Ultrasonic guided wave mixing method for localizing microcracks in tubular structures

Abstract

The invention discloses an ultrasonic guided wave mixing positioning method for microcracks in a tubular structure, comprising: screening a pattern pair, determining a first acoustic signal and a second acoustic signal at two ends of the tubular structure, and further acquiring a first excitation ultrasonic guided wave and a second excitation ultrasonic guided wave; adjusting the excitation time and extracting the acoustic signal; converting the acoustic signal into an electrical signal, the electrical signal passing through a bandpass filter and an oscilloscope, the oscilloscope measuring a total of several time domain signals and averaging them to obtain a time domain signal; extracting the amplitude of the first fundamental wave, the amplitude of the second fundamental wave and the amplitude of the mixing component, calculating the acoustic nonlinear parameter, and obtaining the normalized acoustic nonlinear parameter; drawing a line graph with the position of the center of the mixing area as the x-axis and the normalized acoustic nonlinear parameter as the y-axis, and judging the axial position of the microcrack according to the line graph; making the angle of the receiving point as the polar angle and the normalized acoustic nonlinear parameter as the polar diameter, drawing a polar coordinate image, and judging the circumferential position of the microcrack according to the polar coordinate image.

Description

Technical Field

[0001] The invention belongs to the technical field of ultrasonic detection, and in particular relates to an ultrasonic guided wave mixing positioning method for microcracks in a tubular structure. Background Art

[0002] As one of the five major means of transportation, pipelines are often used to transport gases, liquids or slurries such as natural gas, oil or concrete. During the service process, pressure pipelines are easily impacted or corroded by ramming, traffic loads, geological disasters or the transport medium, and microcracks will be generated. Microcracks will continue to evolve into macro cracks, causing safety accidents such as pipeline leakage. Therefore, before the macro cracks appear, the location of micro cracks and timely and precise maintenance can avoid the occurrence of safety accidents such as leakage and ensure the safe operation of pressure pipelines. Non-destructive testing technology can determine the location of cracks without destroying the pressure pipeline, including ultrasonic testing, eddy current testing, penetrant testing, magnetic particle testing and radiographic testing. Affected by skin effect and capillary effect, eddy current testing, penetrant testing and magnetic particle testing are only sensitive to surface defects or damage. In addition, radiographic testing will produce ionizing radiation. Compared with the above non-destructive testing methods, ultrasonic testing can not only detect defects or damage inside the pressure pipeline, but also does not harm the human body. Compared with the point-by-point detection of ultrasonic body waves, ultrasonic guided waves can propagate over long distances in tubular structures, achieving large-scale and high-efficiency scanning of pressure pipelines. However, traditional ultrasonic guided waves are only sensitive to macro cracks, but cannot detect micro cracks smaller than the wavelength. Second harmonics, as a typical nonlinear acoustic effect, have been used to determine the size of micro cracks. However, the acoustic nonlinear parameters of the second harmonics are the average values ​​of all acoustic nonlinear parameters between the transmitting transducer and the receiving transducer, so the second harmonics cannot determine the extent of micro cracks at a specific location, that is, the location of micro cracks is not achieved. In addition, since nonlinear ultrasonic measurement systems, coupling agents, etc. can all generate second harmonics, this method is difficult to identify the source of second harmonics. Ultrasonic guided wave mixing has certain flexibility in the selection of fundamental and mixing component frequencies, modes and propagation directions, and can overcome the above problems. In summary, the present invention will achieve the characterization and location of micro cracks by means of ultrasonic guided wave mixing in tubular structures before macro cracks appear. Summary of the invention

[0003] In order to solve the above technical problems, the present invention proposes an ultrasonic guided wave mixing positioning method for microcracks in tubular structures. Based on the mixing of axisymmetric modes, the excitation time of the two fundamental waves is adjusted to achieve overall scanning of the tubular structure, perform precise maintenance in a timely manner, avoid further evolution of microcracks and the occurrence of accidents, and ensure the safe operation of pressure pipelines.

[0004] To achieve the above object, the present invention provides an ultrasonic guided wave mixing method for locating microcracks in a tubular structure, comprising:

[0005] Based on the tubular structure to be tested, a cylindrical coordinate system is constructed to determine the frequency, period number and mode of the first fundamental wave and the second fundamental wave by means of matching conditions and dispersion curves, and the mode pair is screened;

[0006] Based on the pattern pair, determining a first acoustic signal and a second acoustic signal at two ends of the tubular structure, and further acquiring a first excitation ultrasonic guided wave and a second excitation ultrasonic guided wave;

[0007] Adjusting the excitation time to move the mixing area from the left side of the tubular structure to the right side, and extracting acoustic signals at specific positions on the left side and the right side of the pipeline;

[0008] The acoustic signal is converted into an electrical signal, the electrical signal is passed through a bandpass filter and an oscilloscope, the oscilloscope measures the time domain signal several times and averages the time domain signal to obtain a time domain signal;

[0009] Performing a fast Fourier transform on the time domain signal, carrying out frequency domain analysis, extracting the amplitude of the first fundamental wave, the amplitude of the second fundamental wave and the amplitude of the mixing component, calculating the acoustic nonlinear parameter, and normalizing the acoustic nonlinear parameter to obtain a normalized acoustic nonlinear parameter;

[0010] Let the x-axis and y-axis represent the center position of the mixing zone and the normalized acoustic nonlinear parameter, draw a line graph, and determine the axial position of the microcrack according to the line graph;

[0011] The excitation time is adjusted to make the mixing zone coincide with the microcrack, the time domain signal is received along the circumference of the pipeline, the normalized acoustic nonlinear parameter is calculated, the angle of the acoustic signal receiving point is used as the polar angle, and the normalized acoustic nonlinear parameter is used as the polar diameter, a polar coordinate image is drawn, and the circumferential position of the microcrack is determined based on the polar coordinate image.

[0012] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the method for constructing a cylindrical coordinate system includes: the origin is located at the left end surface of the first excitation transducer; the r axis is in the left end surface and takes the origin as the starting point; the z axis is along the direction of the pipeline axis and points to the second excitation transducer; is the rotation angle about the r axis.

[0013] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the specific calculation method for constructing the cylindrical coordinate system is:

[0014] According to the Lamé constant, construct the physical equation,

[0015]

[0016] Where σ and ε represent stress and strain, λ and μ represent Lame constants; considering the Lame coefficients of cylindrical coordinates, the geometric equation in the cylindrical coordinate system is established:

[0017]

[0018] Where u is the displacement, substituting the physical equation and geometric equation into the equation of motion, the calculation is as follows:

[0019]

[0020] Where ρ is the density of the material, t is the time; the inner and outer surfaces of the pipe are free, so

[0021]

[0022] Where r1 and r2 are the inner and outer diameters of the pipe.

[0023] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the method for obtaining matching conditions is:

[0024]

[0025] Among them, k a , k b , k - With k + They are the wave vectors of the first fundamental wave, the second fundamental wave, the difference frequency component and the sum frequency component respectively, a is the first fundamental wave, and b is the second fundamental wave.

[0026] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the excitation time is adjusted to move the mixing area from the left side to the right side of the tubular structure, and the intersection of the first fundamental wave and the second fundamental wave includes:

[0027]

[0028] Among them, the first fundamental wave is excited at t=0, the fundamental wave b is excited at t=t0, l is the total length of the pipeline, is the group velocity of the first fundamental wave, is the group velocity of the second fundamental wave, Z1 is the position of the first fundamental wave and the second fundamental wave at the meeting time t1,

[0029] When t=t2, the front end of the second fundamental wave meets the end of the first fundamental wave at z=Z2.

[0030]

[0031] When t=t3, the front end of the first fundamental wave meets the end of the second fundamental wave at z=Z3.

[0032]

[0033] When t=t4, the first fundamental wave and the second fundamental wave are separated at z=Z4.

[0034]

[0035] Among them, l a is the spatial length of the first fundamental wave, l b is the spatial length of the second fundamental wave.

[0036] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the method for calculating the acoustic nonlinear parameters is:

[0037]

[0038] Among them, A a is the amplitude of the first fundamental wave, A b is the amplitude of the second fundamental wave, A ± is the amplitude of the mixing component, and χ is the acoustic nonlinear parameter.

[0039] According to the ultrasonic guided wave mixing positioning method for microcracks in a tubular structure provided by the present invention, the method for obtaining the center position of the mixing area is:

[0040]

[0041] Among them, Z c is the center position of the mixing area, Z1 is the position of the first fundamental wave and the second fundamental wave at the meeting time t1, l b is the spatial length of the second fundamental wave, is the group velocity of the first fundamental wave, l a is the spatial length of the first fundamental wave, is the group velocity of the second fundamental wave.

[0042] Technical effect of the invention: The present invention discloses an ultrasonic guided wave mixing method for locating microcracks in a tubular structure. For tubular structures in service or unused tubular structures, microcracks can be detected and measures can be taken in advance before macro cracks appear or before service, so as to avoid the evolution of microcracks and the occurrence of safety accidents. Compared with the second harmonic in the pipeline, the ultrasonic guided wave mixing method for locating microcracks in the tubular structure avoids the nonlinear interference of the ultrasonic measurement system and realizes the axial and circumferential positioning of the microcracks in the pipeline. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0044] Figure 1 Schematic diagram of the mechanism of ultrasonic guided wave mixing in a tubular structure according to an embodiment of the present invention;

[0045] Figure 2 The displacement field distribution of the interaction between microcracks and ultrasonic guided wave mixing in the tubular structure of the embodiment of the present invention;

[0046] Figure 3 is a graph showing the relationship between the normalized acoustic nonlinear parameter χ' and the center position of the mixing region according to an embodiment of the present invention;

[0047] Figure 4 is a polar coordinate diagram of the normalized acoustic nonlinear parameter χ' according to an embodiment of the present invention;

[0048] Figure 5 A comparison diagram of the measured value and the actual value of the microcrack position in the tubular structure of the embodiment of the present invention;

[0049] Figure 6 The figure is a schematic flow chart of an ultrasonic guided wave mixing positioning method for microcracks in a tubular structure according to an embodiment of the present invention. DETAILED DESCRIPTION

[0050] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0051] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0052] like Figure 6 As shown, in this embodiment, an ultrasonic guided wave mixing method for locating microcracks in a tubular structure is provided, comprising: step S1, selection of a mode pair. For a tubular structure where microcracks may exist, a suitable mode pair is screened out by means of matching conditions and dispersion curves, and the frequency and period number of fundamental wave a and fundamental wave b are determined.

[0053] Step S2, excitation of ultrasonic guided wave mixing in the tubular structure. Excitation transducers a and b are arranged at both ends of the steel pipe, the frequencies and cycles of the acoustic signals a and b are determined, and the ultrasonic guided waves a and b are excited at both ends of the steel pipe by means of a signal generator, a pulse amplifier, the excitation transducers a and b.

[0054] Step S3, when the researcher delays t0 to excite fundamental wave a, the mixing area moves to the left, and when the researcher delays t0 to excite fundamental wave b, the mixing area moves to the right. The present invention changes the excitation time t0, moves the mixing area from the left side of the steel pipe to the right side of the steel pipe in turn, and extracts acoustic signals along the circumference of the pipe.

[0055] Step S4, receiving the acoustic signal. A receiving transducer is arranged between the excitation transducer a and the excitation transducer b, wherein the receiving transducer is close to the excitation transducer a. The receiving transducer converts the acoustic signal into an electrical signal, which passes through a bandpass filter and an oscilloscope. The oscilloscope measures the time domain signal 512 times and averages it, and the above signal is then stored in a USB flash drive.

[0056] Step S5: Analysis of time domain signals. Perform fast Fourier transform (FFT) on the time domain signals stored in the USB flash drive, conduct frequency domain analysis, and extract the amplitude A of the fundamental wave a. a , the amplitude of fundamental wave b is A b and the amplitude A of the mixing component ± , calculate the acoustic nonlinear parameter χ and perform normalization.

[0057] Step S6, determination of the axial position of the microcrack. Let the x-axis and y-axis represent the center position Z of the mixing zone. c and normalized acoustic nonlinear parameter χ, plotting χ and Z c The axial position of the microcrack is determined based on the line graph.

[0058] Step S7-Step S10, determination of the circumferential position of the microcrack. Adjust the excitation time so that the mixing area coincides with the microcrack, receive the time domain signal along the circumference of the pipeline, calculate the normalized acoustic nonlinear parameter, let the angle of the receiving point be the polar angle, the normalized acoustic nonlinear parameter χ be the polar diameter, draw a polar coordinate image, and determine the circumferential position of the microcrack based on the line graph.

[0059] The specific implementation process includes:

[0060] The present invention first constructs a cylindrical coordinate system in the pipeline, with the origin located at the left end surface of the excitation transducer a; the r axis is in the left end surface and takes the origin as the starting point; the z axis is along the direction of the pipeline axis and points to the excitation transducer b; is the rotation angle of the r axis (observed from the opposite direction of the z axis, the rotation direction is counterclockwise).

[0061] According to the Lamé constant, construct the physical equation,

[0062]

[0063] Where σ and ε represent stress and strain, and λ and μ represent Lame constants. Considering the Lame coefficients of cylindrical coordinates, the geometric equations in the cylindrical coordinate system are established.

[0064]

[0065] Where u is the displacement. Substitute the physical equations and geometric equations into the equations of motion, ignoring the body force terms,

[0066]

[0067] Where ρ is the density of the material and t is the time. Since the inner and outer surfaces of the pipe are free,

[0068]

[0069] Where r1 and r2 are the inner diameter and outer diameter of the pipe. According to formula (1), formula (2), formula (3) and formula (4), the dispersion equation of ultrasonic guided waves in the pipe is established, and the dispersion curves of torsional mode, longitudinal mode and bending mode are plotted.

[0070] The present invention uses contact acoustic nonlinearity to reveal the mechanism of ultrasonic guided wave mixing and microcracks. When two ultrasonic guided waves intersect and the mixing zone coincides with the microcrack, the microcrack is periodically stretched and compressed. When the microcrack is stretched and opened, the stiffness decreases and the ultrasonic guided wave cannot pass through the crack. When the microcrack is compressed and opened and closed, the stiffness is normal and the ultrasonic guided wave can pass through the crack. This phenomenon is called the clapping effect. When the frequency is f a and f b When the ultrasonic guided waves intersect, the relationship between stress and strain in the area with microcracks can be expressed as

[0071]

[0072] Among them C 2th is the stiffness of the material, H(*) is the Heaviside unit step function, ε 0 is the initial static contact strain, ε(t) = ε a cos(ω a t)+ε b cos(ω b t). The stress can be divided into a linear part σ L and the nonlinear part σ NL , where the linear part is equal to

[0073] σ L =C 2th [ε a cos(ω a t)+ε b cos(ω b t)] (7)

[0074] The nonlinear part is equal to

[0075] σ NL =H[ε a cos(ω a t)+ε b cos(ω b t)-ε0 ]ΔC[ε a cos(ω a t)+ε b cos(ω b t)] (8)

[0076] In summary, the asymmetry of stiffness leads to the appearance of difference frequency components and sum frequency components.

[0077] According to the vector operation rules, the functional relationship between the fundamental wave and the wave number of the mixing component in ultrasonic guided wave mixing is constructed, that is, the matching criterion

[0078]

[0079] where k a , k b , k - With k + They are the wave vectors of fundamental wave a, fundamental wave b, difference frequency component and sum frequency component respectively.

[0080] With the help of matching condition (9) and dispersion curve, the appropriate mode pair is selected. a -f b and the sum frequency f a +f b The fundamental frequency f should be avoided a or fundamental frequency f b multiples of.

[0081] The present invention changes the position of the mixing zone by adjusting the excitation time of the two fundamental waves, moves the mixing zone along the z-axis, and realizes the overall scanning of the pipeline. Assuming that the total length of the pipeline is l, the fundamental waves a and b are non-dispersive, and the spatial lengths are l a , l b The time t1 and position Z1 when the two fundamental waves meet are:

[0082]

[0083] Among them, fundamental wave a is excited at time t=0, fundamental wave b is excited at time t=t0, and the group velocities of fundamental wave a and fundamental wave b are respectively and

[0084] When t=t2, the front end of fundamental wave b meets the end of fundamental wave a at z=Z2.

[0085]

[0086] When t=t3, the front end of fundamental wave a meets the end of fundamental wave b at z=Z3.

[0087]

[0088] When t=t4, fundamental wave a and fundamental wave b are separated at z=Z4.

[0089]

[0090] Therefore, Z2 and Z3 are the left and right sides of the mixing zone, respectively, and the spatial length ΔZ of the mixing zone is equal to

[0091]

[0092]

[0093] Where Z c It is the center position of the mixing area.

[0094] The nonlinear ultrasonic measurement system involved in the present invention consists of two sets of ultrasonic signal excitation modules and one set of ultrasonic signal receiving modules, and is used to carry out research on ultrasonic guided wave mixing in tubular structures, wherein the signal excitation module consists of a signal generator, a pulse amplifier and an excitation transducer, and the ultrasonic signal receiving module consists of a receiving transducer, a filter and an oscilloscope.

[0095] The present invention is based on the fundamental frequency f a , number of cycles, add Hanning window, design excitation signal a, according to the fundamental frequency f b , number of cycles, add Hanning window, and design excitation signal b. The present invention generates excitation signal a with the help of signal generator a. The signal passes through the pulse amplifier and the excitation transducer, and the converted vibration signal forms ultrasonic guided wave a in the tubular structure. A similar method is also used to excite ultrasonic guided wave b. The two fundamental waves are mixed in a specific area, and the mixed components propagate along the positive or reverse direction of the z-axis. The present invention arranges a receiving transducer on the propagation path of the mixed components. The excitation transducer can be a piezoelectric ceramic array or a piezoelectric ceramic tube, etc., and the receiving transducer can be a piezoelectric ceramic array, a piezoelectric ceramic tube or a laser vibrometer, etc.

[0096] The receiving transducer of the present invention converts the acoustic signal of the mixed frequency component into an electrical signal. The mixed frequency component passes through a bandpass filter and an oscilloscope. The oscilloscope measures the time domain signal 512 times and averages it. The above signal is then stored in a USB flash drive. The present invention then performs coupling, performs 3-5 measurements and stores data, performs fast Fourier transform (FFT) on the time domain signal, performs frequency domain analysis, and extracts the amplitude A of the fundamental wave a. a , the amplitude of fundamental wave b is A b and the amplitude A of the mixing component ± , according to formula (20), the acoustic nonlinear parameter χ is calculated and normalized.

[0097]

[0098] The present invention utilizes machining and a three-point bending test machine to prepare a batch of original samples and samples with microcracks, conducts research on ultrasonic guided wave mixing, performs normalization processing on the acoustic nonlinear parameters of the original samples, and uses the normalized acoustic nonlinear parameter χ to determine the presence or absence of microcracks.

[0099] The present invention aims at a sample with microcracks, excites ultrasonic guided wave mixing in a tubular structure, extracts acoustic signals along the circumference of the pipeline, calculates normalized acoustic nonlinear parameters, takes the angle of the receiving point as the polar angle, and the normalized acoustic nonlinear parameter χ as the polar diameter, draws a polar coordinate image, constructs the relationship between the normalized acoustic nonlinear parameter χ and the microcrack position (along the circumference of the pipeline), and determines the microcrack position. coordinate.

[0100] The present invention also adjusts the position of the mixing zone by changing the excitation time of the two fundamental waves, so that the mixing zone moves along the z-axis, and records the center position Z of the mixing zone. c , extract the acoustic signal and calculate the normalized acoustic nonlinear parameters, let the x-axis and y-axis represent the mixing zone position Z c and normalized acoustic nonlinear parameter χ, plotting χ and Z c The z coordinate of the microcrack is determined based on the relationship between the normalized acoustic nonlinear parameter and the microcrack and the line graph.

[0101] In summary, the present invention uses ultrasonic guided wave mixing in a tubular structure to obtain the z, Coordinates and other information are used to locate and characterize microcracks in pressure pipelines.

[0102] Specific examples of the present invention include:

[0103] The present invention verifies the above method by means of numerical simulation. Finite element simulation software is used to construct a steel pipe with microcracks, stimulate ultrasonic guided wave mixing, extract time domain signals and perform data analysis. The microcracks are determined based on the difference frequency or sum frequency components and their normalized acoustic nonlinear parameters χ'. coordinates and Z coordinates to locate micro cracks in the pipeline. Figure 1 The specific contents of the implementation case are as follows.

[0104] The length, outer diameter and thickness of the steel pipe are 3000mm, 30mm and 2mm respectively, and the density, Young's modulus and Poisson's ratio are 7860kg / m 3 , 1.996×10 11 Pa and 0.29. The present invention constructs a cylindrical coordinate system in a horizontally placed steel pipe, with the origin being the center of the circle on the left side of the steel pipe and the z-axis pointing to the right side of the steel pipe. is the corner of a specific radius on the left side of the steel pipe. According to the nonlinear theory of contact acoustics, the present invention uses the contact acoustics to generate a contact acoustics with a specific radius of 1500 mm and 1500 mm. The long axis of the resection is 4×10 -3 m, short axis is 8×10 -8 The microcracks in the steel pipe are simulated by using an elliptical cylinder with a diameter of 2 mm and a height of 2 mm and setting specific boundary conditions.

[0105] The present invention selects the torsional mode T (0, 1) as the fundamental wave, and excites the fundamental wave a and the fundamental wave b in the form of displacement on the left and right sides of the steel pipe to carry out research on ultrasonic guided wave mixing, wherein the excitation signal of the fundamental wave a is a Hanning windowed sinusoidal signal with 14 cycles and a center frequency of 70 kHz, and the excitation signal of the fundamental wave b is a Hanning windowed sinusoidal signal with 22 cycles and a center frequency of 110 kHz.

[0106] The time step is set to 1×10 -8 s. The steel pipe is divided into region I (z = 0-1400mm), region II (z = 1400-1600mm) and region III (z = 1600-3000mm), where the mesh properties of region I and region III are tetrahedral free with a maximum size of 1mm, and the mesh properties of region II are hexahedral structure with a maximum size of 0.5mm. When microcracks interact with ultrasonic guided wave mixing, the displacement field distribution is as follows Figure 2 shown.

[0107] The present invention changes the position of the mixing region by adjusting the time delay t0, and t0 is set to 0s, 1×10 -4 s, 2×10 -4 s and 3×10 -4 s, where the step size of t0 is 1×10 -4 s. When the researcher delays t0 to excite fundamental wave a, the mixing zone moves to the left, and when the researcher delays t0 to excite fundamental wave b, the mixing zone moves to the right. The present invention calculates the length of the mixing zone, the left side of the mixing zone, the center of the mixing zone, and the right side of the mixing zone based on formula (10) to formula (19) and t0.

[0108] The present invention z=1150mm and Receive the time domain signal at z=1850mm, perform fast Fourier transform, and extract the amplitude A of the fundamental wave a a , the amplitude of fundamental wave b is A b and the amplitude of the mixing component A ± , calculate the acoustic nonlinear parameter χ according to formula (20), and normalize the acoustic nonlinear parameter χ (about the maximum value of the acoustic nonlinear parameter χ max), and obtain the normalized acoustic nonlinear parameter χ'. The present invention uses the position of the center of the mixing area as the x-axis and the normalized acoustic nonlinear parameter χ' as the y-axis to draw a line graph, such as Figure 3 shown.

[0109] In addition, the present invention sets t0 equal to 0s, and makes the mixing zone coincide with the microcrack. At z=1150mm and z=1850mm (located outside the mixing zone), At the beginning, a receiving point is arranged every 15°, with a total of 24 receiving points. The acoustic nonlinear parameter χ is extracted according to the above method. The maximum value of the acoustic nonlinear parameter χ max , perform normalization and obtain the normalized acoustic nonlinear parameter χ', let and the normalized acoustic nonlinear parameter χ' as the polar angle and polar diameter, and a polar coordinate diagram is drawn, as shown in Figure 4 shown.

[0110] The present invention is based on Figure 3 The peak position of the microcrack is determined by Figure 4 The symmetry axis in the microcrack is determined by Coordinates. It should be emphasized that Figure 4 The pattern in the figure is not completely symmetrical. The present invention can only roughly draw the symmetry axis of the pattern. Reducing the step size of t0 can more accurately determine the z coordinate of the microcrack, and increasing the number of receiving points on the circumference can improve the microcrack. In addition, for the normalized acoustic nonlinear parameter χ' of the difference frequency component, the coordinates corresponding to the intersection of the raised part of the pattern and the symmetry axis are the coordinates of the microcrack. The coordinates of the normalized acoustic nonlinear parameter χ' of the sum frequency component are the coordinates of the intersection of the concave part of the pattern and the symmetry axis. coordinate.

[0111] In summary, the present invention can locate microcracks in pipelines and determine the microcracks according to the counter-mixing of T(0,1) mode. coordinates and z coordinates, where The measured value of the coordinates differs from the actual value by 15°, and the measured value of the z coordinate is basically consistent with the actual value. Figure 5 shown.

[0112] The present invention discloses an ultrasonic guided wave mixing method for locating microcracks in a tubular structure. For a tubular structure in service or an unused tubular structure, microcracks are detected and measures are taken in advance before macro cracks appear or before service, so as to avoid the evolution of microcracks and the occurrence of safety accidents. Compared with the second harmonic in the pipeline, the ultrasonic guided wave mixing method for locating microcracks in the tubular structure avoids the nonlinear interference of the ultrasonic measurement system and realizes the axial and circumferential positioning of the microcracks in the pipeline.

[0113] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An ultrasonic guided wave mixing method for locating microcracks in a tubular structure, characterized in that: include: Based on the tubular structure to be tested, a cylindrical coordinate system is constructed to determine the frequency, period number and mode of the first fundamental wave and the second fundamental wave by means of matching conditions and dispersion curves, and the mode pair is screened; Based on the pattern pair, determining a first acoustic signal and a second acoustic signal at two ends of the tubular structure, and further acquiring a first excitation ultrasonic guided wave and a second excitation ultrasonic guided wave; Adjusting the excitation time to move the mixing region from the left side to the right side of the tubular structure, and extracting acoustic signals on the left side of the pipe and the left side of the microcrack, and on the right side of the pipe and the right side of the microcrack; The excitation time is adjusted to move the mixing region from the left side to the right side of the tubular structure, and the intersection of the first fundamental wave and the second fundamental wave includes: (1) (2) Among them, the first fundamental wave is excited at t=0, the second fundamental wave is excited at t=t0, l is the total length of the pipeline, is the group velocity of the first fundamental wave, is the group velocity of the second fundamental wave, is the position of the first fundamental wave and the second fundamental wave at the encounter time t1, When t=t2, the front end of the second fundamental wave meets the end of the first fundamental wave at z = Z2. (3) (4) When t=t3, the front end of the first fundamental wave meets the end of the second fundamental wave at z = Z3. (5) (6) When t=t4, the first fundamental wave and the second fundamental wave are separated at z=Z4. (7) (8) in, is the spatial length of the first fundamental wave, is the spatial length of the second fundamental wave; The method to obtain the center position of the mixing area is: (9) in, is the center position of the mixing area, is the position of the first fundamental wave and the second fundamental wave at the encounter time t1, is the spatial length of the second fundamental wave, is the group velocity of the first fundamental wave, is the spatial length of the first fundamental wave, is the group velocity of the second fundamental wave; The acoustic signal is converted into an electrical signal, the electrical signal is passed through a bandpass filter and an oscilloscope, the oscilloscope measures the time domain signal several times and averages the time domain signal to obtain a time domain signal; Performing a fast Fourier transform on the time domain signal, carrying out frequency domain analysis, extracting the amplitude of the first fundamental wave, the amplitude of the second fundamental wave and the amplitude of the mixing component, calculating the acoustic nonlinear parameter, and normalizing the acoustic nonlinear parameter to obtain a normalized acoustic nonlinear parameter; Let the x-axis and y-axis represent the center position of the mixing zone and the normalized acoustic nonlinear parameter, draw a line graph, and determine the axial position of the microcrack according to the line graph; The excitation time is adjusted to make the mixing zone coincide with the microcrack, the time domain signal is received along the circumference of the pipeline, the normalized acoustic nonlinear parameter is calculated, the angle of the acoustic signal receiving point is used as the polar angle, and the normalized acoustic nonlinear parameter is used as the polar diameter, a polar coordinate image is drawn, and the circumferential position of the microcrack is determined based on the polar coordinate image.

2. The ultrasonic guided wave mixing method for locating microcracks in a tubular structure according to claim 1, characterized in that: The method for constructing a cylindrical coordinate system includes: the origin is located at the left end surface of the first excitation transducer; the r axis is in the left end surface and starts from the origin; the z axis is along the direction of the pipeline axis and points to the second excitation transducer; φ is the rotation angle of the r axis.

3. The ultrasonic guided wave mixing method for locating microcracks in a tubular structure according to claim 1, characterized in that: The specific calculation method for constructing the cylindrical coordinate system is: According to the Lamé constant, construct the physical equation, (10) Where σ and ε represent stress and strain, λ and μ represent Lame constants; considering the Lame coefficients of cylindrical coordinates, the geometric equation in the cylindrical coordinate system is established: (11) Where u is the displacement, substituting the physical equation and geometric equation into the equation of motion, the calculation is as follows: (12) Where ρ is the density of the material, t is the time; the inner and outer surfaces of the pipe are free, so (13) Where r1 and r2 are the inner and outer diameters of the pipe.

4. The ultrasonic guided wave mixing method for locating microcracks in a tubular structure according to claim 1, characterized in that: The method to obtain matching conditions is: (14) Among them, k a , k b , k - With k + They are the wave vectors of the first fundamental wave, the second fundamental wave, the difference frequency component and the sum frequency component respectively, a is the first fundamental wave, and b is the second fundamental wave.

5. The ultrasonic guided wave mixing method for locating microcracks in a tubular structure according to claim 1, characterized in that: The method for calculating acoustic nonlinear parameters is: (15) Among them, A a is the amplitude of the first fundamental wave, A b is the amplitude of the second fundamental wave, A ± is the amplitude of the mixing component, and χ is the acoustic nonlinear parameter.

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