Transducer for exciting fillet weld characteristic guided waves and optimization design method thereof

Abstract

The invention provides a transducer for exciting fillet weld characteristic guided waves and an optimization design method thereof, and relates to the technical field of ultrasonic nondestructive testing, the transducer comprises at least one basic unit, all the basic units are fixedly arranged on the surface of a fillet weld to be detected, each basic unit comprises a first piezoelectric transduction plate, a second piezoelectric transduction plate, a connecting plate and an electrode, the first piezoelectric transduction plate and the second piezoelectric transduction plate are symmetrically arranged at the two ends of the connecting plate, the first piezoelectric transduction plate and the second piezoelectric transduction plate are in cuboid shapes with the same size, the two piezoelectric transduction plates are polarized in the width direction, and the polarization directions are opposite. The electrodes are arranged on the upper surfaces and the lower surfaces of the first piezoelectric transduction plate and the second piezoelectric transduction plate respectively. The body wave suppression type fillet weld characteristic guided wave transducer is constructed through the symmetrically deformed thickness shearing type piezoelectric transduction plate, and the transducer can be arranged on the surface of a fillet weld to monitor crack defects on the surface of the fillet weld in real time.

Description

Technical Field

[0001] The invention relates to the technical field of ultrasonic nondestructive testing, and in particular to a transducer for exciting characteristic guided waves of a fillet weld and an optimization design method thereof. Background Art

[0002] Welding is the most commonly used connection form in large steel structures. Among them, fillet welding, as a welding process, is widely used to connect two steel structure components at right angles or nearly right angles. However, the cross-section of the fillet weld at the joint varies greatly, which is easy to become a stress concentration area. Taking orthotropic steel bridge decks as an example, during the production and service process, fillet welds will have defects of varying degrees, such as pores, inclusions, cracks, etc. Once these defects are formed, they may cause sudden failure of the structure and even seriously affect the service safety of the entire steel structure. Therefore, it is very necessary to conduct timely and effective damage detection of fillet welds. Traditional detection methods such as body wave detection and magnetic particle detection require point-by-point movement for detection, especially when detecting long-distance fillet weld structures such as bridge decks. The efficiency is extremely low, and long-term in-situ online monitoring cannot be achieved. These existing technical means are difficult to meet the needs of modern steel structures for efficient and accurate detection. Studies have shown that the characteristic guided waves existing in fillet welds can propagate along the welds for long distances, which is a very potential method to solve this problem. This kind of fillet weld characteristic waveguide has surface wave characteristics, and its energy is concentrated on the surface of the weld structure, which can identify shallow defects. However, in practical applications, placing a conventional body wave transducer directly on the weld surface often generates significant vertical body waves in the thickness direction, which seriously affects the signal-to-noise ratio.

[0003] In view of the above-mentioned shortcomings of the prior art, there is an urgent need for a transducer for exciting characteristic guided waves of a fillet weld and an optimized design method thereof. Summary of the invention

[0004] The purpose of the present invention is to provide a transducer for exciting characteristic guided waves of fillet welds and an optimization design method thereof to improve the above-mentioned problems. In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is as follows:

[0005] In the first aspect, the present application provides a transducer for exciting characteristic waveguide of a fillet weld, comprising at least one basic unit, all of which are fixedly arranged on the surface of the fillet weld to be detected, each of which comprises a first piezoelectric transducer plate, a second piezoelectric transducer plate, a connecting plate and an electrode, the first piezoelectric transducer plate and the second piezoelectric transducer plate are symmetrically arranged at two ends of the connecting plate, the first piezoelectric transducer plate and the second piezoelectric transducer plate are both rectangular in shape with the same size, and the electrodes are respectively arranged on the upper and lower surfaces of the first piezoelectric transducer plate and the second piezoelectric transducer plate.

[0006] Furthermore, the length direction of the transducer plate in the three-dimensional space coordinate axis is denoted as the x direction, the width direction is denoted as the y direction, and the thickness direction is denoted as the z direction. The polarization direction of the first piezoelectric transducer plate is along the negative direction of the y axis, and the polarization direction of the second piezoelectric transducer plate is along the positive direction of the y axis.

[0007] Furthermore, the length, width and height of the first piezoelectric transducer plate and the second piezoelectric transducer plate are recorded as l×w×h, the center distance between the first piezoelectric transducer plate and the second piezoelectric transducer plate in each basic unit is recorded as Δd, and the transducer plate length l satisfies the condition The width w satisfies the condition w<λ, and the center distance Δd satisfies the condition w≤Δd<λ, where d is the width of the fillet weld to be inspected. λ is the wavelength of the characteristic guided wave, c is the wave velocity of the characteristic guided wave in the weld structure, and f is the vibration frequency that generates the characteristic guided wave.

[0008] Furthermore, the first piezoelectric transducer plate and the second piezoelectric transducer plate both have a thickness shear piezoelectric coefficient d 15 , the thickness shear piezoelectric coefficient d 15 It represents the proportional relationship between the charge density generated by the piezoelectric material under the action of shear stress and the applied stress. The first subscript "1" represents the direction of the charge plane, indicating that the surface generating the charge is perpendicular to the x-axis. The second subscript "5" represents the direction of the applied force, indicating that the shear force is applied in the xz plane.

[0009] Furthermore, the material of the connecting plate is epoxy resin.

[0010] In a second aspect, the present application provides an optimization design method for a transducer for exciting characteristic guided waves of a fillet weld, comprising:

[0011] Acquiring fillet weld dispersion characteristic data, wherein the fillet weld dispersion characteristic data includes geometric dimensions, material properties and boundary conditions of the fillet weld;

[0012] According to the fillet weld dispersion characteristic data, the dispersion characteristic of the waveguide mode in the weld is analyzed to obtain a phase velocity dispersion curve of the characteristic waveguide, and the dispersion curve data and waveform structure data of the characteristic waveguide of the fillet weld are extracted;

[0013] Obtaining preliminary design parameters according to the dispersion curve data and the waveform structure data, wherein the preliminary design parameters include the in-plane size and arrangement of the piezoelectric transducer plate;

[0014] According to the preliminary design parameters, impedance analysis is performed on piezoelectric transducer plates of different thicknesses to obtain an optimized design scheme, wherein the optimized scheme includes a thickness value of the piezoelectric transducer plate and an excitation frequency range;

[0015] According to the optimized design scheme, after optimizing the geometric dimensions and arrangement of the piezoelectric transducer plates, the final design scheme is obtained by optimizing the length, width and plate spacing of the piezoelectric transducer plates to maximize the excitation efficiency and signal-to-noise ratio of the characteristic waveguide.

[0016] The beneficial effects of the present invention are:

[0017] The present invention constructs a body wave suppression type corner weld characteristic waveguide transducer through a thickness shear type piezoelectric transducer plate, which can be set on the surface of the corner weld to monitor the surface crack defects of the weld in real time, and provide technical support for the corner weld structural health monitoring method based on characteristic waveguide. By matching the thickness shear mode of the piezoelectric material with the waveform structure of the characteristic waveguide, a piezoelectric transducer plate with optimal size and polarization direction is designed, ensuring that the transducer plate can excite waveguides that meet the dispersion characteristics requirements on the weld surface, maximizing the excitation efficiency of the waveguide, while reducing unnecessary energy loss and improving the propagation stability of the waveguide.

[0018] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or be understood by implementing the embodiments of the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 It is a schematic structural diagram of a transducer for exciting characteristic guided waves of a fillet weld according to an embodiment of the present invention;

[0021] Figure 2 A schematic diagram of the vibration of the piezoelectric transducer plate and its excitation waveguide in the transducer of the present invention;

[0022] Figure 3 It is a schematic diagram of the cross section of the orthotropic steel bridge deck model;

[0023] Figure 4 The schematic diagram of the cross section of the dispersion curve of the fillet weld of the bridge deck calculated by finite element method;

[0024] Figure 5 The dispersion curve of the fillet weld structure of the bridge deck calculated by finite element method;

[0025] Figure 6The waveform structure of characteristic guided waves in the fillet weld structure of the bridge deck calculated by finite element method;

[0026] Figure 7 It is a schematic diagram of the analysis results of the impedance characteristics of the piezoelectric transducer plate based on finite element;

[0027] Figure 8 Schematic diagram of the experimental setup for exciting ultrasonic waves on a thick plate with a single piezoelectric transducer;

[0028] Fig. 9 Schematic diagram of the experimental setup for exciting ultrasonic waves on a thick plate using a characteristic guided wave transducer;

[0029] Fig.10 Schematic diagram of thickness-direction body wave signal generated by a single piezoelectric transducer plate in the experiment;

[0030] Fig.11 Schematic diagram of the thickness direction body wave signal generated by the characteristic waveguide transducer in the experiment;

[0031] Fig.12 The schematic diagram of the arrangement of the characteristic guided wave transducer in the fillet weld structure of the bridge deck;

[0032] Fig.13 The schematic diagram of the characteristic guided wave transducer exciting the characteristic guided wave signal in the bridge deck fillet weld structure;

[0033] Fig.14 The present invention is a flowchart of a method for optimizing the design of a transducer for exciting characteristic guided waves of a fillet weld as described in an embodiment of the present invention.

[0034] Markings in the figure: 1, basic unit; 11, first piezoelectric transducer plate; 12, second piezoelectric transducer plate; 13, connecting plate; 14, electrode. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0036] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. At the same time, in the description of the present invention, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0037] Embodiment 1:

[0038] This embodiment provides a transducer for exciting characteristic guided waves of a fillet weld, such as Figure 1 As shown, it includes at least one basic unit 1, and all basic units 1 are fixedly arranged on the surface of the fillet weld to be detected, ensuring the close contact between the transducer and the weld, thereby enhancing the excitation efficiency and detection sensitivity of the guided wave. Each basic unit 1 includes a first piezoelectric transducer plate 11, a second piezoelectric transducer plate 12, a connecting plate 13 and an electrode 14. The piezoelectric transducer plate is made of PZT ceramic. The first piezoelectric transducer plate 11 and the second piezoelectric transducer plate 12 are symmetrically arranged at the two ends of the connecting plate 13. The symmetrical arrangement helps to balance the excitation of the guided wave and suppress the generation of body waves on the basis of ensuring the directivity and stability of the characteristic guided wave propagation. The first piezoelectric transducer plate 11 and the second piezoelectric transducer plate 12 are both rectangular parallelepiped shapes of the same size, combined with the geometric characteristics of the weld, so that the transducer can maximize the use of the limited space on the weld surface. The electrodes 14 are respectively arranged on the upper and lower surfaces of the piezoelectric transducer plate. This structural design can achieve efficient guided wave excitation and propagation in structures of limited thickness while maintaining a high signal-to-noise ratio. It is particularly suitable for online monitoring of long-distance fillet welds and shallow defect detection.

[0039] Furthermore, the length direction of the transducer plate in the three-dimensional space coordinate axis is recorded as the x direction, the width direction is recorded as the y direction, and the thickness direction is recorded as the z direction. The definition of the coordinate axis helps to accurately describe the position of the transducer plate in space and its anisotropic electrical properties. In this setting, the polarization direction of the first piezoelectric transducer plate 11 is designed to be along the negative direction of the y axis, while the polarization direction of the second piezoelectric transducer plate 12 is along the positive direction of the y axis. This symmetrical setting of the polarization direction ensures that when the electric field is applied to the transducer plate, the two piezoelectric transducer plates will produce strains in opposite directions, thereby forming a shear mode in the y direction. The formation of this shear mode is the key to exciting characteristic guided waves, because the effective excitation of characteristic guided waves depends on the vibration mode that matches its waveform structure. Through such a polarization setting, the transducer can achieve stable and strong guided wave excitation on the weld surface, while effectively suppressing unnecessary body wave interference in the thickness direction, thereby improving the signal-to-noise ratio of the signal. This design not only improves the propagation efficiency of guided waves, but also ensures the detection accuracy and stability in the complex stress environment of the weld.

[0040] Furthermore, the length, width and height of the first piezoelectric transducer plate 11 and the second piezoelectric transducer plate 12 are denoted as l×w×h, and the center distance between the first piezoelectric transducer plate 11 and the second piezoelectric transducer plate 12 in each basic unit 1 is denoted as Δd. Where d is the width of the fillet weld to be detected. This restriction ensures that the transducer plate can fully cover the cross section of the weld and provide sufficient excitation energy. The width satisfies the condition w<λ, and the center spacing satisfies the condition w≤Δd<λ, where λ is the wavelength of the characteristic waveguide, c is the wave velocity of the characteristic waveguide in the weld structure, and f is the vibration frequency of the characteristic waveguide. The relationship between these parameters ensures that the transducer plate accurately matches the propagation characteristics of the characteristic waveguide when exciting the characteristic waveguide, thereby maximizing the excitation efficiency of the waveguide. This design can not only stimulate stable characteristic waveguides, but also effectively suppress unnecessary wave mode coupling and interference through reasonable plate spacing and size design, ensuring the singleness of the waveguide and the high signal-to-noise ratio of the signal, thereby improving the sensitivity and accuracy of defect detection.

[0041] Furthermore, the first piezoelectric transducer plate 11 and the second piezoelectric transducer plate 12 both have a thickness shear piezoelectric coefficient d 15 , thickness shear piezoelectric coefficient d 15 It represents the proportional relationship between the charge density generated by the piezoelectric material under the shear stress and the applied stress. The first subscript "1" represents the charge surface direction, indicating that the surface generating the charge is perpendicular to the x-axis. The second subscript "5" represents the direction of the applied stress, indicating the shear force applied in the xz plane, corresponding to the polarization setting in the y-axis direction. This design ensures that the transducer can excite characteristic guided waves according to a specific displacement mode.

[0042] Furthermore, the material of the connecting plate 13 is epoxy resin. Epoxy resin is a polymer material with excellent mechanical strength and good bonding properties, and is suitable as a fixed support structure for the piezoelectric transducer plate.

[0043] Preferably, the transducer design and manufacturing process in this embodiment ensures its high efficiency as an actuator through precise process steps. First, when preparing the transducer, the vibration frequency is determined based on the dispersion curve of the characteristic waveguide in the corner weld structure, which provides key parameters for the subsequent geometric design of the transducer. The width of the connecting plate is designed to control the spacing between adjacent piezoelectric transducer plates. This design ensures that the transducer can effectively excite the characteristic waveguide and avoids the decrease in excitation efficiency caused by excessive or insufficient distance. The PZT ceramic material is polarized along the y direction to give it a piezoelectric coefficient d 15This characteristic enables the transducer plate to produce the required thickness shear deformation through the inverse piezoelectric effect when subjected to an electric field. The polarized PZT ceramic is precisely cut into a first piezoelectric transducer plate and a second piezoelectric transducer plate of 2.5mm×12mm×1mm, and placed side by side with an interval of Δd along the y direction to ensure that the polarization directions of the two transducer plates are opposite, so that body waves with opposite phases can be generated during the subsequent excitation process. In addition, the silver glue coating technology is used in the preparation process of the electrode to ensure the good conductivity of the electrode. Finally, the piezoelectric transducer plate is glued to the connecting plate with epoxy resin to ensure its stability and durability.

[0044] Furthermore, in the application of the transducer as an actuator, by connecting the electrodes on the upper and lower surfaces of the piezoelectric transducer plate to a single signal source, the signal source sends an AC signal to simultaneously excite the two piezoelectric transducer plates, such as Figure 2 As shown in the figure, the letter P represents the polarization direction of the piezoelectric transducer plate, and the anti-phase body wave refers to the body wave generated by the two piezoelectric transducers in the thickness direction. Since the polarization directions of the two transducers are opposite, in the inverse piezoelectric effect, the 15 In this mode, the design makes the phases of the body waves generated by the two piezoelectric transducers in the thickness direction opposite. According to the symmetry principle of elastic dynamics, these body waves with opposite phases interfere and destructively in the thickness direction, successfully suppressing the generation of body waves. This destructive interference design improves the excitation purity of the characteristic guided waves, allowing the characteristic guided waves to propagate efficiently in the direction parallel to the polarization. This design not only improves the signal-to-noise ratio of the signal, but also ensures the propagation direction and stability of the guided waves. It is suitable for application in damage detection of complex weld structures and provides reliable technical support for long-term online monitoring.

[0045] Embodiment 2:

[0046] This embodiment provides a method for optimizing the design of a transducer for exciting characteristic guided waves of a fillet weld.

[0047] See also Fig.14 , the figure shows that the method includes steps S100 to S500.

[0048] Step S100, obtaining fillet weld dispersion characteristic data, the fillet weld dispersion characteristic data including geometric dimensions, material properties and boundary conditions of the fillet weld;

[0049] It is understandable that the geometric dimensions determine the shape and thickness variation of the weld, affecting the mode selection and propagation path of the guided wave. Material properties, such as density, elastic modulus, and Poisson's ratio, determine the wave velocity and dispersion relationship in the weld area, which is the basis for calculating the characteristic guided wave propagation velocity and frequency. Boundary conditions, including perfect matching layers and Floquet boundary conditions, determine the accuracy of calculating the characteristic guided wave dispersion characteristics.

[0050] Step S200, according to the fillet weld dispersion characteristic data, by analyzing the dispersion characteristics of the waveguide mode in the weld, a phase velocity dispersion curve of the characteristic waveguide is obtained, and waveform structure data of the characteristic waveguide of the fillet weld is extracted;

[0051] It should be explained that in this process, the phase velocity dispersion curve of the characteristic guided wave is one of the key parameters, which determines the propagation speed of the guided wave in the weld and directly affects the time resolution of the detection. By analyzing the phase velocity dispersion curve, the guided wave mode with the best propagation characteristics in the weld can be identified. At the same time, the analysis of the displacement distribution is used to determine the energy concentration area of ​​the guided wave, especially to identify which parts have larger displacements, indicating that these areas are more likely to excite or detect characteristic guided waves. Further, these analysis results are used to extract the waveform structure data of the characteristic guided wave, that is, the displacement pattern of the guided wave in the weld.

[0052] Step S300, obtaining preliminary design parameters according to the dispersion curve data and the waveform structure data analysis, the preliminary design parameters including the in-plane size and arrangement of the piezoelectric transducer plate;

[0053] It is understandable that the thickness shear mode can produce shear strain in the thickness direction, which is highly matched with the displacement mode of the characteristic waveguide and can effectively stimulate the required waveguide type. In order to achieve this effect, this step analyzes the shear response of different piezoelectric materials in the thickness direction through calculation and simulation, and compares it with the waveform structure of the characteristic waveguide. The matching process not only takes into account the basic properties of the material, but also combines the geometry and material properties of the weld to ensure that the piezoelectric transducer can work effectively under specific operating frequencies and environmental conditions.

[0054] Step S400: performing impedance analysis on piezoelectric transducer plates of different thicknesses according to preliminary design parameters to obtain an optimized design solution, wherein the optimized solution includes a thickness value of the piezoelectric transducer plate and an excitation frequency range.

[0055] On this basis, the excitation frequency is optimized, and the optimal frequency range is selected by taking the intersection of multiple conditions. In this frequency range, the piezoelectric transducer plate can excite the characteristic guided waves in the fillet weld while achieving maximum working efficiency.

[0056] Step S500: According to the optimized design scheme, after optimizing the geometric shape and arrangement of the piezoelectric transducer plate, the length, width and plate spacing of the piezoelectric transducer plate are optimized to maximize the excitation efficiency and signal-to-noise ratio of the characteristic waveguide, thereby obtaining a final design scheme.

[0057] It is understandable that the optimization process focuses on the length, width and plate spacing of the piezoelectric transducer plate. These geometric parameters directly affect the excitation efficiency of the transducer plate for characteristic guided waves. The design of the length and width needs to consider the matching with the wavelength of the characteristic guided wave to ensure that the transducer plate can generate sufficiently strong vibrations on the weld surface to effectively excite the guided wave. In addition, the design of the plate spacing also needs to be precisely controlled to ensure that they can generate body waves with opposite phases while effectively exciting the characteristic guided wave. This design utilizes the symmetry principle of elastic dynamics to suppress the body wave interference in the thickness direction through interference destructive phase, thereby improving the signal-to-noise ratio of the signal. In the optimization process, the methods of numerical simulation and experimental verification are combined. By comparing the guided wave excitation efficiency and signal-to-noise ratio under different geometric parameter combinations, the optimal geometry and arrangement are gradually adjusted and determined. The final optimized design scheme ensures that the transducer can work with the best performance in practical applications, especially in long-distance weld detection and shallow defect identification, the efficiency and stability of the transducer are significantly improved.

[0058] Further, step S200 includes step S210 to step S240.

[0059] Step S210, establishing a finite element model according to the fillet weld dispersion characteristic data, dividing the bridge deck structure into a web, a top plate, a perfectly matched layer and a target area, and setting corresponding boundary conditions to obtain an initial model;

[0060] Step S220, performing meshing processing according to the initial model, dividing the model by using free tetrahedral units, and determining the unit size based on the minimum wavelength of the characteristic waveguide of the fillet weld during the calculation process, to obtain a meshed model;

[0061] Step S230, performing frequency domain analysis processing according to the grid model, applying periodic boundary conditions to simulate the periodic structure of the fillet weld in the axial direction, performing parameterized scanning on the wave number, gradually calculating the corresponding frequency and guided wave mode, and obtaining the frequency domain analysis result;

[0062] Step S240, performing modal screening processing according to the frequency domain analysis results, by analyzing the displacement concentration of the guided wave in the target area, screening out non-dispersive guided wave modes with concentrated energy and suitable for weld defect detection, and obtaining waveform structure data and dispersion curve data of characteristic guided waves in the weld.

[0063] Specifically, the orthotropic steel bridge deck in actual engineering is selected as the research object, such as Figure 3The figure shows a cross-sectional schematic diagram of an orthotropic steel bridge deck structure, where the deck consists of U ribs and a top plate. The dotted box is the research area for finite element calculation of the dispersion characteristics of fillet welds. Since the present invention focuses on the guided wave behavior of the structural weld area, selecting this area can reduce the complexity of the calculation and resource requirements. In addition, since the boundary conditions of the overall structure are difficult to simulate accurately, the boundary condition settings can also be simplified by selecting a part of the structure for analysis.

[0064] like Figure 4 The figure shows a schematic diagram of the model section for calculating the dispersion curve of the fillet weld using the finite element software Comsol frequency domain analysis. The section is divided into four areas, namely the top plate, U rib, perfectly matched layer (PML) and target area. Among them, the top plate and U rib are the main components of the bridge deck structure. The perfectly matched layer is an absorption layer design that can reduce the influence of boundary conditions on the modal analysis results, thereby better analyzing the dispersion characteristics of the guided waves in the structure. Adjusting parameters such as the thickness of the perfectly matched layer can better reduce the reflection of the wave and improve the calculation accuracy. The target area in the section is the covered area under the weld surface, which is used for subsequent retrieval of characteristic guided waves in the fillet weld.

[0065] The finite element structure is meshed using free tetrahedron units. The unit size is related to the minimum wavelength of the characteristic waveguide to be solved. The unit size should satisfy Among them l e is the unit size, λ min is the minimum wavelength. Set the same material parameters as the bridge deck structure in the actual project, including density, elastic modulus, Poisson's ratio, etc. The material is a linear elastic material, and the damping effect is not considered. Apply Floquet periodic boundary conditions to the two end faces of the model perpendicular to the axial direction, and the two faces serve as the source face and the target face respectively. In the frequency domain analysis, the Floquet boundary condition simulates an infinite length structure by assuming that the target weld and its surrounding area are periodically repeated in the axial direction, so that the analysis of a periodic unit can represent the behavior of the entire structure, which is especially suitable for dealing with bridge deck weld structures of approximately infinite length. The specific calculation process is as follows: The displacement control equation is used to ensure the continuity of the entire model in the numerical calculation. The displacement control equation is:

[0066]

[0067] Among them, u dst represents the displacement field of the target surface, u src represents the source surface displacement field, k f represents the periodic wave number defined in the axial direction of the study area, i is an imaginary unit, r dst is the coordinate of the point on the target surface, r srcis the coordinate of the point on the source surface. Based on the harmonic assumption and the principle of virtual work, the characteristic equation under the undamped condition is:

[0068] (K-ω 2 M)u=0;

[0069] Among them, K represents the overall stiffness matrix, M represents the mass matrix, ω represents the eigenfrequency, and u represents the eigenvector.

[0070] Solve the above characteristic equation and set the scanning parameters. The scanning parameters include the scanning interval and step size of the wave number. These parameters determine the distribution of the wave number during the analysis. Selecting the appropriate wave number value can ensure the accuracy of the calculation while avoiding the complexity of the calculation. Scan according to the wave number, where each k i Solve the characteristic equation separately, get the corresponding characteristic frequency, and draw a scatter plot. p =ω / k to calculate the phase velocity of each point, and after processing all data points, the phase velocity dispersion curve is obtained. p is the phase velocity, ω is the characteristic frequency, and k is the wave number.

[0071] The frequency domain analysis results are processed and the target characteristic guided wave modes are obtained through mode screening. The screening criteria are mainly the following aspects: first, the guided wave modes with displacement distribution concentrated in the target area are screened so that they can propagate stably along the weld. The specific method is: the sum of the absolute values ​​of the total displacements of all nodes in the study area is set to S, the sum of the absolute values ​​of the total displacements of all nodes in the target area is set to S′, and their ratio is defined as The values ​​of q under different modes in the results are sorted. The larger the q, the more significant the energy concentration in the target area. The five modes with the largest q are selected to continue to participate in the screening. Second, through eigenvalue analysis, the waveguide mode with a wavelength of l < 10mm is screened, so as to select characteristic waveguides with small defect detection capabilities. Third, it is determined whether the selected waveguide mode is non-dispersive, that is, the phase velocity does not change with frequency, ensuring that the signal is not elongated with the increase of propagation distance.

[0072] After mode screening, the dispersion curve of the characteristic guided wave in the fillet weld structure is obtained. For the convenience of comparison, the dispersion curve is also drawn. 0 The mode is the zero-order symmetrical Lamb wave in the U rib, A 0 The mode is the zero-order antisymmetric Lamb wave in the top plate. Figure 5 As shown in the figure, the characteristic guided wave appears after the frequency of 500kHz. Since the phase velocity does not change with the change of frequency, the characteristic guided wave is non-dispersive. This type of characteristic guided wave is a surface-like mode guided wave, so the non-dispersive characteristics of the characteristic guided wave are not affected by the change of structural size.

[0073] Based on this calculation result, the waveform structure of the characteristic guided wave is extracted. The specific method is as follows: Based on the results of frequency domain analysis, a straight line is taken along the thickness direction of the weld, and the three-directional displacements of all nodes on this line are extracted at the characteristic frequency and wave number corresponding to the target mode. The waveform structure diagram of the characteristic guided wave in the weld is drawn as shown in the figure. Figure 6 As shown. The waveform structures of the characteristic guided waves at different frequencies are compared to verify the accuracy of the obtained waveform structure. It can be observed from the figure that the displacement of the characteristic guided wave is composed of the displacement along the weld direction z and the displacement in the off-plane direction y. Therefore, the characteristic guided wave in the weld can be induced by the displacement vibration in the z direction or the displacement vibration in the y direction.

[0074] Further, step S300 includes step S310 to step S340.

[0075] Step S310, performing displacement analysis processing according to the characteristic waveguide waveform structure data, determining the dominant displacement direction by analyzing the displacement distribution characteristics of the waveguide on the weld surface, and obtaining displacement distribution information;

[0076] Step S320, performing piezoelectric material vibration mode analysis and processing according to the displacement distribution information, and obtaining a piezoelectric material thickness shear mode suitable for characteristic guided wave excitation by selecting a vibration mode that matches the dominant displacement direction;

[0077] Step S330, performing vibration mode matching processing on the piezoelectric transducer plate according to the thickness shear mode of the piezoelectric material, and obtaining a thickness shear type piezoelectric transducer plate by combining the vibration mode of the piezoelectric transducer plate with the waveform structure of the characteristic waveguide;

[0078] Step S340, based on the obtained thickness shear type piezoelectric transducer plate, optimize its in-plane dimensions. By analyzing the geometric structure of the weld and the effective bonding area, optimize the length and width of the transducer plate, and ensure that its shape matches the weld structure, and obtain preliminary design parameters. The preliminary design parameters include the in-plane dimensions and layout of the transducer plate.

[0079] It should be noted that in order to stimulate the target characteristic guided wave in the weld structure, it is necessary to design a suitable piezoelectric ceramic transducer plate for excitation. The specific design method includes: first, the waveform structure is matched with the vibration mode of the piezoelectric transducer plate. In the frequency domain analysis, the waveform structure of the characteristic guided wave shows that on the surface of the weld structure, that is, at the depth = 0mm, the y-direction displacement perpendicular to the weld is the largest, so the characteristic guided wave can be induced by applying the y-direction displacement. When one side of the ceramic piezoelectric sheet is pasted to the structure, its back plate is unconstrained, and it is difficult to generate pure stretching vibration in the structure. Therefore, piezoelectric ceramics cannot be used to excite pure off-surface displacement. It can be seen from the waveform structure that in addition to the y-direction displacement, the z-direction displacement also occupies a large proportion. At the depth = 0mm, the ratio of the y-direction displacement to the z-direction displacement is 7:3. The thickness shear d15 The mode can produce a relatively pure shear displacement by constraining only one side, so d 15 The piezoelectric ceramic plate in thickness shear mode is used to apply displacement vibration in the z direction, thereby inducing characteristic guided waves in the fillet weld.

[0080] Secondly, design the geometry and arrangement of the transducer. Design its size according to the effective bonding area of ​​the weld. For example, if the width of the weld is 20mm, the length l of the piezoelectric transducer plate is required to satisfy 10mm≤l≤20mm so that the shape matches the weld to be measured. Further, consider the waveform structure characteristics, that is, the displacement distribution of the mode on the weld surface along the width direction. According to the displacement distribution of the characteristic waveguide on the weld surface, the length l of the piezoelectric transducer plate is set to 80% of the weld width. This setting can maximize the excitation efficiency. In order to meet the spacing requirements in the width direction, the width of the piezoelectric transducer plate is required to be w<λ, where λ is the wavelength of the target mode waveguide at 1000kHz, λ=2.9mm.

[0081] Further, step S400 includes step S410 to step S430.

[0082] Step S410, performing impedance analysis processing according to the preliminary design parameters, by applying an AC voltage of a preset voltage value to the electrode surface and gradually changing the frequency, measuring the impedance values ​​of the piezoelectric transducer plates with different thicknesses at different frequencies, and obtaining impedance change data of the piezoelectric transducer plates with different thicknesses as the frequency changes;

[0083] Step S420: Screening and processing are performed according to the impedance data under different thicknesses, and the impedance spectrum curves corresponding to the piezoelectric transducer plates with different thicknesses are plotted to screen out the transducer plate thickness values ​​whose resonant frequencies are within the frequency range required for the characteristic waveguide of the weld. The required frequency range needs to be determined according to the actual size of the fillet weld.

[0084] Specifically, according to the phase velocity dispersion curve data of the characteristic guided wave, it is concluded that the frequency of the characteristic guided wave in the weld is above 500kHz. Therefore, the thickness of the piezoelectric transducer plate is optimized with this frequency range as the target, so that the resonant frequency of the piezoelectric transducer plate is above 500kHz. Finite element impedance analysis is performed on piezoelectric transducer plates of different thicknesses. The specific steps are to apply an AC voltage of 1V to the electrode surface, change the frequency, and measure the impedance value of the electrode surface at different frequencies. According to the impedance spectrum curve corresponding to each thickness of the piezoelectric transducer plate, the thickness value corresponding to the resonant frequency greater than 500kHz is selected. The impedance spectrum corresponding to the selected thickness value is as follows: Figure 7 Finally, based on the above optimization design scheme, the thickness of the piezoelectric transducer plate is 1 mm.

[0085] Step S430: Perform frequency matching according to the above analysis. This step comprehensively considers three indicators, performs frequency optimization, obtains an optimized design scheme, and then determines the final excitation frequency range.

[0086] Further, step S430 includes step S431 to step S434.

[0087] Step S431, judging that the frequency of the characteristic guided wave in the weld is above 500 kHz through the dispersion curve data of the characteristic guided wave, and taking the frequency range as the first frequency range;

[0088] Step S432: by processing data through impedance analysis, it is determined that the selected piezoelectric transducer plate with a thickness of 1 mm has several resonance peaks, and the vibration efficiency is maximized when the piezoelectric transducer plate is excited at the frequency corresponding to the resonance peak. Therefore, the frequency corresponding to the resonance peak of the 1 mm piezoelectric transducer plate is recorded as the second frequency range;

[0089] Step S433, by simulating the propagation characteristics of characteristic guided waves in the fillet weld at different frequencies when cracks exist, the propagation stability of characteristic guided waves at different frequencies under defect conditions is analyzed, and the frequency that can still propagate stably in the presence of defects is screened out, which is recorded as the third frequency range;

[0090] Step S434: Obtain a target frequency range by taking the intersection of the first, second and third frequency ranges. Within this frequency range, characteristic guided waves in the fillet weld can be efficiently excited, and defects in the fillet weld can be effectively identified.

[0091] Further, step S500 includes step S510 to step S520.

[0092] Step S510: According to the optimization design scheme, polarization direction optimization processing is performed, by setting the polarization directions of the piezoelectric transducer plates on both sides to be opposite and placing them side by side along the width direction, so as to obtain an optimized polarization direction configuration scheme;

[0093] Step S520, according to the polarization direction configuration scheme, by optimizing the center spacing and width of the piezoelectric transducer plate, the transverse excitation energy is maximized and the vertical body wave suppression effect is optimal, and the plate width and spacing configuration are obtained;

[0094] It should be noted that it is difficult to avoid the generation of body waves when vibrating in a single direction. For structures with limited thickness such as fillet welds, body waves will affect the signal-to-noise ratio. Therefore, this step optimizes the design of the piezoelectric transducer plate so that the final transducer can achieve body wave suppression in the thickness direction, thereby generating characteristic guided waves with a high signal-to-noise ratio in the weld. The specific implementation method is as follows:

[0095] First, let the length, width and height of the first piezoelectric transducer plate be l, w and h respectively, and the polarization direction is along the width w direction. The second piezoelectric transducer plate has the same size as the first piezoelectric transducer plate, and has opposite polarization directions. The first piezoelectric transducer plate and the second piezoelectric transducer plate are placed side by side in the width direction, and epoxy resin is used to connect and control the center spacing of the two piezoelectric transducer plates in the middle. The center spacing of the two piezoelectric transducer plates is less than the wavelength of the characteristic guided wave, and is greater than or equal to the width w of the piezoelectric transducer plate. Through this design, the two piezoelectric transducer plates deform in opposite directions under the excitation of voltage. According to the symmetry principle of elastic dynamics, it can be seen that the body wave formed in the thickness direction always satisfies the conditions of interference destructiveness, thereby suppressing the generation of body waves, thereby forming a weld characteristic guided wave in the direction parallel to the polarization, and obtaining a body wave suppression scheme.

[0096] Furthermore, on this basis, the in-plane size of the piezoelectric transducer plate is optimized by the gradient descent method, where the in-plane size includes the length and width of the piezoelectric transducer plate. The length and width of the transducer plate are adjusted step by step through iteration, so that the final size can most efficiently excite the characteristic guided wave and suppress the vertical body wave.

[0097] Furthermore, on this basis, the body wave attenuation effect of the transducer was experimentally verified, such as Figure 8 , Fig. 9 As shown in the figure, a single piezoelectric transducer plate and a characteristic waveguide piezoelectric transducer unit are arranged on the surface of the thick plate structure, and a piezoelectric transducer plate is attached to the back of the thick plate to receive the body wave signals generated by the two. Fig.10 As shown, the body wave generated by a piezoelectric transducer plate in the thickness direction is received by the back piezoelectric transducer plate, and an obvious body wave signal appears between 15μs and 30μs. Fig.11 This is the signal received by the piezoelectric transducer plate on the back of the body wave generated by a characteristic waveguide piezoelectric transducer unit in the thickness direction. No obvious body wave signal appears at the corresponding moment, indicating that the body wave is successfully suppressed. This method verifies the body wave attenuation effect of the optimized piezoelectric transducer.

[0098] Embodiment 3:

[0099] This embodiment provides a method for using a transducer to excite characteristic guided waves in a bridge deck fillet weld structure, and the specific process is as follows:

[0100] First, a basic unit of a characteristic guided wave transducer is attached to the surface of the fillet weld structure of the bridge deck to be inspected by adhesive. Fig.12 As shown, coupling agent or epoxy resin and other materials are used for bonding. The adhesive should be fully applied to the connecting part to ensure complete contact between the transducer and the weld structure.

[0101] Then the positive electrode of the piezoelectric transducer plate in the transducer is connected to the positive electrode of the power amplifier and the positive electrode of the signal source in turn through a wire, and the negative electrode of the piezoelectric transducer plate in the transducer is connected to the negative electrode of the power amplifier and the negative electrode of the signal source in turn through a wire. The function generator is used to generate signal pulses, and the power amplifier is used to amplify the signal. After the signal is amplified, it is applied to each piezoelectric transducer plate.

[0102] Next, according to the size of the fillet weld structure to be detected, the dispersion curve of the characteristic guided wave is calculated, and then the appropriate excitation frequency is selected, and a pulse signal of the corresponding frequency is applied to the transducer.

[0103] Finally, the transducer receives the pulsed electrical signal and vibrates based on the inverse piezoelectric effect, generating characteristic guided waves in the weld structure. The characteristic guided waves propagate in the weld, and the signal passes through the sensor, and an electrical signal is received based on the piezoelectric effect, such as Fig.13 As shown, it is the characteristic guided wave signal received in the weld. An obvious wave packet appears around 300μs, which can verify the successful excitation and propagation of the characteristic guided wave in the weld.

[0104] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A transducer for exciting characteristic guided waves of a fillet weld, characterized in that: include: At least one basic unit (1), all the basic units (1) are fixedly arranged on the surface of the fillet weld to be detected, each of the basic units (1) comprises a first piezoelectric transducer plate (11), a second piezoelectric transducer plate (12), a connecting plate (13) and an electrode (14), the first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12) are symmetrically arranged at two ends of the connecting plate (13), the first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12) are both in the shape of a rectangular parallelepiped with the same size, and the electrodes (14) are respectively arranged on the upper and lower surfaces of the first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12).

2. The transducer for exciting characteristic guided waves of fillet welds according to claim 1, characterized in that: The length direction of the transducer plate in the three-dimensional space coordinate axis is denoted as the x direction, the width direction is denoted as the y direction, and the thickness direction is denoted as the z direction. The polarization direction of the first piezoelectric transducer plate is along the negative direction of the y axis, and the polarization direction of the second piezoelectric transducer plate is along the positive direction of the y axis.

3. The transducer for exciting characteristic guided waves of fillet welds according to claim 2, characterized in that: The length, width and height dimensions of the first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12) are recorded as l×w×h, the center distance between the first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12) in each basic unit (1) is recorded as Δd, and the length satisfies the condition The width satisfies the condition w<λ, and the center spacing satisfies the condition w≤Δd<λ, where d is the width of the fillet weld to be tested. λ is the wavelength of the characteristic guided wave, c is the wave velocity of the characteristic guided wave in the weld structure, and f is the vibration frequency that generates the characteristic guided wave.

4. The transducer for exciting characteristic guided waves of fillet welds according to claim 3, characterized in that: The first piezoelectric transducer plate (11) and the second piezoelectric transducer plate (12) both have a thickness shear piezoelectric coefficient d 15 , the thickness shear piezoelectric coefficient d 15 It represents the proportional relationship between the charge density generated by the piezoelectric material under the action of shear stress and the applied stress. The first subscript "1" represents the direction in which the charge is generated by the crystal, indicating that the surface on which the charge is generated is perpendicular to the x-axis. The second subscript "5" represents the direction in which the force is applied, indicating that the shear force is applied in the xz plane.

5. The transducer for exciting characteristic guided waves of fillet welds according to claim 1, characterized in that: The material of the connecting plate (13) is epoxy resin.

6. An optimization design method for a transducer for exciting characteristic guided waves of a fillet weld, characterized in that: include: Acquiring fillet weld dispersion characteristic data, wherein the fillet weld dispersion characteristic data includes geometric dimensions, material properties and boundary conditions of the fillet weld; According to the fillet weld dispersion characteristic data, the dispersion characteristic of the waveguide mode in the weld is analyzed to obtain a phase velocity dispersion curve of the characteristic waveguide, and the dispersion curve data and waveform structure data of the characteristic waveguide of the fillet weld are extracted; Obtaining preliminary design parameters according to the dispersion curve data and the waveform structure data, wherein the preliminary design parameters include the in-plane size and arrangement of the piezoelectric transducer plate; According to the preliminary design parameters, impedance analysis is performed on piezoelectric transducer plates of different thicknesses to obtain an optimized design scheme, wherein the optimized scheme includes a thickness value of the piezoelectric transducer plate and an excitation frequency range; According to the optimized design scheme, after optimizing the geometric shape and arrangement of the piezoelectric transducer plates, the final design scheme is obtained by optimizing the length, width and plate spacing of the piezoelectric transducer plates to maximize the excitation efficiency and signal-to-noise ratio of the characteristic waveguide.

7. The method for optimizing the design of a transducer for exciting characteristic waveguide of a fillet weld according to claim 6, characterized in that: According to the fillet weld dispersion characteristic data, after analyzing the dispersion characteristics of the waveguide mode in the weld, the phase velocity dispersion curve and displacement distribution of the characteristic waveguide are obtained, and the dispersion curve data and waveform structure data of the characteristic waveguide of the fillet weld are extracted, including: A finite element model is established based on the fillet weld dispersion characteristic data, and an initial model is obtained by dividing the bridge deck structure into a web, a top plate, a perfectly matched layer and a target area, and setting corresponding boundary conditions; Performing meshing processing according to the initial model, dividing the model by using free tetrahedral units, and determining the unit size based on the minimum wavelength of the characteristic waveguide of the fillet weld during the calculation process, thereby obtaining a meshed model; Performing frequency domain analysis processing according to the grid model, applying periodic boundary conditions to simulate the periodic structure of the fillet weld in the axial direction, performing parameterized scanning on the wave number, gradually calculating the corresponding frequency and guided wave mode, and obtaining the frequency domain analysis result; Modal screening is performed based on the frequency domain analysis results. By analyzing the displacement concentration of the guided wave in the target area, non-dispersive guided wave modes with concentrated energy suitable for weld defect detection are screened out, and waveform structure data and dispersion curve data of characteristic guided waves in the weld are obtained.

8. The method for optimizing the design of a transducer for exciting characteristic waveguide of a fillet weld according to claim 6, characterized in that: Preliminary design parameters are obtained according to the dispersion curve data and the waveform structure data. The preliminary design parameters include the in-plane size and arrangement of the piezoelectric transducer plate, including: Performing displacement analysis processing according to the waveform structure data, determining the dominant displacement direction by analyzing the displacement distribution characteristics of the guided wave on the weld surface, and obtaining displacement distribution information; According to the displacement distribution information, a piezoelectric material vibration mode analysis process is performed, and a piezoelectric material thickness shear mode suitable for characteristic guided wave excitation is obtained by selecting a vibration mode that matches the dominant displacement direction; According to the thickness shear mode of the piezoelectric material, the vibration mode matching process of the piezoelectric transducer plate is performed, and the thickness shear type piezoelectric transducer plate is obtained by combining the vibration mode of the piezoelectric transducer plate with the waveform structure of the characteristic waveguide; According to the obtained thickness shear type piezoelectric transducer plate, its in-plane size is optimized. By analyzing the geometric structure of the weld and the effective bonding area, the length and width of the transducer plate are optimized, and its shape is ensured to match the weld structure. The preliminary design parameters are obtained, which include the in-plane size and layout of the transducer plate.

9. The method for optimizing the design of a transducer for exciting characteristic waveguide of a fillet weld according to claim 6, characterized in that: According to the preliminary design parameters, impedance analysis was performed on piezoelectric transducer plates of different thicknesses to obtain an optimized design solution, including: Perform impedance analysis according to the preliminary design parameters, apply an AC voltage with a preset voltage value to the electrode surface, and gradually change the frequency to measure the impedance values ​​of piezoelectric transducer plates with different thicknesses at different frequencies, so as to obtain impedance change data of piezoelectric transducer plates with different thicknesses as the frequency changes; The impedance data under different thicknesses are screened and processed. By drawing the impedance spectrum curves corresponding to piezoelectric transducer plates with different thicknesses, the transducer plate thickness values ​​whose resonant frequencies are in the frequency range required for the characteristic waveguide of the weld are screened out. The required frequency range is determined according to the actual size of the fillet weld.

10. The method for optimizing the design of a transducer for exciting characteristic waveguide of a fillet weld according to claim 6, characterized in that: According to the optimization design scheme, after optimizing the geometric dimensions and arrangement of the piezoelectric transducer plate, the length, width and plate spacing of the piezoelectric transducer plate are optimized to maximize the excitation efficiency and signal-to-noise ratio of the characteristic waveguide, and the final design scheme is obtained, including: According to the optimization design scheme, a polarization direction optimization process is performed, by setting the polarization directions of the piezoelectric transducer plates on both sides to be opposite and placing them side by side along the width direction, thereby obtaining an optimized polarization direction configuration scheme; According to the polarization direction configuration scheme, the center spacing and width of the piezoelectric transducer plates are optimized to maximize the lateral excitation energy and optimize the vertical body wave suppression effect, thereby obtaining the plate width and spacing configuration; The piezoelectric transducer plate spacing is optimized according to the bulk wave suppression scheme, and the plate spacing configuration is obtained by adjusting the center spacing between the two piezoelectric transducer plates to be smaller than the wavelength of the characteristic waveguide and greater than or equal to the width of the piezoelectric transducer plates.

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