A nondestructive testing method of thin-walled structures using ultrasonic guided waves based on flexible ultrasonic sensors

Abstract

The present invention proposes a thin-walled structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensors, and the steps are: conducting guided wave focusing sound field simulation, obtaining the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array, determining the design scheme of the flexible ultrasonic sensor array and the focusing excitation beam angle threshold; preparing the flexible ultrasonic sensor array according to the design scheme of the flexible ultrasonic sensor array; testing all channel electroacoustic parameters of the flexible ultrasonic sensor array, and returning if the relative error exceeds 10%; conducting a thin-walled structure guided wave sound field directivity test on the flexible ultrasonic sensor array, and adjusting the guided wave phased array focusing scheme if the beam angle is greater than the focusing excitation beam angle threshold; and using the flexible ultrasonic sensor array to perform defect detection on the thin-walled structure. The present invention has high structural integration, thin structure, small volume, and flexibility, and is suitable for defect detection and long-term online monitoring of thin-walled structures in narrow detection spaces, and has strong adaptability to the surface flatness of the structure.

Description

Technical Field

[0001] The invention relates to the technical field of industrial ultrasonic detection sensors, and in particular to a thin-wall structure ultrasonic guided wave nondestructive detection method based on a flexible ultrasonic sensor. Background Art

[0002] Thin-walled structures are widely used in important fields such as aerospace, petrochemicals, and transportation. Defects such as damage and corrosion in the structure will not only reduce the remaining life of the structure, but may even cause catastrophic accidents. Effective nondestructive testing and online monitoring technology are of great significance to structural integrity evaluation and safe service. Ultrasonic guided wave nondestructive testing technology has advantages in large-area detection and online monitoring. It can propagate over long distances in the structure and is sensitive to minor damage. It is an important nondestructive testing method, especially for detection of areas with limited space to be detected and areas that cannot be reached by sensors. The advantages of ultrasonic guided wave detection technology are more obvious. There are two methods to excite ultrasonic guided waves: electromagnetic sensors and piezoelectric sensors. Piezoelectric sensors are easier to integrate into multi-channel arrays for dynamic focusing of the guided wave acoustic field. For thin-walled structures, through the combination of array layout design and signal processing technology, specific modes of ultrasonic guided waves can be effectively excited to achieve accurate detection of the target area.

[0003] In the prior art, for nondestructive testing and online monitoring of thin-walled complex structures in important components, traditional ultrasonic sensors are usually rigid and difficult to fit complex or curved structures. Flexible ultrasonic sensors are light in weight, thin in thickness, and have good conformability and flexibility, which makes them very convenient during installation and use, reducing the load impact of the sensor on the structure being tested. In recent years, they have developed rapidly in the fields of industrial nondestructive testing and medical ultrasound. Flexible ultrasonic guided wave sensing technology is used for health monitoring and defect detection of complex or curved structures. It can fit tightly to targets with curved or irregular surfaces such as blades, pipes, and thin-walled structures, ensuring effective excitation and detection of guided waves. Flexible ultrasonic guided wave sensors are combined with phased array technology to achieve beamforming and focusing through electronic control of multiple sensor units, further improving detection sensitivity and resolution, thereby achieving high-precision detection of thin-walled structures. Integrated into thin-walled structures, real-time structural health monitoring can be achieved, defects can be discovered before potential problems become serious, and safety can be improved while reducing maintenance costs.

[0004] For example, the invention patent with application number 202210227382.6 relates to a design and preparation method of a flexible piezoelectric micro-machined ultrasonic transducer array, in which N flexible and plastic micro-ultrasonic transducer units are arranged into an array, and two adjacent flexible and plastic micro-ultrasonic transducer units are connected together by a curved interconnection structure, which is also a signal line interconnected between the flexible and plastic micro-ultrasonic transducer units. The curved interconnection structure enables the flexible piezoelectric micro-machined ultrasonic transducer array to have a large bending function. The method provided by the invention can prepare a flexible micro-electromechanical system (MEMS) ultrasonic transducer (MUT) array based on thin film materials, which can be used for low-power environmental signal detection and sensing. However, the preparation method of the flexible ultrasonic transducer array is complex and has high equipment requirements. In addition, compared with the present application, the ultrasonic transducer array prepared by micro-machining technology usually has low excitation energy. If it is used to excite ultrasonic guided waves in a thin plate structure, the propagation distance is short and the detection range is small. Summary of the invention

[0005] In view of the technical problems that it is difficult to detect defects in thin-walled structures and the arrangement of sensors on the surfaces of complex-shaped structures is limited, the present invention proposes a method for nondestructive testing of thin-walled structures using ultrasonic guided waves based on flexible ultrasonic sensors, which realizes focused excitation of the guided wave acoustic field in thin-walled structures. The flexible ultrasonic sensor array has a thin and flexible structure and can realize nondestructive testing of defects in narrow detection spaces, surfaces of complex-shaped structures, and internal defects of thin-walled structures, or can be arranged on the surface of the structure for long-term online monitoring.

[0006] In order to achieve the above object, the technical solution of the present invention is implemented as follows: a method for nondestructive testing of thin-walled structures by ultrasonic guided waves based on a flexible ultrasonic sensor, the steps of which are as follows:

[0007] Step 1: Set the flexible ultrasonic sensor array at the center of the thin-wall structure and perform guided wave focusing acoustic field simulation to obtain the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array, and determine the design scheme of the flexible ultrasonic sensor array and the focusing excitation beam angle threshold according to the parameters in the guided wave phased array focusing excitation scheme;

[0008] Step 2: Building a micron-level high-precision processing and preparation platform, and preparing a flexible ultrasonic sensor array according to the design scheme of the flexible ultrasonic sensor array;

[0009] Step 3: Test the electroacoustic parameters of all channels of the flexible ultrasonic sensor array. If the relative error exceeds 10%, return to step 2, otherwise proceed to step 4;

[0010] Step 4: Perform a thin-wall structure guided wave acoustic field directivity test on the flexible ultrasonic sensor array. If the beam angle is less than the focusing excitation beam angle threshold of step 1, proceed to step 5. Otherwise, adjust the guided wave phased array focusing scheme to continue the thin-wall structure guided wave acoustic field directivity test.

[0011] Step 5: Use flexible ultrasonic sensor array to detect defects in thin-walled structures.

[0012] Preferably, the method for obtaining the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array is: designing an array element arrangement scheme of the flexible ultrasonic sensor array, designing different array element arrangement forms, changing the arrangement mode and quantity of the sensor array, setting the load application point to simulate the excitation of each array element in the flexible ultrasonic sensor array, and obtaining the guided wave focusing excitation scheme of the flexible ultrasonic sensor array;

[0013] The method for setting the load application point to simulate the excitation of each array element in the flexible ultrasonic sensor array is: applying time-delay excitation to the array element to form a focused sound beam; setting multiple point probes on the upper surface of the thin-wall structure to analyze the waveform of sound propagation; at least two probes are on the same straight line with the center of the array element.

[0014] Preferably, the method for determining the focused excitation beam angle threshold is as follows: multiple periodic Hanning window modulation signals are set, a probe is used as the focus, a phased array excitation signal is applied to each array element, the center of the sensor array is used as the origin, and the maximum signal amplitude of each point is extracted to draw a directivity diagram on a circular area with a radius equal to the distance between the probe and the center of the sensor array. The azimuth of the probe is taken as the 0° direction. The maximum focused excitation beam angle when the sensor array has excellent directivity in the directivity diagram is the focused excitation beam angle threshold.

[0015] Preferably, each array element of the flexible ultrasonic sensor array includes a piezoelectric material, a positive electrode and a negative electrode, the positive electrode and the negative electrode are respectively arranged on the upper side and the lower side of the piezoelectric material, the positive electrode and the negative electrode are both adhered to the piezoelectric material by conductive silver glue, the positive electrodes of all array elements are independently connected, the negative electrode is designed as an island bridge structure, and a flexible packaging layer is provided on the outside of the positive electrode and the negative electrode.

[0016] Preferably, the thin-walled structure is an aluminum plate, and the flexible packaging layer is a PDMS packaging layer;

[0017] The piezoelectric material is preferably a 1-3 composite piezoelectric material with a center frequency of 600kHz, and the difference between the acoustic impedance of the piezoelectric material and the acoustic impedance of the aluminum plate is less than 3-5 Mrayl; the positive electrode is composed of four layers of electrode materials stacked in sequence, and both the positive electrode and the negative electrode are copper foil coated with polyimide.

[0018] Preferably, the electroacoustic parameter test includes impedance test, frequency response test and radiated sound field scanning test, and the results of the electroacoustic parameter test include resonant frequency, electrical impedance, electromechanical coupling coefficient, center frequency, bandwidth, peak-to-peak value of reflected signal, spatial peak-time peak sound intensity, and spatial average time-domain peak sound intensity;

[0019] All array elements are tested one by one. If the relative error of the same parameters between the array elements exceeds 10%, they are returned to the processing and preparation stage, and new array elements are prepared to replace the array elements with excessive relative errors. The electroacoustic parameter test is re-performed until the relative error of the same parameters between the array elements does not exceed 10%.

[0020] Preferably, the impedance test obtains the relationship between the frequency and electrical impedance of each array element, and the impedance test uses an impedance analyzer to measure the electrical impedance, phase angle, resonant frequency, and electromechanical coupling coefficient of each array element of the flexible ultrasonic sensor;

[0021] The frequency response test uses a pulse reflection method to test the frequency response of all array elements of the sensor array to obtain the center frequency, bandwidth, and peak-to-peak value of the reflected signal of all array elements;

[0022] The radiated sound field scanning test scans the radiated sound field of all array elements of the flexible ultrasonic sensor, including a plane parallel to the acoustic axis of the array element and a plane perpendicular to the acoustic axis of the array element, and obtains the sound pressure distribution obtained by scanning in the direction parallel to the acoustic axis of the sensor array element and the sound pressure distribution obtained by scanning in the direction perpendicular to the acoustic axis of the sensor array element, respectively.

[0023] Preferably, the maximum width of the -3dB region of each sensor array element parallel to the acoustic axis is required to be no more than 4.5mm, and the maximum width of the -6dB region is required to be no more than 6mm; the width of the -3dB region of each sensor array element perpendicular to the acoustic axis is required to be no more than 4mm, and the width of the -6dB region is required to be no more than 5mm;

[0024] The spatial peak time peak sound intensity I SPTP is a sound intensity parameter with space as a variable, the spatial average time domain peak sound intensity I SATP It is the sound intensity parameter related to the area of ​​the -6dB zone;

[0025] The calculation method of the spatial peak time peak sound intensity is: cumulatively integrating the continuous time domain signal p(t) received by the hydrophone at each scanning point to obtain the square integral of the pulse sound pressure of the time domain signal p i ; In the time domain signal, the pulse sound pressure reaches 0.1p i The time when the pulse sound pressure reaches 0.9p is recorded as t1. i The time is recorded as t2, and the pulse duration is calculated as: t d =1.25(t2-t1); using the square integral of the pulse sound pressure pi and pulse duration t d Calculate the pulse sound pressure level p p ; Assuming that the sound wave propagates in a plane wave-like manner, according to the pulse sound pressure level p p Calculate the time peak sound intensity I tp , calculate the time domain signal of all scanning results in the hydrophone scanning plane to obtain the time peak sound intensity I tp The maximum value of the spatial distribution is recorded as the spatial peak time peak sound intensity I SPTP ;

[0026] The spatial peak time peak sound intensity I SPTP The maximum value is used as a reference, and a -6dB threshold is set. In the scanning plane, the area covered by the scanning points with an area higher than the -6dB threshold is recorded as A, and the spatial average time domain peak sound intensity is calculated.

[0027] Preferably, the thin-walled structure guided wave acoustic field directivity test utilizes a multi-channel ultrasonic system to apply time-delayed excitation flexible ultrasound to focus the sensor array, and the excitation signal propagates along the thin-walled structure to achieve focused control of the guided wave acoustic field; a laser vibrometer scanning system is used to collect ultrasonic guided wave signals on the surface of the thin-walled structure for sensor array directivity research, and the ultrasonic guided wave signals at the focus are obtained, and the guided wave signal mode recognition is performed in combination with the dispersion curve in the thin-walled structure.

[0028] Preferably, according to the performance test results of each element of the flexible ultrasonic sensor array, the average value of the center frequency of each element is selected as the center frequency of the array excitation, the distance R mm from the center of the sensor array is the acoustic focus point, the center of the sensor array is taken as the origin, the focus distance R mm is taken as the radius scan to extract the maximum amplitude of the vibration signal on the scanning radius, and the directivity diagram is drawn in the polar coordinate system. The beam angle is less than the focusing excitation beam angle threshold, and the ultrasonic guided wave signal propagation has an obvious focusing direction;

[0029] The time domain signal at the focus is transformed by continuous wavelet transform, and the waveguide dispersion curve in the thin-walled structure is solved by numerical method. The continuous wavelet transform result is compared with the dispersion curve in the thin-walled structure. The wave velocity of the time domain signal is close to the wave velocity of the A0 mode, indicating that the waveguide acoustic field has a good focusing effect.

[0030] The defect detection method comprises the following steps: using a numerical method to solve dispersion curves of symmetric modes and antisymmetric modes in a thin-walled structure, extracting time domain signals and performing ultrasonic guided wave modal analysis, performing continuous wavelet transform on the time domain signals, and comparing the continuous wavelet transform results with dispersion curve results. The propagation velocity of the guided wave signal is close to the group velocity corresponding to the A0 mode, and the signal generated by the flexible ultrasonic transducer array in the thin-walled structure is mainly the antisymmetric mode A0 mode; setting the excitation focus at the edge of the thin-walled structure, receiving the reflected signal from the edge, and the amplitude of the received signal of each channel is close; arranging the flexible ultrasonic sensor array at a distance of R mm from the through hole, using the same excitation reception, and the amplitude of the reflected signal becomes low due to the influence of the through hole acoustic scattering.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention is committed to the nondestructive detection and long-term online monitoring technology of thin-walled structures based on flexible ultrasonic sensor arrays, especially the nondestructive detection of narrow detection spaces and thin-walled structures with complex shapes, and aims to achieve focused excitation of specific guided wave modes by designing and optimizing the layout of flexible piezoelectric sensor arrays, signal excitation schemes, and mode selection strategies. The flexible ultrasonic sensor array has the structural characteristics of high structural integration, thin structure, small volume, and bendability, and is suitable for defect detection and long-term online monitoring of narrow detection spaces and thin-walled structures. It has strong adaptability to the surface flatness of the structure and can be expanded to nondestructive detection of thin-walled structures with complex shapes. The present invention designs the arrangement form of the flexible piezoelectric sensor surface array by finite element simulation method, and obtains the guided wave focusing excitation scheme of the sensor surface array at the same time; builds a micron-level high-precision processing and preparation platform for the sensor surface array, and prepares a flexible ultrasonic sensor; performs electroacoustic parameter tests on all array elements of the flexible ultrasonic sensor to ensure the consistency of each array element; excites the flexible ultrasonic sensor according to the guided wave focusing excitation scheme; tests the directivity of the sensor array guided wave sound field in the thin plate structure; and detects defects in the thin plate structure. This invention will provide a theoretical basis and technical support for the application of flexible ultrasonic sensors in thin-wall defect detection and structural health monitoring, and promote the widespread application of ultrasonic guided wave technology in new flexible structures, providing a new solution for structural health monitoring and industrial safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 It is a flow chart of the present invention.

[0034] Figure 2 This is a simulation model diagram of the guided wave focusing of a flexible ultrasonic sensor in a thin-wall structure.

[0035] Figure 3 This is a simulation result diagram of the waveguide focusing directivity of a flexible ultrasonic sensor in a thin-wall structure.

[0036] Figure 4 This is a structural diagram of a flexible piezoelectric ultrasonic sensor array.

[0037] Figure 5 It is the reflection signal diagram of the edge of the thin plate structure.

[0038] Figure 6 This is the reflection signal diagram of the circular hole defect in the thin plate structure.

[0039] Figure 7 It is a peak-to-peak statistical diagram of the reflection signal of a circular hole defect in a thin plate structure. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] like Figure 1 As shown, a nondestructive testing method for thin-walled structures using ultrasonic guided waves based on flexible ultrasonic sensors is used. The arrangement of the flexible ultrasonic sensor array is designed using the finite element simulation method, and the guided wave focusing excitation scheme of the sensor surface array is obtained at the same time; a micron-level high-precision processing and preparation platform for the sensor surface array is built to prepare the flexible ultrasonic sensor; the electroacoustic parameter test of all array elements of the flexible ultrasonic sensor is carried out to ensure the consistency of each array element; the flexible ultrasonic sensor is excited according to the guided wave focusing excitation scheme; the directivity of the sensor array guided wave sound field is tested in the thin plate structure; and the defects in the thin plate structure are detected. The specific steps include:

[0042] Step 1: Set the flexible ultrasonic sensor array at the center of the thin-wall structure, perform guided wave focusing acoustic field simulation on the flexible ultrasonic sensor array, obtain the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array, and determine the design scheme and focusing excitation beam angle threshold of the flexible ultrasonic sensor array based on the parameters in the guided wave phased array focusing excitation scheme.

[0043] In this embodiment, the flexible ultrasonic sensor array is a flexible piezoelectric sensor surface array. The arrangement form of the flexible piezoelectric sensor array is simulated and designed, mainly including guided wave focusing sound field simulation and sensor array design. Figure 2As shown, a thin-walled aluminum plate structure is established in COMSOL software, a flexible piezoelectric sensor array is arranged at the center of the thin-walled aluminum plate structure, an array element arrangement scheme of the piezoelectric sensor array is designed, different array element arrangement forms are designed, the arrangement mode and number of the sensor array are changed, and the load application point is set to simulate the excitation of each array element in the ultrasonic sensor array, and the guided wave focusing excitation scheme of the sensor array is obtained. In this embodiment, the volume of the thin aluminum plate structure is 360×360×1.5mm. Time-delay excitation is applied to the array element to form a focused sound beam. A plurality of point probes are set on the upper surface of the thin-walled aluminum plate structure to analyze the waveform of sound propagation. The guided wave phased array focusing excitation scheme includes the number of array elements, the size of the array element, the spacing between the array elements, and the delayed excitation time between the array elements, wherein the number of array elements, the size of the array element, and the spacing between the array elements are used as the preparation scheme of the flexible piezoelectric sensor array.

[0044] In the center area of ​​the aluminum plate, a 4×8 piezoelectric sensor array is preferably used to apply a load to simulate the excitation of each element in the ultrasonic sensor array. In this embodiment, the element size is 2×2×2.5 mm, the element center distance is 3 mm, the excitation signal frequency is 600 kHz, and probes A and B are set to extract the simulation signal on the focus path. The distances between probe A and probe B and the center of the excitation sensor are 50 mm and 100 mm, respectively. The center of the piezoelectric sensor array, probe A, and probe B are as follows: Figure 2 As shown in the figure, they are set on the same straight line. Set a 5-cycle Hanning window modulation signal, take probe B as the focus, apply a phased array excitation signal to each array element, take the center of the sensor array as the origin, extract the maximum signal amplitude of each point on the circumference of a circle with a radius of 100 mm (that is, the circumference where probe B is located), and draw a directivity diagram, as shown in the figure below. Figure 3 As shown, the 0° direction is the position of probe B, the focused excitation beam angle is less than 60°, and the sensor array has excellent directivity.

[0045] Step 2: Build a micron-level high-precision processing and preparation platform, and prepare a flexible ultrasonic sensor array according to the design scheme of the flexible ultrasonic sensor array obtained in step 1.

[0046] like Figure 4As shown in the figure, the flexible piezoelectric sensor surface array is composed of piezoelectric material, positive electrode, negative electrode, and PDMS packaging layer. The piezoelectric material is preferably 1-3 composite piezoelectric material (mainly composed of PZT5H and epoxy resin), the center frequency is about 600kHz, the acoustic impedance of the material is 15Mrayl, which is close to the acoustic impedance of the aluminum plate (17.1Mrayl), and no acoustic matching layer is set on the sensor surface. The positive electrode is stacked in sequence by four layers of electrode materials to achieve independent connection of the positive electrodes of all array elements. The negative electrode is designed as an island bridge structure. The positive electrode and the negative electrode are both made of copper foil coated with polyimide (PI), and PI is mainly used to strengthen the copper foil structure. The thickness of PI is 20μm, and the thickness of copper foil is 20μm. The positive electrode and the negative electrode are respectively arranged on the upper and lower sides of the piezoelectric material. The positive electrode and the negative electrode are both attached to the piezoelectric material through conductive silver glue, and the outer sides of the positive electrode and the negative electrode are provided with PDMS packaging layer. The sensor structure is designed as a 4-layer structure, and each array element can realize independent excitation and reception of ultrasonic signals. The sensor array as a whole is soft and bendable, and can be arranged on the surface of complex curved structures such as industrial pipelines and structures in narrow spaces. The copper foil is used to form positive and negative electrodes and connected to the connector using shielded signal harnesses, thereby realizing the connection between the flexible ultrasonic sensor array and the multi-channel ultrasonic control system.

[0047] According to the design scheme of the flexible piezoelectric sensor array obtained by simulation, the array elements and electrodes of the flexible ultrasonic sensor surface array are prepared using the piezoelectric sensor micron-level high-precision processing and preparation platform. The micron-level high-precision processing platform mainly includes a dicing machine and a laser cutting machine. Each array element is cut by a dicing machine (array element size 2×2×2.5mm), and the positive electrode and negative electrode are cut by a laser cutting machine. Conductive silver glue is used to glue the positive electrode and negative electrode to the positive and negative surfaces of the piezoelectric material, respectively, to form a flexible piezoelectric sensor surface array. The flexible piezoelectric sensor surface array is encapsulated by PDMS (polydimethylsiloxane).

[0048] Step 3: Test the electroacoustic parameters of all channels of the flexible ultrasonic sensor array. If the relative error exceeds 10%, return to step 2, otherwise proceed to step 4.

[0049] The electroacoustic parameter test of the ultrasonic sensor array ensures the consistency of each array element and increases the operability of the sound beam focusing. The electroacoustic parameter test includes impedance test to obtain the relationship between the frequency and electrical impedance of each array element. The impedance analyzer is used to measure the electrical impedance, phase angle, resonant frequency, and electromechanical coupling coefficient of each array element of the sensor. The frequency at which the impedance reaches the minimum value is called the resonant frequency, and the resonant frequency is denoted by f. r The frequency at which the impedance reaches its maximum value is called the anti-resonance frequency, and the anti-resonance frequency is denoted by f a The resonant frequency is f r =571±11kHz, the impedance corresponding to the resonant frequency of the array element is Z@f r=1437±16Ω. Using the resonant frequency f r and anti-resonance frequency f a Calculate the electromechanical coupling coefficient k in the thickness direction t =0.51±0.02. Electromechanical coupling coefficient k t The calculation formula is:

[0050]

[0051] The pulse reflection method is used to test the center frequency, frequency response, and bandwidth of all array elements of the sensor array. The pulse reflection method is used to test the frequency response of all array elements of the sensor array, and the center frequency, bandwidth, and peak-to-peak value of all channels are obtained, which are f c =576±11kHz, BW=21.58±2.17%, V p-p =59.38±3.71mV, the performance indicators of each array element are close, and the consistency of the sensor array is guaranteed.

[0052] The radiated acoustic field of all elements of the piezoelectric sensor was scanned and tested using a hydrophone and an acoustic field scanning system, including the plane parallel to the acoustic axis of the sensor element and the plane perpendicular to the acoustic axis of the sensor element. The acoustic parameters of the sensor elements were measured underwater using the acoustic field scanning system, and the radiated acoustic field of the sensor was received underwater using a hydrophone with a scanning step of 0.5 mm. The scanning scheme was set to obtain the sound pressure distribution obtained by scanning in the direction parallel to the acoustic axis of the sensor element and the sound pressure distribution obtained by scanning in the direction perpendicular to the acoustic axis of the sensor element. Due to the wide energy area, the energy of the detection sound field generated by the synchronous excitation of each element is not concentrated, and the defect reflection signal loses its positioning significance. Therefore, it is required that the maximum width of the -3dB area of ​​each sensor element parallel to the acoustic axis is not more than 4.5mm, and the maximum width of the -6dB area is not more than 6mm; it is required that the width of the -3dB area of ​​each sensor element perpendicular to the acoustic axis is not more than 4mm, and the width of the -6dB area is not more than 5mm. The wide energy area causes the energy of the detection sound field generated by the synchronous excitation of each element to be not concentrated, and the defect reflection signal loses its positioning significance. The focused excitation of the guided wave phased array will improve the defect location accuracy of the sensor array, and the consistency between the array elements will improve the focusing accuracy of the phased array.

[0053] The sound field scanning data contains more abundant sensor performance information. The sound pressure distribution in the scanning plane is worth noting. The spatial peak time peak sound intensity I SPTP is a sound intensity parameter with space as a variable, the spatial average time domain peak sound intensity I SATPIt is a sound intensity parameter related to the area of ​​the -6dB region and is a key indicator for analyzing and summarizing the key characteristics of sound intensity in both time and space dimensions. Define the continuous time domain signal received by the hydrophone at each scanning point as p(t), and calculate the square integral of the pulse sound pressure of the time domain signal p according to formula (2): i for:

[0054]

[0055] In the time domain signal, the pulse sound pressure reaches 0.1p i The time when the pulse sound pressure reaches 0.9p is recorded as t1. i The time is recorded as t2, and the pulse duration t is defined d for:

[0056] t d = 1.25(t2-t1) (3)

[0057] Pulse sound pressure level p p for:

[0058]

[0059] Assuming that the sound wave propagates in a plane wave-like manner, the time peak sound intensity I is obtained. tp , where ρ is the density of water, c is the speed of sound in water, and the time peak sound intensity I tp for:

[0060]

[0061] The time domain calculation results are extended to space, and the time domain signals of all scanning results in the hydrophone scanning plane are calculated to obtain the time peak sound intensity I tp The maximum value of the spatial distribution is recorded as the spatial peak time peak sound intensity

[0062] I SPTP = max[I tp (x,y)] (6)

[0063] Where (x, y) represents the coordinates of each point in the scanning plane. Calculate the peak-to-peak sound intensity of all array elements, I SPTP =25.27±1.59mW / cm 2 . The spatial peak time peak sound intensity I SPPP The maximum value of is used as a reference, and a -6dB threshold is set. In the scanning plane, the area covered by the scanning points with an area higher than the -6dB threshold is recorded as A, and I is defined at the same time. SATP is the spatially averaged time-domain peak sound intensity, and

[0064]

[0065] Calculate and obtain the spatial average time domain peak sound intensity of each array element, I SATP =10.96±0.95mW / cm 2 .

[0066] The above electroacoustic parameter test results, including resonant frequency, electrical impedance, electromechanical coupling coefficient, center frequency, bandwidth, reflected signal peak-to-peak value, spatial peak-time peak sound intensity, and spatial average time-domain peak sound intensity, a total of 8 test parameters, will be used as the criterion for good array consistency in the future. All array elements are tested one by one, and the relative error of the same parameters between each array element is no more than 10% as the criterion for good array consistency. If the relative error of a parameter exceeds 10%, it will return to the micron-level high-precision processing and preparation link, prepare new array elements to replace the array elements with excessive errors, and re-test the electroacoustic parameters until the relative error of the same parameters between each array element does not exceed 10%.

[0067] Step 4: Perform a thin-wall structure guided wave acoustic field directivity test on the flexible ultrasonic sensor array. If the beam angle is less than the focusing excitation beam angle threshold of step 1, proceed to step 5. Otherwise, adjust the guided wave phased array focusing scheme to continue the thin-wall structure guided wave acoustic field directivity test.

[0068] The directivity test of the guided wave acoustic field of thin-walled structures uses a multi-channel ultrasonic system to focus the sensor array by applying a time-delayed excitation method. The excitation signal propagates along the thin-walled structure to achieve focused control of the guided wave acoustic field. The laser vibrometer collects ultrasonic guided wave signals on the surface of the thin-walled structure for the directivity study of the sensor array, obtains the ultrasonic guided wave signal at the focus, and combines the dispersion curve in the thin-walled structure to perform guided wave signal mode identification.

[0069] The flexible piezoelectric ultrasonic sensor array is arranged on an aluminum plate of 1000×500×1.5mm. The size of the thin-wall structure does not affect the directivity test of the flexible ultrasonic sensor array. It is connected to a multi-channel ultrasonic excitation system to achieve focused excitation and propagation of the guided wave signal. The guided wave mode and focusing effect of the flexible piezoelectric ultrasonic sensor array excited in the aluminum plate are verified by measuring with a laser vibrometer scanning system (Polytec 500). During the test, according to the performance test results of each element of the flexible piezoelectric ultrasonic sensor array, the average value of the center frequency of each element was selected as the center frequency of the array excitation, that is, the excitation frequency was set to 576kHz, and the acoustic focusing point was 100mm away from the center of the sensor. A scanning scheme with the center of the sensor as the origin and the focal distance of 100mm as the radius was set. The maximum amplitude of the vibration signal on the scanning radius was extracted, and the directivity diagram was drawn in the polar coordinate system. The beam angle index requirements were consistent with those in the simulation of step one. The beam angle was less than 60°, and the ultrasonic guided wave signal propagation had an obvious focusing direction, which verified the reliability of the simulation scheme of step one and clarified the propagation direction of the ultrasonic guided wave in the aluminum plate, providing a basis for the interpretation of the received signal in the defect detection experiment. Consistent with the simulation signal analysis method, a continuous wavelet transform is performed on the time domain signal at the focus, and a numerical method is used to solve the guided wave dispersion curve in the aluminum plate. The continuous wavelet transform result is compared with the dispersion curve in the aluminum plate. The time domain signal wave velocity is 2650m / s, which is still close to the A0 mode wave velocity, indicating that the flexible piezoelectric ultrasonic sensor array designed in this embodiment excites a guided wave sound field dominated by the A0 mode in an aluminum plate with a thickness of 1.5mm. The directivity test results show that the guided wave sound field has a good focusing effect, which provides a basis for the subsequent thin-walled structure ultrasonic guided wave nondestructive testing method.

[0070] Step 5: Use flexible ultrasonic sensor array to perform defect detection test on thin-walled structure.

[0071] Thin-walled structure defect detection verification: on an aluminum plate with a thickness of 1.5 mm, a through hole with a diameter of 10 mm is tested to verify the application performance of the sensor array.

[0072] Identify the modes of guided wave signals propagating in thin-walled structures, improve the resolution of sensor array defect detection, and improve the accuracy of defect location. First, the dispersion curves of the symmetric mode and antisymmetric mode in the 1.5mm thick aluminum plate are solved by numerical method. The time domain signal of probe B is extracted and ultrasonic guided wave modal analysis is performed. The time domain signal of probe B is subjected to continuous wavelet transform. The continuous wavelet transform results are compared with the dispersion curve results. The propagation speed of the guided wave signal is 2850m / s, which is close to the group velocity (2900m / s) corresponding to the A0 mode under the condition of frequency-thickness product of 0.9MHz·mm. Therefore, it shows that the signal generated by the flexible ultrasonic transducer array in the aluminum plate is mainly the antisymmetric mode A0 mode, and the propagation speed is 2850m / s.

[0073] To test the defects of thin-walled structures, a flexible ultrasonic sensor array is placed on an aluminum plate with a circular through hole of 10 mm in diameter. First, the flexible ultrasonic sensor array is placed 100 mm away from the edge of the aluminum plate, and the excitation focus is set on the edge of the aluminum plate. The 8 array elements in the first row are used as receiving channels to receive the reflected signal from the edge. The amplitude of the received signals of each channel is close. The signal of the first channel is listed, and the results are as follows: Figure 5 The flexible ultrasonic sensor array is placed 100 mm away from the through hole, and the same excitation and receiving scheme is adopted. The receiving signals of the 8 channels are shown in Figure 6 As shown, the peak-to-peak value of the received signal of each channel is plotted on Figure 7 As shown, compared to Figure 5 The edge reflection signal is affected by the through-hole acoustic scattering, and the reflection signal amplitude becomes lower. Among the 8 receiving channels, channels 4 and 5 are in the middle of the linear array, and the received signal amplitude is the largest. The remaining channels decrease in turn according to the distance. The data results contain defect location information, which is consistent with the actual experimental settings.

[0074] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for nondestructive testing of thin-walled structures using ultrasonic guided waves based on flexible ultrasonic sensors, characterized in that: The steps are as follows: Step 1: Set the flexible ultrasonic sensor array at the center of the thin-wall structure and perform guided wave focusing acoustic field simulation to obtain the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array, and determine the design scheme of the flexible ultrasonic sensor array and the focusing excitation beam angle threshold according to the parameters in the guided wave phased array focusing excitation scheme; Step 2: Building a micron-level high-precision processing and preparation platform, and preparing a flexible ultrasonic sensor array according to the design scheme of the flexible ultrasonic sensor array; Step 3: Test the electroacoustic parameters of all channels of the flexible ultrasonic sensor array. If the relative error exceeds 10%, return to step 2, otherwise proceed to step 4; Step 4: Perform a thin-wall structure guided wave acoustic field directivity test on the flexible ultrasonic sensor array. If the beam angle is less than the focusing excitation beam angle threshold of step 1, proceed to step 5. Otherwise, adjust the guided wave phased array focusing scheme to continue the thin-wall structure guided wave acoustic field directivity test. Step 5: Using a flexible ultrasonic sensor array to detect defects in thin-walled structures; The method for obtaining the guided wave phased array focusing excitation scheme of the flexible ultrasonic sensor array is as follows: designing an array element arrangement scheme of the flexible ultrasonic sensor array, designing different array element arrangement forms, changing the arrangement mode and quantity of the sensor array, setting a load application point to simulate the excitation of each array element in the flexible ultrasonic sensor array, and obtaining the guided wave focusing excitation scheme of the flexible ultrasonic sensor array; The method of setting the load application point to simulate the excitation of each array element in the flexible ultrasonic sensor array is as follows: applying time-delayed excitation to the array element to form a focused sound beam; setting multiple point probes on the upper surface of the thin-walled structure to analyze the waveform of sound propagation; at least two probes are on the same straight line with the center of the array element; The defect detection method comprises the following steps: using a numerical method to solve dispersion curves of symmetric modes and antisymmetric modes in a thin-walled structure, extracting time domain signals and performing ultrasonic guided wave modal analysis, performing continuous wavelet transform on the time domain signals, and comparing the continuous wavelet transform results with dispersion curve results. The propagation velocity of the guided wave signal is close to the group velocity corresponding to the A0 mode, and the signal generated by the flexible ultrasonic transducer array in the thin-walled structure is mainly the antisymmetric mode A0 mode; setting the excitation focus at the edge of the thin-walled structure, receiving the reflected signal from the edge, and the amplitude of the received signal of each channel is close; arranging the flexible ultrasonic sensor array at a distance of R mm from the through hole, using the same excitation reception, and the amplitude of the reflected signal becomes low due to the influence of the through hole acoustic scattering.

2. The thin-wall structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensor according to claim 1 is characterized in that: The method for determining the focused excitation beam angle threshold is as follows: multiple periodic Hanning window modulation signals are set, a probe is used as the focus, a phased array excitation signal is applied to each array element, the center of the sensor array is used as the origin, and the maximum signal amplitude of each point is extracted on a circular area with a radius equal to the distance between the probe and the center of the sensor array to draw a directivity diagram, the azimuth of the probe is used as the 0° direction, and the maximum focused excitation beam angle when the sensor array has excellent directivity in the directivity diagram is the focused excitation beam angle threshold.

3. The thin-wall structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensor according to claim 1 or 2, characterized in that: Each array element of the flexible ultrasonic sensor array includes a piezoelectric material, a positive electrode and a negative electrode. The positive electrode and the negative electrode are respectively arranged on the upper side and the lower side of the piezoelectric material. The positive electrode and the negative electrode are both adhered to the piezoelectric material by conductive silver glue. The positive electrodes of all array elements are independently connected, and the negative electrode is designed as an island bridge structure. Flexible packaging layers are provided on the outside of the positive electrode and the negative electrode.

4. The thin-wall structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensor according to claim 3 is characterized in that: The thin-walled structure is an aluminum plate, and the flexible packaging layer is a PDMS packaging layer; The piezoelectric material is a 1-3 composite piezoelectric material with a center frequency of 600kHz, and the difference between the acoustic impedance of the piezoelectric material and the acoustic impedance of the aluminum plate is less than 3-5 Mrayl; the positive electrode is composed of four layers of electrode materials stacked in sequence, and the positive electrode and the negative electrode are both copper foils coated with polyimide.

5. The method for nondestructive testing of thin-walled structures by ultrasonic guided waves based on flexible ultrasonic sensors according to claim 4, characterized in that: The electroacoustic parameter test includes impedance test, frequency response test and radiated sound field scanning test, and the results of the electroacoustic parameter test include resonant frequency, electrical impedance, electromechanical coupling coefficient, center frequency, bandwidth, peak-to-peak value of reflected signal, spatial peak-to-time peak sound intensity, and spatial average time-domain peak sound intensity; All array elements are tested one by one. If the relative error of the same parameters between the array elements exceeds 10%, they are returned to the processing and preparation stage, and new array elements are prepared to replace the array elements with excessive relative errors. The electroacoustic parameter test is re-performed until the relative error of the same parameters between the array elements does not exceed 10%.

6. The method for nondestructive testing of thin-walled structures by ultrasonic guided waves based on flexible ultrasonic sensors according to claim 5, characterized in that: The impedance test obtains the relationship between the frequency and electrical impedance of each array element. The impedance test uses an impedance analyzer to measure the electrical impedance, phase angle, resonant frequency, and electromechanical coupling coefficient of each array element of the flexible ultrasonic sensor; The frequency response test uses a pulse reflection method to test the frequency response of all array elements of the sensor array to obtain the center frequency, bandwidth, and peak-to-peak value of the reflected signal of all array elements; The radiated sound field scanning test scans the radiated sound field of all array elements of the flexible ultrasonic sensor, including a plane parallel to the acoustic axis of the array element and a plane perpendicular to the acoustic axis of the array element, and obtains the sound pressure distribution obtained by scanning in the direction parallel to the acoustic axis of the sensor array element and the sound pressure distribution obtained by scanning in the direction perpendicular to the acoustic axis of the sensor array element, respectively.

7. The thin-wall structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensor according to claim 6 is characterized in that: The maximum width of the -3dB area of ​​each sensor array element parallel to the acoustic axis is required to be no more than 4.5mm, and the maximum width of the -6dB area is required to be no more than 6mm; the width of the -3dB area of ​​each sensor array element perpendicular to the acoustic axis is required to be no more than 4mm, and the width of the -6dB area is required to be no more than 5mm; The spatial peak time peak sound intensity I SPTP is a sound intensity parameter with space as a variable, the spatial average time domain peak sound intensity I SATP It is the sound intensity parameter related to the area of ​​the -6dB zone; The calculation method of the spatial peak time peak sound intensity is: cumulatively integrating the continuous time domain signal p(t) received by the hydrophone at each scanning point to obtain the square integral of the pulse sound pressure of the time domain signal p i ; In the time domain signal, the pulse sound pressure reaches 0.1p i The time when the pulse sound pressure reaches 0.9p is recorded as t1. i The time is recorded as t2, and the pulse duration is calculated as: t d =1.25(t2-t1); using the square integral of the pulse sound pressure p i and pulse duration t d Calculate the pulse sound pressure level p p ;set up The sound wave propagates in a plane wave-like manner. According to the pulse sound pressure level p p Calculate the time peak sound intensity I tp , calculate the time domain signal of all scanning results in the hydrophone scanning plane to obtain the time peak sound intensity I tp The maximum value of the spatial distribution is recorded as the spatial peak time peak sound intensity I SPTP ; The spatial peak time peak sound intensity I SPTP The maximum value is used as a reference, and a -6dB threshold is set. In the scanning plane, the area covered by the scanning points with an area higher than the -6dB threshold is recorded as A, and the spatial average time domain peak sound intensity is calculated.

8. The method for nondestructive testing of thin-walled structures by ultrasonic guided waves based on flexible ultrasonic sensors according to any one of claims 4 to 7, characterized in that: The thin-walled structure guided wave acoustic field directivity test utilizes a multi-channel ultrasonic system to apply time-delayed excitation flexible ultrasound to focus excite the sensor array, and the excitation signal propagates along the thin-walled structure to achieve focused regulation of the guided wave acoustic field; a laser vibration scanning system is used to collect ultrasonic guided wave signals on the surface of the thin-walled structure for sensor array directivity research, and ultrasonic guided wave signals at the focus are obtained, and the guided wave signal mode recognition is performed in combination with the dispersion curve in the thin-walled structure.

9. The thin-wall structure ultrasonic guided wave nondestructive testing method based on flexible ultrasonic sensor according to claim 8 is characterized in that: According to the performance test results of each element of the flexible ultrasonic sensor array, the average value of the center frequency of each element is selected as the center frequency of the array excitation, and the acoustic focus point is R mm away from the center of the sensor array. The center of the sensor array is taken as the origin, and the focus distance R mm is taken as the radius scan to extract the maximum amplitude of the vibration signal on the scanning radius. The directivity diagram is drawn in the polar coordinate system. The beam angle is less than the focusing excitation beam angle threshold, and the ultrasonic guided wave signal propagation has obvious focusing direction. The time domain signal at the focus is subjected to continuous wavelet transform, and the guided wave dispersion curve in the thin-walled structure is solved by a numerical method. The continuous wavelet transform result is compared with the dispersion curve in the thin-walled structure. The wave velocity of the time domain signal is close to the A0 mode wave velocity, indicating that the guided wave acoustic field has a good focusing effect.

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