A method for pulsed eddy current testing of the depth of a blind hole in a metal under lift-off perturbation conditions

Abstract

This invention relates to the field of nondestructive testing technology, and more particularly to a pulsed eddy current testing method for detecting the depth of blind holes in metals under lift-off disturbance conditions. The invention includes the following steps: S1: A conductor specimen without blind holes is tested using a pulsed eddy current tester, and the induced electromotive force output signal is used as a blank control group. This invention can be used for pulsed eddy current testing of the depth of blind holes in metals under lift-off disturbance conditions, breaking through the limitations of traditional methods on lift-off distance. It has strong adaptability to lift-off disturbances and can accurately detect within a large lift-off range. The detection accuracy is improved through characteristic curve conversion and standard curve comparison, and comprehensive defect information can be obtained. In practical applications, it plays an important role in industrial production quality control and equipment maintenance safety inspection, enabling rapid and accurate detection of blind hole depth, ensuring product quality and equipment safety, improving production efficiency, and reducing costs.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, specifically to a method for detecting the depth of blind holes in conductors based on pulsed eddy currents. Background Technology

[0002] Based on the principle of electromagnetic induction, the pulsed eddy current (PEC) detection method uses a pulsed current to pass through an excitation coil, which generates a pulsed magnetic field. This generates pulsed eddy currents (transient eddy currents) in the conductor specimen within the magnetic field. The magnetic field generated by the pulsed eddy current induces a voltage signal that changes with time on the detection coil, thereby achieving the detection purpose.

[0003] Typically, the pulsed eddy current method determines the parameters and defects of the material under test based on the amplitude of the response signal and the zero-crossing time. However, this criterion is very sensitive to the intensity of the excitation signal and the lift-off disturbance, and is usually only applicable to the detection of metal defects with a lift-off of less than 5 mm and a fixed lift-off condition.

[0004] Based on this, a method for detecting the depth of blind holes in conductors based on pulsed eddy currents is proposed, providing a technical solution to the above problems.

[0005] Lift-off disturbance refers to the change in distance between the probe and the surface of the conductor specimen during pulsed eddy current testing. This distance change significantly affects the test results because it alters the distribution and attenuation characteristics of the eddy current. As the lift-off distance increases, the intensity of the eddy current weakens, and the shape of its attenuation time constant curve changes, thus affecting the reliability of the feature quantities extracted based on these curves for detecting defects such as blind hole depth. Summary of the Invention

[0006] The purpose of this invention is to provide a method for detecting the depth of a blind hole in a conductor based on pulsed eddy currents, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for detecting the depth of a conductor blind hole based on pulsed eddy currents, comprising the following steps:

[0008] Step 1: The conductor specimen without blind holes was tested using a pulsed eddy current detector, and the induced electromotive force output signal ε0(t) was obtained as a blank control group.

[0009] Step 2: For N conductor specimens of the same specification with known blind hole depths, a pulsed eddy current detector is used to perform tests above and behind the blind holes, respectively, to obtain the corresponding N surface blind hole induced electromotive force output signals ε. i1 (t) and the output signal ε of the induced electromotive force of N subsurface blind holes i2 (t), (i = 1, 2, 3…N);

[0010] Step 3: Calculate ε0(t) and ε... i1 (t) and ε i2 (t) is transformed into characteristic curves τ0(t) and τ based on the apparent time constant of eddy current decay. i1 (t) and τ i2 (t), (i = 1, 2, 3…N);

[0011] Step 4: Put τ i1 The differential time constant curves of N surface blind holes are obtained by (t)-τ0(t), and their minimum values ​​A are extracted. i1 ,(i=1,2,3…N), τ i2 The differential time constant curves of N subsurface blind holes are obtained by (t)-τ0(t), and their minimum values ​​A are extracted. i2 ,(i=1,2,3…N);

[0012] Step 5: Place A i1 -A i2 Obtain the feature quantity of the difference between N minimum values, and then use A i1 -A i2 Plot a standard curve for the depth of the blind hole.

[0013] Step 6: Repeat steps S2 to S4 for specimens with unknown blind hole depths within 50 mm of the disturbance, obtain the characteristic quantity A1-A2 of the minimum difference between surface blind holes and subsurface blind holes, and finally determine the depth of the blind hole to be tested by comparing it with the standard curve.

[0014] Furthermore, the pulsed eddy current detector includes a pulsed current transmitter, a detection probe, and a signal acquisition unit. Both the pulsed current transmitter and the signal acquisition unit are connected to the detection probe, and the signal feedback terminal of the signal acquisition unit is used to connect to a computer.

[0015] Furthermore, the detection probe includes a transmitting coil and a receiving coil, the signal output terminal of the pulse current transmitter is connected to the transmitting coil, and the signal acquisition terminal of the signal acquisition machine is connected to the receiving coil.

[0016] Furthermore, in the third step, the induced electromotive force output signal ε(t) is converted into a characteristic curve τ based on the apparent time constant of eddy current decay. ATC The conversion formula for (t) is:

[0017]

[0018] In the formula, d represents the differential sign, and t is the reciprocal of the sampling frequency of the signal acquisition machine, i.e., the sampling time step.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] This invention can be used for pulsed eddy current detection of the depth of metal blind holes under lift-off disturbance conditions. It breaks through the limitations of traditional methods on lift-off distance, has strong adaptability to lift-off disturbances, and can accurately detect within a large lift-off range. The detection accuracy is improved by characteristic curve conversion and standard curve comparison, and comprehensive defect information can be obtained. In practical applications, it plays an important role in industrial production quality control and equipment maintenance safety inspection. It can quickly and accurately detect the depth of blind holes, ensure product quality and equipment safety, improve production efficiency, and reduce costs. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the pulse eddy current detection principle of the present invention;

[0022] Figure 2 This invention provides the PEC characteristic curve based on the apparent time constant of induced eddy current decay.

[0023] Figure 3 This is a model diagram of the pulsed eddy current test specimen of the present invention;

[0024] Figure 4 This is a schematic diagram of the differential apparent time constant curves of surface defects and subsurface defects in this invention.

[0025] Figure 5 Feature A of the present invention i1 -A i2 Schematic diagram showing the variation of blind hole depth;

[0026] Figure 6 This is a schematic diagram of the connection of the detection probe of the present invention. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The apparent time constant of eddy current decay is an important parameter describing the decay characteristics of eddy currents in a conductor. During pulsed eddy current testing, when a pulsed current is applied to the excitation coil to generate a pulsed magnetic field, the pulsed eddy currents generated in the conductor specimen gradually decay over time. The apparent time constant of eddy current decay reflects the rate of this decay and is related to factors such as the conductor's material, geometry, and internal defects. By converting the induced electromotive force output signal to obtain a characteristic curve based on the apparent time constant of eddy current decay, the internal conditions of the conductor can be analyzed more intuitively, providing important information for detecting defects such as blind hole depth.

[0029] Please refer to the following: Figures 1-6 A method for detecting the depth of a blind hole in a conductor based on pulsed eddy currents includes the following steps:

[0030] Step 1: The conductor specimen without blind holes was tested using a pulsed eddy current detector, and the induced electromotive force output signal ε0(t) was obtained as a blank control group.

[0031] Step 2: For N conductor specimens of the same specification with known blind hole depths, a pulsed eddy current detector is used to perform tests above and behind the blind holes, respectively, to obtain the corresponding N surface blind hole induced electromotive force output signals ε. i1 (t) and the output signal ε of the induced electromotive force of N subsurface blind holes i2 (t), (i = 1, 2, 3…N);

[0032] Step 3: Calculate ε0(t) and ε... i1 (t) and ε i2 (t) is transformed into characteristic curves τ0(t) and τ based on the apparent time constant of eddy current decay. i1 (t) and τ i2 (t), (i=1,2,3…N), such as Figure 2 As shown;

[0033] The conversion formula is as follows:

[0034]

[0035] In the formula, d represents the differential sign, and t is the reciprocal of the sampling frequency of the signal acquisition machine, i.e., the sampling time step.

[0036] Step 4: Put τ i1 The differential time constant curves of N surface blind holes are obtained by (t)-τ0(t), and their minimum values ​​A are extracted. i1 ,(i=1,2,3…N), τ i2 The differential time constant curves of N subsurface blind holes are obtained by (t)-τ0(t), and their minimum values ​​A are extracted. i2 ,(i=1,2,3…N);

[0037] Step 5: Place A i1 -A i2 Obtain the feature quantity of the difference between N minimum values, and then use A i1 -A i2 Plot a standard curve for the depth of the blind hole.

[0038] Step 6: Repeat steps S2 to S4 for specimens with unknown blind hole depths within 50mm of the disturbance, and obtain the characteristic quantity A1-A2, which is the difference between the minimum values ​​of surface blind holes and subsurface blind holes. By comparing this value with the standard curve, the depth of the blind hole to be measured is finally determined. Step 3: Convert the induced electromotive force output signal ε(t) into a characteristic curve τ based on the eddy current decay time constant. ATC (t);

[0039] Please see Figure 6 In this embodiment, the pulsed eddy current detector includes a pulsed current transmitter, a detection probe, and a signal acquisition unit;

[0040] Here, the detection probe includes a transmitting coil and a receiving coil. The signal output terminal of the pulse current transmitter is connected to the transmitting coil, the signal acquisition terminal of the signal acquisition machine is connected to the receiving coil, and the signal feedback terminal of the signal acquisition machine is used to connect to a computer.

[0041] In this embodiment, an aluminum plate is selected as the conductor specimen, such as Figure 1 As shown, the PEC sensor located on the z = 100 mm plane consists of a transmitting coil and a receiving coil with radii of 50 mm and 20 mm, respectively. A 400 mm × 400 mm × 10 mm metal plate is placed on the XOY plane. A 60 × 10 × d mm blind hole parallel to the y-axis is set on the upper surface of the metal plate, centered at (x, y) = 0, 60 mm. The depth d of the blind hole is 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm, respectively.

[0042] Eight standard aluminum plate specimens with known blind hole depths were set up. One aluminum plate serving as the control group had no blind hole defects. The blind hole depths of the other seven standard aluminum plate specimens were 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm, respectively. All other parameters of the eight standard aluminum plate specimens were the same. A pulsed eddy current detector was used to detect the blind holes above and on the back of the seven standard aluminum plate specimens, as well as at the location of the blind holes in the blank control group aluminum plate. The induced electromotive force output signal ε(t) of the 15 standard aluminum plate specimens was obtained accordingly.

[0043] The induced electromotive force output signals ε(t) of the 15 standard aluminum plate specimens were converted into the apparent time constant curve τ based on eddy current decay. ATC (t), 14 τ with blind hole defects ATC (t) compared with the control group τ without blind hole defects ATC (t) Subtraction yields 14 differential characteristic curves, and the minimum value A of the differential characteristic curves corresponding to the 7 types of surface blind holes is determined. i1 The minimum value of the differential characteristic curves corresponding to the 7 types of subsurface blind holes A i2; see specific curves. Figure 4 ;

[0044] The induced electromotive force output signal ε(t) of the standard conductor specimen is converted into a curve τ based on the apparent time constant of eddy current decay. ATC The conversion formula for (t) is:

[0045]

[0046] The solution based on the eddy current decay time constant curve τ is obtained through integral calculation. ATC (t), to reduce the impact of signal noise on the solution accuracy;

[0047] Based on the blind hole depth and corresponding characteristic value A of 7 standard aluminum plate specimens i1 -A i2 The depth of the blind hole and the characteristic value A of the standard conductor specimen were obtained by fitting. i1 -A i2 Relationship curve; see details Figure 5 ;

[0048] The induced electromotive force output signal of the aluminum plate specimen under test on the z=50mm, 60mm, and 80mm plane (taking 3mm, 5mm, and 8mm deep blind holes as examples in this case) was measured using a pulse eddy current detector, and the induced electromotive force output signal of the aluminum plate specimen under test was converted into a differential characteristic curve of the aluminum plate specimen under test based on the eddy current decay time constant.

[0049] Substituting the difference A1-A2 between the minimum value A1 of the differential characteristic curve corresponding to the surface blind hole and the minimum value A2 of the differential characteristic curve corresponding to the subsurface blind hole obtained in step four into the blind hole depth of the standard conductor specimen obtained in step five, we can find the relationship between A1 and A2. i1 -A i2 Compare them in the relationship curve, such as Figure 5 As shown, the depth of the blind hole in the conductor specimen under test is obtained.

[0050] Lift-off disturbances significantly alter the curve τ of the eddy current decay time constant. ATC The shape of (t) is altered, thereby destroying the minimum value A of the difference characteristic curve. i1 The reliability of the detection of blind hole depth by equal feature quantities.

[0051] Depend on Figure 4 It can be seen that, under the same lift-off parameters, the difference A between the minimum values ​​of surface blind holes and subsurface blind holes is... i1 -A i2 It increases monotonically with increasing blind hole depth, from Figure 5 It can be seen that the difference A of the minimum characteristic quantities under the same blind hole depth condition is... i1 -A i2The impact of lift parameters is relatively minor. Taking blind holes with depths of d=3mm, 5mm, and 8mm as examples, data under an 80mm lift condition is plotted as a red "○", data under a 70mm lift condition is displayed as a black "+", data under a 60mm lift condition is designated as a green "☆", and data under a 50mm lift condition is marked as a pink "□". Figure 5 It can be seen that the minimum difference of the differential D-ATC curves for the same blind hole depth is relatively concentrated under the five lift-off conditions. This characteristic makes it possible to quantify the blind hole depth under large lift-off disturbances.

[0052] Taking a 3mm deep blind hole as an example, the maximum shift of its minimum difference under lift-off disturbance is +5.26μs, while the distance between the minimum difference characteristic of a 4mm deep blind hole and that of a 3mm deep blind hole under 100mm lift-off is +8.96μs. Therefore, the shift of the characteristic caused by lift-off disturbance is less than 60% of the difference of the characteristic corresponding to a 1mm blind hole depth change. Therefore, the difference in the minimum time of the differential D-ATC curve can be used as a characteristic for the detection of blind hole depth in aluminum plates under large lift-off disturbance background.

[0053] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the depth of a blind hole in a conductor based on pulsed eddy currents, characterized in that, Includes the following steps: S1: A pulsed eddy current detector is used to test the conductor specimen without blind holes to obtain the induced electromotive force output signal. As a blank control group; S2: For N conductor specimens of the same specification with known blind hole depths, a pulsed eddy current detector is used to perform tests above and behind the blind holes, respectively, to obtain the corresponding N surface blind hole induced electromotive force output signals. and N subsurface blind hole induced electromotive force output signals , ( =1,2,3…N); S3: respectively , and Through expression Converted to a characteristic curve based on the apparent time constant of eddy current decay. , and , ( =1,2,3…N), where The sign indicates the differential, and t is the reciprocal of the sampling frequency of the signal acquisition machine, i.e., the sampling time step. S4: Will Obtain the differential time constant curves for N surface blind holes and extract their minimum values. ,( =1,2,3…N), Obtain the differential time constant curves for N subsurface blind holes and extract their minimum values. ,( =1,2,3…N); S5: Will Obtain the feature quantity of the difference between N minimum values, and then... Plot a standard curve for blind hole depth; S6: Repeat S2 to S4 for specimens with unknown blind hole depths within 50 mm of the disturbance, and obtain the characteristic quantity of the difference between the minimum values ​​of surface blind holes and subsurface blind holes. The depth of the blind hole to be tested is finally determined by comparing it with the standard curve.

2. The method for detecting the depth of a conductor blind hole based on pulsed eddy currents according to claim 1, characterized in that, The pulsed eddy current detector includes a pulsed current transmitter, a detection probe, and a signal acquisition unit. Both the pulsed current transmitter and the signal acquisition unit are connected to the detection probe, and the signal feedback terminal of the signal acquisition unit is used to connect to a computer.

3. The method for detecting the depth of a conductor blind hole based on pulsed eddy currents according to claim 2, characterized in that, The detection probe includes a transmitting coil and a receiving coil. The signal output terminal of the pulse current transmitter is connected to the transmitting coil, and the signal acquisition terminal of the signal acquisition machine is connected to the receiving coil.

Images