JPS6314905B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multi-frequency eddy current flaw detection method using a plurality of coils and a multi-frequency eddy current flaw detection device.

Multi-frequency eddy current flaw detection technology focuses on the fact that the penetration depth of eddy current in eddy current flaw detection depends on the test frequency, increases the amount of information obtained at once by using multiple test frequencies, and combines them. This is an attempt to reduce various noise factors, separate superimposed factors, estimate defect locations, etc.
It is used for inspections of nuclear power plants during their service life. This flaw detection method can be broadly classified into a mixing method and a switching method.

As shown in Fig. 1, the mixing method uses oscillators 1 1 , ₁ with different test frequencies f ₁ , f ₂ , ...
_The mixer 2 mixes excitation currents from the respective oscillators 1 ₁ , 1 ₂ - 1n to drive a single detection coil 3, and the detection coil 3 is provided with a band pass corresponding to each test frequency. Filter 4 ₁ , 4 ₂ ~
Single frequency flaw detector 5 ₁ for each frequency via 4n,
5 ₂ to 5n, and each of these flaw detectors 5 ₁ , 5 ₂ to 5
flaw detection outputs at a plurality of test frequencies can be obtained from n at the same time. The switching method is
As shown in FIG. 2, the test frequency of each single frequency flaw detector 5 ₁ , 5 ₂ to 5n is switched by the multiplexer 7 according to the timing of the switching timing generator 6, and the single detection coil 3 is time-divided. It is used for

As described above, in the conventional flaw detection method, a method is adopted in which a single detection coil 3 is used and this detection coil 3 is driven at a plurality of test frequencies. However, the conventional multi-frequency eddy current flaw detection method has the following problems when identifying defect positions.
For example, as shown in Fig. 3, artificial defects 8,
Sample 10 of 4 mm thick stainless steel plate with test frequency 10KHz and 100KHz 2
Perform frequency eddy current flaw detection. However, a mixing method is used here, and a single standard comparison type with a coil length of 2 mm is used for the detection coil 3. When the detection coil 3 is moved in the direction of the arrow, 10KHz,
The phase detection output at 100KHz is as shown in Figure 4 A and B. At 10 KHz, the penetration depth δ due to the skin effect of the electromagnetic field is about 4 mm, so not only defect 9 but also defect 8 is detected. On the other hand, at 100 KHz, only defect 9 was detected at a penetration depth δ of approximately 1.3 mm.
Therefore, if only the defect 9 on the back side on the same side as the detection coil 3 is to be detected, a single 100KHz is sufficient, and if only the defect 8 on the front side is to be detected, 10KHz and 100KHz for the same standard defect should be detected in advance. By calculating the sensitivity ratio k and setting it as V ₁₀ ×k−V ₁₀₀ to eliminate the defect 9 on the back surface, it becomes possible to detect only the defect 8 on the front surface. However, as is clear from the waveforms in FIG. 4, when the test frequencies are different, signals from the same factor end up changing not only the sensitivity but also the signal width. Generally, the lower the frequency, the wider the signal width, but this differs depending on the frequency due to the skin effect, which spreads the electromagnetic field within the conductor when the detection coil 3 is close to the conductor. This is due to this. Therefore, V ₁₀ ×k−V ₁₀₀ (k=1.3 in this case)
Even if the difference is determined, a difference remains as shown in the waveform of FIG. 4c, and it becomes impossible to detect the defect 8 only on the surface. To put it simply, with the same detection coil 3, the same factors are perceived differently depending on the frequency, so even if they are combined, it cannot be completely eliminated.

The above has described an example of defect position identification, but similar problems arise in removing and reducing noise factors as long as a single coil is used. In addition, in the case of a single coil, the bridge circuit for balancing is also single, so balance can only be achieved at one test frequency, and other test frequencies are left unbalanced. . If the test frequencies are relatively close, an unbalanced amplifier can still fall within the dynamic range of the amplifier, but as the ratio of the test frequencies increases, when one test frequency is balanced, the other test frequency will become saturated. There was a fear that it would end. For the above reasons, the conventional multi-frequency eddy current flaw detection method can only obtain a frequency ratio of 2 to 3 times. For this reason, the penetration depth ratio is limited to 1.4 to 1.7 times, and there is also the problem that a wide discrimination range cannot be achieved. Furthermore, since it is a single coil, the number of turns is the same, and there is a large difference in sensitivity depending on the test frequency, making it impossible to achieve high sensitivity in flaw detection.

In view of these conventional problems, the present invention is provided for the purpose of reducing differences in sensitivity due to differences in contributions from the same factor due to differences in test frequencies, and improving accuracy and sensitivity of signal analysis. The first feature is the multi-frequency eddy current, which applies excitation currents of multiple different test frequencies to the detection coil, generates eddy currents in the test material, and detects defects. In the flaw detection method, a plurality of detection coils to which excitation currents of different test frequencies are applied are used corresponding to each excitation current, and the response range of each eddy current generated on the surface of the test material by each excitation current is determined. Conditions such as the coil length, coil diameter, and number of turns of each detection coil are determined according to each test frequency so that the values are approximately equal, and the excitation current corresponding to each detection coil is applied to each detection coil during flaw detection. The difference in output from a single frequency flaw detector connected to each sensing coil is determined to eliminate noise factors between different test frequencies. The second feature is that the multi-frequency eddy current flaw detection device applies excitation currents of multiple different test frequencies to the detection coil, generates eddy currents in the test material, and detects defects. , excitation currents of different test frequencies are applied separately, and the response ranges of eddy currents generated on the surface of the test material by the excitation currents are approximately equal,
A plurality of detection coils with conditions such as coil length, coil diameter, number of turns, etc. determined according to each test frequency are provided corresponding to each excitation current, and a plurality of single frequency flaw detectors are connected to each detection coil. and analysis means for determining the difference in phase detection output from each of the flaw detectors,
and display means for displaying the analysis results from the analysis means, and each of the flaw detectors is provided with a bandpass filter or a correlator for each test frequency that passes only a specific test frequency corresponding to each detection coil. be.

Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments. In FIG. 5, ₁₁ is a test material;
2 ₂ to 12n are a plurality of detection coils provided for each test frequency, and each of the detection coils 12 ₁ ,
12 ₂ to 12n are single frequency eddy current flaw detectors 13 ₁ , 1
3 ₂ to 13n are connected to each other. Each of the detection coils 12 ₁ , 12 ₂ to 12n is wound around the same bobbin, and their respective response ranges on the surface of the test material 11 are approximately equal at each test frequency f ₁ , f ₂ , to fn. In addition, the coil length, coil diameter, and number of turns are determined so that optimal sensitivity can be obtained for each test frequency f ₁ , f ₂ , ~fn. In other words, for interpolated and through-type coils, the range in which the defect responds is determined by the coil length, but that range also changes depending on the test frequency, and in general, the higher the frequency, the smaller the penetration depth and the narrower the response range. Therefore, by making the length of the high-frequency detection coil slightly longer in advance to compensate for the above-mentioned influence, the response range of each test frequency is matched. In addition, with probe-type coils, the range in which defects respond is determined by the coil diameter, and that range also changes depending on the test frequency. Match the response range of each test frequency by adjusting . Therefore, the contribution from the noise factor in the test material 11 is equal to each detection coil 12 ₁ , 12 ₂ to 12n, and it is possible to prevent changes in signal width due to differences in test frequencies, and to eliminate noise in the signal processing process. factors can be removed.

Each single frequency eddy current flaw detector 13 ₁ , 13 ₂ ~ 13n
are the oscillators 14 ₁ , 14 of each test frequency f ₁ , f ₂ to fn
Bridge circuits 15 ₁ , 15 ₂ -15n that balance the detection coils 12 ₁ , 12 ₂ -12n for each test frequency _{f 1} , f ₂ , -fn, each test frequency f ₁ , f ₂ , Bandpass filter 1 that passes only ~fn
6 ₁ , 16 ₂ to 16n, and phase detector 17 ₁ , 1
7 ₂ to 17n. Each detection coil 12 ₁ ,
If 12 ₂ and 12n are wound on the same bobbin,
Even when the coils are moved, the arrangement parameters such as changes in the distance between the test material 11 and each of the detection coils 12 ₁ , 12 ₂ to 12n can be made the same, but the excitation current of each test frequency can be used to simultaneously Detection coil 12 ₁ , 1
When 2 ₂ to 12n are driven, the detection coils 12 ₁ ,
Mutual inductance occurs between 12 ₂ and 12n,
The i-th sensing coil 12i has 1 to (i-1),
An induced voltage is generated by the excitation current of each test frequency flowing through the (i+1) to n-th detection coils. By removing this with the bandpass filter 16i, it becomes possible to perform phase detection on the component of the test frequency fi, and the detection outputs Xi, Yi(i
=1, 2,...n). These are each connected to an amplifier 18.
After multiplying X ₁ , 18Y ₁ to 18X _o , 18Y _o by a proportional constant determined in advance to obtain KiXi, liYi, the adder/subtractor 19 performs a linear operation to obtain A= _o 〓 ⁱ⁼¹ (kiXi
+liYi). Note that ki and li are proportionality constants determined in advance from standard samples. The calculation result obtained by the adder/subtracter 19 is displayed in waveform on a display 20 such as a pen recorder.

In the above embodiment, the output of each bridge circuit 15i is correlated with the sine wave signal sin2πfit and cosine wave signal cos2πfit output from the oscillator 14i, as shown in FIG. 6, instead of passing through the bandpass filter 16i and the phase detector 17i. A similar function can be obtained by inputting the data into the device 21. Note that the correlator 21 includes multipliers 22Xi, 22Yi and an integrator 23.
Xi, 23Yi.

Next, another embodiment for realizing the method of the present invention is shown in FIG. This includes a plurality of detection coils 12 ₁ , 12 ₂ to 12 provided for each test frequency f ₁ , f ₂ . . . fn.
One variable frequency eddy current flaw detector 24 for n
The oscillation frequency of the variable frequency oscillator 14 is sequentially switched by the timing pulse from the switching timing generator 25, and the signal switching unit 26 accordingly switches between the predetermined detection coils 12 ₁ , 12 ₂ -
12n and the corresponding test frequencies f ₁ , f ₂ ,...fn
It is designed to distribute the . Therefore, different test frequencies f ₁ , f ₂ ,...
A phase detection output with respect to fn is obtained, which is sequentially passed through sample and hold units 27X ₁ , 27Y ₁ , 27X ₂ ,
By sampling and holding at 27Y ₂ to 27X _o and 27Y _o , the detection outputs of each test frequency X ₁ , Y ₁ ,
…Obtain X _o and Y _o . The process after this sample hold is the same as described above. The eddy current flaw detector 24 includes an oscillator 14, a bridge circuit 15, and a phase detector 16.

Next, an example will be described in which the present invention is applied to the inspection of a single heat exchanger material during its service life.

Test material: Heat exchanger copper alloy tube outer diameter 25.4mm
Wall thickness: 1.5mm Detection target: vertical flaw (artificial defect, width: 0.1mm)
(Depth 0.3mm Length 100mm) Noise factor…Internal corrosion Test frequency…20KHz 100KHz Detection coil…Interpolation type Self-comparison Multiple coils
A 20KHz coil (length 50mm) is placed around the outer circumference of a 100KHz coil (length 54mm).
mm) concentrically.

The flaw detector is for 20KHz as shown in Figure 8.
Single frequency eddy current flaw detector 13a, 13 for 100KHz
Use b respectively. In this case, for example, the detection coil 12a on the 20KHz side is connected to the detection coil 12b on the 100KHz side due to mutual inductance.
Since a 100KHz component is induced, the bridge output signal is passed through a 20KHz bandpass filter 16a.
The 100 KHz component is removed, and the changed amplitude and phase of the 20 KHz component are adjusted by the amplifier 28a and phase shifter 29a, respectively, and input to the phase detector 16a.
The same goes for the 100KHz side. Of the phase detection outputs, Y ₂₀ and Y ₁₀₀ are used here to calculate k ₂₀ Y ₂₀ +k ₁₀₀ Y ₁₀₀ using an adder/subtractor 19, and the result is input to a display 20 such as a pen recorder to display the calculated waveform. Display.

20KHz and 100KHz flaw detectors 13a and 13, respectively
By adjusting the phase of b so that the backlash component of the detection coils 12a and 12b becomes horizontal, each Y signal no longer contains the backlash component. Therefore, when observing each Y signal (Y ₂₀ , Y ₁₀₀ ),
As shown in Figure 9, the 20KHz Y signal a also
Large fluctuations due to internal corrosion are also seen in the 100 KHz Y signal b, and artificial defects on the external surface are caused by the 20 KHz Y signal a.
It's also buried inside. Therefore, k ₂₀ Y ₂₀ +K ₁₀₀ Y ₁₀₀
When (k ₂₀ and k ₁₀₀ are constants determined in advance), an artificial defect portion d appears as shown in the top signal c.

On the other hand, when a similar analysis is performed using a single coil using a mixing method as in the conventional two-frequency eddy current method, the 100KHz sensing coil 1
2b cannot be made slightly longer than the 20KHz detection coil 12a, so as shown in Figure 10, the 100KHz Y signal b is sensitive to corrosion components.
It is different from the 20KHz Y signal a. Since the contribution of corrosion components is originally very large, a slight difference in signal width in a portion where the slope of the signal waveform is large will result in a large spurious signal as a result of calculation.

FIG. 11 shows another example of analysis according to the present invention. This example is for a sample containing an artificial defect with a width of 0.1 mm, a depth of 0.5 mm, and a length of 100 mm.

For example, as shown in Fig. 12, a slit 31 with a depth of 0.5 mm is formed on the inner circumferential surface of a defect-free stainless steel pipe material 30, and this is detected by the interpolation type standard comparison method as internal corrosion to be removed. In this case, since stainless steel is a non-magnetic material with a specific resistance of 70 μΩ·cm, the penetration depth δ of the eddy current at that time is as follows.

When f=5KHz δ=6.0mm When f=10KHz δ=4.2mm When f=100KHz δ=1.3mm Therefore, 5KHz is adopted to detect external flaws, and 100KHz is adopted to remove internal noise. You can use these and combine them.

The signal waveforms when the slit 31 was inspected at test frequencies of 5 KHz and 100 KHz are as shown in FIGS. 13A and 13B. However, the coil length is the same at 30mm.
As can be seen from this, as the frequency ratio increases, the signal width varies considerably and becomes narrower on the high frequency side. Therefore, by making the coil length on the high frequency side slightly longer than that on the low frequency side, the result will be as shown in Figure c.
It can be matched with the signal width on the low frequency side. In this case, just lengthen the 100KHz side by 5mm.

As detailed in the examples above, in the method of the present invention,
A plurality of sensing coils to which excitation currents of different test frequencies are applied are used corresponding to each excitation current, and the response ranges of each eddy current generated on the surface of the test material are approximately equal due to each excitation current. According to each test frequency, the coil length, coil diameter,
Conditions such as the number of turns are determined, and during flaw detection, each excitation current corresponding to the detection coil is applied to each detection coil, and the difference in output from the single frequency flaw detector connected to each detection coil is determined. , since this noise factor between different test frequencies is removed, S/
Pseudo signals are not generated due to calculations that should bring about effects such as improvement of N, and flaw detection accuracy can be improved.

In the apparatus of the present invention, excitation currents of different test frequencies are applied separately, and the excitation currents are applied according to each test frequency so that the response ranges of eddy currents generated on the surface of the test material by the excitation currents are approximately equal. A plurality of detection coils with fixed conditions such as coil length, coil diameter, number of turns, etc. are provided corresponding to each excitation current, and a plurality of single frequency flaw detectors connected to each detection coil, and each of the flaw detectors. and a display means for displaying the analysis results from the analysis means. Since it is equipped with a filter or correlator for each test frequency and multiple detection coils corresponding to each different test frequency, it is possible to design the optimum coil for each test frequency, and high-sensitivity flaw detection is possible. be. In addition, since multiple detection coils are installed, multiple bridge circuits can be installed independently.
Each test frequency can be balanced independently. Furthermore, since balance is possible by combining arbitrary test frequencies, flaw detection can be performed at arbitrary depths.

[Brief explanation of the drawing]

Figures 1 and 2 are block diagrams showing conventional examples;
3 is an explanatory diagram of the same, FIG. 4 is a waveform diagram of the same, FIG. 5 is a block diagram showing one embodiment of the present invention, FIGS. 6 and 7 are block diagrams showing other embodiments, and FIG. The figure is a block diagram showing an example of application of the present invention, Figures 9 and 11 are diagrams showing an example of signal analysis of the present invention, Figure 10 is a diagram showing an example of conventional signal analysis, and Figure 12 is a diagram showing an example of signal analysis of the present invention. The explanatory diagram, FIG. 13, is a waveform diagram thereof. DESCRIPTION OF SYMBOLS 11... Test material, 12... Detection coil, 13... Single frequency eddy current flaw detector, 14... Oscillator, 15... Bridge circuit, 16... Phase detector, 19... Adder/subtractor, 20
...Indicator, 21...Correlator, 26...Signal switch.

Claims (1)

[Scope of Claims] 1 In the multi-frequency eddy current flaw detection method in which a plurality of excitation currents with different test frequencies are applied to a detection coil and eddy currents are generated in the test material to detect defects, excitation currents with different test frequencies are used. A plurality of detection coils to which current is applied are used in correspondence with each of the excitation currents,
Conditions such as the coil length, coil diameter, and number of turns of each detection coil are determined according to each test frequency so that the response range of each eddy current generated on the surface of the test material by each excitation current is approximately equal. Then, during flaw detection, apply an excitation current corresponding to each detection coil to each detection coil, find the difference in output from the single frequency flaw detector connected to each detection coil, and calculate the difference between the different test frequencies. A multi-frequency eddy current flaw detection method using a multiple coil method, which is characterized by the removal of noise factors. 2. In a multi-frequency eddy current flaw detection device that detects defects by applying multiple types of excitation currents with different test frequencies to the detection coil and generating eddy currents in the test material, the excitation currents with different test frequencies are applied separately. , and so that the response ranges of eddy currents generated on the surface of the test material by the excitation current are approximately equal,
A plurality of detection coils with conditions such as coil length, coil diameter, number of turns, etc. determined according to each test frequency are provided corresponding to each excitation current, and a plurality of single frequency flaw detectors are connected to each detection coil. and analysis means for determining the difference in phase detection output from each of the flaw detectors,
and a display means for displaying the analysis results from the analysis means, and each of the flaw detectors is provided with a bandpass filter or a correlator for each test frequency that passes only a specific test frequency corresponding to each detection coil. Features multi-frequency eddy current flaw detection equipment.