A method for detecting lining collapse of bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing

Abstract

A bimetallic composite pipe liner collapse detection method based on magnetic saturation pulsed eddy current and signal processing consists of two parts: a pulsed eddy current method based on magnetic saturation and a signal processing method with late correlation coefficient as an adaptive feature quantity. When implementing this method, the outer carbon steel of the bimetallic composite pipe is first magnetized by a magnetic yoke until magnetic saturation, thereby reducing the magnetic resistance between the outer carbon steel and the inner stainless steel, so that the magnetic field and eddy current field generated by the pulsed eddy current can penetrate the outer carbon steel and enter the inner stainless steel. Then, the original pulsed eddy current signal is subjected to low-pass filtering, independent component analysis, and Gaussian filtering to eliminate high-frequency noise respectively. , power frequency interference and random noise, and then the filtered pulse eddy current late signal of the bimetallic composite pipe without liner collapse is used as the reference signal, and the Pearson correlation coefficient of the filtered pulse eddy current late signal of the bimetallic composite pipe to be tested and the reference signal is calculated, and the characteristic quantity is called the late correlation coefficient, and finally the characteristic quantity and the corresponding calibration curve are used to characterize the degree of liner collapse of the bimetallic composite pipe; the method of the present invention proposes an optimized pulse eddy current method and an adaptive characteristic quantity, which realizes the detection of liner collapse of bimetallic composite pipe, and has high theoretical value and engineering application value.

Description

Technical Field

[0001] The present invention relates to the technical field of bimetallic composite pipe liner collapse detection, and in particular to a bimetallic composite pipe liner collapse detection method based on magnetic saturation pulse eddy current and signal processing. Background Art

[0002] Today, oil and gas resources are widely used in various fields, including industry, agriculture, and the military. Sufficient oil and gas resources can drive a country's economic development and improve people's living standards. Natural gas reception and storage are important peak-shaving measures in my country. The next 10 years will be a peak period for gas storage construction and a period of strategic opportunity. Accelerating the replenishment of insufficient natural gas storage capacity, the construction of a large number of gas storage facilities is imperative.

[0003] The intake and exhaust pipes are crucial components of a gas storage structure, and ensuring their integrity is crucial to ensuring its integrity. These pipes are typically bimetallic composite tubular structures, consisting of a carbon steel outer structural tube and a stainless steel inner functional tube. The two tube layers are simply bonded together, not welded. Therefore, after long-term service, the bimetallic composite pipe structures of gas storage inlets and outlets may develop defects such as structural cracks, corrosion defects, and lining collapse. While structural cracks and corrosion defects can be detected through conventional eddy current testing, magnetic flux leakage testing, and ultrasonic testing, lining collapse has long been a bottleneck and difficulty in practical engineering testing. Because the two layers of the bimetallic composite pipe are simply bonded together, not welded, the bond between the two layers is weak, resulting in severe ultrasonic reflection at the interface, making ultrasonic testing impossible. Furthermore, because the stainless steel inner lining is a non-ferromagnetic material, magnetic flux leakage testing is also not feasible. However, due to the large difference in magnetic permeability of the two metal materials, there is a large magnetic resistance at the interface, which makes it difficult for the magnetic field to penetrate the carbon steel layer to reach the stainless steel layer. Therefore, conventional eddy current or pulsed eddy current testing cannot be used.

[0004] By analyzing the limitations of the above-mentioned conventional detection methods, it can be seen that developing a new inspection technology based on integrity management, using pulsed eddy current technology for detection on the basis of reducing the magnetic resistance between the double-layer pipes, and seeking new adaptive characteristic quantities are the key to realizing the collapse detection of the lining of bimetallic composite pipes. Summary of the Invention

[0005] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to propose a bimetallic composite pipe lining collapse detection method based on magnetic saturation pulsed eddy current and signal processing to solve the problem that the collapse of the bimetallic composite pipe lining is difficult to detect. It is a bimetallic composite pipe lining collapse detection method based on a magnetic saturation pulsed eddy current method and a signal processing method with a late correlation coefficient as a characteristic quantity.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A method for detecting lining collapse of bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing includes the following steps:

[0008] Step 1: Build a magnetic saturation pulsed eddy current detection experimental platform, which includes a magnetic yoke, a high-power signal source, a pulsed eddy current detection probe, a filter amplifier, a signal acquisition card, and a computer. First, place the magnetic yoke on the outer wall of the bimetallic composite pipe and magnetize the outer carbon steel until it is magnetically saturated, so that the induced eddy current penetrates deeply into the inner stainless steel layer of the bimetallic composite pipe, enhancing the detection capability of the pulsed eddy current detection method. Then, connect the output end of the high-power signal source to the excitation coil of the pulsed eddy current detection probe, then place the pulsed eddy current detection probe between the magnetic yoke and the bimetallic composite pipe, connect the detection coil to the input end of the filter amplifier, then connect the output end of the filter amplifier to the signal acquisition card, and finally connect the signal acquisition card to the computer.

[0009] Step 2: First, use the pulsed eddy current testing experimental platform in step 1 to test the reference specimen (bimetallic composite pipe without lining collapse). It is necessary to ensure that the design wall thickness of the two-layer pipe of the reference specimen is consistent with the bimetallic composite pipe to be tested; according to the design wall thickness of the two-layer pipe in the reference specimen, set the parameters of the excitation signal emitted by the high-power signal source, including waveform, excitation frequency, excitation amplitude, excitation wave number, etc., and then set the parameters of the filter amplifier, including amplification factor and low-pass filter frequency, and finally set the signal acquisition card parameters, including sampling frequency and cutoff voltage; pass the excitation signal into the excitation of the pulsed eddy current detection probe. The coil is then detected, and the corresponding detection signal is obtained in the detection coil. After the detection signal passes through the filter amplifier and the signal acquisition card, the original pulsed eddy current detection signal U0(t) corresponding to the reference specimen detection point is obtained in the computer; the original pulsed eddy current detection signal U0(t) is processed in sequence by low-pass filtering, independent component analysis and Gaussian filtering to eliminate high-frequency noise, power frequency interference and random noise in the signal respectively, and the filtered pulsed eddy current detection signal U1(t) is obtained; the terminal signal of U1(t) is extracted to obtain the benchmark pulsed eddy current late signal u1(t), and this is used as the benchmark signal;

[0010] Step 3: Using the bimetallic composite pipe to be tested (a bimetallic composite pipe whose liner collapse is unknown) as the test object, repeat the operation in step 2 to obtain the pulsed eddy current late signal u2(t) of the bimetallic composite pipe to be tested;

[0011] Step 4: Calculate the Pearson correlation coefficient between the pulsed eddy current late signal u2(t) of the bimetallic composite tube to be tested and the pulsed eddy current late signal u1(t) of the reference. The expression of the Pearson correlation coefficient is shown in formula (1).

[0012]

[0013] where ρ XY is the Pearson correlation coefficient between the two variables, cov(X,Y) is the covariance between the two variables, D(X) and D(Y) are the variances of the two variables, is the standard deviation of the two variables, and E(X) and E(Y) are the means of the two variables. The calculated Pearson correlation coefficient is the characteristic quantity for detecting whether the bimetallic composite pipe has liner collapse, which is called the late correlation coefficient. This characteristic quantity is used to characterize the degree of liner collapse of the bimetallic composite pipe. The smaller the late correlation coefficient, the greater the degree of liner collapse. The maximum late correlation coefficient is 1, at which point the bimetallic composite pipe does not have liner collapse.

[0014] Step 5: Prepare calibration specimens of bimetallic composite pipes with different degrees of collapse, and use the above steps 1 to 4 to measure the calibration curve d = f (ρ) on the calibration specimen, where d represents the degree of liner collapse of the bimetallic composite pipe, and ρ represents the late correlation coefficient of the adaptive characteristic quantity; in subsequent testing, the late correlation coefficient ρ0 obtained at the test point of the bimetallic composite pipe to be tested is substituted into the calibration curve d = f (ρ), and the liner collapse of the bimetallic composite pipe to be tested can be calculated.

[0015] Preferably, the yoke in step 1 is essentially a permanent magnet or electromagnet with a strong magnetic field. When in use, it is placed on the outer wall of the bimetallic composite pipe and covers the pulsed eddy current detection probe. The function of the magnetic yoke is to magnetize the outer carbon steel of the bimetallic composite pipe until magnetic saturation, reduce its magnetic permeability, and make its magnetic permeability close to the magnetic permeability of the inner stainless steel material, thereby reducing the influence of the interfacial magnetic resistance, so that the magnetic field can penetrate the carbon steel layer to reach the stainless steel layer. In addition, in the process of reducing the magnetic permeability of the carbon steel by magnetization, the skin effect of the eddy current is also reduced, and the skin depth is increased. Therefore, this method can eventually make the induced eddy current penetrate deeply into the inner stainless steel layer of the bimetallic composite pipe, enhancing the detection capability of the pulsed eddy current detection method. This method is named the magnetic saturation pulsed eddy current method. In addition, the key to selecting the magnetic yoke is to ensure that the outer carbon steel is magnetized to magnetic saturation. Considering the factors of portability and ease of use, permanent magnet yokes are more commonly used. However, when the diameter and thickness of the outer carbon steel pipe are large, a magnetic yoke with stronger magnetization ability is required. At this time, a permanent magnet yoke or electromagnet with a larger permanent magnet volume and a larger number is required.

[0016] Preferably, the pulsed eddy current testing probe in step 1 can be optimized in the following aspects:

[0017] Optimization 1: Optimize the parameters of the excitation coil and detection coil, including the inner diameter, outer diameter, height, wire diameter, reference turns, etc.

[0018] Optimization 2: Optimize the magnetic core, including the material, shape, size, etc.

[0019] Optimization 3: Design electrical and magnetic shielding structures around the excitation coil and detection coil to weaken the influence of the primary magnetic field generated by the excitation coil on the total magnetic field, increase the proportion of the secondary magnetic field excited by eddy currents in the total magnetic field, and ultimately enhance the detection signal. Furthermore, after optimization, the electromagnetic shielding pattern around the coil is designed to be: the magnetic shielding structure is inside, and the electrical shielding structure is outside.

[0020] Optimization 4: The excitation coil and detection coil of the pulsed eddy current detection probe can be placed coaxially or off-axis.

[0021] Optimization 5: Integrate the pulsed eddy current detection probe with the magnetic yoke to avoid interference with the signal caused by probe vibration, thereby improving the stability of the detection signal and test results.

[0022] Preferably, the late correlation coefficient in step 4 is an adaptive feature quantity proposed for the magnetic saturation pulse eddy current detection of bimetallic composite pipe liner collapse. The late period refers to a signal at the end of the detection signal within a cycle. The proportion of the late signal length in the total signal length and the selection position of the late signal can be selected according to the actual situation of the signal. The correlation coefficient refers to the Pearson correlation coefficient, which reflects the correlation between the detection signal of the bimetallic composite pipe to be tested and the detection signal of the non-collapsed bimetallic composite pipe.

[0023] Compared with the prior art, the advantages of the present invention are as follows:

[0024] 1) Combined with the pulsed eddy current detection method based on magnetic saturation, the method of the present invention can magnetize the outer carbon steel of the bimetallic composite pipe until magnetic saturation is reached, reducing its magnetic permeability so that its magnetic permeability is close to that of the stainless steel material, thereby reducing the influence of the magnetic resistance between the interfaces, allowing the magnetic field to penetrate the carbon steel layer to reach the stainless steel layer. In addition, in the process of reducing the magnetic permeability of the carbon steel by magnetization, the skin effect of the eddy current is also reduced, and the skin depth is increased. Based on the above analysis, the method of the present invention greatly improves the penetration ability of the induced eddy current compared to the traditional pulsed eddy current method, allowing the induced eddy current to enter the inner lining stainless steel layer of the bimetallic composite pipe more, thereby improving the limit of penetrating the thickness of the outer carbon steel layer and enhancing the detection ability of the inner lining collapse. In addition, thanks to the magnetic saturation effect of the magnetic yoke, the method of the present invention does not need to use a very low excitation frequency, thereby greatly shortening the detection time and improving the detection efficiency.

[0025] 2) A signal processing method using the late correlation coefficient as a characteristic quantity is combined. Compared with the traditional characteristic quantity selected for pulsed eddy current detection, the late correlation coefficient focuses on the overall change of the late signal compared with the reference signal, increases the sensitivity to the difference between similar signals, and is less affected by various types of noise. Therefore, the method of the present invention can better reflect the degree of collapse of the lining of the bimetallic composite pipe, greatly enhances the detection capability of the lining collapse, and is an adaptive and stable feature extraction method.

[0026] 3) The actual application of the gas storage facility has verified that among all the current methods for detecting lining collapse of bimetallic composite pipes, the method proposed in the present invention is the only method that can effectively detect lining collapse, and the detection effect is significantly better than other methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The present invention is a schematic diagram and a flow chart of various steps when performing liner collapse detection on a bimetallic composite pipe.

[0028] Figure 2a This is a schematic diagram of the off-axis pulsed eddy current detection probe optimized by the present invention. Figure 2b Schematic diagram of the coaxially placed pulsed eddy current detection probe optimized for the present invention.

[0029] Figure 3a The following diagrams show the effects of the present invention applied to bimetallic composite pipes of 6mm carbon steel and 2mm stainless steel. Figure 3b The diagram shows the effect of applying the present invention on a bimetallic composite pipe of 8mm carbon steel + 2mm stainless steel. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0031] The difficulty in detecting lining collapse in bimetallic composite pipes using traditional pulsed eddy current methods lies primarily in the significant difference in magnetic permeability between carbon steel and stainless steel. Consequently, the magnetic resistance at the interface between the two is significant, and only a minimal amount of the magnetic field generated by eddy currents can penetrate the stainless steel, making it difficult to induce eddy currents in the stainless steel layer. Furthermore, due to the skin effect, carbon steel has a high magnetic permeability, resulting in a very low penetration depth of eddy currents in carbon steel. Based on this fact, a magnetic saturation pulsed eddy current method was first developed. This method uses a magnetic yoke to magnetize the carbon steel outer layer of the bimetallic composite pipe until it reaches magnetic saturation, lowering its magnetic permeability to a value close to that of stainless steel. This reduces the influence of the interfacial magnetic resistance and allows the magnetic field to penetrate the carbon steel layer to the stainless steel layer. Furthermore, as the magnetic permeability of the carbon steel is reduced by magnetization, the skin effect of the eddy currents is also reduced, increasing the skin depth. This method ultimately allows the induced eddy currents to penetrate deeply into the stainless steel layer of the bimetallic composite pipe.

[0032] The characteristic quantities extracted by conventional pulsed eddy current detection methods are generally peak values, late slopes, etc., which are mostly local characteristics of the signal. In the detection of bimetallic composite pipes, which have a double-layer structure with a certain magnetic resistance in the middle, they are not sensitive to changes in the inner stainless steel layer. The method of the present invention has developed a signal processing method with the late correlation coefficient as an adaptive characteristic quantity, which overcomes the above difficulties, selects the overall characteristics of a longer section of the signal, and directly performs correlation analysis with the baseline signal of the lining without collapse, showing the difference between the signal to be tested and the baseline signal to the greatest extent. In addition, this method can adaptively extract the late signal of the signal and adaptively calculate the Pearson correlation coefficient between it and the baseline signal, realizing high-precision and high-efficiency detection of lining collapse of bimetallic composite pipes.

[0033] like Figure 1 As shown, the specific implementation steps of the method of the present invention are: first, a magnetic yoke is placed on the outer wall of the reference specimen (a bimetallic composite pipe without liner collapse), and the outer carbon steel is magnetized until magnetic saturation. Then, the pulsed eddy current detection probe shown in FIG2 is placed between the magnetic yoke and the bimetallic composite pipe, close to the outer wall of the bimetallic composite pipe, and then as shown in FIG2 Figure 1 As shown, a square wave excitation signal is generated by a high-power excitation source and applied to the excitation coil of the pulsed eddy current detection probe. Subsequently, the detection coil of the pulsed eddy current detection probe generates a corresponding detection signal. The detection signal is amplified by an amplifier and collected by a signal acquisition card to obtain the pulsed eddy current detection original signal; the original signal is subjected to low-pass filtering, independent component analysis and Gaussian filtering to eliminate high-frequency noise, power frequency interference and random noise in the signal respectively, to obtain a filtered pulsed eddy current detection signal; the terminal section of the filtered pulsed eddy current detection signal is taken to obtain a pulsed eddy current late signal, which is used as a reference signal; then the above steps are repeated on the bimetallic composite pipe to be tested to obtain the pulsed eddy current late signal of the bimetallic composite pipe to be tested, and the Pearson correlation coefficient between the pulsed eddy current late signal and the reference signal is calculated. The smaller the correlation coefficient, the greater the degree of lining collapse. The maximum correlation coefficient is 1, and at this time, there is no lining collapse in the bimetallic composite pipe; finally, combined with the calibration curve, the specific collapse degree of the bimetallic composite pipe to be tested is calculated.

[0034] The following combination Figure 1 、 Figure 2a and Figure 2b 、 Figure 3a and Figure 3b The steps and effects of the present invention are further described in detail in the following specific embodiments.

[0035] A method for detecting lining collapse of a bimetallic composite pipe based on magnetic saturation pulsed eddy current and signal processing comprises the following steps:

[0036] Step 1: If Figure 1As shown, a magnetic saturation pulse eddy current detection experimental platform is built. The magnetic saturation pulse eddy current detection experimental platform includes a magnetic yoke (permanent magnet yoke or electromagnet yoke), a high-power signal source that can generate arbitrary waveforms, a pulse eddy current detection probe, a low-noise filter amplifier, a high-precision signal acquisition card, and a laptop computer; first, the magnetic yoke is placed on the outer wall of the bimetallic composite pipe, and the outer carbon steel is magnetized until magnetic saturation, and then the output end of the high-power signal source is connected to the excitation coil of the pulse eddy current detection probe, and then the pulse eddy current detection probe is placed between the magnetic yoke and the bimetallic composite pipe, and the detection coil is connected to the input end of the filter amplifier, and then the output end of the filter amplifier is connected to the signal acquisition card, and finally the signal acquisition card is connected to the laptop computer.

[0037] The yoke is essentially a permanent magnet or electromagnet with a strong magnetic field. When used, it is placed on the outer wall of the bimetallic composite pipe. Its function is to magnetize the outer carbon steel layer of the bimetallic composite pipe until it is magnetically saturated, reducing its magnetic permeability to a value close to that of the inner stainless steel layer. This reduces the influence of the interfacial magnetic resistance and allows the magnetic field to penetrate the carbon steel layer and reach the stainless steel layer. In addition, the process of reducing the magnetic permeability of the carbon steel by magnetization also reduces the skin effect of the eddy current and increases the skin depth. Therefore, this method can ultimately allow the induced eddy current to penetrate deeply into the inner stainless steel layer of the bimetallic composite pipe, enhancing the detection capability of the pulsed eddy current testing method. In addition, the key to selecting a yoke is to ensure that the outer carbon steel layer is magnetized to magnetic saturation. Considering the factors of portability and ease of use, permanent magnet yokes are more commonly used. However, when the diameter and thickness of the outer carbon steel pipe are larger, a yoke with stronger magnetization is required. In this case, a permanent magnet yoke or electromagnet with larger permanent magnets and a larger number of permanent magnets is required.

[0038] In addition, pulsed eddy current detection probes such as Figure 2a and Figure 2b As shown in the figure, compared with the traditional pulsed eddy current detection probe, the following aspects are optimized: ① Optimization of the parameters of the excitation coil and the detection coil, including the inner diameter, outer diameter, height, wire diameter, reference number of turns, etc. of the coil. Figure 2a The specific parameters of the probe are as follows: the excitation coil has an inner diameter of 8-15mm, an outer diameter of 18-25mm, a height of 10-20mm, a wire diameter of 0.4-1.5mm, and a reference number of turns of 100-2000. The detection coil has an inner diameter of 4-10mm, an outer diameter of 10-20mm, a height of 10-20mm, a wire diameter of 0.05-0.5mm, and a reference number of turns of 3000-30000. Figure 2bThe specific parameters of the probe are as follows: the excitation coil has an inner diameter of 4-10mm, an outer diameter of 10-20mm, a height of 10-20mm, a wire diameter of 0.4-1.5mm, and a reference number of turns of 100-2000. The detection coil has an inner diameter of 11-21mm, an outer diameter of 21-30mm, a height of 10-20mm, a wire diameter of 0.05-0.5mm, and a reference number of turns of 3000-30,000. ② Optimize the magnetic core, including its material, shape, and dimensions. The specific parameters are as follows: The magnetic core material of both the excitation coil and the detection coil is a ferrite core, and both are cylindrical in shape. Figure 2a The bottom diameter of the magnetic core of the excitation coil of the probe is 8-15mm and the height is 10-20mm. The bottom diameter of the magnetic core of the detection coil is 4-10mm and the height is 10-20mm. Figure 2b The excitation coil and detection coil of the probe share a magnetic core, the bottom diameter of which is 4-10mm and the height is 10-20mm. Figure 2a 、 Figure 2b As shown, electric and magnetic shielding structures are designed around the excitation coil and the detection coil to weaken the influence of the primary magnetic field generated by the excitation coil on the total magnetic field, increase the proportion of the secondary magnetic field excited by the eddy current in the total magnetic field, and ultimately enhance the detection signal. And after optimization, the electromagnetic shielding mode around the coil is designed as follows: the magnetic shielding structure is inside and the electric shielding structure is outside. ④ The excitation coil and the detection coil of the pulsed eddy current detection probe can be placed coaxially or off-axis. Figure 2a and Figure 2b In addition to optimizing the pulsed eddy current test probe separately, the pulsed eddy current test probe and the magnetic yoke can also be integrated into a design to avoid interference with the signal caused by probe vibration, thereby improving the stability of the test signal and test results;

[0039] Step 2: First exploit Figure 1The magnetic saturation pulse eddy current platform shown is used to test the reference specimen (a bimetallic composite pipe without lining collapse). It is necessary to ensure that the design wall thickness of the two-layer pipe of the reference specimen is consistent with the bimetallic composite pipe to be tested; according to the design wall thickness of the two-layer pipe in the reference specimen, the parameters of the excitation signal emitted by the high-power signal source are set, including waveform, excitation frequency, excitation amplitude, excitation wave number, etc., and then the parameters of the filter amplifier are set, including amplification factor and low-pass filter frequency, and finally the signal acquisition card parameters are set, including sampling frequency and cutoff voltage; the excitation signal is passed into the excitation coil of the pulse eddy current detection probe, and then the corresponding detection signal will be obtained in the detection coil, and the detection signal will be obtained. After the measured signal passes through the filter amplifier and the signal acquisition card, the original pulsed eddy current detection signal U0(t) corresponding to the reference test piece detection point will be obtained in the laptop computer; the original pulsed eddy current detection signal U0(t) is processed in sequence by low-pass filtering, independent component analysis and Gaussian filtering to eliminate high-frequency noise, power frequency interference and random noise in the signal respectively, and obtain the filtered pulsed eddy current detection signal U1(t); the terminal signal of U1(t) is extracted as the reference pulsed eddy current late signal u1(t), and the proportion of the late signal length in the total signal length and the selection position of the late signal can be selected according to the actual situation of the signal;

[0040] Step 3: Using the bimetallic composite pipe to be tested (a bimetallic composite pipe whose liner collapse is unknown) as the test object, repeat the operation in step 2 to obtain the pulsed eddy current late signal u2(t) of the bimetallic composite pipe to be tested;

[0041] Step 4: Calculate the Pearson correlation coefficient between the pulsed eddy current late signal u2(t) of the bimetallic composite tube to be tested and the reference pulsed eddy current late signal u1(t) to reflect the correlation between the late signal of the bimetallic composite tube to be tested and the late signal of the non-collapsed bimetallic composite tube. The expression of the Pearson correlation coefficient is shown in formula (1).

[0042]

[0043] where ρ XY is the Pearson correlation coefficient between the two variables, cov(X,Y) is the covariance between the two variables, D(X) and D(Y) are the variances of the two variables, is the standard deviation of the two variables, and E(X) and E(Y) are the means of the two variables. The calculated Pearson correlation coefficient is the characteristic quantity for detecting whether the bimetallic composite pipe has liner collapse, which is called the late correlation coefficient. This characteristic quantity is used to characterize the degree of liner collapse of the bimetallic composite pipe. The smaller the late correlation coefficient, the greater the degree of liner collapse. The maximum late correlation coefficient is 1, at which point the bimetallic composite pipe does not have liner collapse.

[0044] Step 5: Prepare calibration specimens of bimetallic composite pipes with different degrees of collapse, and use the above steps 1 to 4 to measure the calibration curve d = f (ρ) on the calibration specimen, where d represents the degree of liner collapse of the bimetallic composite pipe, and ρ represents the late correlation coefficient of the adaptive characteristic quantity; in subsequent testing, the late correlation coefficient ρ0 obtained at the test point of the bimetallic composite pipe to be tested is substituted into the calibration curve d = f (ρ), and the liner collapse of the bimetallic composite pipe to be tested can be calculated.

[0045] In the embodiment, the method of the present invention is applied to a bimetallic composite pipe to be tested of 6mm carbon steel + 2mm stainless steel and a bimetallic composite pipe to be tested of 8mm carbon steel + 2mm stainless steel, respectively, to detect whether the inner lining of the two pipes has collapsed. A total of four test pieces are required, including a non-collapsed reference test piece of 6mm carbon steel + 2mm stainless steel, a non-collapsed reference test piece of 8mm carbon steel + 2mm stainless steel, and the above two test pieces. The yoke and the pulse eddy current detection probe are first placed on the non-collapsed reference test piece, and the method of the invention is used to measure 20 times continuously. Then, they are placed on the test piece and measured 20 times continuously. Then, they are placed back on the non-collapsed reference test piece and measured 20 times continuously. Finally, they are placed on the test piece and measured 20 times continuously. The reason for multiple and repeated measurements is to prove the stability of the method of the present invention. The experimental results are as follows. Figure 3a (6mm carbon steel + 2mm stainless steel), Figure 3b (8mm carbon steel + 2mm stainless steel) are shown in the figure. The light-colored portions of the two figures are the measurement results on the non-collapsed benchmark specimen, so the late correlation coefficient is approximately 1. The dark-colored portions are the measurement results on the test specimens. The late correlation coefficients are significantly less than 1, indicating that both test specimens have liner collapse. Therefore, the method of the present invention can effectively detect liner collapse in bimetallic composite pipes and has good application and development prospects.

Claims

1. A method for detecting liner collapse in bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing, characterized by: The steps include: Step 1: Build a magnetic saturation pulsed eddy current detection experimental platform, which includes a magnetic yoke, a high-power signal source, a pulsed eddy current detection probe, a filter amplifier, a signal acquisition card, and a computer. First, place the magnetic yoke on the outer wall of the bimetallic composite pipe and magnetize the outer carbon steel until it is magnetically saturated, so that the induced eddy current penetrates deeply into the inner stainless steel layer of the bimetallic composite pipe, enhancing the detection capability of the pulsed eddy current detection method. Then, connect the output end of the high-power signal source to the excitation coil of the pulsed eddy current detection probe, then place the pulsed eddy current detection probe between the magnetic yoke and the bimetallic composite pipe, connect the detection coil to the input end of the filter amplifier, then connect the output end of the filter amplifier to the signal acquisition card, and finally connect the signal acquisition card to the computer. Step 2: First, use the pulsed eddy current detection experimental platform in step 1 to detect the benchmark specimen, that is, the bimetallic composite pipe without lining collapse. It is necessary to ensure that the design wall thickness of the two-layer pipe of the benchmark specimen is consistent with the bimetallic composite pipe to be tested; according to the design wall thickness of the two-layer pipe in the benchmark specimen, set the parameters of the excitation signal emitted by the high-power signal source, including waveform, excitation frequency, excitation amplitude and excitation wave number, and then set the parameters of the filter amplifier, including amplification factor and low-pass filter frequency, and finally set the parameters of the signal acquisition card, including sampling frequency and cutoff voltage; the excitation signal is passed into the excitation coil of the pulsed eddy current detection probe, and then the corresponding detection signal will be obtained in the detection coil. After the detection signal passes through the filter amplifier and the signal acquisition card, the original pulsed eddy current detection signal U0(t) corresponding to the detection point of the benchmark specimen will be obtained in the computer; the original pulsed eddy current detection signal U0(t) is processed in sequence by low-pass filtering, independent component analysis and Gaussian filtering to eliminate high-frequency noise, power frequency interference and random noise in the signal, and obtain the filtered pulsed eddy current detection signal U1(t); Extract the terminal signal of U1(t) to obtain the reference pulse eddy current late signal u1(t), and use it as the reference signal; Step 3: Using the bimetallic composite pipe to be tested, i.e., the bimetallic composite pipe whose liner collapse is unknown, as the test object, repeat the operation in step 2 to obtain the pulsed eddy current late signal u2(t) of the bimetallic composite pipe to be tested; Step 4: Calculate the Pearson correlation coefficient between the pulsed eddy current late signal u2(t) of the bimetallic composite tube to be tested and the pulsed eddy current late signal u1(t) of the reference. The expression of the Pearson correlation coefficient is shown in formula (1). where ρ XY is the Pearson correlation coefficient between the two variables, cov(X,Y) is the covariance between the two variables, D(X) and D(Y) are the variances of the two variables, is the standard deviation of the two variables, E(X) and E(Y) are the means of the two variables; The calculated Pearson correlation coefficient is a characteristic quantity for detecting whether the bimetallic composite pipe has liner collapse, which is called the late correlation coefficient. This characteristic quantity is used to characterize the degree of liner collapse of the bimetallic composite pipe. The smaller the late correlation coefficient, the greater the degree of liner collapse. The maximum late correlation coefficient is 1, at which point the bimetallic composite pipe does not have liner collapse. Step 5: Prepare calibration specimens of bimetallic composite pipes with different degrees of collapse, and use the above steps 1 to 4 to measure the calibration curve d = f (ρ) on the calibration specimen, where d represents the degree of liner collapse of the bimetallic composite pipe, and ρ represents the late correlation coefficient of the adaptive characteristic quantity; in subsequent testing, the late correlation coefficient ρ0 obtained at the test point of the bimetallic composite pipe to be tested is substituted into the calibration curve d = f (ρ), that is, the liner collapse of the bimetallic composite pipe to be tested is calculated.

2. The method for detecting liner collapse of bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing according to claim 1 is characterized in that: The magnetic yoke in step 1 is a permanent magnet or electromagnet with a strong magnetic field. When in use, it is placed on the outer wall of the bimetallic composite pipe and covers the pulsed eddy current detection probe. The function of the magnetic yoke is to magnetize the outer carbon steel of the bimetallic composite pipe until magnetic saturation, reduce its magnetic permeability, and make its magnetic permeability close to the magnetic permeability of the inner stainless steel material, thereby reducing the influence of the interfacial magnetic resistance and allowing the magnetic field to penetrate the carbon steel layer to reach the stainless steel layer. In addition, in the process of reducing the magnetic permeability of the carbon steel by magnetization, the skin effect of the eddy current is also reduced and the skin depth is increased. Therefore, this method can ultimately enable the induced eddy current to penetrate deeply into the inner stainless steel layer of the bimetallic composite pipe, enhancing the detection capability of the pulsed eddy current detection method. This method is named the magnetic saturation pulsed eddy current method. In addition, the key to selecting the magnetic yoke is to ensure that the outer carbon steel is magnetized to magnetic saturation. Considering the factors of portability and ease of use, a permanent magnet yoke is used. However, when the diameter and thickness of the outer carbon steel pipe are large, a magnetic yoke with stronger magnetization ability is required. In this case, a permanent magnet yoke or electromagnet with a larger permanent magnet volume and a larger number is required.

3. The method for detecting liner collapse of bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing according to claim 1 is characterized in that: The pulsed eddy current test probe in step 1 is optimized in the following aspects: Optimization 1: Optimize the parameters of the excitation coil and detection coil, including the inner diameter, outer diameter, height, wire diameter, and reference number of turns of the coil; Optimization 2: Optimize the magnetic core, including the material, shape, and size of the core; Optimization 3: Design electrical and magnetic shielding structures around the excitation coil and detection coil to weaken the influence of the primary magnetic field generated by the excitation coil on the total magnetic field, increase the proportion of the secondary magnetic field excited by eddy currents in the total magnetic field, and ultimately enhance the detection signal. Furthermore, after optimization, the electromagnetic shielding pattern around the coil is designed to be: the magnetic shielding structure is inside, and the electrical shielding structure is outside. Optimization 4: The excitation coil and detection coil of the pulsed eddy current testing probe can be placed coaxially or off-axis; Optimization 5: Integrate the pulsed eddy current detection probe with the magnetic yoke to avoid interference with the signal caused by probe vibration, thereby improving the stability of the detection signal and test results.

4. The method for detecting liner collapse of bimetallic composite pipes based on magnetic saturation pulsed eddy current and signal processing according to claim 1, characterized in that: The late correlation coefficient in step 4 is an adaptive feature quantity proposed for the magnetic saturation pulse eddy current detection of bimetallic composite pipe liner collapse. The late period refers to a signal at the end of the detection signal within a cycle. The proportion of the late signal length in the total signal length and the selection position of the late signal are selected according to the actual situation of the signal. The correlation coefficient refers to the Pearson correlation coefficient, which reflects the correlation between the detection signal of the bimetallic composite pipe to be tested and the detection signal of the bimetallic composite pipe without collapse.

Images