A method for discriminating noise intervals of a pulsed eddy current detection signal

Abstract

This invention provides a method for identifying noise intervals in pulsed eddy current (PDC) detection signals, belonging to the field of nondestructive testing (NDT) technology. It can at least partially solve the problems of reduced signal-to-noise ratio and the impact of noise data on the accuracy of quantitative analysis in the later stages of PDC detection. Based on the magnetic field attenuation rate signal acquired during PDC detection, the method performs a logarithmic transformation on the signal amplitude, then performs inverse linear fitting starting from the last sampling time corresponding to the signal, calculating the slope sequence point by point, and applying multi-level mean filtering to the slope sequence. By determining the time position at which the smoothed slope sequence first reaches or crosses a preset threshold, the starting position of the invalid signal interval, dominated by noise, is determined. Using the identification results, invalid signal intervals can be removed from subsequent data processing, thereby improving the stability and reliability of PDC detection in the detection of clad metal structures.

Description

Technical Field

[0001] This invention belongs to the field of nondestructive testing technology, specifically a method for determining the noise range of a pulse eddy current detection signal. Background Technology

[0002] Metal pressure vessels, pressure pipelines, and boilers are widely used in industries such as petroleum, chemical, and power. To meet requirements for corrosion protection, heat preservation, or thermal insulation, the surfaces of these devices are typically covered with thick non-conductive or weakly conductive cladding layers. Under long-term exposure to high temperatures, high pressures, and corrosive media, the metal substrate is prone to thinning and corrosion defects, seriously affecting the safe operation of the equipment. Therefore, there is an urgent need for a non-destructive testing (NDT) technique that can effectively inspect metal structures without damaging the cladding layer. Pulsed eddy current testing technology is widely used for wall thickness measurement and defect detection of clad metal structures due to its advantages such as non-contact operation, insensitivity to cladding layers, and rich spectral information. This technology uses square wave or bipolar pulses to excite coils, and after the excitation is turned off, it acquires the secondary magnetic field signal generated by the attenuation of eddy currents in the specimen, and evaluates the metal wall thickness or defects based on the magnetic field attenuation characteristics. Related studies have shown that the eddy current magnetic field decays exponentially in the time domain, and its attenuation rate is closely related to the metal wall thickness.

[0003] However, in practical applications of pulsed eddy current testing, the received signal amplitude is extremely small in the later stages, making it susceptible to electronic noise and environmental interference, resulting in a significant decrease in the signal-to-noise ratio. When the signal tail is dominated by noise components, the attenuation characteristics of the eddy current magnetic field are no longer significant. If this data is still used for wall thickness calculation or quantitative defect analysis, it can easily introduce large errors, reducing the accuracy and reliability of the detection results. Existing technologies mostly use empirical truncation or fixed time windows to remove signal tail noise, lacking objective criteria based on the physical and statistical characteristics of the signal, making it difficult to adapt to the detection needs under different wall thicknesses and operating conditions. Therefore, we propose a method for discriminating the noise range of pulsed eddy current detection signals. Summary of the Invention

[0004] The present invention aims to at least solve one of the technical problems existing in the prior art, and provides a method for determining the noise range of pulse eddy current detection signals.

[0005] This invention provides a method for determining the noise range of a pulsed eddy current detection signal. The method is based on the magnetic field attenuation rate signal acquired during pulsed eddy current detection and includes the following steps:

[0006] S1: Perform a logarithmic transformation on the amplitude of the magnetic field attenuation rate signal;

[0007] S2: Starting from the last sampling time corresponding to the magnetic field attenuation rate signal, select a preset number of continuous signal data points for linear fitting to calculate the slope value, and move point by point towards the starting direction of the magnetic field attenuation rate signal according to a preset step size to form a slope sequence corresponding to each time node.

[0008] S3: At least three different window lengths with increasing window lengths are used sequentially to perform multi-level mean filtering on the slope sequence to obtain a smoothed slope sequence.

[0009] S4: Determine the starting position of the invalid signal interval dominated by noise by taking the time position when the smoothed slope sequence first reaches or crosses the preset threshold.

[0010] Further, in step S1, the logarithmic transformation is to take the natural logarithm or common logarithm of the amplitude of the magnetic field attenuation rate signal.

[0011] Specifically, in step S2, the number of data points of the continuous signal used for the straight line fitting ranges from 20 to 50.

[0012] Specifically, in step S2, the preset step size is equal to the time interval corresponding to one sampling point of the magnetic field attenuation rate signal.

[0013] Preferably, in step S3, the multi-level mean filtering includes at least three mean filtering processes.

[0014] Specifically, the window length used in each mean filtering process increases sequentially.

[0015] Furthermore, in step S4, the preset threshold is a zero value, or a predetermined threshold range that includes a zero value.

[0016] Furthermore, in step S4, the first time the preset threshold is reached or crossed refers to the time point at which the sign of the smoothed slope sequence changes from the zero value side to the other side.

[0017] Furthermore, the discrimination method is applied in the pulsed eddy current nondestructive testing process of metal structures with cladding layers.

[0018] Specifically, the invalid signal interval is used to distinguish invalid data intervals of the received signal in the pulse eddy current detection.

[0019] The beneficial effects of this invention are as follows:

[0020] Based on the slope variation characteristics of the magnetic field attenuation rate signal, this invention achieves automatic identification of noise-dominant regions in pulsed eddy current detection signals through logarithmic transformation, slope fitting, and multi-level mean filtering, avoiding the uncertainty caused by manual truncation. By performing reverse analysis from the later stages of the signal and obtaining the slope sequence point by point, the invention can accurately identify the time position of the transition from effective eddy current signal to noise signal, improving the stability and reliability of noise region identification. Using the identified invalid signal regions for subsequent data processing helps reduce the impact of noise data on wall thickness measurement or defect quantitative analysis results, thereby enhancing the engineering practicality of pulsed eddy current detection. Attached Figure Description

[0021] Figure 1 The flowchart illustrates the steps of a method for determining the noise range of a pulsed eddy current detection signal according to a specific embodiment of the present invention.

[0022] Figure 2 The magnetic field attenuation rate curve of a 3mm thick steel plate is shown in the pulse eddy current detection signal noise range discrimination method according to a specific embodiment of the present invention.

[0023] Figure 3 The magnetic field attenuation rate curve of a 6mm thick steel plate is shown in the pulse eddy current detection signal noise range discrimination method according to a specific embodiment of the present invention.

[0024] Figure 4 The magnetic field attenuation rate curve of a 9mm thick steel plate is shown in the pulse eddy current detection signal noise range discrimination method according to a specific embodiment of the present invention.

[0025] Figure 5 The magnetic field attenuation rate curve of a 12mm thick steel plate is shown in the pulse eddy current detection signal noise range discrimination method according to a specific embodiment of the present invention.

[0026] Figure 6 The slope value, smoothing value, and noise start position diagram of the magnetic field attenuation curve of a 3mm thick steel plate are shown in a specific embodiment of the present invention for the method of discriminating the noise range of pulse eddy current detection signal.

[0027] Figure 7 The magnetic field attenuation curve slope, smoothing value, and noise start position diagram of a 6mm thick steel plate for a method of discriminating the noise range of a pulsed eddy current detection signal according to a specific embodiment of the present invention.

[0028] Figure 8 The slope value, smoothing value and noise start position diagram of the magnetic field attenuation curve of a 9mm thick steel plate are shown in a specific embodiment of the present invention for the method of discriminating the noise range of pulse eddy current detection signal.

[0029] Figure 9The magnetic field attenuation curve slope, smoothing value, and noise start position diagram of a 12mm thick steel plate for a method of discriminating the noise range of a pulsed eddy current detection signal according to a specific embodiment of the present invention.

[0030] Figure 10 This is a schematic diagram illustrating the working principle of pulse eddy current detection excitation and reception signals in a coated pipeline, which is a method for determining the noise range of a pulse eddy current detection signal according to a specific embodiment of the present invention.

[0031] Figure 11 This is a schematic diagram illustrating the principle of pulse eddy current detection in a coated pipeline, illustrating a method for determining the noise range of a pulse eddy current detection signal according to a specific embodiment of the present invention.

[0032] The components are: 1. Pipeline; 2. Covering filler; 3. Covering skin; 4. Processing module. Detailed Implementation

[0033] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] like Figure 1 , Figure 10 As shown in the figure, a method for determining the noise range of a pulsed eddy current detection signal is provided by a specific embodiment of the present invention. The method is implemented based on the magnetic field attenuation rate signal acquired in pulsed eddy current detection and includes the following steps:

[0035] S1: Perform a logarithmic transformation on the amplitude of the magnetic field attenuation rate signal;

[0036] S2: Starting from the last sampling time corresponding to the magnetic field attenuation rate signal, select a preset number of continuous signal data points for linear fitting to calculate the slope value, and move point by point in the starting direction of the magnetic field attenuation rate signal according to a preset step size to form a slope sequence corresponding to each time node.

[0037] S3: At least three different mean filtering methods with increasing window lengths are used sequentially to perform multi-level mean filtering on the slope sequence to obtain a smoothed slope sequence.

[0038] S4: Determine the starting position of the invalid signal interval dominated by noise by using the time position when the smoothed slope sequence first reaches or crosses the preset threshold.

[0039] In this embodiment, the magnetic field attenuation rate refers to the rate at which the secondary magnetic field decreases over time due to the gradual attenuation of induced eddy currents within the tested metal structure after the pulse excitation signal is turned off. In other words, the magnetic field attenuation rate characterizes the rate of change of the secondary magnetic field on the time axis, and its numerical characteristics can reflect the eddy current diffusion and attenuation process. Since the wall thickness, conductivity, permeability, cladding conditions, probe coupling state, and detection parameters of the tested object vary, the attenuation process of the secondary magnetic field differs. Therefore, the magnetic field attenuation rate can serve as an important time-domain characteristic quantity characterizing the state of the tested structure. Correspondingly, the magnetic field attenuation rate signal refers to the time-series response signal that the receiving unit senses and outputs after the excitation signal is turned off, reflecting the change in the magnetic field attenuation rate. This signal can be represented as a discrete data sequence arranged in the order of sampling time, used to characterize the dynamic changes in the magnetic field attenuation process at different time points. In this invention, the magnetic field attenuation rate signal serves as the foundational data for subsequent logarithmic transformation, slope fitting, multi-level mean filtering, and threshold discrimination. In this embodiment, the magnetic field attenuation rate, i.e., the rate at which the magnetic field decays over time, is measured using Faraday's law of electromagnetic induction. The rate of change of magnetic flux over a small area is measured using a coil to determine the rate of change of the magnetic field within that area. According to Faraday's law of electromagnetic induction, the magnitude of the induced electromotive force (EMF) or voltage in the coil at a given moment characterizes the rate of change of magnetic flux at that location, thus equivalently reflecting the attenuation rate of the magnetic field in that region.

[0040] Furthermore, in the pulsed eddy current detection process, the magnetic field attenuation rate signal typically reflects the effective eddy current response in the early stages, exhibiting a large amplitude and a clear attenuation pattern. As time progresses, the signal amplitude gradually decreases, becoming more susceptible to electronic noise, environmental interference, and system background noise, leading to a decrease in the signal-to-noise ratio and gradually entering a noise-dominated range. Since this attenuation process generally exhibits an approximately exponential decay characteristic, a logarithmic transformation of the magnetic field attenuation rate signal amplitude can convert the original nonlinear attenuation relationship into an approximately linear one. This facilitates obtaining the local slope at each time point through sliding straight line fitting, and using the trend of the slope sequence to identify the effective signal range and the noise-dominated range. Furthermore, the magnetic field attenuation rate curve refers to the curve formed by representing the magnetic field attenuation rate signal on the time axis according to the sampling time sequence, used to characterize the change process of the magnetic field attenuation rate signal over time. The curve generally reflects the attenuation trend of the eddy current magnetic field from strong to weak. In the early stage, it mainly reflects the effective eddy current response. In the later stage, as the signal amplitude decreases, it is more susceptible to electronic noise, environmental interference, and system background noise, thus gradually entering the noise-dominated interval. In this invention, the magnetic field attenuation rate curve is the basis for subsequent noise interval discrimination. Specifically, the magnetic field attenuation rate signal amplitude is first logarithmically transformed to convert the originally exponentially decaying signal into an approximately linear change form. Then, starting from the last sampling time, continuous signal data is used to perform linear fitting and the fitting window is moved point by point to form a slope sequence corresponding to each time node. Subsequently, the slope sequence is subjected to multi-level mean filtering to obtain a smoothed slope sequence. Finally, based on the time position corresponding to the first time the smoothed slope sequence reaches or crosses the preset threshold, the starting position of the noise-dominated invalid signal interval is determined.

[0041] It should also be noted that this invention focuses not only on the amplitude at a single point in time, but on the change process of the magnetic field attenuation rate signal throughout the entire sampling time domain. By fitting the slope sequence point by point backward from the last sampling time, and performing multi-level mean filtering on the slope sequence, the attenuation trend can be extracted more stably. When the smoothed slope sequence first reaches or crosses a preset threshold, it can be considered that the magnetic field attenuation rate signal gradually transitions from being dominated by effective eddy current response to being dominated by noise, thereby determining the starting position of the invalid signal interval.

[0042] Specifically, such as Figure 11As shown, in step S1, by performing a logarithmic transformation on the amplitude of the magnetic field attenuation rate signal, the signal, which originally exhibited exponential attenuation characteristics, can be transformed into an approximately linear change form. This facilitates subsequent analysis of the signal attenuation characteristics using a linear fitting method, improving the stability and reliability of the slope calculation. Pulse eddy current detection is a non-destructive testing method based on the principle of electromagnetic induction. By applying a pulse current to the excitation coil, eddy currents are excited in the clad pipe being tested, and the attenuation characteristics of the eddy current magnetic field are detected to determine the pipe wall thickness or defect status under the cladding. Clad metal structural components generally include a pipe 1, a cladding filler 2, and a cladding skin 3. The detection signal is generally sent to the processing module 4 for processing.

[0043] Furthermore, in step S2, the reverse fitting process is performed starting from the last sampling time corresponding to the magnetic field attenuation rate signal, so that the slope calculation is performed preferentially from the later stage of the signal, which is beneficial to identifying the change characteristics of the signal transitioning from the effective eddy current response to the noise-dominated state.

[0044] Furthermore, by moving the fitting window point by point towards the starting direction of the signal and calculating the corresponding slope value at each time node, a complete slope sequence can be formed, thereby reflecting the changing trend of the magnetic field attenuation rate signal over the entire time range.

[0045] Based on the above basic implementation method, in step S1, the logarithmic transformation is to take the natural logarithm or common logarithm of the amplitude of the magnetic field attenuation rate signal.

[0046] Specifically, in practical applications, the natural logarithm or commonly used logarithm can be selected for transformation according to data processing habits or the needs of the computing platform. This selection does not affect the basic principle and discrimination effect of the present invention in distinguishing noise intervals.

[0047] In one specific implementation, in step S2, the number of data points of the continuous signal used for linear fitting ranges from 20 to 50; in step S2, the preset step size is equal to the time interval corresponding to one sampling point of the magnetic field attenuation rate signal.

[0048] In this embodiment, selecting the number of data points within the above range can ensure the stability of slope calculation while taking into account the time resolution requirements, thereby avoiding the introduction of additional errors due to an excessively large or small fitting window.

[0049] Specifically, when the preset step size is the time interval corresponding to one sampling point, the slope change can be analyzed point by point, which is helpful for accurately locating the time position corresponding to the slope change feature.

[0050] In another specific embodiment, in step S3, the multi-level mean filtering includes at least three mean filtering processes; the window length used in each mean filtering process increases sequentially.

[0051] Specifically, by using mean filtering with different window lengths and increasing them step by step, random fluctuations in the slope sequence can be effectively suppressed while preserving its overall trend, thus obtaining a smoother and more stable slope curve.

[0052] In another specific embodiment, in step S4, the preset threshold is zero, or a predetermined threshold range containing zero; in step S4, the first time the preset threshold is reached or crossed refers to the time point at which the sign of the smoothed slope sequence changes from the zero side to the other side; the discrimination method is applied in the pulsed eddy current nondestructive testing process of a clad metal structure; the invalid signal range is used to distinguish invalid data ranges of the received signal in pulsed eddy current testing.

[0053] Furthermore, by comparing the smoothed slope sequence with a preset threshold, the effective signal region dominated by eddy current response and the invalid signal region dominated by noise can be objectively distinguished in the signal.

[0054] Furthermore, in subsequent wall thickness assessment or defect quantitative analysis, eliminating data corresponding to invalid signal intervals helps improve the accuracy and stability of pulsed eddy current detection results.

[0055] In another specific embodiment, such as Figure 2 , Figure 3 , Figure 4 , Figure 5 As shown, the object under inspection is a section of steel pipe with an outer coating layer, the thickness of which is 100 mm. A pulsed eddy current testing device is used to inspect the steel pipe. During the inspection, a pulsed square wave excitation signal is applied to the excitation coil. After the excitation signal is turned off, the magnetic field attenuation rate signal generated by the attenuation of eddy currents in the pipe is acquired through the receiving coil. The magnetic field attenuation rate signal is acquired at a fixed sampling frequency, resulting in discrete sampled data arranged in chronological order. The data duration is 0.3 s, and a total of 1536 sampling points are collected.

[0056] Furthermore, the acquired magnetic field attenuation rate signal is then processed according to the method of the present invention, specifically including the following steps:

[0057] First, the amplitude of the magnetic field attenuation rate signal is logarithmically transformed. In this embodiment, the amplitude of the magnetic field attenuation rate signal is taken as the natural logarithm to reduce the dynamic range of the signal and enhance its linearity.

[0058] Then, starting from the last sampling time corresponding to the magnetic field attenuation rate signal, 30 consecutive sampling points are selected as a fitting window. The signal data within the window is fitted with a straight line to calculate the corresponding slope value. Subsequently, the fitting window is moved point by point towards the signal starting direction according to the time interval corresponding to a sampling point, and the slope value corresponding to each time node is calculated in turn to form a slope sequence.

[0059] Next, the slope sequence is subjected to multi-stage mean filtering. In this embodiment, the slope sequence is subjected to three mean filtering processes with window lengths of 49 points, 99 points, and 149 points, respectively, and the window length of each filtering process increases sequentially to obtain a smoothed slope sequence.

[0060] Finally, the smoothed slope sequence is compared with a preset threshold. In this embodiment, the preset threshold is set to zero. When the smoothed slope sequence changes from a positive value to a negative value or from a negative value to a positive value, the corresponding time point is determined as the time point when the slope sequence first reaches or crosses the preset threshold, and this time point is determined as the starting position of the invalid signal interval dominated by noise.

[0061] Furthermore, such as Figure 6 , Figure 7 , Figure 8 , Figure 9 The figure shows the slopes of the attenuation curves at different nodes for wall thicknesses of 3mm, 6mm, 9mm, and 12mm, as well as the slope values ​​after mean filtering. From the first intersection point of the curve after the third mean filtering (thin black line) and the zero line, the noise zones for 3mm, 6mm, 9mm, and 12mm from the start time are 0.063s, 0.076s, 0.107s, and 0.155s, respectively. Comparison with the original magnetic field attenuation curves demonstrates the rationality of the method.

[0062] To aid in a better understanding of the present invention, a more comprehensive and specific embodiment is described. In this embodiment, the present invention provides a method for discriminating the noise range of a pulsed eddy current detection signal. The method is implemented based on the magnetic field attenuation rate signal acquired during pulsed eddy current detection and includes the following steps:

[0063] S1: Perform a logarithmic transformation on the amplitude of the magnetic field attenuation rate signal;

[0064] S2: Starting from the last sampling time corresponding to the magnetic field attenuation rate signal, select a preset number of continuous signal data points for linear fitting to calculate the slope value, and move point by point in the starting direction of the magnetic field attenuation rate signal according to a preset step size to form a slope sequence corresponding to each time node.

[0065] S3: At least three different mean filtering methods with increasing window lengths are used sequentially to perform multi-level mean filtering on the slope sequence to obtain a smoothed slope sequence.

[0066] S4: Determine the starting position of the invalid signal interval dominated by noise by using the time position when the smoothed slope sequence first reaches or crosses the preset threshold.

[0067] In this embodiment, in step S1, the logarithmic transformation is to take the natural logarithm or common logarithm of the amplitude of the magnetic field attenuation rate signal; in step S2, the number of data points of the continuous signal used for linear fitting ranges from 20 to 50; in step S2, the preset step size is equal to the time interval corresponding to one sampling point of the magnetic field attenuation rate signal; in step S3, the multi-level mean filtering includes at least three mean filtering processes; the window length used in each mean filtering process increases sequentially.

[0068] Specifically, in step S4, the preset threshold is zero, or a predetermined threshold range containing zero; in step S4, the first time the preset threshold is reached or crossed refers to the time point at which the sign of the smoothed slope sequence changes from the zero side to the other side; the discrimination method is applied in the pulsed eddy current nondestructive testing process of a metal structure with a cladding layer; the invalid signal range is used to distinguish invalid data ranges of the received signal in pulsed eddy current testing.

[0069] In summary, the embodiments disclosed herein have at least the following technical effects:

[0070] Based on the slope change characteristics of the magnetic field attenuation rate signal, this invention performs logarithmic transformation, slope fitting and multi-level mean filtering on the received signal, and combines it with preset threshold criteria to automatically identify the starting position of the invalid signal interval dominated by noise, thus avoiding the subjectivity and uncertainty brought about by manual experience truncation or fixed time window methods.

[0071] By performing a logarithmic transformation on the magnetic field attenuation rate signal, the exponential attenuation relationship is transformed into an approximately linear relationship, so that the slope calculation can directly reflect the attenuation characteristics of the eddy current magnetic field, and the effective eddy current signal and noise signal can be distinguished from the noise signal from the physical mechanism, thus improving the accuracy of noise interval discrimination.

[0072] By employing a multi-level mean filtering method with at least three different window lengths and increasing window lengths to smooth the slope sequence, the overall trend of change can be preserved while suppressing random fluctuations, making the threshold criterion more stable and reducing the occurrence of misjudgments.

[0073] This invention performs reverse slope analysis starting from the later stage of the signal and obtains the complete slope sequence by moving the fitting window point by point. It does not depend on fixed time parameters and can adapt to pulse eddy current detection signals under different wall thicknesses, different cladding layer conditions and different sampling parameters, thus having strong adaptability.

[0074] By removing invalid signal intervals from subsequent wall thickness assessments or quantitative defect analyses, the interference of noise data on the calculation results is reduced, thereby improving the reliability and engineering practical value of pulsed eddy current testing in the inspection of clad metal structures.

[0075] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A method for determining the noise range of a pulsed eddy current detection signal, characterized in that, The discrimination method is implemented based on the magnetic field attenuation rate signal acquired in pulsed eddy current detection. The magnetic field attenuation rate signal refers to the time-series response signal that the receiving unit senses and outputs after the excitation signal is turned off, reflecting the change in the magnetic field attenuation rate. The method includes the following steps: S1: Perform a logarithmic transformation on the amplitude of the magnetic field attenuation rate signal; S2: Starting from the last sampling time of the magnetic field attenuation rate signal, a preset number of continuous signal data points are selected for linear fitting to calculate the slope value. The data is then moved point by point towards the starting direction of the magnetic field attenuation rate signal according to a preset step size to form a slope sequence corresponding to each time node. The number of data points of the continuous signal used for linear fitting ranges from 20 to 50. The preset step size is equal to the time interval corresponding to one sampling point of the magnetic field attenuation rate signal. S3: The slope sequence is subjected to multi-level mean filtering by using at least three different window lengths in sequence with the window length increasing, to obtain a smoothed slope sequence. The multi-level mean filtering includes at least three mean filtering processes, and the window length used in each mean filtering process increases sequentially. S4: Determine the starting position of the invalid signal interval dominated by noise by the time position corresponding to the first time the smoothed slope sequence reaches or crosses the preset threshold. The preset threshold is zero or a predetermined threshold interval containing zero. The first time the preset threshold is reached or crossed refers to the time point when the sign of the smoothed slope sequence changes from the zero side to the other side.

2. The method for determining the noise range of a pulsed eddy current detection signal according to claim 1, characterized in that, In step S1, the logarithmic transformation is to take the natural logarithm or common logarithm of the amplitude of the magnetic field attenuation rate signal.

3. The method for determining the noise range of a pulsed eddy current detection signal according to claim 1, characterized in that, The discrimination method is applied in the pulsed eddy current nondestructive testing process of metal structures with cladding layers.

4. The method for determining the noise range of a pulsed eddy current detection signal according to any one of claims 1 to 3, characterized in that, The invalid signal interval is used to distinguish invalid data intervals of the received signal in the pulse eddy current detection.

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