Circumferential positioning method for microcracks in pipe based on ultrasonic guided wave second harmonics

Abstract

According to the in-pipe microcrack circumferential positioning method based on the ultrasonic guided wave second harmonic, the second harmonic is generated by means of the fundamental wave of the axial symmetry mode, circumferential positioning of the microcrack in a tubular structure is achieved according to the mode and the amplitude of the second harmonic, and the microcrack in the pipeline can be quickly positioned before the microcrack of the pipeline develops into a macroscopic crack. The microcracks can be found in time and accurately positioned, so that the pipeline can be accurately maintained in time, further evolution of the microcracks and accidents are avoided, safe operation of the pressure pipeline is guaranteed, and the time cost and the economic cost of manual troubleshooting can be reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to a method for circumferentially locating microcracks in a pipe based on second harmonic of ultrasonic guided waves. Background Art

[0002] As one of the five major means of transportation, pipelines are often used to transport gases, liquids and slurries, such as natural gas, liquefied petroleum gas and oil. During the service process, pressure pipelines are easily affected by impact, corrosion and other factors, and cracks are prone to initiation. Microcracks will continue to evolve into macrocracks, causing safety accidents such as pipeline leakage, fire and explosion. Therefore, before macrocracks appear, locating microcracks and performing precise maintenance in time can avoid the occurrence of the above safety accidents and ensure the safe operation of pressure pipelines.

[0003] Nondestructive testing technology can determine the location of cracks without destroying pressure pipes. Currently, the most widely used nondestructive testing technologies include ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing and radiographic testing. Compared with magnetic particle testing, penetrant testing and eddy current testing, ultrasonic testing can penetrate pressure pipes and detect internal damage. Compared with radiographic testing, ultrasonic testing has no radiation and is harmless to the human body. It is a safe and effective nondestructive testing method. Among them, point-by-point scanning based on body waves as an ultrasonic testing method can determine the displacement distribution of defects or damage in pressure pipes. However, for large-sized structures such as plates, pipes or rails, more points need to be detected and the amount of data processing in the later stage is large, so point-by-point scanning is inefficient and costly. Ultrasonic guided waves can overcome the above shortcomings, propagate over long distances in rails, plate structures or tubular structures, and efficiently determine the location of defects or damage. However, traditional ultrasonic guided waves mainly evaluate macroscopic defects or damage based on physical phenomena such as reflection, transmission or refraction, with the help of linear acoustic parameters (such as amplitude, sound velocity and attenuation coefficient), but are unable to detect changes in interfaces without obvious acoustic reflections such as fatigue, creep, microcracks and plastic deformation.

[0004] Compared with traditional ultrasonic guided waves, nonlinear ultrasonic guided waves mainly study higher-order harmonics (including higher harmonics, difference frequency components, and sum frequency components) with frequencies different from the fundamental frequency, and are more sensitive to defects or damages that are much smaller than the wavelength. Among them, microstructural changes such as dislocations, precipitates, microcracks or micropores have a significant effect on the amplitude of higher-order harmonics, but have little effect on the fundamental wave. Therefore, nonlinear ultrasonic guided waves can propagate over long distances and detect microstructural changes, and have gradually attracted the interest of scholars in recent decades.

[0005] As a typical nonlinear acoustic effect, the second harmonic of ultrasonic guided waves has been used to determine the size of microcracks in plate structures, but less involved in the microstructural changes in tubes.

[0006] Due to the dispersion, multi-mode and nonlinearity, the excitation of the second harmonic of ultrasonic guided waves in pipelines is also difficult. In addition, the interaction mechanism between the second harmonic of ultrasonic guided waves and microcracks in pipelines is still unclear, especially the functional relationship between the size and position of the microcrack zone and the second harmonic mode and amplitude has not been established. Summary of the invention

[0007] The present invention is made to solve the above-mentioned problems, and its purpose is to provide a nondestructive testing method that can characterize and circumferentially locate microcracks in pipes by means of the second harmonic of ultrasonic guided waves before macro cracks appear. The present invention takes into account the advantages of ultrasonic guided waves and nonlinear ultrasound, generates second harmonics by means of the fundamental wave of the axisymmetric mode, and realizes the circumferential positioning of microcracks in tubular structures according to the mode and amplitude of the second harmonic, and proposes a method for circumferential positioning and characterization of microcracks in tubular structures. The method can accurately locate microcracks in tubular structures, so that precise maintenance can be carried out in a timely manner based on the positioning information, avoiding the further evolution of microcracks and the occurrence of accidents, and ensuring the safe operation of pressure pipelines.

[0008] Specifically, the present invention adopts the following technical solutions:

[0009] The present invention provides a nondestructive testing method based on ultrasonic guided wave second harmonic, which is used to characterize and circumferentially locate cracks in a pipeline, and is characterized by: step S1, based on the physical parameters and geometric parameters of the pipeline, establishing a dispersion equation of the ultrasonic guided wave in the pipeline, and obtaining dispersion curves of the torsional mode and the longitudinal mode; step S2, based on the dispersion curve, selecting a corresponding mode and an excitation frequency; step S3, based on the mode and the excitation frequency, using a nonlinear ultrasonic measurement system to measure the pipeline, wherein the nonlinear ultrasonic measurement system generates an ultrasonic guided wave of a fixed frequency, the fundamental wave propagates in the pipeline to form a second harmonic, and the nonlinear ultrasonic measurement system receives the second harmonic on its propagation path; step S4, based on the received second harmonic, calculating and analyzing to obtain a normalized acoustic nonlinear parameter, and based on the normalized acoustic nonlinear parameter or the second harmonic, obtaining the position information and size information of the microcrack in the pipeline.

[0010] The method for circumferential positioning of microcracks in a pipe based on the second harmonic of ultrasonic guided waves provided by the present invention may also have such a technical feature, wherein step S1 includes the following sub-steps: step S1-1, constructing a cylindrical coordinate system based on the pipeline; step S1-2, constructing a physical equation of the pipeline according to the physical parameters; step S1-3, constructing a geometric equation of the pipeline in the cylindrical coordinate system according to the geometric parameters and the Lame coefficient of the cylindrical coordinate system; step S1-4, substituting the physical equation and the geometric equation into the motion equation in the cylindrical coordinate system, and ignoring the body force term , obtaining the wave equation of the ultrasonic guided wave in the pipeline; step S1-5, based on the interface continuity and free boundary conditions of the pipeline, forming the boundary conditions of the second harmonic in the pipeline; step S1-6, according to the linear terms of the wave equation and the boundary conditions, establishing the dispersion equation of the ultrasonic guided wave in the pipeline, and obtaining the dispersion curves of the torsional mode and the longitudinal mode; step S1-7, based on the nonlinear terms of the wave equation and the boundary conditions, using the second-order perturbation method and the orthogonal mode expansion method, establishing the mathematical expression of the second harmonic in the pipeline.

[0011] The method for circumferential positioning of microcracks in a pipe based on the second harmonic of ultrasonic guided waves provided by the present invention may also have such a technical feature, wherein the nonlinear ultrasonic measurement system includes a signal generator and an excitation transducer for generating the ultrasonic guided waves, and in step S1-1, the origin of the cylindrical coordinate system is located at the left end surface of the excitation transducer, the r axis is within the left end surface, takes the origin as the starting point, and the z axis points to the right side of the excitation transducer along the axial direction of the pipeline, is the rotation angle of the r axis. In step S1-2, the physical equation is:

[0012]

[0013] In the formula, σ represents stress, σ rr represents the radial normal stress, represents shear stress; ε represents strain, represents radial strain, ε zz represents the axial linear strain, represents shear strain; λ and μ represent Lame constants. In step S1-3, the geometric equation is:

[0014]

[0015] Where u is the displacement, u r represents the radial displacement, represents the circumferential displacement. In step S1-4, the wave equation is:

[0016]

[0017] Wherein, ρ is the density of the pipeline material, t is the time, and in step S1-5, the boundary condition is:

[0018]

[0019] In step S1-7, the mathematical expression of the second harmonic in the pipeline includes:

[0020] u=u1+u2

[0021]

[0022] In the formula, u1 represents the displacement of the fundamental wave, u2 represents the displacement of the second harmonic, and A m (z) represents the amplitude of the second harmonic, cc represents the high-order term, and for the m-mode with a frequency of 2ω, represents the displacement along r and distribution.

[0023] The method for circumferential positioning of microcracks in a pipe based on the second harmonic of ultrasonic guided waves provided by the present invention may also have such a technical feature, wherein the mathematical expression of the second harmonic in the pipe is A m (z) is expressed as:

[0024]

[0025]

[0026]

[0027]

[0028]

[0029] In the formula, σ m and σ n represents the stress tensor of the m-order and n-order modes, v m and v n represents the velocity vector of the m-order and n-order modes, n x and n z Represents the unit vectors in the x and z directions, k n * represents the wave number of the second harmonic, and k represents the wave number of the fundamental wave.

[0030] The method for circumferential positioning of microcracks in a tube based on ultrasonic guided wave second harmonic provided by the present invention may also have such a technical feature, wherein, in step S2, based on the phase velocity matching condition, the non-zero energy flow criterion and the dispersion curve, a suitable mode pair is screened, and a Hanning window is added according to the frequency and period number of the fundamental wave to design an excitation signal, thereby obtaining the mode and the excitation frequency.

[0031] The method for circumferential positioning of microcracks in a pipe based on ultrasonic guided wave second harmonic provided by the present invention may also have such a technical feature, wherein the nonlinear ultrasonic measurement system includes a receiving transducer and an oscilloscope, the receiving transducer is used to receive the acoustic signal of the second harmonic and convert it into an electrical signal, the oscilloscope is used to record the time domain signal of the second harmonic, and in step S4, the measured time domain signal is subjected to frequency domain analysis, the amplitude A1 of the fundamental wave and the amplitude A2 of the second harmonic are extracted, and the acoustic nonlinear parameter β is calculated based on the following formula:

[0032]

[0033] A normalized processing is performed on the microcrack to obtain a normalized acoustic nonlinear parameter β', and based on the normalized acoustic nonlinear parameter β' or the second harmonic, the coordinates and size of the microcrack on the pipeline are obtained.

[0034] The method for circumferential positioning of microcracks in a pipe based on ultrasonic guided wave second harmonic provided by the present invention may also have such a technical feature, wherein, in step S4, a series of notches of different sizes are prefabricated on multiple spare pipes of the same model as the pipe, the microcracks are prepared on the spare pipes using a three-point bending tester, and then the notches are cut off by machining, thereby forming a batch of the samples with microcracks of different lengths, and the multiple samples are measured using the nonlinear ultrasonic measurement system to obtain the normalized acoustic nonlinear parameter β' or the functional relationship between the second harmonic and the microcrack size.

[0035] The method for circumferential positioning of microcracks in a pipe based on ultrasonic guided wave second harmonic provided by the present invention may also have such a technical feature, wherein the nonlinear ultrasonic measurement system includes a piezoelectric ceramic array for receiving the second harmonic, and in step S4, nonlinear ultrasonic measurement is performed to extract the acoustic signal of each unit in the piezoelectric ceramic array and calculate the normalized acoustic nonlinear parameter β' or the distribution of the second harmonic, and establish the normalized acoustic nonlinear parameter β' or the second harmonic and the microcrack area. The relationship between the coordinates and the number of microcracks is determined by controlling other variables unchanged. Different numbers of notches are prefabricated along the circumference of the spare pipeline. The microcracks are prepared by means of a three-point bending tester. The notches are then cut off by machining to form a batch of samples with different numbers of microcracks.

[0036] Function and Effect of the Invention

[0037] According to the method for circumferential positioning of microcracks in a pipe based on the second harmonic of ultrasonic guided waves of the present invention, the second harmonic is generated by means of the fundamental wave of the axisymmetric mode, and the circumferential positioning of the microcracks in the tubular structure is achieved according to the mode and amplitude of the second harmonic. The microcracks in the pipeline can be discovered and accurately positioned in time before they develop into macro cracks, so that the pipeline can be accurately maintained in time, and the further evolution of the microcracks and the occurrence of accidents can be avoided, thereby ensuring the safe operation of the pressure pipeline and reducing the time cost and economic cost of manual inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a flow chart of a method for circumferentially locating microcracks in a tube based on second harmonic of ultrasonic guided waves in an embodiment of the present invention;

[0039] Figure 2 is a diagram of an excitation signal and a selected mode in an embodiment of the present invention;

[0040] Figure 3 is a stress cloud diagram of the second harmonic of ultrasonic guided waves in an embodiment of the present invention;

[0041] Figure 4 It is a typical time domain signal and spectrum diagram of the axial displacement of the reflected wave in the embodiment of the present invention;

[0042] Figure 5 It is a typical time domain signal and spectrum diagram of the axial displacement of the transmission wave in the embodiment of the present invention;

[0043] Figure 6 is a circumferential distribution diagram of the axial displacement amplitude of the second harmonic of the reflected wave in an embodiment of the present invention;

[0044] Figure 7 It is a circumferential distribution diagram of the axial displacement amplitudes of the fundamental wave and the second harmonic of the transmitted wave in the embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the method for circumferential positioning of microcracks in a tube based on the second harmonic of ultrasonic guided waves of the present invention is specifically described below in combination with embodiments and drawings.

[0046] <Example>

[0047] This embodiment provides a method for circumferentially locating microcracks in a pipe based on the second harmonic of ultrasonic guided waves, which is used to circumferentially locate microcracks in a pipe before macro cracks appear. Microcracks in a pipe refer to cracks with a width less than 10 μm.

[0048] In the method of this embodiment, a nonlinear ultrasonic measurement system is used to measure the pipeline. The nonlinear ultrasonic measurement system is composed of a signal generator, a pulse amplifier, an excitation transducer, a receiving transducer, a filter, an oscilloscope, etc., wherein the excitation transducer and the receiving transducer can be selected from piezoelectric ceramic arrays, piezoelectric ceramic tubes, etc.

[0049] Figure 1 It is a flow chart of the method for circumferential positioning of microcracks in a tube based on the second harmonic of ultrasonic guided waves in this embodiment.

[0050] like Figure 1 As shown, the method comprises the following steps:

[0051] Step S1, based on the physical parameters and geometric parameters of the pipeline, establish the dispersion equation of the ultrasonic guided wave in the pipeline, and obtain the dispersion curves of the torsional mode and the longitudinal mode.

[0052] Step S2: selecting a corresponding mode and excitation frequency based on the dispersion curve.

[0053] Step S3, based on the selected mode and excitation frequency, the pipeline is measured using a nonlinear ultrasonic measurement system, wherein the nonlinear ultrasonic measurement system generates an ultrasonic guided wave of a fixed frequency, wherein the fundamental wave propagates in the pipeline to form a second harmonic, and the nonlinear ultrasonic measurement system receives the second harmonic on its propagation path.

[0054] Step S4, performing calculation and analysis based on the received second harmonic to obtain normalized acoustic nonlinear parameters, and obtaining location information and size information of microcracks in the pipeline based on the normalized acoustic nonlinear parameters or the second harmonic.

[0055] The above steps will be described in detail below.

[0056] Step S1, based on the physical parameters and geometric parameters of the pipeline, establish the dispersion equation of the ultrasonic guided wave in the pipeline, and obtain the dispersion curves of the torsional mode and the longitudinal mode.

[0057] Step S1 specifically includes the following sub-steps:

[0058] Step S1-1, constructing a cylindrical coordinate system in the pipeline.

[0059] The origin is located at the left end face of the excitation transducer; the r axis is in the left end face and takes the origin as the starting point; the z axis is along the direction of the pipeline axis and points to the right side of the excitation transducer; is the rotation angle of the r axis (observed from the opposite direction of the z axis, the rotation direction is counterclockwise).

[0060] Step S1-2, constructing the physical equation of the pipeline according to the physical parameters of the pipeline, as follows:

[0061]

[0062] In the formula, σ represents stress, σ rr represents the radial normal stress, represents shear stress; ε represents strain, represents radial strain, ε zz represents the axial linear strain, represents shear strain; λ and μ represent Lame constants.

[0063] Step S1-3, according to the geometric parameters of the pipeline and the Lame coefficients of the cylindrical coordinate system, establish the geometric equation of the pipeline in the cylindrical coordinate system:

[0064]

[0065] Where u is the displacement, u r represents the radial displacement, Represents the circumferential displacement.

[0066] Step S1-4, substituting the above physical equations and geometric equations into the motion equations in the cylindrical coordinate system and ignoring the body force term, the wave equation of nonlinear ultrasound in the pipeline is obtained as follows:

[0067]

[0068] Where ρ is the density of the pipe material and t is the time.

[0069] Step S1-5, based on the interface continuity and free boundary conditions of the pipeline, the boundary conditions of the second harmonic of the ultrasonic guided wave in the pipeline are formed.

[0070] That is, since the inner and outer surfaces of the pipe are free,

[0071]

[0072] Where 2r1 is the inner diameter of the pipe and 2r2 is the outer diameter of the pipe.

[0073] Step S1-6, according to the above formula (1), formula (2), formula (3) and formula (4), establish the dispersion equation of ultrasonic guided waves in the pipeline, and draw the dispersion curves of the torsional mode and the longitudinal mode.

[0074] That is, in this embodiment, the high-order terms of the geometric equations in the cylindrical coordinate system are considered, and according to the characteristics of the orthogonal coordinate system and the isotropic material, the Landau and Lifshitz's third-order elastic constants are introduced to construct the high-order terms in the physical equations. With the help of the Lame coefficients, the Second Piola-Kirchhoff stress tensor and the First Piola-Kirchhoff stress tensor are established in sequence; based on the nonlinear strain-displacement relationship, the stress-strain relationship and the differential equation of motion in the cylindrical coordinate system, the body terms are ignored and the wave equation of nonlinear ultrasound in the steel pipe is established; based on the interface continuity and the free boundary conditions, the boundary conditions of the second harmonic of ultrasonic guided waves in the steel pipe are formed.

[0075] Step S1-7, based on the wave equation and boundary conditions of the second harmonic of the ultrasonic guided wave in the pipeline, a mathematical expression of the second harmonic in the pipeline is established using the second-order perturbation method and the orthogonal mode expansion method.

[0076] The expressions of the second-order perturbation method and the orthogonal mode expansion method are:

[0077] u=u1+u2 (5)

[0078]

[0079] In the formula, u1 represents the displacement of the fundamental wave, u2 represents the displacement of the second harmonic, and A m (z) represents the amplitude of the second harmonic, cc represents the high-order term, and for the m-mode with a frequency of 2ω, represents the displacement along r and distribution.

[0080] Among them, A m (z) can be expressed as:

[0081]

[0082]

[0083]

[0084]

[0085]

[0086] In the formula, σ m and σ n represents the stress tensor of the m-order and n-order modes, v m and v n represents the velocity vector of the m-order and n-order modes, n x and n z Represents the unit vectors in the x and z directions, kn * represents the wave number of the second harmonic, and k represents the wave number of the fundamental wave.

[0087] Step S2: selecting a corresponding mode and excitation frequency based on the dispersion curve.

[0088] Among them, based on the phase velocity matching condition, the non-zero energy flow criterion and the above dispersion curve, the appropriate mode pair is screened. In the phase velocity dispersion curve, there is a horizontal area of ​​the L(0,2) mode, and there is a suitable mode pair to generate a second harmonic with a cumulative effect. In addition, the fundamental waves of the torsional mode and the longitudinal mode can only generate the second harmonic of the longitudinal mode.

[0089] In this embodiment, a Hanning window is added according to the frequency and period number of the fundamental wave to design the excitation signal.

[0090] Step S3, based on the selected mode and excitation frequency, the pipeline is measured using a nonlinear ultrasonic measurement system, wherein the nonlinear ultrasonic measurement system generates an ultrasonic guided wave of a fixed frequency, the fundamental wave of the ultrasonic guided wave propagates in the pipeline to form a second harmonic, and the nonlinear ultrasonic measurement system receives the second harmonic on its propagation path.

[0091] Specifically, the signal generator generates a corresponding excitation signal based on the selected mode pair and excitation frequency. The excitation signal is converted into a vibration signal through a pulse amplifier and an excitation transducer. The vibration signal forms an ultrasonic guided wave of a fixed frequency in the pipeline, and the fundamental wave propagates in the tubular structure of the pipeline to form a second harmonic with a cumulative effect.

[0092] Step S4, performing calculation and analysis based on the received second harmonic to obtain normalized acoustic nonlinear parameters, and obtaining location information and size information of microcracks in the pipeline based on the normalized acoustic nonlinear parameters or the second harmonic.

[0093] Wherein, a receiving transducer is arranged on the propagation path of the second harmonic, and the receiving transducer converts the acoustic signal of the second harmonic into an electrical signal. In this embodiment, the receiving transducer is a piezoelectric ceramic array, which is arranged along the circumference of the pipeline. During measurement, the acoustic signal of each unit in the piezoelectric ceramic array is extracted and the distribution of the normalized acoustic nonlinear parameter β' or the second harmonic displacement is calculated.

[0094] The second harmonic is passed through a bandpass filter and an oscilloscope, and the oscilloscope measures multiple time domain signals and averages them. In this embodiment, the oscilloscope measures the time domain signal 512 times and averages them, and stores the time domain signal in a USB flash drive, recouples the excitation transducer and the receiving transducer, and then performs 2-4 measurements and stores the data.

[0095] Then, the measured time domain signal is subjected to frequency domain analysis. In this embodiment, with the help of origin software, the parameters such as signal input and sampling interval are set, the time domain signal is subjected to fast Fourier transform (FFT), the frequency domain analysis is carried out, the amplitude A1 of the fundamental wave and the amplitude A2 of the second harmonic are extracted, the acoustic nonlinear parameter β is calculated based on the following formula, and normalization is performed:

[0096]

[0097] The normalized acoustic nonlinear parameter is recorded as β'. Based on the normalized acoustic nonlinear parameter β' and the second harmonic, the microcrack area on the pipeline is obtained. Coordinates and dimensions.

[0098] The functional relationship between the normalized acoustic nonlinear parameter β' or the second harmonic and the crack size and position can be obtained by performing the above measurement on multiple samples.

[0099] In this embodiment, other variables are controlled unchanged, a series of notches of different sizes are prefabricated on a plurality of spare pipes of the same model as the above-mentioned pipe, microcracks are prepared on the plurality of spare pipes respectively using a three-point bending test machine, and then the notches are cut off by machining to form a batch of specimens with microcracks of different lengths. The plurality of specimens are measured using a nonlinear ultrasonic measurement system to obtain a functional relationship between a normalized acoustic nonlinear parameter β' or a second harmonic and the size of the microcracks.

[0100] While keeping other variables constant, different numbers of notches are prefabricated along the circumference of the spare pipe, microcracks are prepared by means of a three-point bending test machine, and the notches are cut off by machining to form a batch of samples with different numbers of microcracks. Nonlinear ultrasonic measurement is performed to extract the acoustic signal of each unit in the piezoelectric ceramic array and calculate the distribution of the normalized acoustic nonlinear parameter β' or the second harmonic, and establish the normalized acoustic nonlinear parameter β' or the second harmonic and the microcrack area. Functional relationship between coordinates and the number of microcracks.

[0101] As described above, the method of this embodiment uses the second harmonic of ultrasonic guided waves to obtain the microcrack area in the pipeline. The information such as coordinates, microcrack size and number of microcracks is used to characterize and circumferentially locate microcracks in pressure pipelines.

[0102] In this embodiment, the above method is verified through finite element simulation experiments.

[0103] The pipe is a 304 stainless steel round pipe with a length of 1000 mm, an outer diameter of 30 mm, an inner diameter of 26 mm, and a density of 7860 kg / m 3 , the Young's modulus is 1.996×1011 Pa, Poisson's ratio is 0.29.

[0104] When constructing the cylindrical coordinate system, the pipe is placed roughly horizontally, with the center of the left end of the pipe as the origin, and the z-axis points to the right along the axial direction of the pipe. The rotation angle of the specific radius of the left end of the pipe is

[0105] Microcracks were simulated on the pipe. Specifically, according to the nonlinear theory of contact acoustics, the major and minor axes were 5×10 -3 m and 2×10 -8 m, and completely cut off the ellipse along the thickness direction of the pipeline. The cut-off part is regarded as the microcrack on the pipeline, and the contact properties of the master and slave surfaces are set to construct the microcrack model. In the simulation experiment, the above method is used to cut off the microcrack at z = 500 mm on the pipeline. arrive One microcrack is constructed every 10°, and a total of 7 microcracks are arranged.

[0106] Figure 2 2 is a diagram showing the excitation signal and the selected mode in this embodiment.

[0107] like Figure 2 As shown in the simulation experiment, the excitation signal is a 200kHz short sine pure tone with a Hanning window, and a normal displacement is applied to the left end face of the pipe. The amplitude is 1×10 -7 m.

[0108] According to the formula The time step is chosen to be 1×10 -8 s, and the time length is selected as 5×10 -5 s. The present invention divides the pipeline into region I (z = 0-400 mm), region II (z = 400-600 mm) and region III (z = 600-1000 mm). According to the formula The cell shape and technology in the mesh control properties of area I and area III are tetrahedron and free, and the maximum cell mesh size is 1mm. The cell shape and technology in the mesh control properties of area II are hexahedron and structure, and the maximum cell size is 0.5mm.

[0109] In addition, in the simulation experiment, the geometric nonlinearity in the simulation model is turned off. This helps to determine the source of nonlinearity.

[0110] In order to avoid mutual interference between the reflected wave and the transmitted wave in the received signal, in the simulation experiment, at z = 250 mm and z = 750 mm, Initially, an acoustic signal is received every 15°, with a total of 25 receiving points.

[0111] Figure 3Graph 1 is a stress cloud diagram of the second harmonic of the ultrasonic guided wave in this embodiment.

[0112] When the ultrasonic guided wave in the pipeline passes through the area with microcracks, the stress cloud diagram is as follows Figure 3 shown, and Figure 3 In the figure, the area where microcracks exist on the pipe is locally enlarged.

[0113] Figure 4 3 is a typical time domain signal and spectrum diagram of the reflected wave in this embodiment, showing a typical time domain signal and spectrum diagram of wave packet 3 at z=250 mm.

[0114] like Figure 4 As shown in the figure, by analyzing the group velocity, it can be seen that wave packet 1 is the L(0,2) mode and wave packet 2 is the L(0,1) mode. Compared with the L(0,2) mode, the energy of the L(0,1) mode is relatively small. Therefore, in the simulation experiment, the influence of the L(0,1) mode is ignored. FFT processing was performed on wave packet 1, and no second harmonic was found. The ultrasonic guided wave in the pipeline encountered microcracks and reflected, so there was wave packet 3 in the received signal. The reflected wave encountered the left end surface and reflected again, passing through the receiving point and forming wave packet 4. In addition, the transmitted wave was reflected on the right end surface, passing through the receiving point and forming wave packet 5, and then reflected on the left end surface to form wave packet 6. FFT processing was performed on wave packets 3, 4, 5, and 6, and second harmonics were found. The analysis results show that microcracks are the main source of second harmonics of ultrasonic guided waves.

[0115] Figure 5 : is a typical transmission wave time domain signal and spectrum diagram in this embodiment, wherein the typical ultrasonic guided wave time domain signal and the spectrum diagram of wave packet 1 at z = 750 mm are shown, wherein Figure 5 (a) is the time domain signal diagram, Figure 5 (b) is the spectrum of wave packet 1.

[0116] like Figure 5 As shown in the figure, the ultrasonic guided wave passes through the microcrack, forms wave packet 1 at the receiving point, reflects at the right end face, and then forms wave packet 2 at the receiving point. In addition, the reflected wave of the microcrack is reflected by the left end face and the right end face, and forms wave packet 3 and wave packet 4 at the receiving point. FFT processing is performed on wave packet 1, wave packet 2, wave packet 3 and wave packet 4, and the second harmonic, third harmonic and even fourth harmonic are found. Therefore, when the ultrasonic guided wave passes through the microcrack, high-order harmonics are formed.

[0117] Figure 6 3 is a circumferential distribution diagram of the axial displacement of the second harmonic of the reflected wave in this embodiment.

[0118] like Figure 6As shown in the figure, in the simulation experiment, at z = 250 mm, the displacement is received along the z-axis every 15° around the pipe, and a total of 24 signals are received. The wave packet 3 in the time domain signal is processed by FFT to obtain the amplitude of the second harmonic, and the relationship between the angle and the amplitude of the second harmonic is plotted, that is, Figure 6 It can be seen that the fundamental wave of the axisymmetric mode passes through the microcrack and is reflected, generating the second harmonic of the bending mode. In addition, when φ = 15°, the amplitude of the second harmonic reaches the maximum. This angle deviates by 15° from the center of the microcrack area.

[0119] Figure 7 is the circumferential distribution diagram of the axial displacement of the fundamental wave and the second harmonic of the transmitted wave in this embodiment, where Figure 7 (a) is the circumferential distribution diagram of the axial displacement of the fundamental wave of the transmitted wave. Figure 7 (b) is the circumferential distribution diagram of the second harmonic axial displacement.

[0120] like Figure 7 As shown in the figure, in the simulation experiment, at z = 750 mm, 24 signals are received using the same method. The wave packet 1 in the time domain signal is processed by FFT, and the relationship between the angle and the fundamental amplitude, and the angle and the second harmonic amplitude are plotted, that is, Figure 7 It can be seen that when the fundamental wave of the axisymmetric mode passes through the microcrack, no mode conversion occurs, but the second harmonic of the bending mode is generated. Therefore, the fundamental wave of the axisymmetric mode is not sensitive to microcracks, and the amplitude of the second harmonic is The angle reaches its maximum when , which is exactly consistent with the center of the crack zone. and When , the amplitude of the second harmonic is significantly higher than that at other angles.

[0121] Based on the above simulation experiments and corresponding analysis, it can be seen that the method of this embodiment is based on nonlinear ultrasonic guided waves in the pipeline, and realizes circumferential positioning of microcracks according to the amplitude and mode of the second harmonics of the reflected wave and the transmitted wave, among which the second harmonic of the transmitted wave is more sensitive.

[0122] In this embodiment, parts not described in detail are well-known technologies in the art.

[0123] Example Function and Effect

[0124] According to the method and device for circumferential positioning of microcracks in a pipe based on the second harmonic of ultrasonic guided waves provided in this embodiment, the second harmonic is generated by means of the fundamental wave of the axisymmetric mode, and the circumferential positioning of the microcracks in the tubular structure is achieved according to the mode and amplitude of the second harmonic. The microcracks in the pipeline can be detected in time before they develop into macro cracks, so that the pipeline can be accurately maintained in time, and the further evolution of the microcracks and the occurrence of accidents can be avoided, thereby ensuring the safe operation of the pressure pipeline and reducing the time cost and economic cost of manual inspection.

[0125] The above embodiments are only used to illustrate specific implementation modes of the present invention, and the present invention is not limited to the description scope of the above embodiments.

[0126] In the above embodiment, the method is used to characterize and locate microcracks in a 304 stainless steel round tube of a given size. In fact, the method can also be used to characterize and locate microcracks in pipes of other materials and sizes.

[0127] In the above embodiment, a series of notches of different sizes are prefabricated on a plurality of spare pipes of the same model by machining and three-point bending, microcracks are prepared by means of a three-point bending test machine, and then the notches are cut off by machining to form a batch of specimens with microcracks of different lengths and numbers. In an alternative solution, other methods in the prior art may also be used to prepare specimens with microcracks of different lengths and numbers.

Claims

1. A method for circumferentially locating microcracks in a pipe based on second harmonic of ultrasonic guided waves, which is used to characterize and locate microcracks in a pipe, characterized in that: The following steps are involved: Step S1, based on the physical parameters and geometric parameters of the pipeline, establishing a dispersion equation of ultrasonic guided waves in the pipeline, and obtaining dispersion curves of torsional mode and longitudinal mode; Step S2, selecting a corresponding mode and excitation frequency based on the dispersion curve; Step S3, based on the mode and the excitation frequency, using a nonlinear ultrasonic measurement system to measure the pipeline, wherein the nonlinear ultrasonic measurement system generates an ultrasonic guided wave of a fixed frequency, the fundamental wave of the ultrasonic guided wave propagates in the pipeline to form a second harmonic, and the nonlinear ultrasonic measurement system receives the second harmonic on its propagation path; Step S4, performing calculation and analysis based on the received second harmonic to obtain normalized acoustic nonlinear parameters, and obtaining position information and size information of the microcracks in the pipeline based on the normalized acoustic nonlinear parameters or the second harmonic.

2. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 1, Features: Wherein, step S1 includes the following sub-steps: Step S1-1, constructing a cylindrical coordinate system based on the pipeline; Step S1-2, constructing a physical equation of the pipeline according to the physical parameters; Step S1-3, constructing a geometric equation of the pipeline in the cylindrical coordinate system according to the geometric parameters and the Lame coefficients of the cylindrical coordinate system; Step S1-4, substituting the physical equation and the geometric equation into the motion equation in the cylindrical coordinate system and ignoring the body force term to obtain the wave equation of the ultrasonic guided wave in the pipeline; Step S1-5, forming the boundary condition of the second harmonic in the pipeline based on the interface continuity and free boundary condition of the pipeline; Step S1-6, establishing a dispersion equation of the ultrasonic guided wave in the pipeline according to the wave equation and the linear terms of the boundary conditions, and obtaining dispersion curves of the torsional mode and the longitudinal mode; Step S1-7, based on the nonlinear terms of the wave equation and the boundary conditions, using the second-order perturbation method and the orthogonal mode expansion method, establish a mathematical expression for the second harmonic in the pipeline.

3. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 2, characterized in that: in, The nonlinear ultrasonic measurement system comprises a signal generator and an excitation transducer for generating the ultrasonic guided wave, In step S1-1, the origin of the cylindrical coordinate system is located at the left end surface of the excitation transducer, the r axis is in the left end surface, takes the origin as the starting point, and the z axis points to the right side of the excitation transducer along the axial direction of the pipeline. is the rotation angle of the r axis, In step S1-2, the physical equation is: In the formula, σ represents stress, σ rr represents the radial normal stress, represents shear stress; ε represents strain, represents radial strain, ε zz represents the axial linear strain, represents shear strain; λ and μ represent Lame constants, In step S1-3, the geometric equation is: Where u is the displacement, u r represents the radial displacement, represents the circumferential displacement, In step S1-4, the wave equation is: Where ρ is the density of the pipe material, t is the time, In step S1-5, the boundary conditions are: In step S1-7, the mathematical expression of the second harmonic in the pipeline includes: u=u1+u2 In the formula, u1 represents the displacement of the fundamental wave, u2 represents the displacement of the second harmonic, and A m (z) represents the amplitude of the second harmonic, cc represents the high-order term, and for the m-mode with a frequency of 2ω, represents the displacement along r and distribution.

4. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 3, characterized in that: in, In the mathematical expression of the second harmonic in the pipeline, A m (z) is expressed as: In the formula, σ m and σ n represents the stress tensor of the m-order and n-order modes, v m and v n represents the velocity vector of the m-order and n-order modes, n x and n z Represents the unit vectors in the x and z directions, k n * represents the wave number of the second harmonic, and k represents the wave number of the fundamental wave.

5. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 1, characterized in that: in, In step S2, based on the phase velocity matching condition, the non-zero energy flow criterion and the dispersion curve, a suitable mode pair is screened, and a Hanning window is added according to the frequency and period number of the fundamental wave to design an excitation signal, thereby obtaining the mode and the excitation frequency.

6. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 1, characterized in that: in, The nonlinear ultrasonic measurement system includes a receiving transducer and an oscilloscope. The receiving transducer is used to receive the acoustic signal of the second harmonic and convert it into an electrical signal. The oscilloscope is used to record the time domain signal of the second harmonic. In step S4, the measured time domain signal is subjected to frequency domain analysis to extract the amplitude A1 of the fundamental wave and the amplitude A2 of the second harmonic, and the acoustic nonlinear parameter β is calculated based on the following formula: And normalize it to obtain the normalized acoustic nonlinear parameter β', Based on the normalized acoustic nonlinear parameter β' or the second harmonic, the coordinates and size of the microcracks on the pipeline are obtained.

7. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 6, characterized in that: in, In step S4, the nonlinear ultrasonic measurement system is used to measure a plurality of samples, thereby obtaining a functional relationship between the normalized acoustic nonlinear parameter β' or the second harmonic and the microcrack size. A series of notches of different sizes are prefabricated on a plurality of spare pipes of the same model as the pipe, the microcracks are prepared on the spare pipes using a three-point bending tester, and the notches are then cut off by machining, thereby forming a batch of the specimens with microcracks of different lengths.

8. The method for circumferential positioning of microcracks in a pipe based on second harmonic of ultrasonic guided waves according to claim 7, characterized in that: in, The nonlinear ultrasonic measurement system comprises a piezoelectric ceramic array for receiving the second harmonic. In step S4, nonlinear ultrasonic measurement is performed to extract the acoustic signal of each unit in the piezoelectric ceramic array and calculate the normalized acoustic nonlinear parameter β' or the distribution of the second harmonic, and establish the normalized acoustic nonlinear parameter β' or the second harmonic and the microcrack area. The relationship between coordinates and the number of microcracks, While keeping other variables unchanged, different numbers of notches are prefabricated along the circumference of the spare pipe, the microcracks are prepared by means of a three-point bending tester, and the notches are then cut off by machining to form a batch of samples with different numbers of microcracks.

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