CN112505138A - Pulse eddy current detection method for air gap with multilayer structure - Google Patents
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Abstract
A pulse eddy current detection method for air gaps of a multilayer structure belongs to the field of electromagnetic nondestructive detection. Firstly, acquiring a pulse eddy current detection signal of a differential relative probe, and extracting lift-off cross point signal characteristics under different probe lift-off distances; then changing the size of the air gap of the standard test piece to obtain the lift-off cross point signal characteristics under the condition of different multi-layer structure air gaps; then fitting an air gap measurement curve of the amplitude and time parameters of the lift-off intersection point which change along with the thickness; detecting a multilayer structure with unknown air gap size, acquiring the signal characteristics of the lift-off intersection point, and extracting the amplitude and time parameters of the lift-off intersection point; and finally, substituting the amplitude and the time parameter of the lift-off intersection point obtained under the condition of unknown air gap size into the air gap measurement curve obtained under the condition of the standard test piece to obtain the air gap size parameter under the corresponding condition, thus carrying out quantitative evaluation on the air gap with the multilayer structure. The detection device is convenient to use and good in detection effect.
Description
Technical Field
The invention relates to a pulse eddy current detection method, in particular to a pulse eddy current detection method of a multilayer structure air gap, which is suitable for the field of electromagnetic nondestructive detection.
Background
In the fields of aerospace, nuclear industry and the like, multilayer structures are widely applied, are used for bearing the negative impact pressure of external gas or the internal expansion pressure so as to enhance the mechanical property and improve the bearing capacity, and are key bearing basic components. Generally, the multilayer structure is a double-layer conductive structure and is covered with a non-conductive coating, so that the multilayer structure can be used under the conditions of pressure bearing and vibration, meanwhile, the corrosion problem caused by the environment is also caused, and the abrasion among the multilayer structures causes air gaps among the multilayer structures, further more serious corrosion and abrasion are caused, and the safety and the reliability of the multilayer structure are seriously influenced. The system can cause failure damage under the condition of long-term service, and can cause serious safety accidents and casualties when in serious condition. Therefore, it is necessary to periodically detect the air gap of the multi-layer structure.
Compared with other nondestructive detection methods, the pulse eddy current detection is an electromagnetic nondestructive detection method without digesting the surface non-conductive coating, and can realize the detection of the multilayer structure under the service condition. When a pulse excitation signal is applied to the probe excitation coil, an eddy current field is generated in the multilayer structure to be detected, a secondary magnetic field is generated in the multilayer structure in a reverse direction, and the change of the secondary magnetic field is influenced by the air gaps among the multilayer structures, so that the air gaps of the multilayer structure to be detected can be evaluated by measuring the change of the secondary magnetic field through the detection coil.
However, in practical inspection, the pulsed eddy current inspection signal is susceptible to surface non-conductive coatings or dirt, causing the distance between the probe and the multilayer structure to change, and further disturbing the inspection signal, reducing the inspection accuracy. The effect caused by the change of the lift-off distance from the probe to the multilayer structure is called as the lift-off effect or the lift-off effect. In addition to the surface non-conductive coating or fouling effects described above, lift-off effects can also occur from probe jitter, tilt, and pressure variations between the probe and the multi-layer structure. The lift-off effect interferes with or masks useful detection information, and seriously affects the detection result.
Disclosure of Invention
Aiming at the defects of the technology, the pulse eddy current detection method for the air gap of the multilayer structure is convenient to use, good in detection effect, capable of inhibiting the influences of the non-conductive coating on the surface of the multilayer structure, the dirt on the surface and the probe shaking, capable of improving the detection precision of the air gap of the multilayer structure and capable of accurately evaluating the integrity of the multilayer structure.
In order to realize the technical purpose, the pulse eddy current detection device used in the method for detecting the air gap with the multilayer structure comprises a probe, a function generator, a signal conditioning module, a data acquisition card and a computer, wherein the probe is a coil probe with a magnetic core and comprises the magnetic core, an excitation coil is arranged on the outer side of the magnetic core, a detection coil a and a detection coil b with different lift-off heights are respectively arranged on the outer side of the excitation coil, the distance between the detection coil a and the detection coil b can be adjusted according to the detection requirement, and the distance between the detection coil b and the bottom end of the probe is adjustable; the magnetic core can be replaced by magnetic cores of different materials according to detection requirements;
the method comprises the following steps:
a, firstly, adjusting the position distance between a detection coil a and a detection coil b in a probe according to requirements;
b, placing the probe in the air without contacting any test piece with a multilayer structure, collecting voltage response signals of the detection coil a and the detection coil b, and storing the voltage signals obtained under the air condition as reference signals;
c, selecting a multilayer structure test piece made of non-ferromagnetic materials, knowing material parameters and air gap size, then placing a probe on the multilayer structure test piece with the known material parameters and the air gap size, detecting the current multilayer structure test piece by using a pulse eddy current detection device, and obtaining voltage response signals of a detection coil a and a detection coil b of the multilayer structure test piece with the known material parameters and the air gap size;
d, subtracting the reference signal from the voltage response signal acquired by the detection coil a and the detection coil b obtained in the step c, so as to respectively obtain a difference signal between the detection coil a signal and the detection coil a reference signal of the current multilayer structure test piece and a difference signal between the detection coil b signal and the detection coil b reference signal of the current multilayer structure test piece;
e calculating time and amplitude parameters of the differential signal intersection point of the detection coil a and the detection coil b in the step d:
f, changing the size of the air gap of the multilayer structure test piece in the step c, keeping other conditions unchanged, and repeating the steps c-e, so as to obtain the amplitude value and the time parameter of the cross point of the difference signals between the detection coil a and the detection coil b of the non-ferromagnetic material multilayer structure test piece with different sizes of the air gap and the reference signal;
g, fitting the amplitude and the time parameters of the signal cross point obtained in the step f with the air gaps of different sizes of the corresponding multilayer structure test piece to obtain a measurement curve, and obtaining a signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the air gaps;
h, detecting the multilayer structure test piece with known material parameters and the unknown air gap size of the non-ferromagnetic material by using a pulse eddy current detection device, and repeating the steps b to e to obtain the signal cross point amplitude and the time parameter information of the multilayer structure test piece with the unknown air gap size at present;
i, substituting the amplitude parameter of the signal cross point obtained in the step h into the amplitude parameter measurement curve of the signal cross point obtained in the step g for calculation, and substituting the time parameter of the signal cross point obtained in the step h into the approximate linear section of the time parameter measurement curve obtained in the step g for calculation if the calculated size of the unknown air gap of the tested multilayer structure exceeds the calculation range defined by the approximate linear section of the amplitude parameter measurement curve of the signal cross point, wherein the calculation result is the size of the air gap of the multilayer structure test piece; if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is not exceeded, the calculation result is directly output, namely the size of the air gap of the multilayer structure test piece, and if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is exceeded, the calculation result is invalid.
j, when the selected multilayer structure test piece is made of ferromagnetic materials, placing a probe on the multilayer structure test piece with known material parameters and air gap size, and detecting the multilayer structure test piece made of ferromagnetic materials by using a pulse eddy current detection device to obtain voltage response signals of a detection coil a and a detection coil b of the multilayer structure test piece with known material parameters and air gap size; then, respectively subtracting the reference signals from the obtained voltage response signals collected by the detection coil a and the detection coil b, so as to respectively obtain the difference signals of the detection coil a signal and the reference signal of the current multilayer structure test piece and the difference signals of the detection coil b signal and the reference signal; then, by the formula
Calculating a standard difference value of differential signals of the detection coil a and the detection coil b; in the formula, xiFor the ith sample point data of the pulsed eddy current signal,
the average value of all sampling point data of the pulse eddy current signal is obtained, and n is the number of sampling points;
k, respectively subtracting the corresponding standard difference value calculated in the step j from the difference signal of the detection coil a and the detection coil b to obtain a secondary difference signal of the detection coil a and the detection coil b;
l, extracting the cross point characteristics of the secondary differential signals of the detection coil a and the detection coil b, and calculating the amplitude and time parameters of the signal cross point;
m, changing the size of the air gap of the multilayer structure test piece in the step j, keeping other conditions unchanged, and repeating the step j to the step l to obtain the amplitude value and the time parameter of the cross point of the detection signal of the multilayer structure test piece under the condition of different sizes of the air gap;
n, fitting the signal cross point amplitude and the time parameter obtained in the step m with the corresponding air gaps of the test pieces with different multilayer structures to obtain a measurement curve, and obtaining a secondary difference signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the size of the air gaps;
o, detecting the multilayer structure test piece with known material parameters and unknown air gap size of the ferromagnetic material by using a pulse eddy current detection device, and repeating the steps j to l to obtain the signal cross point amplitude and time parameter information of the multilayer structure test piece with the unknown air gap size;
p, substituting the amplitude parameter of the signal intersection point obtained in the step o into the amplitude parameter measurement curve of the secondary differential signal intersection point obtained in the step n for calculation, and substituting the time parameter of the signal intersection point obtained in the step o into the approximate linear section of the time parameter measurement curve obtained in the step n for calculation if the calculated unknown air gap size of the tested multilayer structure exceeds the approximate linear section defining calculation range of the amplitude parameter measurement curve of the signal intersection point, wherein the calculation result is the air gap size of the multilayer structure test piece; if the amplitude parameter does not exceed the approximate linear section range of the signal intersection point amplitude parameter measurement curve, directly outputting a calculation result, namely the air gap size of the multilayer structure test piece.
The distance between the detection coil a and the detection coil b of the probe is adjusted to be not more than the height of the probe at most, and the distance between the detection coil b and the bottom end of the probe is not more than half of the height of the probe at most.
The step of calculating the time and amplitude parameters of the differential signal crossing points in step e is: firstly, subtracting the data of the differential signal curves of the detection coil a and the detection coil b obtained in the step d to obtain a cross point, obtaining a time parameter corresponding to a zero value, and taking the time parameter at the falling edge of the differential signal of the detection coil a or the detection coil b as the time parameter of the signal cross point; then, the acquired time parameter is substituted into the differential signal curve of the detection coil a or the detection coil b, and the amplitude parameter of the signal intersection is calculated.
Has the advantages that: due to the adoption of the scheme, compared with the conventional nondestructive testing method, the measuring device and the measuring method do not need to digest the non-conductive coating or surface dirt of the multilayer structure, can effectively inhibit the influence of the lift-off change of the probe, and improve the measurement precision of the air gap of the multilayer structure and the reliability of the measurement result. Compared with other eddy current methods, the pulse eddy current detection method can realize effective measurement of the air gap of the multilayer structure, can inhibit the lift-off effect of the probe, and improves the measurement range and precision of the air gap of the multilayer structure. On the other hand, the evaluation of the integrity of the multi-layer structure is mostly performed from the aspects of defect detection and thickness measurement, and there are few reports focusing on the evaluation of the structural integrity by the air gap of the multi-layer structure. Therefore, the method for evaluating the quality of the multilayer structure by measuring the air gap of the multilayer structure through the signal cross point characteristic of the pulse eddy current probe can provide a novel method for evaluating the quality of the multilayer structure, also provides a method for detecting the pulse eddy current by inhibiting the lift-off effect of the probe, and further enriches and expands the application of the pulse eddy current detection technology in the evaluation of the integrity of the multilayer structure.
The signal cross point characteristic of the differential probe can effectively inhibit the lift-off effect, eliminate the influence of a non-conductive coating or surface dirt of the multilayer structure, realize the measurement of the air gap of the multilayer structure and further improve the pulse eddy current detection precision of the air gap of the multilayer structure.
The problem of probe lift-off effect caused by different thicknesses of the non-conductive coating when the air gap with the multi-layer structure of the non-conductive coating is detected is solved, and the purpose of the invention is achieved.
The advantages are that: the method can effectively eliminate the influence of a multilayer-structure non-conductive coating or surface dirt, can effectively inhibit the lift-off effect caused by the change of the lift-off distance between the probe and the test piece in the pulse eddy current detection process through the signal cross point characteristic, and obtains the signal cross point characteristic of the differential probe immune to the lift-off effect. When the detection of the air gap with the multilayer structure is carried out, the influence caused by the lifting change of the probe can be effectively inhibited, and the pulse eddy current detection precision and the pulse eddy current detection efficiency are further improved.
Drawings
FIG. 1 is a block diagram of a pulsed eddy current inspection system of the present invention.
FIG. 2 is a schematic diagram of a pulsed eddy current inspection probe according to the present invention.
FIG. 3 is a signal cross point signature of a differential probe according to the present invention.
FIG. 4 is a flow chart of a pulsed eddy current testing method for air gaps with a multi-layer structure according to the present invention.
Fig. 5 is a signal diagram of the cross-point characteristics of the signals of the ferromagnetic material acquired by the differential detection probe.
Fig. 6 is a graph of the measurement of signal cross-over point as a function of air gap for a multilayer structure.
In the figure: 1-probe, 2-function generator, 3-signal conditioning module, 4-data acquisition card, 5-computer, 6-exciting coil, 7-magnetic core, 8-detection coils a and 9-and detection coil b.
Detailed Description
Embodiments of the invention are further described below with reference to the accompanying drawings:
as shown in fig. 1 and fig. 2, the pulse eddy current testing method for air gaps with a multilayer structure of the present invention uses a pulse eddy current testing device comprising a probe 1, a function generator 2, a signal conditioning module 3, a data acquisition card 4 and a computer 5, wherein the probe 1 is a coil probe with a magnetic core and comprises a magnetic core 7, an excitation coil 6 is arranged outside the magnetic core 7, a detection coil a8 and a detection coil b9 with different lift-off heights are respectively arranged outside the excitation coil 6, the distance between the detection coil a8 and the detection coil b9 is adjustable according to the testing requirements, and the distance between the detection coil b9 and the bottom end of the probe 1 is adjustable; the magnetic core 7 can be made of different materials according to detection requirements; the distance between the detection coil a8 and the detection coil b9 of the probe 1 is adjusted to be not more than the height of the probe at most, and the distance between the detection coil b9 and the bottom end of the probe 1 is not more than half of the height of the probe at most.
As shown in FIG. 3, the steps of the pulsed eddy current testing method for air gap with multi-layer structure of the present invention are as follows:
a, firstly, adjusting the position distance between a detection coil a8 and a detection coil b9 in the probe 1 according to requirements;
b, placing the probe 1 in the air without contacting with any multi-layer structure test piece, collecting voltage response signals of the detection coil a8 and the detection coil b9, and storing the voltage signals obtained under the air condition as reference signals;
c, selecting a multi-layer structure test piece made of non-ferromagnetic materials, knowing material parameters and air gap size, then placing the probe 1 on the multi-layer structure test piece with the known material parameters and air gap size, detecting the current multi-layer structure test piece by using a pulse eddy current detection device, and obtaining voltage response signals of a detection coil a8 and a detection coil b9 of the multi-layer structure test piece with the known material parameters and air gap size;
d, subtracting the reference signal from the voltage response signals acquired by the detection coil a8 and the detection coil b9 obtained in the step c, so as to respectively obtain a difference signal between the detection coil a8 signal and the detection coil a8 reference signal of the current multilayer structure test piece and a difference signal between the detection coil b9 signal and the detection coil b9 reference signal of the current multilayer structure test piece;
e calculating the time and amplitude parameters of the differential signal intersection point of the detection coil a8 and the detection coil b9 in the step d: the steps of time and amplitude parameters of the crossing point are: firstly, subtracting the data of the differential signal curves of the detection coil a8 and the detection coil b9 obtained in the step d to obtain an intersection point, obtaining a time parameter corresponding to a zero value, and taking the time parameter at the falling edge of the differential signal of the detection coil a8 or the detection coil b9 as the time parameter of the signal intersection point; then, the acquired time parameters are brought into a differential signal curve of the detection coil a8 or the detection coil b9, and the amplitude parameter of the signal intersection point is calculated; fig. 4 is a signal diagram of a cross-point characteristic of a non-ferromagnetic material signal acquired by a differential detection probe.
f, changing the size of the air gap of the multilayer structure test piece in the step c, keeping other conditions unchanged, and repeating the steps c-e, so as to obtain the amplitude value and the time parameter of the cross point of the difference signals between the detection coil a8 and the detection coil b9 of the non-ferromagnetic material multilayer structure test piece with different sizes of the air gap and the reference signal;
g, fitting the amplitude and the time parameters of the signal cross point obtained in the step f with the air gaps of different sizes of the corresponding multilayer structure test piece to obtain a measurement curve, and obtaining a signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the air gaps; fig. 6 is a graph of the measurement of signal cross-over point as a function of air gap for a multilayer structure.
h, detecting the multilayer structure test piece with known material parameters and the unknown air gap size of the non-ferromagnetic material by using a pulse eddy current detection device, and repeating the steps b to e to obtain the signal cross point amplitude and the time parameter information of the multilayer structure test piece with the unknown air gap size at present;
i, substituting the amplitude parameter of the signal cross point obtained in the step h into the amplitude parameter measurement curve of the signal cross point obtained in the step g for calculation, and substituting the time parameter of the signal cross point obtained in the step h into the approximate linear section of the time parameter measurement curve obtained in the step g for calculation if the calculated size of the unknown air gap of the tested multilayer structure exceeds the calculation range defined by the approximate linear section of the amplitude parameter measurement curve of the signal cross point, wherein the calculation result is the size of the air gap of the multilayer structure test piece; if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is not exceeded, the calculation result is directly output, namely the size of the air gap of the multilayer structure test piece, and if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is exceeded, the calculation result is invalid.
j when the selected multilayer structure test piece is made of ferromagnetic materials, placing the probe 1 on the multilayer structure test piece with known material parameters and air gap size, detecting the multilayer structure test piece made of ferromagnetic materials by using a pulse eddy current detection device, and obtaining voltage response signals of the detection coil a8 and the detection coil b9 of the multilayer structure test piece with known material parameters and air gap size; then, the obtained voltage response signals collected by the detection coil a8 and the detection coil b9 are respectively subtracted by the reference signal, so that a difference signal of the detection coil a8 signal and the reference signal of the current multilayer structure test piece and a difference signal of the detection coil b9 signal and the reference signal are respectively obtained; then, by the formula
Calculating the standard deviation value of the differential signals of the detection coil a8 and the detection coil b 9; in the formula, xiFor the ith sample point data of the pulsed eddy current signal,
the average value of all sampling point data of the pulse eddy current signal is obtained, and n is the number of sampling points; fig. 5 is a signal diagram of the cross-point characteristics of the signals of the ferromagnetic material acquired by the differential detection probe.
k, subtracting the corresponding standard difference value calculated in the step j from the difference signal of the detection coil a8 and the detection coil b9 respectively to obtain secondary difference signals of the detection coil a8 and the detection coil b 9;
l, extracting the cross point characteristics of the secondary difference signals of the detection coil a8 and the detection coil b9, and calculating the amplitude and time parameters of the signal cross point;
m, changing the size of the air gap of the multilayer structure test piece in the step j, keeping other conditions unchanged, and repeating the step j to the step l to obtain the amplitude value and the time parameter of the cross point of the detection signal of the multilayer structure test piece under the condition of different sizes of the air gap;
n, fitting the signal cross point amplitude and the time parameter obtained in the step m with the corresponding air gaps of the test pieces with different multilayer structures to obtain a measurement curve, and obtaining a secondary difference signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the size of the air gaps;
o, detecting the multilayer structure test piece with known material parameters and unknown air gap size of the ferromagnetic material by using a pulse eddy current detection device, and repeating the steps j to l to obtain the signal cross point amplitude and time parameter information of the multilayer structure test piece with the unknown air gap size;
p, substituting the amplitude parameter of the signal intersection point obtained in the step o into the amplitude parameter measurement curve of the secondary differential signal intersection point obtained in the step n for calculation, and substituting the time parameter of the signal intersection point obtained in the step o into the approximate linear section of the time parameter measurement curve obtained in the step n for calculation if the calculated unknown air gap size of the tested multilayer structure exceeds the approximate linear section defining calculation range of the amplitude parameter measurement curve of the signal intersection point, wherein the calculation result is the air gap size of the multilayer structure test piece; if the amplitude parameter does not exceed the approximate linear section range of the signal intersection point amplitude parameter measurement curve, directly outputting a calculation result, namely the air gap size of the multilayer structure test piece.
Firstly, a detection signal of the differential probe in the air is obtained as a reference signal, and data is stored.
And then detecting the multilayer structure with known material parameters and air gap size to obtain a pulse eddy current detection signal of the multilayer structure.
Then, the detection signal and the reference signal of the multilayer structure are differentiated to obtain the pulsed eddy current differential signal shown in fig. 3. If the tested piece is a ferromagnetic material test piece, the difference between the differential signal and the standard difference value of the differential signal is calculated again, so as to obtain the signal cross point characteristic of the pulse eddy current secondary differential signal, as shown in fig. 5.
As the air gap of the multilayer structure increases, the signal crossover point features exhibit a regularly directed change, as shown in fig. 5. If the approximate linear section range of the measurement curve shown in fig. 6 is known, when a test piece of the air gap of the unknown multilayer structure to be measured is detected, only the signal cross point characteristic needs to be obtained, and the size of the air gap of the multilayer structure to be measured can be evaluated through the amplitude and the time parameter of the cross point characteristic.
Claims (4)
1. A pulse eddy current detection method for air gaps of a multilayer structure is characterized in that: the used pulse eddy current detection device comprises a probe (1), a function generator (2), a signal conditioning module (3), a data acquisition card (4) and a computer (5), wherein the probe (1) is a coil probe with a magnetic core and comprises a magnetic core (7), an excitation coil (6) is arranged on the outer side of the magnetic core (7), a detection coil a (8) and a detection coil b (9) with different lift-off heights are respectively arranged on the outer side of the excitation coil (6), the distance between the detection coil a (8) and the detection coil b (9) is adjustable according to detection requirements, and the distance between the detection coil b (9) and the bottom end of the probe (1) is adjustable; the magnetic core (7) can be replaced by magnetic cores of different materials according to detection requirements;
the method comprises the following steps:
a, firstly, adjusting the position distance of a detection coil a (8) and a detection coil b (9) in a probe (1) according to requirements;
b, placing the probe (1) in the air without contacting any test piece with a multilayer structure, acquiring voltage response signals of the detection coil a (8) and the detection coil b (9), and storing the voltage signals acquired under the air condition as reference signals;
c, selecting a multilayer structure test piece made of non-ferromagnetic materials, knowing material parameters and air gap size, then placing a probe (1) on the multilayer structure test piece with the known material parameters and the air gap size, detecting the current multilayer structure test piece by using a pulse eddy current detection device, and obtaining voltage response signals of a detection coil a (8) and a detection coil b (9) of the multilayer structure test piece with the known material parameters and the air gap size;
d, subtracting the reference signal from the voltage response signals acquired by the detection coil a (8) and the detection coil b (9) obtained in the step c, so as to respectively obtain a difference signal between the detection coil a (8) signal and the detection coil a (8) reference signal of the current multilayer structure test piece and a difference signal between the detection coil b (9) signal and the detection coil b (9) reference signal of the current multilayer structure test piece;
e calculating time and amplitude parameters of the differential signal intersection point of the detection coil a (8) and the detection coil b (9) in the step d:
f, changing the size of the air gap of the multilayer structure test piece in the step c, keeping other conditions unchanged, and repeating the steps c-e, so as to obtain the amplitude value and the time parameter of the cross point of the difference signals between the detection coil a (8) and the detection coil b (9) of the non-ferromagnetic material multilayer structure test piece with different sizes of the air gap and the reference signal;
g, fitting the amplitude and the time parameters of the signal cross point obtained in the step f with the air gaps of different sizes of the corresponding multilayer structure test piece to obtain a measurement curve, and obtaining a signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the air gaps;
h, detecting the multilayer structure test piece with known material parameters and the unknown air gap size of the non-ferromagnetic material by using a pulse eddy current detection device, and repeating the steps b to e to obtain the signal cross point amplitude and the time parameter information of the multilayer structure test piece with the unknown air gap size at present;
i, substituting the amplitude parameter of the signal cross point obtained in the step h into the amplitude parameter measurement curve of the signal cross point obtained in the step g for calculation, and substituting the time parameter of the signal cross point obtained in the step h into the approximate linear section of the time parameter measurement curve obtained in the step g for calculation if the calculated size of the unknown air gap of the tested multilayer structure exceeds the calculation range defined by the approximate linear section of the amplitude parameter measurement curve of the signal cross point, wherein the calculation result is the size of the air gap of the multilayer structure test piece; if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is not exceeded, the calculation result is directly output, namely the size of the air gap of the multilayer structure test piece, and if the range of the approximate linear section of the amplitude parameter measurement curve of the signal intersection point is exceeded, the calculation result is invalid.
2. The method of pulsed eddy current inspection of air gaps in a multi-layer structure of claim 1, wherein:
j, when the selected multilayer structure test piece is made of ferromagnetic materials, placing the probe (1) on the multilayer structure test piece with known material parameters and air gap size, detecting the multilayer structure test piece made of ferromagnetic materials by using a pulse eddy current detection device, and obtaining voltage response signals of a detection coil a (8) and a detection coil b (9) of the multilayer structure test piece with the currently known material parameters and the air gap size; then, respectively subtracting the reference signals from the obtained voltage response signals collected by the detection coil a (8) and the detection coil b (9), thereby respectively obtaining the difference signals of the detection coil a (8) signal and the reference signal and the difference signals of the detection coil b (9) signal and the reference signal of the current multilayer structure test piece; then, by the formula
Calculating a standard difference value of differential signals of the detection coil a (8) and the detection coil b (9); in the formula, xiFor the ith sample point data of the pulsed eddy current signal,
the mean value of all sampling points of the pulse eddy current signal is obtained, and n is the samplingCounting;
k, subtracting the corresponding standard difference value calculated in the step j from the difference signal of the detection coil a (8) and the detection coil b (9) respectively to obtain a secondary difference signal of the detection coil a (8) and the detection coil b (9);
l, extracting the cross point characteristics of the secondary difference signals of the detection coil a (8) and the detection coil b (9), and calculating the amplitude and time parameters of the signal cross point;
m, changing the size of the air gap of the multilayer structure test piece in the step j, keeping other conditions unchanged, and repeating the step j to the step l to obtain the amplitude value and the time parameter of the cross point of the detection signal of the multilayer structure test piece under the condition of different sizes of the air gap;
n, fitting the signal cross point amplitude and the time parameter obtained in the step m with the corresponding air gaps of the test pieces with different multilayer structures to obtain a measurement curve, and obtaining a secondary difference signal cross point amplitude parameter measurement curve and a time parameter measurement curve which change along with the size of the air gaps;
o, detecting the multilayer structure test piece with known material parameters and unknown air gap size of the ferromagnetic material by using a pulse eddy current detection device, and repeating the steps j to l to obtain the signal cross point amplitude and time parameter information of the multilayer structure test piece with the unknown air gap size;
p, substituting the amplitude parameter of the signal intersection point obtained in the step o into the amplitude parameter measurement curve of the secondary differential signal intersection point obtained in the step n for calculation, and substituting the time parameter of the signal intersection point obtained in the step o into the approximate linear section of the time parameter measurement curve obtained in the step n for calculation if the calculated unknown air gap size of the tested multilayer structure exceeds the approximate linear section defining calculation range of the amplitude parameter measurement curve of the signal intersection point, wherein the calculation result is the air gap size of the multilayer structure test piece; if the amplitude parameter does not exceed the approximate linear section range of the signal intersection point amplitude parameter measurement curve, directly outputting a calculation result, namely the air gap size of the multilayer structure test piece.
3. The method of pulsed eddy current inspection of air gaps in a multi-layer structure of claim 1, wherein: the distance between the detection coil a (8) and the detection coil b (9) of the probe (1) is adjusted to be not more than the height of the probe at most, and the distance between the detection coil b (9) and the bottom end of the probe (1) is not more than half of the height of the probe at most.
4. The method of pulsed eddy current inspection of air gaps in a multi-layer structure of claim 1, wherein: the step of calculating the time and amplitude parameters of the differential signal crossing points in step e is: firstly, subtracting the data of the differential signal curves of the detection coil a (8) and the detection coil b (9) obtained in the step d to obtain an intersection point, obtaining a time parameter corresponding to a zero value, and taking the time parameter at the falling edge of the differential signal of the detection coil a (8) or the detection coil b (9) as the time parameter of the signal intersection point; then, the acquired time parameter is substituted into a differential signal curve of the detection coil a (8) or the detection coil b (9), and an amplitude parameter of the signal intersection is calculated.
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