and the coil lifting distance is set to 0.2mm. The coil excitation currents i1 and i2 are both 0.3MHz Tone-burst signals with an amplitude of 20A and a direction in the x-axis direction. The phase of i2 is delayed by 1 / 4 cycle. The main parameters of the simulation model are shown in Table 1.

[0074] Table 1 Parameters of three-dimensional simulation model

[0075]

[0076] Figure 7 The simulation results of the displacement of the particle on the plate surface at 35us to 50us are given. Only the right side of the EMAT has a guided wave propagating to the right, while the small amplitude wave propagating to the left on the left side is very weak and can be ignored, that is, the guided wave propagating to the left is greatly attenuated; in addition, Figure 8 The figure shows the y component of the particle displacement on the y=0 section at 52μs. The displacement of the guided wave particle is antisymmetric along the thickness direction of the plate, that is, there is only the antisymmetric mode SH1 wave, and no symmetric mode SH0 wave is generated. The simulation results show that the EMAT proposed in this paper can excite unidirectional single-mode SH1 guided waves.

[0077] Select two mass points P and P' with a distance of 60 mm from the center of EMAT and a depth of 0.01 mm. The displacement signal in the y direction is as follows: Fig. 9As shown in Figure 2, the received signal at the weakened side point p' is 6.1% of the received signal at the enhanced side point P, so it can be determined that the EMAT in this paper can excite the unidirectional and single-mode SH1 wave at 300kHz.

[0078] In order to verify the effect of the new EMAT on the unidirectional single-mode SH1, according to Fig.10 The experimental system was built as shown for verification. The dual output channels and output delay function of the high-energy pulse test system are used to output two pulse currents with a center frequency of 0.3MHz and a phase difference of 90 degrees. After impedance matching, it is connected to each coil array of the double-sided EMAT. The received signal is connected to the receiving end of the test system after high-pass filtering and preamplifier. First, two points x=80mm and 100mm are taken as receiving points along the wave propagation direction on the right side of the EMAT to measure the wave velocity. The signal time difference between the two points is 73.6μs. The calculated wave velocity is 2717m / s, and the relative error with the theoretical SH1 wave group velocity of 2654m / s is 2.4%. It can be seen that the ultrasonic wave excited by the EMAT is the SH1 wave. Then, two points equidistant from the left side (weakening side) and the right side (enhancing side) of the excited EMAT are selected as receiving points. The received waveform is shown as follows Fig.11 As shown in the figure, the amplitude of the wave propagating to the right is much larger than that of the wave propagating to the left. The experiment verifies that the electromagnetic ultrasonic transducer proposed in this paper can excite unidirectional and single-mode SH1 waves.

[0079] In the description of the present invention, it should be noted that for directional words, such as the terms "center", "horizontal", "vertical",

[0080] The directions, lengths, widths, thicknesses, up, down, front, back, left, right, vertical, horizontal, top, bottom, inside, outside, clockwise, and counterclockwise, etc., indicating directions and positional relationships, are based on the directions or positional relationships shown in the drawings and are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the devices or elements referred to must have a specific direction, be constructed and operated in a specific direction, and should not be construed as limiting the specific protection scope of the present invention.

[0081] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or inherent to these processes, methods, products or apparatuses.

[0082] Note that the above are only preferred embodiments of the present invention and the principles of the application technology. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention is described in more detail through the above embodiments, the present invention is not limited to the specific embodiments described herein, and may include more other effective embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. An electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves, characterized in that: It comprises an aluminum plate and an EMAT-A structure and an EMAT-B structure which are placed on the upper and lower surfaces of the aluminum plate and have the same structure. The EMAT-A structure comprises an electromagnetic ultrasonic excitation unit; the electromagnetic ultrasonic excitation unit is composed of four rectangular permanent magnet units with the same structure and a coil, the four rectangular permanent magnet units are arranged in rows along the length direction of the aluminum plate in sequence, and each rectangular permanent magnet unit is arranged in a vertical row of two rectangular permanent magnets; the rectangular permanent magnet units between two adjacent rectangular permanent magnet units are spaced 1 / 4 wavelength apart; The coil is an alternating coil array, that is, each coil is composed of an enameled wire wound on a single rectangular permanent magnet; one end of the coil in each rectangular permanent magnet unit is recorded as the head end, and the other end is recorded as the tail end, and the winding direction of the coils on the two rectangular permanent magnets in each rectangular permanent magnet unit is one clockwise and the other counterclockwise; In each electromagnetic ultrasonic excitation unit, the coil tail ends of two rectangular permanent magnet units separated by a rectangular permanent magnet unit are connected to each other, and the first sections are respectively connected to the pulse excitation current to achieve the excitation of unidirectional single-mode SH1 waves through wave interference; the EMAT-B structure is the same as the EMAT-A structure.

2. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to claim 1, characterized in that: The connection between the coil and the pulse excitation current in the electromagnetic ultrasonic excitation unit is specifically as follows: from left to right, when the coil head end of the first rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the first pulse excitation current, the coil head end of the third rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the first pulse excitation current; at the same time, the coil head end of the first rectangular permanent magnet unit in the EMAT-B structure is connected to the input end of the first pulse excitation current, and the coil head end of the third rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the first pulse excitation current; at this time, the coil head end of the second rectangular permanent magnet unit in the EMAT-A structure is connected to the input end of the second pulse excitation current, and the coil head end of the fourth rectangular permanent magnet unit in the EMAT-A structure is connected to the output end of the second pulse excitation current; at the same time, the coil head end of the second rectangular permanent magnet unit in the EMAT-B structure is connected to the output end of the second pulse excitation current, and the coil head end of the fourth electromagnetic ultrasonic excitation unit in the EMAT-B structure is connected to the input end of the second pulse excitation current.

3. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to claim 1, characterized in that: There are multiple electromagnetic ultrasonic excitation units, which are arranged in a row, and the connection relationship between the corresponding coil and the corresponding pulse excitation current in each electromagnetic ultrasonic excitation unit is the same.

4. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to any one of claims 1 to 3, characterized in that: It also includes a fixing groove for fixing the relative position of the rectangular permanent magnet to remain unchanged. The fixing groove is arranged on the upper and lower surfaces of the aluminum plate corresponding to the rectangular permanent magnet.

5. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to claim 4, characterized in that: The adjacent grooves are x Direction distance Δ D for λ / 4, slot gap Δ d 0.5mm, y Direction gap Δ h 2mm; the notch on the side Δ s 1mm, used to adjust the winding direction of the coil.

6. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to claim 4, characterized in that: The fixing groove adopts two 3D printed R4600 nylon frame structures.

7. The electromagnetic ultrasonic transducer for exciting unidirectional single-mode SH1 guided waves according to claim 4, characterized in that: Excitation frequency f is 0.3MHz, the phase velocity c p is 3621 m / s, the group velocity is 2654 m / s, then the wavelength λ = c p / f =12mm.

8. An excitation system for an electromagnetic ultrasonic transducer for exciting a unidirectional single-mode SH1 guided wave according to claim 1, characterized in that: It includes a high-energy pulse test system, a PC, an oscilloscope, an impedance matching module, a preamplifier, a high-pass filter and a receiving probe; the first output end of the high-energy pulse test system is used to output a first pulse excitation current through an impedance matching circuit, and the second output end is used to output a second pulse excitation current through an impedance matching circuit, which are respectively used to be connected to an electromagnetic ultrasonic transducer for exciting a unidirectional single-mode SH1 waveguide; the receiving probe is arranged on an aluminum plate, and is connected to the receiving end of the high-energy pulse test system through a high-pass filter and a preamplifier in turn; the PC and the oscilloscope are respectively connected to the high-energy pulse test system.

9. A method for exciting an electromagnetic ultrasonic transducer for exciting a unidirectional single-mode SH1 guided wave based on claim 1, characterized in that: The steps include: A: Excite the single-mode SH1 wave: A pulse excitation current is passed through the coil. Under the action of the alternating magnetic pole magnetic field, the Lorentz force generated in the skin layer of the aluminum plate. At this time, the Lorentz force and displacement at the symmetrical positions on the upper and lower sides of the aluminum plate are opposite, which can effectively suppress the generation of the symmetrical mode SH0. At the same time, the excitation of the asymmetric mode SH1 wave is enhanced, that is, the single-mode SH1 wave is excited; B: Stimulate the unidirectional single-mode SH1 wave: adjust the second pulse excitation current i 2 than the first pulse excitation current i 1 Delay 1 / 4 cycle, thus generating an edge in the coil x SH1 waves in the positive and negative directions of the axis; f represents the excitation signal frequency, c Represents the wave phase velocity; the wave signal on the right is enhanced due to interference, while the interference destructive effect on the left makes the wave signal almost zero; that is, a single-mode SH1 wave that only propagates to the right is generated; C: When it is necessary to excite a single-mode SH1 wave propagating to the left, change the first excitation current of the coil i 1 and 2 excitation current i 2, the delay sequence, that is, the first pulse excitation current i 1 to second pulse excitation current i 2 is delayed by 1 / 4 cycle, thus obtaining a unidirectional single-mode SH1 wave propagating to the left.

10. A simulation method for an electromagnetic ultrasonic transducer for exciting a unidirectional single-mode SH1 guided wave according to claim 1, characterized in that: The steps include: S1: Establish a coordinate system and set the solver: establish a three-dimensional rectangular coordinate system with the row arrangement direction of the permanent magnets, i.e. the SH1 waveguide propagation direction, as the x-axis, the vertical arrangement direction as the y-axis, and the thickness direction of the aluminum plate as the z-axis; add a steady-state solver to solve steady-state variables, and add a transient solver to solve time-varying variables related to the pulse excitation current; S2: Build geometric models and add physical fields and material properties: Build geometric models of the air solution domain, permanent magnet, coil, and aluminum plate, and set their respective material properties; use the AC / DC module in the finite element simulation software to build the static magnetic field of the permanent magnet and the dynamic magnetic field of the coil, and add the magnetic field module and solid mechanics module to the physical field of the aluminum plate; S3: Set solution conditions and mesh generation: In order to eliminate the influence of reflected waves, except for the upper and lower end faces of the aluminum plate, the remaining four end faces are set as low-reflection boundary conditions; in order to reduce the calculation time, the mesh generation of the air domain and the permanent magnet is appropriately coarsened, and the mesh generation of the coil and the surface of the aluminum plate near the coil is appropriately refined; S4: Solving the simulation model and post-processing the data: In order to ensure the convergence and accuracy of the transient solution, the pre-processor of the iterative processor is used to improve the convergence speed of the calculation. The transient calculation time step cannot be set too large; after the solution is completed, the relevant data is post-processed and analyzed.

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