Metal pipeline defect detection method based on time-frequency domain electromagnetic method

Abstract

The invention provides a metal pipeline defect detection method based on a time-frequency domain electromagnetic method, which comprises the following steps of: (1) establishing a time-frequency domain dual-mode electromagnetic sensor test system with parallel time-frequency domain coils, and respectively measuring and acquiring electromagnetic response parameters through two modes; (2) measuring by a frequency domain method to obtain a function relationship of the depth dn of the near-surface small-size defect with respect to the surface size r of the small-size defect and an induced voltage difference value Ufd; (3) measuring by a time domain method to obtain a function relationship between the attenuation rate k and the thickness D; and (4) performing detection optimization on different types of defects through data fusion of a time domain method and a frequency domain method.

Description

Technical Field

[0001] The invention belongs to the technical field of non-destructive testing and relates to a measuring method based on the electromagnetic induction principle, which is used for non-contact measurement of defects near and far surfaces of metals. Background Art

[0002] For marine or buried oil and gas pipelines, the inner and outer surfaces of the pipelines are affected by the transport medium and the environment, and are prone to corrosion damage to varying degrees, affecting the transportation safety and service life of the pipelines. Due to the complex external environment of the pipeline, inspectors hope to be able to carry defect detection sensors on pipeline robots to measure defects inside the pipeline. Traditional defect detection methods have their own limitations and scope of application, and are difficult to detect defects with large size spans and far-surface defects.

[0003] The eddy current electromagnetic method is based on the principle of electromagnetic induction. When a coil with an alternating current is close to the metal specimen to be tested, an alternating electromagnetic field will be generated around the coil. This electromagnetic field will produce changing eddy currents on the surface of the specimen to be tested, and the eddy currents will produce a magnetic field that hinders the change of the coil's magnetic field. This process will cause the impedance of the eddy current probe to change. This change is related to the electrical conductivity, magnetic permeability and surface state of the specimen to be tested. When there are defects on the surface of the metal specimen to be tested, its electrical conductivity and magnetic permeability will change, which in turn affects the distribution of eddy currents at that location. By detecting the change in eddy current impedance or the amplitude of the probe, the surface state of the specimen to be tested can be detected. Due to the advantages of non-contact, non-invasive, low-cost and portability, the eddy current detection method has received widespread attention in the detection of metal pipeline defects.

[0004] Eddy current electromagnetic method mainly includes time domain electromagnetic method (i.e. transient electromagnetic method TEM) and frequency domain electromagnetic method (FDEM). The difference is that in frequency domain electromagnetic method, one or more harmonics of frequency are used for excitation to obtain secondary induced magnetic field, and the size, location and shape of defects are determined by analyzing the impedance change or the amplitude and phase of the induced voltage signal; while transient electromagnetic method obtains attenuated secondary induced magnetic field through pulse or square wave excitation, and detects defects in metal pipes by analyzing the change of induced voltage signal over time.

[0005] However, transient electromagnetic method and frequency domain electromagnetic method each have their limitations. Transient electromagnetic method can detect large-area corrosion defects, but it is difficult to detect defects smaller than the probe size; frequency domain electromagnetic method can only characterize the location and size of small-sized defects on the metal surface, and cannot measure large-area defects and far-surface defects. Considering the complexity of metal pipeline defect characteristics, dual-mode measurement and data fusion combined with time domain and frequency domain can measure defects of different sizes, and can also characterize far-surface defects, thereby obtaining more comprehensive defect information. Summary of the invention

[0006] The present invention designs a metal defect detection method based on the time-frequency domain electromagnetic method, which measures the metal pipe or metal plate to be tested by the time-domain method and the frequency-domain method respectively, obtains the electromagnetic response parameters in the time-domain and frequency-domain, fuses the time-frequency domain data, and classifies the defect signal characteristics of different scales and near and far surfaces, thereby inverting the defects, realizing a detection method that is applicable to a wide range of sizes and can measure far-surface defects. Its technical solution is as follows:

[0007] A metal pipeline defect detection method based on time-frequency domain electromagnetic method comprises the following steps:

[0008] (1) Establish a time-frequency domain dual-mode electromagnetic sensor test system with parallel time-frequency domain coils, and obtain electromagnetic response parameters by measuring the two modes respectively;

[0009] (2) Frequency domain method measurement to obtain the depth d of small size defects near the surface n On the surface size r of small defect and the induced voltage difference U fd Functional relationship of

[0010] (3) Using the time domain method to measure, we can obtain the functional relationship between the attenuation rate k and the thickness D;

[0011] (4) Data fusion of time domain method and frequency domain method for detection optimization of different types of defects:

[0012] 1) Calibration parameter U fdmin , that is, there is a minimum voltage difference near the surface defect. During the scanning process, when U fd >U fdmin When , it is judged that there is a near-surface defect at the corresponding position, otherwise, it is judged that there is no near-surface defect;

[0013] 2) Using the frequency domain method to measure U fd >U fdmin The coordinate range of the near-surface defect is obtained by the surface size r, and the diameter of the frequency domain receiving coil is set to d coil , there are two cases:

[0014] (a) According to the size of the frequency domain receiving coil, determine the threshold coefficient h, if r≤hd coil , then the near-surface defect is considered to be a small-size defect, and the near-surface defect depth is d obtained by frequency domain method measurement. n ;

[0015] For a position with no defects or only small defects on the near surface, the defect depth on the far surface is:

[0016] d f =DD t

[0017] Among them, d fis the depth of the far-surface defect of the metal pipe, D is the standard thickness of the defect-free metal pipe, and D t The thickness of the metal pipe at the position detected by the time domain method;

[0018] (b) If r>hd coil , then the near-surface defect is considered to be a large-size defect, and the near-surface defect depth is calculated based on the time domain method measurement results:

[0019] d n =DD t -d f

[0020] Among them, d n is the depth of the near-surface defect of the metal pipe, D is the standard thickness of the defect-free metal pipe, and D t is the thickness of the metal pipe at that location detected by the time domain method; d f is the depth of the defect on the far surface of the metal pipe, and the d at this position is determined by the continuity of the defect. f Same as nearby locations.

[0021] Furthermore, the method of step (2) is:

[0022] For the metal pipe to be tested, the changes in the induced voltage of the frequency domain receiving coil at different positions are obtained by scanning. If there is a near-surface defect, the induced voltage difference U fd The voltage changes with the scanning coordinates. The extreme point of the voltage change during the scanning process is the defect center position. According to the induced voltage difference U fd The changing coordinate range determines the defect surface size;

[0023] Assume the depth of the small defect near the surface is d n , whose surface size is r, measures the near-surface small size of different surface sizes and depths, and collects the corresponding induced voltage difference U fd Calibrate and fit the depth d of small defects near the surface n On the surface size r of small defect and the induced voltage difference U fd The functional relationship of .

[0024] Furthermore, in step (2), the induced voltage difference U of the frequency domain receiving coil is obtained. fd The method is:

[0025] Determine the standard thickness of a metal pipe without defects. Assume that the induced voltage value of the frequency domain receiving coil is U st ;

[0026] For the metal pipe to be tested, the induced voltage value of the frequency domain receiving coil is assumed to be U f ;

[0027] The induced voltage difference between the two is U fd =U f -U st .

[0028] Furthermore, the depth d of the small-sized defect near the surface n On the surface size r of small defect and the induced voltage difference U fd The functional relationship is:

[0029]

[0030] In the formula, the constant a ij is the fitting coefficient, m, n are U fd and the highest power of r.

[0031] Furthermore, in the frequency domain measurement of step (2), the frequency domain coil is composed of an excitation coil and a receiving coil, and is scanned along the extension direction of the pipeline at a certain height from the pipe wall; during detection, the signal generator gives the excitation coil a high-frequency sinusoidal alternating signal to excite the harmonic magnetic field, forming an induced eddy current in the metal pipe under test, and the secondary induced magnetic field generated by the eddy current forms a secondary induced current in the receiving coil, and the induced voltage amplitude of the receiving coil is measured;

[0032] Furthermore, the method of step (3) is:

[0033] Assuming the attenuation rate of the induced voltage signal in the time domain is k, and the thickness of the metal pipe is D, the metal pipes of different thicknesses are measured, the corresponding attenuation rates are calibrated, and the functional relationship between the attenuation rate k and the thickness D is obtained by fitting;

[0034] The time domain method coil is used to scan the metal pipe to be tested, and the attenuation rate k at different positions is obtained. t , and then calculate the thickness D of the metal pipe at different positions t .

[0035] Furthermore, the functional relationship between the attenuation rate k and the thickness D in step (3) is:

[0036] D=g(k)=ak b

[0037] Where a and b are fitting coefficients.

[0038] Furthermore, in step (3), the time domain method coil is composed of an excitation coil and a receiving coil, with the excitation coil inside and the receiving coil outside, and is scanned along the extension direction of the pipeline while maintaining a certain lifting height from the pipe wall; during detection, the signal generator gives a low-frequency pulse signal to the excitation coil to excite the decaying pulse induced magnetic field, forming an induced eddy current in the metal pipeline under test, and the secondary induced magnetic field generated by the eddy current forms a secondary induced current in the receiving coil, and a curve of the induced voltage signal of the receiving coil decaying with time is measured.

[0039] Furthermore, h=2.

[0040] The present invention adopts the above technical solution and has the following advantages:

[0041] (1) Using the information from dual-mode measurements of time-domain electromagnetic and frequency-domain electromagnetic, a time-domain and frequency-domain electromagnetic measurement model for metal pipeline defects was established, which enables the measurement of near-surface defects and large-size defects far from the surface, thus broadening the detectable range compared with traditional detection methods.

[0042] (2) Through the organic fusion of time-frequency domain electromagnetic measurement data, it is possible to measure the situation where defects exist on both the near surface and the far surface at the same time, and obtain a more complete and comprehensive metal pipeline defect thickness distribution map.

[0043] (3) The parameter information obtained by the two modes of the time-frequency domain electromagnetic method can compensate each other, thereby greatly improving the reliability and accuracy of the detection data. This method is simple to operate, has good portability, and can be measured online. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a structural block diagram of the metal pipeline defect detection system using the time-frequency dual-mode electromagnetic method provided by the present invention.

[0045] Figure 2 This is a schematic diagram of the sensor coil structure for measuring metal specimens using the time domain and frequency domain coils of the measurement method of the present invention.

[0046] Figure 3 A curve diagram showing the influence of defect depth on detection voltage difference in frequency domain electromagnetic method is provided for an embodiment of the present invention.

[0047] Figure 4 A detection voltage curve diagram of test pieces with different thicknesses in a time domain electromagnetic method is provided for an embodiment of the present invention.

[0048] Figure 5 A schematic diagram is provided for an embodiment of the present invention when large-area near-surface defects and far-surface defects exist simultaneously.

[0049] Figure 6 A flow chart of detecting defects of metal specimens using a time-frequency dual-mode electromagnetic method is provided for an embodiment of the present invention. DETAILED DESCRIPTION

[0050] The present invention proposes a metal pipeline defect detection method based on time-frequency domain electromagnetic method, which is suitable for the detection of large-diameter ferromagnetic metal pipelines or flat plates. Based on the theoretical model of measuring metal pipeline defects by frequency domain electromagnetic method and time domain electromagnetic method, a metal pipeline defect detection model of time-frequency domain electromagnetic method is obtained through experimental calibration and data fusion. The present invention is described in detail below in conjunction with the accompanying drawings and embodiments:

[0051] Figure 1 It is a structural diagram of a metal specimen defect detection system based on the time-frequency domain electromagnetic method, including a signal generating device, a power amplifier, a coil, a signal processing module, a data acquisition module and a parameter calculation module;

[0052] Figure 2 (a) and (b) are schematic diagrams of the spatial distribution of the metal specimen measured by the frequency domain and time domain coils, respectively. The excitation coil and receiving coil used in the frequency domain method have the same structure, with about 100 turns; the excitation coil in the time domain method is inside and the receiving coil is outside, with 800 and 1000 turns respectively.

[0053] Figure 6 This is a flow chart of the time-frequency dual-mode electromagnetic method for detecting defects in metal specimens. The following is a specific description of the metal pipeline defect detection method based on the time-frequency domain electromagnetic method.

[0054] The detection method includes the following steps:

[0055] Step 1: The theoretical model of frequency domain electromagnetic method for detecting small-sized defects near the surface is

[0056]

[0057] Among them, d n is the depth of the defect near the surface of the metal pipe, U fd is the difference between the frequency domain detection voltage at this position and the detection voltage of the defect-free standard specimen, r is the size of the near-surface defect, a ij It is a constant coefficient calibrated by optimization algorithms such as the least squares method.

[0058] The theoretical model of the time-domain electromagnetic method for detecting the thickness of ferromagnetic metal specimens can be found in the paper: Chen Xingle, Lei Yinzhao. Time-domain approximation of the induced voltage of the pulsed eddy current field on a ferromagnetic plate[J]. Transactions of the Chinese Society of Electrotechnical Engineering, 2015, 30(10): 14-19. DOI: 10.19595 / j.cnki.1000-6753.tces.2015.10.003.

[0059] The theoretical model is:

[0060]

[0061] Among them, u e (t) is the time domain solution of the eddy current induced voltage of the receiving coil under the falling edge pulse excitation, i′ f (t) is the time derivative of the falling edge of the excitation current, “*” indicates the convolution operation on time, λ i is the ith positive root of the transcendental equation J1(ρx)=0, ξ tk and ck It is the equation tanξ+2ξ / μ r λ i D = 0 and cosξ-2ξ / μ r λ i D=kth positive root of 0. C d (λ i ) and C p (λ i ) are the coil coefficients of the excitation coil and the detection coil respectively. D, ρ, σ, μ are the thickness, density, electrical conductivity and magnetic permeability of the ferromagnetic flat specimen respectively, where μ = μ0μ r , μ0, μ r are the vacuum permeability and relative permeability respectively.

[0062] When the relative magnetic permeability μ r >>1, and the eddy current diffuses to the far surface of the specimen (late stage, t>>0), by simplifying equations (2) and (3), we can get the approximate equation of the late induced voltage signal:

[0063]

[0064] in,

[0065]

[0066] τ d =μσD 2 / π 2 (6)

[0067] From equations (5) and (6), we can see that U e is a function related to the material and the lift-off height, and has nothing to do with time, τ d is the eddy current diffusion time constant of the defined ferromagnetic plate. Take the logarithm of both sides of equation (6) and then take the derivative with respect to time, then take the absolute value and substitute equation (6) to obtain:

[0068]

[0069] Where k is the later attenuation rate of the induced voltage signal, which is represented by the absolute value of the signal attenuation slope in a single logarithmic coordinate system, π 2 / μσ is a fixed value, so the theoretical model of metal specimen thickness and attenuation rate can be obtained

[0070] D=g(k)=ak b (8)

[0071] Where D is the thickness of the ferromagnetic metal specimen, k is the later attenuation rate of the induced voltage signal, and a and b are constants measured by experimental calibration.

[0072] Step 2: Measure the standard metal specimen through frequency domain electromagnetic method and time domain electromagnetic method to obtain experimental calibration parameters.

[0073] Schematic diagram of frequency domain electromagnetic method detection Figure 2 (a) As shown. The coil and the metal specimen to be tested are kept at a certain lifting height, and a sinusoidal signal with an amplitude of 10V and a frequency of 100kHz is passed through the excitation coil. The induced magnetic field generated by it generates eddy currents in the metal specimen to be tested. The secondary induced magnetic field generated by the eddy currents generates an induced voltage signal in the receiving coil. The differential signal at both ends of the receiving coil is input into the signal processing module, and after collection, it is analyzed and processed to obtain the frequency domain detection voltage at that position.

[0074] The frequency domain electromagnetic method is used to measure the defect-free standard thickness metal specimen and obtain the standard frequency domain detection voltage value U st A series of metal specimens with near-surface defects of different sizes and depths were made, and the corresponding detection voltage value U was obtained by frequency domain electromagnetic method. f , and then get the induced voltage difference

[0075] U fd =U f -U st (9)

[0076] Through experiments, we get U fd With different sizes r, depth d n The defect relationship correspondence table is used to fit the function, so as to calculate the formula (1) d n =f(U fd ,r) specific expression. Figure 3 The figure shows the effect of defect depth on the detection voltage difference in the frequency domain electromagnetic method obtained by COMSOL finite element simulation, where the horizontal axis is the near-surface defect depth d n , the vertical axis is the induced voltage difference U fd , the defects used in the simulation are circular defects with the same surface size r.

[0077] The schematic diagram of time domain electromagnetic method detection is as follows Figure 2(b) As shown. The coil and the metal specimen to be tested are kept at a certain lifting height, and a pulse signal with an amplitude of 50V, a frequency of 1Hz, and a duty cycle of 10% is passed through the excitation coil, which generates a decaying pulse induced magnetic field, and a decaying eddy current is generated in the metal specimen to be tested. The secondary induced magnetic field generated by the eddy current generates a rapidly decaying induced voltage signal in the receiving coil. The differential signal at both ends of the receiving coil is input into the signal processing module, and after collection, it is analyzed and processed to obtain the time domain detection voltage change curve of the position over time.

[0078] Defect-free metal specimens of different standard thicknesses were measured using time domain electromagnetic method. Figure 4 The figure shows the curve of the induced voltage changing with time during time-domain electromagnetic method detection obtained by COMSOL finite element simulation. The four curves are the detection voltage curves when the thickness of the metal specimen to be tested is 10, 12, 14, and 16 mm. Figure 4 It can be seen that when the induced voltage decays to the late stage, it decays approximately linearly in the single logarithmic coordinate system. The absolute value of the slope of the signal in the late stage in the single logarithmic coordinate system is taken as the decay rate k, and the corresponding relationship between the decay rate k and the thickness D is obtained. The parameters a and b in formula (8) are obtained by fitting.

[0079] Step 3: Measure the metal specimen to be tested by time domain electromagnetic method. Use the time domain electromagnetic method coil to scan the test piece to obtain the attenuation rate k at different positions t , and then calculate the thickness D of the metal specimen at different positions t =g(k t ). In the time-domain electromagnetic method, the influence of small defects near the surface on the attenuation rate is extremely small and can be ignored, that is, the thickness change caused by large-area corrosion thinning is measured.

[0080] Step 4: Measure the metal specimen to be tested by frequency domain electromagnetic method. Use the frequency domain electromagnetic method coil to scan the test piece to obtain the detection voltage difference U at different positions. fd . Calibrate a parameter U fdmin is the minimum voltage difference at which a near-surface defect is considered to exist, U fdmin Determined by the detection sensitivity of the device. fd >U fdmin When , there is a near-surface defect at this position, otherwise there is no near-surface defect. fd >U fdmin The coordinate range of the near-surface defect is r, and the diameter of the frequency domain coil is d coil .

[0081] Step 5: Based on the data obtained by time domain electromagnetic method and frequency domain electromagnetic method, the following classification discussion is carried out:

[0082] (a) If r≤2d coil, then the near-surface defect at this position is considered to be a small-size defect, and the near-surface defect depth d n Determined by formula (1). For the position where there is no defect on the near surface or only small defects, the defect depth on the far surface is:

[0083] d f =DD t (10)

[0084] Among them, d f is the depth of the far-surface defect of the metal specimen, D is the standard thickness of the defect-free metal specimen, and D t The thickness of the metal specimen at this position is obtained by time domain method.

[0085] (b) If r>2d coil , then the near-surface defect at this position is considered to be a large-size defect. Figure 5 The example of the coexistence of large near-surface defects and far-surface defects is discussed. Figure 5 As shown, the standard thickness D = 10 mm, there is a large area near-surface defect with a depth of 2 mm in the bd section, and there is a large area far-surface defect with a depth of 2 mm in the ce section.

[0086] During the dual-mode detection in the time-domain and frequency-domain, the thickness D of each position is determined after the time-domain method is used. t , that is, the ab and ef segments are 10 mm thick, the bc and de segments are 8 mm thick, and the cd segment is 6 mm thick.

[0087] Section bc: The frequency domain method determined that there was a large area near-surface defect in section bd, and there was no far-surface defect in section ab. According to the continuity of large-area corrosion defects, it was considered that section bc should be a large area near-surface defect. n =DD t =2mm.

[0088] cd segment: the cd frequency domain detection voltage has no significant change compared with the bc segment (the difference is less than U fdmin ), indicating that the thickness of the near-surface defect in segment cd does not change compared with segment bc, that is, the thickness change is caused by the far-surface defect. n =2mm,d f =DD t -d n =2mm.

[0089] de segment: The frequency domain method determines that there is no large area near-surface defect in de segment, so the thickness change of de segment is caused by the far-surface defect, that is, d f =DD t =2mm.

[0090] Among them, d nis the depth of the defect near the surface of the metal specimen, d f is the depth of the far-surface defect of the metal specimen, D is the standard thickness of the defect-free metal pipe, and D t The thickness of the metal pipe at this location is detected by the time domain method.

Claims

1. A metal pipeline defect detection method based on time-frequency domain electromagnetic method, comprising the following steps: (1) Establish a time-frequency domain dual-mode electromagnetic sensor test system with parallel time-frequency domain coils, and obtain electromagnetic response parameters by measuring the two modes respectively; (2) Frequency domain method measurement to obtain the depth d of small size defects near the surface n On the surface size r of small defect and the induced voltage difference U fd Functional relationship of (3) Using the time domain method to measure, we can obtain the functional relationship between the attenuation rate k and the thickness D; (4) Data fusion of time domain method and frequency domain method for detection optimization of different types of defects: 1) Calibration parameter U fdmin , that is, there is a minimum voltage difference near the surface defect. During the scanning process, when U fd >U fdmin When , it is judged that there is a near-surface defect at the corresponding position, otherwise, it is judged that there is no near-surface defect; 2) Using the frequency domain method to measure U fd >U fdmin The coordinate range of the near-surface defect is obtained by the surface size r, and the diameter of the frequency domain receiving coil is set to d coil , there are two cases: (a) According to the size of the frequency domain receiving coil, determine the threshold coefficient h, if r≤hd coil , then the near-surface defect is considered to be a small-size defect, and the near-surface defect depth is d obtained by frequency domain method measurement. n ; For a position with no defects or only small defects on the near surface, the defect depth on the far surface is: d f =D-D t in, d f is the depth of the far-surface defect of the metal pipe, D is the standard thickness of the defect-free metal pipe, and D t The thickness of the metal pipe at the position detected by the time domain method; (b) If r>hd coil , then the near-surface defect is considered to be a large-size defect, and the near-surface defect depth is calculated based on the time domain method measurement results: d n =D-D t -d f Among them, d n is the depth of the near-surface defect of the metal pipe, D is the standard thickness of the defect-free metal pipe, and D t is the thickness of the metal pipe at that location detected by the time domain method; d f It is the depth of defects far from the surface of the metal pipe.

2. The metal pipeline defect detection method according to claim 1, characterized in that: The method of step (2) is: For the metal pipe to be tested, the changes in the induced voltage of the frequency domain receiving coil at different positions are obtained by scanning. If there is a near-surface defect, the induced voltage difference U fd The voltage changes with the scanning coordinates. The extreme point of the voltage change during the scanning process is the defect center position. According to the induced voltage difference U fd The changing coordinate range determines the defect surface size; Assume the depth of the small defect near the surface is d n , whose surface size is r, measures the near-surface small size of different surface sizes and depths, and collects the corresponding induced voltage difference U fd Calibrate and fit the depth d of small defects near the surface n On the surface size r of small defect and the induced voltage difference U fd The functional relationship of .

3. The metal pipeline defect detection method according to claim 2, characterized in that: In step (2), the induced voltage difference U of the frequency domain receiving coil is obtained fd The method is: Determine the standard thickness of a metal pipe without defects. Assume that the induced voltage value of the frequency domain receiving coil is U st ; For the metal pipe to be tested, the induced voltage value of the frequency domain receiving coil is assumed to be U f ; The induced voltage difference between the two is U fd =U f -U st .

4. The metal pipeline defect detection method according to claim 1, characterized in that: Depth d of small defects near the surface n On the surface size r of small defect and the induced voltage difference U fd The functional relationship is: In the formula, the constant a ij is the fitting coefficient, m, n are U fd and the highest power of r.

5. The metal pipeline defect detection method according to claim 1, characterized in that: In the frequency domain measurement of step (2), the frequency domain coil is composed of an excitation coil and a receiving coil, and is scanned along the extension direction of the pipeline while maintaining a certain lifting height from the pipe wall. During detection, the signal generator gives the excitation coil a high-frequency sinusoidal alternating signal to excite a harmonic magnetic field, thereby forming an induced eddy current in the metal pipeline being measured. The secondary induced magnetic field generated by the eddy current forms a secondary induced current in the receiving coil, and the induced voltage amplitude of the receiving coil is measured.

6. The metal pipeline defect detection method according to claim 1, characterized in that: The method of step (3) is: Assuming the attenuation rate of the induced voltage signal in the time domain is k, and the thickness of the metal pipe is D, the metal pipes of different thicknesses are measured, the corresponding attenuation rates are calibrated, and the functional relationship between the attenuation rate k and the thickness D is obtained by fitting; The time domain method coil is used to scan the metal pipe to be tested, and the attenuation rate k at different positions is obtained. t , and then calculate the thickness D of the metal pipe at different positions t .

7. The metal pipeline defect detection method according to claim 1, characterized in that: The functional relationship between the attenuation rate k and the thickness D in step (3) is: D=g(k)=ak b Where a and b are fitting coefficients.

8. The metal pipeline defect detection method according to claim 1, characterized in that: In step (3), the time domain method coil is composed of an excitation coil and a receiving coil, with the excitation coil inside and the receiving coil outside, and is scanned along the extension direction of the pipeline at a certain height from the pipe wall; during detection, the signal generator gives a low-frequency pulse signal to the excitation coil to excite the decaying pulse induced magnetic field, forming an induced eddy current in the metal pipeline under test, and the secondary induced magnetic field generated by the eddy current forms a secondary induced current in the receiving coil, and a curve of the induced voltage signal of the receiving coil decaying with time is measured.

9. The metal pipeline defect detection method according to claim 1, characterized in that: h＝2。

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