Structural surface defect depth monitoring device and preparation and monitoring method thereof

Abstract

The invention relates to the technical field of structure health monitoring, in particular to a structure surface defect depth monitoring device and a preparation and monitoring method thereof.The structure surface defect depth monitoring device comprises a plurality of combined units of Rayleigh wave emitters and Rayleigh wave receiver arrays, and the Rayleigh wave emitters and the Rayleigh wave receiver arrays are prepared or adhered to the surface of a to-be-detected structure in situ; the Rayleigh wave receiver array is located between the Rayleigh wave emitter and the surface defect, and the Rayleigh wave emitter comprises a first direct-writing type lead-free piezoelectric ceramic layer and an interdigital electrode located on the first direct-writing type lead-free piezoelectric ceramic layer. The Rayleigh wave receiver array comprises a second direct-writing type lead-free piezoelectric ceramic layer and a comb-shaped electrode located on the second direct-writing type lead-free piezoelectric ceramic layer. According to the invention, accurate, reliable and low-cost detection and monitoring of the depth of the surface defect of the curved surface structure are realized without manual scanning and optical sight.

Description

Technical Field

[0001] This invention relates to the field of structural health monitoring technology, and in particular to a device for monitoring the depth of structural surface defects, and its preparation and monitoring methods. Background Technology

[0002] In practical engineering structures with complex surfaces, such as oil and gas pipelines, welded joints, and metal plates, components often develop surface cracks and corrosion pits during service due to factors such as stress corrosion, fatigue damage, or manufacturing defects. These surface defects directly compromise the integrity of the structure and threaten its service safety. If the depth of these defects is not detected in time and accurately assessed, it can easily lead to catastrophic accidents. Therefore, achieving accurate, reliable, and low-cost monitoring of the depth of structural surface defects has become a key technical challenge in the field of structural health assessment and operation and maintenance.

[0003] Currently, common methods for defect depth assessment include Ultrasonic Phased Array (UPA) technology, which typically uses shear waves to manually scan and image the target area. However, this method has significant limitations: firstly, it relies on manual scanning, resulting in low detection efficiency and requiring highly skilled operators; secondly, when dealing with complex geometric structures such as curved pipes and weld joints, the deployment of the UPA is difficult, and the imaging quality is easily affected by the structural morphology, making it difficult to guarantee detection accuracy. Furthermore, the shear waves used in UPA have weak propagation capabilities on structural surfaces, resulting in low sensitivity for detecting near-surface defects and difficulty in accurately identifying shallow problems.

[0004] In comparison, Rayleigh waves, as elastic surface waves with highly concentrated energy on the material surface, have higher sensitivity for detecting defects such as surface cracks and corrosion. At the same time, Rayleigh waves also have good surface propagation capabilities, making them suitable for the detection of various complex structural components.

[0005] Most existing Rayleigh wave detection methods require the use of laser interferometers and Rayleigh wave transducers to analyze defect echoes in order to accurately assess defect depth. However, laser interferometers are not only expensive, but also require unobstructed optical paths, making them unsuitable for engineering structures with obstructions, dense components, or non-visible areas. Furthermore, if multiple ultrasonic sensor arrays are used to monitor Rayleigh waves, the sensors will produce significant scattering effects on the waves, causing rapid attenuation of Rayleigh wave energy, which limits their practical application.

[0006] Therefore, there is an urgent need for a Rayleigh wave surface defect monitoring device and method that does not require unobstructed vision, does not require manual scanning, and can adapt to curved surfaces, in order to solve the above-mentioned problems of the existing technology. Summary of the Invention

[0007] The purpose of this invention is to provide a device for monitoring the depth of surface defects in a structure, as well as its preparation and monitoring method, so as to achieve accurate, reliable, and low-cost detection and monitoring of the depth of surface defects in curved structures without the need for manual scanning or optical sight.

[0008] To achieve the above objectives, the present invention provides a structural surface defect depth monitoring device, comprising a combination unit of several Rayleigh wave transmitters and Rayleigh wave receiver arrays. The Rayleigh wave transmitters and Rayleigh wave receiver arrays are prepared in situ or adhered to the surface of the structure to be tested and arranged along a direction collinear with the surface defect. The Rayleigh wave receiver array is located between the Rayleigh wave transmitters and the surface defect. The Rayleigh wave transmitter includes a first direct-write lead-free piezoelectric ceramic layer and interdigitated electrodes located on the first direct-write lead-free piezoelectric ceramic layer. The Rayleigh wave receiver array includes a second direct-write lead-free piezoelectric ceramic layer and comb-shaped electrodes located on the second direct-write lead-free piezoelectric ceramic layer.

[0009] Preferably, the Rayleigh wave transmitter has a longitudinally telescopic or thickness-shearing structure.

[0010] Preferably, the comb electrodes of the Rayleigh wave receiver array include at least four groups of equally spaced dispersed electrodes.

[0011] Preferably, the dispersing electrode is elongated, with an aspect ratio greater than 5, and its length direction is perpendicular to the propagation direction of the Rayleigh wave. The width of the dispersing electrode is less than half of the Rayleigh wave reflection wavelength corresponding to the smallest surface defect being monitored.

[0012] The method for preparing the above-mentioned surface defect depth monitoring device includes the following steps: S101. Weigh potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide, mix and ball-mill, dry, pre-calcine, and sieve to obtain lead-free piezoelectric ceramic precursor powder. S102. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer one is prepared in situ on the surface of the structure to be tested, in a direction that is located on the side of the surface defect and collinear with the surface defect. Interdigitated electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer one to form a Rayleigh wave emitter for exciting incident Rayleigh waves. S103. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer two is prepared in situ between the Rayleigh wave emitter and the surface defect on the surface of the structure to be tested. Comb-shaped electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer two to form a Rayleigh wave receiver array for receiving reflected Rayleigh waves.

[0013] Preferably, the general chemical formula of the lead-free piezoelectric ceramic in S101 is: [(K 0.5 Na 0.5 ) 1-x Lix ]Nb 1-y Ta y O3, where x = 0 - 0.1, y = 0 - 0.1.

[0014] Preferably, in S102 and S103, before in-situ preparation, a stacked structure of NiCrAlY transition layer and yttrium oxide stabilized zirconia thermal barrier layer is first prepared on the surface of the structure to be tested to form a transition thermal barrier layer. On the transition thermal barrier layer, a direct-write lead-free piezoelectric ceramic layer one and a direct-write lead-free piezoelectric ceramic layer two are prepared in-situ respectively.

[0015] The monitoring method of the above-mentioned structural surface defect depth monitoring device includes the following steps: S201. An incident Rayleigh wave signal directed towards the surface defect is emitted on the surface of the structure to be tested through a Rayleigh wave transmitter, and the reflected Rayleigh wave signal reflected back from the surface defect is received through a Rayleigh wave receiver array. S202. The incident Rayleigh wave signal and the reflected Rayleigh wave signal in S201 are enhanced by using a delay superposition algorithm; S203. Calculate the depth of surface defects based on the enhanced incident Rayleigh wave signal and the reflected Rayleigh wave signal.

[0016] Preferably, in S201, the structure to be tested is a planar structure, a tubular structure, or a curved structure. The structure to be tested includes a weld area, and the surface defects are located near the weld area. The surface defects are surface cracks or surface corrosion.

[0017] Preferably, the delay superposition algorithm in S202 includes the following processing steps: S2021, determining the arrival time of the incident Rayleigh wave signal and the reflected Rayleigh wave signal; S2022. Perform a fast Fourier transform on the incident Rayleigh wave signal and the reflected Rayleigh wave signal to obtain the spectrum; S2023. The spectrum obtained in S2022 is superimposed to obtain the enhanced incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal.

[0018] Therefore, the present invention, employing the above-mentioned structural surface defect depth monitoring device and its preparation and monitoring method, has the following beneficial effects: (1) This invention breaks through the technical bottlenecks of traditional UPA technology, such as manual scanning, low detection efficiency and poor adaptability to complex structures. It is especially suitable for structures with complex surfaces such as welds and pipes, and realizes highly sensitive, non-contact and line-of-sight-free accurate monitoring of the depth of surface defects. (2) The present invention uses a direct-write lead-free piezoelectric ceramic functional layer for Rayleigh wave excitation and reception. A high-density ceramic layer is deposited in situ on the surface of the structure through a high-speed thermal spraying process. Combined with micro-nano electrode patterning technology, interdigitated and comb-shaped electrode arrays are constructed, avoiding the problems of complex wiring and matching difficulties of traditional sensors. This realizes the integration of device functions and the construction of in-situ sensing capabilities of structural components. (3) The delay superposition algorithm used in this invention enhances the incident and reflected Rayleigh wave signals. Combined with the signal acquisition by the multi-point distributed electrode array, it significantly improves the signal-to-noise ratio and time-frequency clarity of the surface defect echo signal. It can effectively suppress environmental noise and coupling interference, realize high-resolution inversion of defect depth, and is suitable for micro-cracks and corrosion pits with minimum feature size down to the sub-millimeter level. (4) The lead-free piezoelectric ceramic used in this invention [(K 0.5 Na 0.5 ) 1-x Li x ]Nb 1-y Ta y O3 has excellent piezoelectric properties and a piezoelectric range of 250-350 °C. Its high-temperature performance, combined with in-situ heat treatment, can further improve crystallization quality and meet the long-term service requirements of various complex industrial environments. (5) The monitoring method and device constructed by the present invention do not require masks or lines of sight and can be constructed in a fully automated manner. They are applicable to various engineering structural scenarios such as planar structures, curved components and weld areas, and have good process compatibility, scalability and industrial application prospects.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a structural surface defect depth monitoring device according to Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the direct-write lead-free piezoelectric ceramic layer and stacking structure in Embodiment 5 of the present invention; Figure 3 This is a schematic diagram of the delayed superposition processing in Embodiment 6 of the present invention. Figure 3 (a) in the diagram shows the principle of the delay superposition algorithm to enhance the main signal. Figure 3 (b) in the diagram is a schematic of the delay superposition algorithm for eliminating noise signals; Figure 4 This is a schematic diagram of the monitoring device of the present invention, which uses three different receivers. Figure 4 (a) is a schematic diagram of a monitoring device whose receiver is a direct-write lead-free piezoelectric ceramic layer Rayleigh wave receiver array. Figure 4The receiver in (b) is a schematic diagram of a monitoring device for an ultrasonic transducer array of bulk lead-free piezoelectric ceramics; Figure 4 (c) is a schematic diagram of the receiver being the monitoring device of the laser interferometer; Figure 5 The diagram shows that different receivers can detect significant peak reflection coefficients within the effective frequency band. Figure 5 (a) shows the peak reflection coefficient of a Rayleigh wave receiver array with direct-write lead-free piezoelectric ceramic. Figure 5 (b) shows the peak reflection coefficient of the ultrasonic transducer array made of bulk lead-free piezoelectric ceramics. Figure 5 (c) shows the peak reflection coefficient of the laser interferometer receiving array; Figure 6 A schematic diagram showing the differences in incident Rayleigh wave signals detected by three different receivers; Figure 7 This is a schematic diagram of application example 1 of the present invention, which applies the monitoring device to monitor the pipeline structure; Figure 8 This is the time-frequency domain joint analysis diagram of Application Example 1 of the present invention; Figure 9 This is a schematic diagram of application example 2 of the present invention, which applies the monitoring device to a curved structure; Figure 10 This is the time-frequency domain joint analysis diagram of Application Example 2 of the present invention.

[0021] Figure label: 1. Rayleigh wave transmitter; 2. Rayleigh wave receiver array; 3. Direct-write lead-free piezoelectric ceramic layer one; 4. Interdigitated electrode; 5. Direct-write lead-free piezoelectric ceramic layer two; 6. Comb electrode; 7. Structure under test; 8. Surface defect; 9. NiCrAlY transition layer; 10. NiCrAlY+YSZ composite coating; 11. YSZ thermal barrier layer. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0023] A surface defect depth monitoring device includes a combination unit of several Rayleigh wave transmitters and Rayleigh wave receiver arrays. The Rayleigh wave transmitters and Rayleigh wave receiver arrays are prepared in situ or adhered to the surface of the structure to be tested and arranged in a direction collinear with the surface defect. The Rayleigh wave receiver array is located between the Rayleigh wave transmitters and the surface defect. The Rayleigh wave transmitter includes a first direct-write lead-free piezoelectric ceramic layer and interdigitated electrodes located on the first direct-write lead-free piezoelectric ceramic layer. The Rayleigh wave receiver array includes a second direct-write lead-free piezoelectric ceramic layer and comb-shaped electrodes located on the second direct-write lead-free piezoelectric ceramic layer.

[0024] Preferably, the Rayleigh wave transmitter has a longitudinally telescopic or thickness-shearing structure.

[0025] Preferably, the comb electrodes of the Rayleigh wave receiver array include at least four groups of equally spaced dispersed electrodes.

[0026] Preferably, the Rayleigh wave receiver array is located close to the surface to be monitored, between the Rayleigh wave transmitter and the defect on the surface to be measured, in order to obtain a clearer Rayleigh wave signal with contrast between the incident and reflected signals from the defect.

[0027] Preferably, the dispersing electrode is elongated, with an aspect ratio greater than 5, and its length direction is perpendicular to the propagation direction of the Rayleigh wave. The width of the dispersing electrode is less than half the Rayleigh wave reflection wavelength corresponding to the smallest surface defect being monitored. This improves the resolution of defect echoes.

[0028] More preferably, the number of electrodes in each group of dispersed electrodes is 3 to 5.

[0029] The method for preparing the above-mentioned surface defect depth monitoring device includes the following steps: S101. Weigh potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide, mix and ball-mill, dry, pre-calcine, and sieve to obtain lead-free piezoelectric ceramic precursor powder. S102. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer one is prepared in situ on the surface of the structure to be tested, in a direction that is located on the side of the surface defect and collinear with the surface defect. Interdigitated electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer one to form a Rayleigh wave emitter for exciting incident Rayleigh waves. S103. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer two is prepared in situ between the Rayleigh wave emitter and the surface defect on the surface of the structure to be tested. Comb-shaped electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer two to form a Rayleigh wave receiver array for receiving reflected Rayleigh waves.

[0030] Preferably, the general chemical formula of the lead-free piezoelectric ceramic in S101 is: [(K 0.5 Na0.5 ) 1-x Li x ]Nb 1-y Ta y O3, where x = 0 - 0.1, y = 0 - 0.1.

[0031] In a further preferred embodiment, potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide are weighed according to the molar ratio of the general chemical formula for lead-free piezoelectric ceramics, with potassium carbonate and sodium carbonate in excess by 5% to compensate for the volatilization of K and Na elements, thereby ensuring the stability of the components during sintering and spraying.

[0032] In a further preferred embodiment, the ball milling in S101 is performed by using anhydrous ethanol as the milling medium in a nylon tank, using zirconia balls as the milling balls, and a ball-to-material ratio of 8:1 to obtain a mixed powder.

[0033] In a further preferred embodiment, after drying in S101, the material is placed in a corundum crucible, and then the corundum crucible is placed in a muffle furnace and heated to 750-1000 °C. Preheat for 3-6 hours.

[0034] More preferably, the particle size obtained after sieving in S101 is 20-50 mm. m-type lead-free piezoelectric ceramic precursor powder.

[0035] Preferably, in S102 and S103, before in-situ preparation, a stacked structure of NiCrAlY transition layer and yttrium oxide stabilized zirconia thermal barrier layer is first prepared on the surface of the structure to be tested to form a transition thermal barrier layer. On the transition thermal barrier layer, a direct-write lead-free piezoelectric ceramic layer one and a direct-write lead-free piezoelectric ceramic layer two are prepared in-situ respectively.

[0036] Furthermore, the purpose of preparing the NiCrAlY transition layer is to roughen the surface of the structure to be monitored, specifically by using plasma spraying with an inert argon gas flow rate of 80-100 L. The hydrogen flow rate is 15-25L / min. The spraying voltage is 150-200V, the spraying current is 300-350A, the spraying distance is 80-100mm, and a thickness of 50-100mm is formed on the surface of the structure to be monitored. m NiCrAlY transition layer.

[0037] Furthermore, the purpose of preparing the yttrium-stabilized zirconia (YSZ) thermal barrier layer is to maintain better adhesion between the first and second direct-write lead-free piezoelectric ceramic layers and the structure to be monitored, and to ensure that the direct-write lead-free piezoelectric ceramic layer can withstand higher operating temperatures. Specifically, a plasma spraying method is used, with an inert argon gas flow rate of 80-100 L / min. The hydrogen flow rate is 10-15 L / min. The spraying voltage is 100-150V, the spraying current is 200-250A, and the spraying distance is 80-100mm. First, spray 50-100mm of the coating onto the NiCrAlY transition layer. A NiCrAlY+YSZ composite coating of m is then sprayed with 100-150 μm. A YSZ thermal barrier layer with a thickness of 150-250 μm is ultimately formed. m is a transitional thermal barrier layer.

[0038] Preferably, both the in-situ preparation of the first and second direct-write lead-free piezoelectric ceramic layers are performed using plasma spraying, with an argon gas flow rate of 80-100 L. The hydrogen flow rate is 10-15 L / min. The spraying voltage is 80-100V, the spraying current is 150-200A, and the spraying distance is 80-100mm, forming a 150-200mm thick layer on the surface of the transition thermal barrier. m-type direct-write lead-free piezoelectric ceramic layer one and direct-write lead-free piezoelectric ceramic layer two.

[0039] Preferably, the spraying temperature for in-situ preparation of the first and second direct-write lead-free piezoelectric ceramic layers is controlled at 1100°C (by adjusting the heat source power, i.e., the spraying voltage and spraying current parameters). ~1300 The deposition rate (adjusted by the type of process gas, gas flow rate, and powder feed rate, etc.) ranges from 10 to 100. m min.

[0040] Alternatively, the first and second direct-write lead-free piezoelectric ceramic layers can be heat-treated with plasma or flame to improve their crystallinity and thus enhance their piezoelectric properties.

[0041] In a further preferred embodiment, in S102 and S103, the printing is one of inkjet printing, microdroplet printing, or magnetron sputtering.

[0042] More preferably, the electrode materials for the interdigitated electrodes and comb electrodes are gold, silver, or silver-palladium alloys.

[0043] This invention uses a plasma spraying direct writing process to deposit a piezoelectric ceramic functional layer on the surface of the structure to be monitored in situ, and uses micro-electrode forming technology to directly construct an interdigitated transmitter and a comb-shaped receiver array on the functional layer, thereby realizing the directional excitation and multi-point reception of Rayleigh waves.

[0044] The monitoring method of the above-mentioned structural surface defect depth monitoring device includes the following steps: S201. An incident Rayleigh wave signal directed towards the surface defect is emitted on the surface of the structure to be tested through a Rayleigh wave transmitter, and the reflected Rayleigh wave signal reflected back from the surface defect is received through a Rayleigh wave receiver array. S202. The incident Rayleigh wave signal and the reflected Rayleigh wave signal in S201 are enhanced by using the Delay-and-Sum algorithm. S203. Calculate the depth of surface defects based on the enhanced incident Rayleigh wave signal and the reflected Rayleigh wave signal.

[0045] Preferably, in S201, the structure to be tested is a planar structure, a tubular structure, or a curved structure. The structure to be tested includes a weld area, and the surface defects are located near the weld area. The surface defects are surface cracks or surface corrosion.

[0046] Preferably, the incident Rayleigh wave signal emitted in S201 propagates along a direction collinear with the surface defect of the structure.

[0047] Preferably, the delay superposition algorithm in S202 includes the following processing steps: S2021. Determine the arrival times of the incident Rayleigh wave signal and the reflected Rayleigh wave signal; S2022. Perform a fast Fourier transform on the incident Rayleigh wave signal and the reflected Rayleigh wave signal to obtain the spectrum; S2023. The spectrum obtained in S2022 is superimposed to obtain the enhanced incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal.

[0048] In a further preferred embodiment, in S2023, the spectrum obtained in S2022 is subjected to time delay compensation and superposition enhancement processing to obtain the incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal with improved signal-to-noise ratio. The time compensation accuracy is less than 1 / 10 of the Rayleigh wave period, ensuring that the superimposed and enhanced waveform is clear and has high positioning accuracy.

[0049] In a further preferred embodiment, in S203, based on the enhanced incident Rayleigh wave signal and the reflected Rayleigh wave signal, key characteristic parameters such as time delay difference and energy attenuation are extracted. Combined with Rayleigh wave velocity and wavelength information, the depth and spatial location of the surface defect are calculated to achieve defect depth assessment.

[0050] The present invention provides a monitoring method for a structural surface defect depth monitoring device, which is applicable to various industrial structural surfaces such as planar structures, curved structures, and weld areas, and is suitable for structural health monitoring scenarios with different curvatures and materials.

[0051] This invention employs lead-free piezoelectric ceramic materials and a direct-write deposition process to construct an array of Rayleigh wave transmitters and receivers in situ on the surface of the structure to be monitored. The Rayleigh wave transmitters and receivers are arranged according to the Rayleigh wave propagation path. By precisely controlling the thickness of the lead-free piezoelectric ceramic layer, the electrode structure, and the element spacing, highly directional and sensitive Rayleigh wave excitation and reception are achieved. Finally, a delay-superposition algorithm is used to enhance the incident and reflected Rayleigh wave signals, enabling precise monitoring of the surface defect depth of complex-shaped structural components.

[0052] Mechanism of the invention: Rayleigh waves are elastic wave modes with energy concentrated on the surface of materials. They are highly sensitive to defects such as near-surface cracks and corrosion, and can propagate along curved surfaces, making them suitable for various complex surface structures. This invention avoids the problems of manual scanning, limited line of sight, and complex sensor deployment in traditional UPA (Ultra-Proof-of-Action) testing by directly constructing an array of Rayleigh wave transmitters and receivers on the structural surface. Simultaneously, through array-based reception and delay superposition algorithms, the signal-to-noise ratio of the defect echo signal can be effectively improved, significantly enhancing the detection accuracy of defect depth.

[0053] Example 1 like Figure 1 As shown, the present invention provides a structural surface defect depth monitoring device, including a combination unit of several Rayleigh wave transmitters 1 and Rayleigh wave receiver arrays 2. The Rayleigh wave transmitters 1 and Rayleigh wave receiver arrays 2 are prepared in situ or adhered to the surface of the structure 7 to be tested, and arranged in a direction collinear with the surface defect 8. The Rayleigh wave receiver arrays 2 are located between the Rayleigh wave transmitters 1 and the surface defect 8.

[0054] Rayleigh wave transmitter 1 includes a direct-write lead-free piezoelectric ceramic layer 3 and interdigitated electrodes 4 located on the direct-write lead-free piezoelectric ceramic layer 3. The structure of Rayleigh wave transmitter 1 is either longitudinally stretched or thickness-sheared. Rayleigh wave receiver array 2 includes a second direct-write lead-free piezoelectric ceramic layer 5 and comb-shaped electrodes 6 located on the second direct-write lead-free piezoelectric ceramic layer 5, featuring lightweight, high integration, and high conformality.

[0055] Rayleigh wave receiver array 2 is located close to the surface to be monitored, between Rayleigh wave transmitter 1 and the surface defect 8, to obtain a clearer Rayleigh wave signal with contrast between the incident and reflected signals from the defect. The comb-shaped electrodes 6 of Rayleigh wave receiver array 2 include at least four sets of equally spaced dispersed electrodes.

[0056] The dispersive electrodes are elongated strips with an aspect ratio greater than 5, and their length direction is perpendicular to the propagation direction of the Rayleigh wave, enabling the detection of ultrasonic signals at different locations on the structural surface. The width of the dispersive electrodes in the Rayleigh wave receiver array 2 is determined by the minimum crack depth to be monitored, and its width is less than half the corresponding Rayleigh wave reflection wavelength. When half the wavelength of the Rayleigh wave is less than the width of the dispersive electrode and propagates along the electrode width direction, interference effects will weaken the detection signal. To avoid this problem, the width of the dispersive electrodes must be less than half the Rayleigh wave wavelength corresponding to the minimum monitored crack depth, and the number of electrodes in each group is 3 to 5.

[0057] Example 2 This invention provides a method for preparing a device for monitoring the depth of structural surface defects, comprising the following steps: S101. Weigh potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide, mix and ball-mill, dry, pre-calcine, and sieve to obtain lead-free piezoelectric ceramic precursor powder.

[0058] S102. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer 3 is prepared in situ on the surface of the structure to be tested 7 in a direction that is on the side of the surface defect 8 and collinear with the surface defect 8. Interdigitated electrodes 4 are printed on the surface of the direct-write lead-free piezoelectric ceramic layer 3 to form a Rayleigh wave emitter 1 for exciting incident Rayleigh waves.

[0059] S103. Using the lead-free piezoelectric ceramic precursor powder of S101, an in-situ direct-write lead-free piezoelectric ceramic layer 2 5 is prepared on the surface of the structure under test 7 between the Rayleigh wave transmitter 1 and the surface defect 8. A comb-shaped electrode 6 is printed on the surface of the direct-write lead-free piezoelectric ceramic layer 2 5 to form a Rayleigh wave receiver array 2 for receiving reflected Rayleigh waves.

[0060] In S102 and S103, before in-situ preparation, a stacked structure of NiCrAlY transition layer 9 and yttrium oxide stabilized zirconium oxide thermal barrier layer is first prepared on the surface of the structure to be tested 7 to form a transition thermal barrier layer. On the transition thermal barrier layer, a direct-write lead-free piezoelectric ceramic layer 1 3 and a direct-write lead-free piezoelectric ceramic layer 2 5 are prepared in-situ respectively.

[0061] The purpose of preparing the NiCrAlY transition layer 9 is to roughen the surface of the structure to be monitored, specifically by plasma spraying with an inert argon gas flow rate of 90 L / min. min, hydrogen flow rate is 20L The spraying voltage was 180V, the spraying current was 320A, and the spraying distance was 90mm, forming a 75mm thick coating on the surface of the structure to be monitored. m NiCrAlY transition layer 9.

[0062] The purpose of preparing the yttrium-stabilized zirconia (YSZ) thermal barrier layer is to maintain better adhesion between the direct-write lead-free piezoelectric ceramic layer 3 and the direct-write piezoelectric ceramic layer 5 and the structure to be monitored, and to ensure that the two direct-write lead-free piezoelectric ceramic layers can withstand higher operating temperatures. Specifically, a plasma spraying method was used, with an inert argon gas flow rate of 90 L / min. min, hydrogen flow rate is 12L The spraying voltage was 120V, the spraying current was 225A, and the spraying distance was 90mm. First, 75% of the coating was sprayed onto the NiCrAlY transition layer 9. A NiCrAlY+YSZ composite coating of 10 μm was then sprayed with 125 μm. The YSZ thermal barrier layer 11, with a final thickness of 200 μm, is formed. m is a transitional thermal barrier layer.

[0063] Both the in-situ preparation of the direct-write lead-free piezoelectric ceramic layer 1 (3) and the direct-write lead-free piezoelectric ceramic layer 2 (5) were carried out using plasma spraying. The flow rate of the inert gas argon was 90 L / min, and the flow rate of hydrogen was 12 L / min. The spraying voltage was 90V, the spraying current was 175A, and the spraying distance was 90mm, forming a 175mm thick layer on the surface of the transition thermal barrier layer. m is a direct-write lead-free piezoelectric ceramic layer 13 and a direct-write lead-free piezoelectric ceramic layer 25.

[0064] The spraying temperature of the in-situ prepared direct-write lead-free piezoelectric ceramic layer 3 and direct-write lead-free piezoelectric ceramic layer 5 was controlled at 1200°C by adjusting the heat source power, i.e., the spraying voltage and spraying current parameters. Between these points, the deposition rate (adjusted by the process gas type, gas flow rate, and powder feed rate, etc.) is 50. m min.

[0065] The direct-write lead-free piezoelectric ceramic layer 3 and the direct-write lead-free piezoelectric ceramic layer 5 are heat-treated with plasma or flame to improve their crystallinity and thus enhance their piezoelectric properties.

[0066] The printing is inkjet printing, and the electrode material for the interdigital electrode 4 and the comb electrode 6 is gold.

[0067] Example 3 The difference from Example 2 is that when preparing the NiCrAlY transition layer 9 by plasma spraying, the flow rate of the inert argon gas is 80 L. min, hydrogen flow rate is 15L The spraying voltage was 150V, the spraying current was 300A, and the spraying distance was 80mm, forming a 50mm thick layer on the surface of the structure to be monitored. m NiCrAlY transition layer 9.

[0068] When preparing the yttrium-stabilized zirconia (YSZ) thermal barrier layer using plasma spraying, the flow rate of the inert gas argon is 80 L. min, hydrogen flow rate is 10L The spraying voltage was 100V, the spraying current was 200A, and the spraying distance was 80mm. First, 50 μL of the coating was sprayed onto the NiCrAlY transition layer 9. A NiCrAlY+YSZ composite coating of 10 μm was then sprayed with 100 μm. The YSZ thermal barrier layer 11, with a final thickness of 150 μm, is formed. m is a transitional thermal barrier layer.

[0069] When in-situ fabrication of direct-write lead-free piezoelectric ceramic layer 3 and direct-write lead-free piezoelectric ceramic layer 5 using plasma spraying, the flow rate of the inert gas argon is 80 L. min, hydrogen flow rate is 10L The spraying voltage was 80V, the spraying current was 150A, and the spraying distance was 80mm, forming a 150mm thick layer on the surface of the transition thermal barrier layer. The direct-write lead-free piezoelectric ceramic layer 3 and the direct-write lead-free piezoelectric ceramic layer 5 are constructed. The spraying temperature (controlled by adjusting the heat source power, i.e., the spraying voltage and spraying current parameters) is maintained at 1100°C. Between these points, the deposition rate (adjusted by the process gas type, gas flow rate, and powder feed rate, etc.) is 10. m min.

[0070] The printing is done by micro-droplet spraying. The electrode material for the interdigitated electrode 4 and the comb electrode 6 is silver.

[0071] Example 4 The difference from Example 2 is that when preparing the NiCrAlY transition layer 9 by plasma spraying, the flow rate of the inert argon gas is 100 L. min, hydrogen flow rate is 25L The spraying voltage was 200V, the spraying current was 350A, and the spraying distance was 100mm, forming a 100mm thick layer on the surface of the structure to be monitored. m NiCrAlY transition layer 9.

[0072] When preparing the yttrium-stabilized zirconia (YSZ) thermal barrier layer using plasma spraying, the flow rate of the inert gas argon is 100 L. min, hydrogen flow rate is 15L The spraying voltage was 150V, the spraying current was 250A, and the spraying distance was 100mm. First, 100mm of the coating was sprayed onto the NiCrAlY transition layer 9. A NiCrAlY+YSZ composite coating of 10 μm was then sprayed with 150 μm. The YSZ thermal barrier layer 11, with a final thickness of 250 μm, is formed. m is a transitional thermal barrier layer.

[0073] When in-situ fabrication of direct-write lead-free piezoelectric ceramic layer 3 and direct-write lead-free piezoelectric ceramic layer 5 using plasma spraying, the flow rate of the inert gas argon is 100 L. min, hydrogen flow rate is 15L The spraying voltage was 100V, the spraying current was 200A, and the spraying distance was 100mm, forming a 200mm thick layer on the surface of the transition thermal barrier layer. The direct-write lead-free piezoelectric ceramic layer 3 and the direct-write lead-free piezoelectric ceramic layer 5 are constructed. The spraying temperature (controlled by adjusting the heat source power, i.e., the spraying voltage and spraying current parameters) is maintained at 1300°C. Between these points, the deposition rate (adjusted by the process gas type, gas flow rate, and powder feed rate, etc.) is 100. m min.

[0074] The printing is done by magnetron sputtering. The electrode materials for the interdigitated electrodes 4 and the comb electrodes 6 are silver-palladium alloys.

[0075] Example 5 This invention provides a direct-write lead-free piezoelectric ceramic layer for a structural surface defect depth monitoring device according to Embodiment 1, comprising a first direct-write lead-free piezoelectric ceramic layer 3 and a second direct-write lead-free piezoelectric ceramic layer 5. The direct-write lead-free piezoelectric ceramic layer can form a strong bond with the structure to be monitored and can operate stably in a temperature range from room temperature to 300°C. The general chemical formula of lead-free piezoelectric ceramic is: [(K... 0.5 Na 0.5 ) 1-x Li x ]Nb 1-y Ta y O3, where x = 0 - 0.1, y = 0 - 0.1.

[0076] The above-mentioned direct-write lead-free piezoelectric ceramic layer is prepared by the following steps: S1011. Weigh potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide raw materials according to the molar ratio of the general chemical formula for lead-free piezoelectric ceramics. Potassium carbonate and sodium carbonate are in excess by 5% to compensate for the volatilization of K and Na elements, so as to ensure the stability of the components during sintering and spraying.

[0077] S1012. The raw materials are put into a nylon can, anhydrous ethanol is used as the ball milling medium, zirconia balls are used as the grinding balls, the ball-to-material ratio is 8:1, and the mixture is ball milled to obtain a mixed powder.

[0078] S1013. After drying the mixed powder, place it in an alumina crucible, then place the alumina crucible in a muffle furnace and heat it at 850°C. Pre-calcined for 4.5 hours, then sieved to obtain particles with a size of 35. m-distribution lead-free piezoelectric ceramic precursor powder.

[0079] S1014. First, a NiCrAlY transition layer 9 and a yttrium oxide-stabilized zirconia thermal barrier layer are stacked on the surface of the structure to be tested 7 to form a transition thermal barrier layer.

[0080] S1015. Lead-free piezoelectric ceramic precursor powder is prepared in situ on the surface of the transition thermal barrier layer by plasma spraying to obtain a direct-write lead-free piezoelectric ceramic layer.

[0081] The stacked structure is a gradient transition layer, designed to enhance the bonding force between the direct-write lead-free piezoelectric ceramic layer and the structure under test, and to reduce stress mismatch at the metal-ceramic interface. Specifically, this applies to structure 7 under test. NiCrAlY transition layer 9 NiCrAlY+YSZ composite coating 10 YSZ thermal barrier layer 11 / lead-free piezoelectric ceramic layer, such as Figure 2 As shown. This gradient transition stacking structure effectively controls the difference in the interfacial thermal expansion coefficients to not exceed 1.5. 10 -6 This enables a tight bond between the direct-write lead-free piezoelectric ceramic layer and the structure to be monitored, significantly reducing stress mismatch and improving service life.

[0082] By controlling the Li and Ta contents (i.e., the values ​​of parameters x and y) in the direct-write lead-free piezoelectric ceramic layer composition, five types of direct-write lead-free piezoelectric ceramic layers were prepared: x=0, y=0; x=0.05, y=0; x=0.05, y=0.05; x=0.10, y=0.05; x=0.10, y=0.10. These layers were then applied at different temperatures (25°C). 100 200 300 Its piezoelectric properties were tested under the following conditions.

[0083] Since the direct-write lead-free piezoelectric ceramic layer is a coating configuration, with one side being a non-free end face, a laser vibrometer is used to measure the vibration displacement δ of the free end surface under an applied excitation voltage U, according to the formula: d 33,e =d dU, calculate the effective piezoelectric coefficient d 33,e The piezoelectric properties were evaluated, and the results are shown in Table 1.

[0084] Table 1. Piezoelectric properties of direct-write lead-free piezoelectric ceramic layers with different compositions at different temperatures. As shown in Table 1, component (K) 0.5 Na 0.5 ) 0.95 Li 0.05 Nb 0.95 Ta 0.05 O3 (i.e., x=0.05, y=0.05) at 25 Up to 300 It exhibits excellent piezoelectric properties across a wide temperature range. Therefore, this lead-free piezoelectric ceramic material is suitable for preparing direct-write piezoelectric ceramic layers and can be widely applied in the field of structural health monitoring.

[0085] Example 6 A monitoring method for a structural surface defect depth monitoring device includes the following steps: S201. An incident Rayleigh wave signal pointing towards the surface defect 8 is emitted on the surface of the structure under test 7 by a Rayleigh wave transmitter 1, and the reflected Rayleigh wave signal reflected back from the surface defect 8 is received by a Rayleigh wave receiver array 2; the emitted incident Rayleigh wave signal propagates in a direction collinear with the surface defect 8 of the structure.

[0086] Rayleigh waves propagate along the surface of a structure, with their energy concentrated primarily within a depth range equivalent to their wavelength. When the propagating Rayleigh wave reaches the edge of a surface defect, some of its energy is reflected; the ratio of the reflected wave amplitude to the incident wave amplitude is defined as the reflection coefficient. When the defect depth... With Rayleigh wave wavelength The reflection coefficient reaches its maximum value when the ratio is 0.45. Therefore, the defect depth and the frequency of the reflected Rayleigh wave can be correlated using the following formula: In the formula, The Rayleigh wave speed is a constant of 2.95 km. s; This is the Rayleigh wave frequency.

[0087] According to this formula, the wavelength of the reflected Rayleigh wave can be measured. or frequency The depth of surface defects can be calculated by distribution. .

[0088] S202. The incident and reflected Rayleigh wave signals in S201 are enhanced using a delay-and-sum algorithm to improve the signal-to-noise ratio of the incident and reflected Rayleigh wave signals. Since the spacing between the dispersed electrodes is consistent, this algorithm can be effectively applied to this method, including the following processing steps: S2021. Determine the arrival times of the incident Rayleigh wave signal and the reflected Rayleigh wave signal.

[0089] S2022. Perform a fast Fourier transform on the incident Rayleigh wave signal and the reflected Rayleigh wave signal to obtain the spectrum.

[0090] S2023. Perform time delay compensation and superposition enhancement processing on the spectrum obtained in S2022 to obtain the incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal with improved signal-to-noise ratio. The schematic diagram is as follows. Figure 3 As shown, the time compensation accuracy is less than 1 / 10 of the Rayleigh wave period, ensuring that the superimposed and enhanced waveform is clear and has high positioning accuracy.

[0091] S203. Based on the enhanced incident Rayleigh wave signal and reflected Rayleigh wave signal, key characteristic parameters such as time delay difference and energy attenuation are extracted. Combined with Rayleigh wave velocity and wavelength information, the depth and spatial location of surface defect 8 are calculated to achieve defect depth assessment.

[0092] To verify the effectiveness of the monitoring device and method of the present invention, it was compared with a laser interferometer and a Rayleigh wave receiving array based on bulk lead-free piezoelectric ceramics.

[0093] Four sets of interdigitated electrodes 4, printed on a direct-write piezoelectric ceramic layer, were used as Rayleigh wave transmitters 1, with an excitation signal of -50Vp and a pulse width of 385ns. The structural specimen was a 40mm thick aluminum alloy block, with a 1mm deep slot machined to simulate surface defects 8. The Rayleigh wave receivers employed the following three types of receivers: Figure 4 As shown in (a), there is a Rayleigh wave receiver array 2 consisting of four comb-shaped electrodes 6 printed on a direct-write lead-free piezoelectric ceramic layer. Figure 4 As shown in (b) above, an array of four ultrasonic transducers based on bulk lead-free piezoelectric ceramics. Figure 4 As shown in (c), the laser interferometer samples at four points.

[0094] The incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal after processing by the delay superposition algorithm for the above three types of receivers, as well as the calculated reflection coefficient Cr curve, are shown below. Figure 5 As shown, the reflection coefficient is obtained by the ratio of the reflected signal amplitude to the incident signal amplitude, and the peak value is searched within the -6dB bandwidth of the incident wave to determine the defect depth.

[0095] Depend on Figure 5It can be seen that the Rayleigh wave receiver array 2 based on direct-write lead-free piezoelectric ceramics and the laser interferometer receiver array can detect significant reflection coefficient peaks within the effective frequency band, indicating the reliability of these two monitoring methods. However, no significant reflection coefficient peaks were detected within the effective frequency band for the ultrasonic transducer array based on bulk lead-free piezoelectric ceramics. This is because the amplitude of the reflected Rayleigh wave signal is low, which stems from the high attenuation rate when using a bulk lead-free piezoelectric ceramic ultrasonic transducer as a Rayleigh wave receiver.

[0096] By comparing the differences in the incident Rayleigh wave signals detected by the three different receivers, the amplitude of the incident Rayleigh wave signals was recorded and its attenuation rate at four test points was calculated. Figure 6 As shown in the figure, the ultrasonic transducer array based on bulk lead-free piezoelectric ceramics exhibits the highest signal attenuation rate, while the Rayleigh wave receiver array 2 based on a direct-write lead-free piezoelectric ceramic layer has an attenuation rate only slightly higher than that of the non-contact laser interferometer. The high attenuation of the ultrasonic transducer array based on bulk lead-free piezoelectric ceramics is mainly due to its large size, which leads to energy loss in the Rayleigh wave sensitive area. Simultaneously, its low mechanical damping narrows the detection bandwidth, thus reducing the measurable range of defect depth. The performance of the three different receivers in measuring surface defect 8 is compared, and the results are shown in Table 2.

[0097] Table 2. Performance comparison of three different receivers for measuring surface defects at a depth of 1 mm. As shown in Table 2, the Rayleigh wave receiver array 2 of the direct-write lead-free piezoelectric ceramic layer of the present invention is comparable to the laser interferometer in terms of incident wave bandwidth. Although it is slightly lower than the laser interferometer in terms of crack depth measurement accuracy, it is lower in cost and does not depend on line-of-sight conditions. It has the characteristics of deep penetration and long-distance propagation, and is therefore suitable for widespread application in engineering structural health monitoring.

[0098] Application Example 1 The surface defect depth monitoring device of Example 1 was applied to monitor the pipeline structure, and the monitoring method was the same as that of Example 6.

[0099] like Figure 7 As shown, three sets of Rayleigh wave transmitters 1 and Rayleigh wave receiver arrays 2 are arranged at equal intervals along the circumference of the pipe structure surface to form full circumferential coverage. The Rayleigh wave excited by the Rayleigh wave transmitter 1 propagates along the axial direction of the pipe structure; when it encounters a surface defect 8 or a near-surface defect 8, the Rayleigh wave is reflected back to the Rayleigh wave receiver array 2 and is finally converted into an electrical signal through the piezoelectric effect.

[0100] The received signal is processed using joint time-frequency domain analysis to obtain the arrival time and spectral characteristics of the incident and reflected Rayleigh wave signals, such as... Figure 8 As shown.

[0101] The propagation time difference is obtained by comparing the arrival times of the incident Rayleigh wave signal and the reflected Rayleigh wave signal. Based on known Rayleigh wave group velocities g According to the formula Determine the axial distance from surface defect 8 to Rayleigh wave receiver array 2. Furthermore, by combining the detection results of the three Rayleigh wave receiver arrays 2, triangulation is performed, which allows for the calculation of the three-dimensional position parameters (axial position) of the surface damage in the tube coordinate system. x Circumferential angle Radial depth d ).

[0102] The aforementioned three sets of Rayleigh wave transmitters 1 and Rayleigh wave receiver arrays 2, arranged at equal intervals along the circumference, combined with a time-frequency domain processing flow, can achieve full-circumferential defect detection and online monitoring of tubular structures. The spatial inversion accuracy for damage locations is better than 2 mm, and it can identify surface and subsurface defects 8 with a minimum feature size of approximately 0.5 mm. This verifies the effectiveness of the monitoring device and method of the present invention in improving detection coverage and positioning accuracy in tubular structure scenarios.

[0103] Application Example 2 The surface defect depth monitoring device of Example 1 was applied to monitor a curved structure with a certain curvature, and the monitoring method was the same as that of Example 6.

[0104] like Figure 9 As shown, the Rayleigh wave transmitter 1 is located in the vertical region of the curved structure, while the Rayleigh wave receiver array 2 is arranged in the curved region of the curved structure.

[0105] Both the Rayleigh wave transmitter 1 and the Rayleigh wave receiver array 2 are fabricated using direct-write lead-free piezoelectric ceramics, possessing a high degree of conformity to the surfaces of complex engineering structures. Therefore, in-situ fabrication and stable bonding can be achieved directly on structural surfaces with curvature and corners. With this arrangement, Rayleigh waves can propagate efficiently along curved structural surfaces and maintain good signal characteristics after passing through curved areas, thus ensuring that damage reflection signals can be accurately acquired and analyzed, such as... Figure 10 The results of the joint time-frequency domain analysis are shown. This verifies the applicability of the monitoring device and method of the present invention to complex curved surface structures. It overcomes the limitation of traditional arrays being difficult to deploy in curved regions, enabling in-situ monitoring of curved structures and providing a feasible technical path for real-time health status detection of complex structural components.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A device for monitoring the depth of structural surface defects, characterized in that: The device comprises a combination unit consisting of several Rayleigh wave transmitters and Rayleigh wave receiver arrays. The Rayleigh wave transmitters and Rayleigh wave receiver arrays are fabricated in situ or adhered to the surface of the structure under test and arranged in a direction collinear with the surface defects. The Rayleigh wave receiver array is located between the Rayleigh wave transmitters and the surface defects. The Rayleigh wave transmitter includes a first direct-write lead-free piezoelectric ceramic layer and interdigitated electrodes located on the first direct-write lead-free piezoelectric ceramic layer. The Rayleigh wave receiver array includes a second direct-write lead-free piezoelectric ceramic layer and comb-shaped electrodes located on the second direct-write lead-free piezoelectric ceramic layer.

2. The structural surface defect depth monitoring device according to claim 1, characterized in that: Rayleigh wave transmitters have either a longitudinally telescopic or a thickness-shearing structure.

3. The structural surface defect depth monitoring device according to claim 1, characterized in that: The comb electrodes of the Rayleigh wave receiver array include at least four groups of equally spaced dispersed electrodes.

4. The structural surface defect depth monitoring device according to claim 3, characterized in that: The dispersion electrode is elongated and strip-shaped with an aspect ratio greater than 5. Its length direction is perpendicular to the propagation direction of the Rayleigh wave, and the width of the dispersion electrode is less than half of the Rayleigh wave reflection wavelength corresponding to the smallest surface defect being monitored.

5. A method for preparing a structural surface defect depth monitoring device as described in any one of claims 1-4, characterized in that: Includes the following steps: S101. Weigh potassium carbonate, sodium carbonate, lithium carbonate, niobium pentoxide, and tantalum dioxide, mix and ball-mill, dry, pre-calcine, and sieve to obtain lead-free piezoelectric ceramic precursor powder. S102. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer one is prepared in situ on the surface of the structure to be tested, in a direction that is located on the side of the surface defect and collinear with the surface defect. Interdigitated electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer one to form a Rayleigh wave emitter for exciting incident Rayleigh waves. S103. Using the lead-free piezoelectric ceramic precursor powder of S101, a direct-write lead-free piezoelectric ceramic layer two is prepared in situ between the Rayleigh wave emitter and the surface defect on the surface of the structure to be tested. Comb-shaped electrodes are printed on the surface of the direct-write lead-free piezoelectric ceramic layer two to form a Rayleigh wave receiver array for receiving reflected Rayleigh waves.

6. The method for preparing a structural surface defect depth monitoring device according to claim 5, characterized in that: The general chemical formula for lead-free piezoelectric ceramics in S101 is: [(K 0.5 Na 0.5 ) 1-x Li x ]Nb 1-y Ta y O3, where x = 0 - 0.1, y = 0 - 0.

1.

7. The method for preparing a structural surface defect depth monitoring device according to claim 5, characterized in that: In S102 and S103, before in-situ preparation, a stacked structure of NiCrAlY transition layer and yttrium oxide-stabilized zirconia thermal barrier layer is first prepared on the surface of the structure to be tested to form a transition thermal barrier layer. On the transition thermal barrier layer, direct-write lead-free piezoelectric ceramic layer one and direct-write lead-free piezoelectric ceramic layer two are prepared in-situ respectively.

8. The monitoring method of the structural surface defect depth monitoring device as described in any one of claims 1-4, characterized in that: Includes the following steps: S201. An incident Rayleigh wave signal directed towards the surface defect is emitted on the surface of the structure to be tested through a Rayleigh wave transmitter, and the reflected Rayleigh wave signal reflected back from the surface defect is received through a Rayleigh wave receiver array. S202. The incident Rayleigh wave signal and the reflected Rayleigh wave signal in S201 are enhanced by using a delay superposition algorithm; S203. Calculate the depth of surface defects based on the enhanced incident Rayleigh wave signal and the reflected Rayleigh wave signal.

9. The monitoring method of the structural surface defect depth monitoring device according to claim 8, characterized in that: In S201, the structure to be tested is a planar structure, a tubular structure, or a curved structure. The structure to be tested includes a weld area, and the surface defects are located near the weld area. The surface defects are surface cracks or surface corrosion.

10. The monitoring method of the structural surface defect depth monitoring device according to claim 8, characterized in that: The delay superposition algorithm in S202 includes the following processing steps: S2021, determine the arrival time of the incident Rayleigh wave signal and the reflected Rayleigh wave signal; S2022. Perform a fast Fourier transform on the incident Rayleigh wave signal and the reflected Rayleigh wave signal to obtain the spectrum; S2023. The spectrum obtained in S2022 is superimposed to obtain the enhanced incident Rayleigh wave spectrum signal and the reflected Rayleigh wave spectrum signal.

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