JP2006118902A - Defect height evaluation method by eddy current testing - Google Patents

Abstract

【Task】
It was difficult to evaluate the height of defects formed on the surface of a material having a plate thickness greater than the penetration depth.
[Solution]
From the amplitude data when eddy current flaw measurement is performed on two or more different frequency conditions for the same defect, obtain a numerical value where the effect of the electrical conductivity of the crack surface of the defect is sufficiently smaller than the effect of the defect size. By evaluating the height of the defect, it was possible to evaluate the height of the defect formed on the surface of the material having a thickness greater than the penetration depth.
According to the present invention, in the defect height evaluation by eddy current flaw detection, the defect height can be quickly evaluated from the eddy current flaw measurement result, and no special skill is involved. effective.
In addition, since the defect evaluation procedure is simple, the potential for evaluation errors is low, which is effective in improving the reliability of inspection.
Moreover, since no device other than the device used for general eddy current flaw detection is required, capital investment is unnecessary, and there is an effect in terms of cost.
[Selection] Figure 1

Description

  The present invention relates to a technique related to height evaluation of defects by eddy current flaw detection.

  Conventional height evaluation of defects by eddy current flaw detection is mainly applied to the inspection of heat transfer tubes in pressurized water nuclear power plants.

  In a material having a plate thickness equal to or less than the skin depth of the magnetic flux generated from the eddy current flaw detection probe, such as a heat transfer tube, the phase change corresponding to the height of the defect can be obtained by eddy current flaw inspection. Defect height can be evaluated from phase change. Here, the skin depth is a depth at which the magnetic flux is attenuated to 1 / e times the surface, and is expressed by Equation [2].

  However, in a material having a plate thickness equal to or greater than the skin depth, it is difficult to evaluate the height of the defect from the change in phase because a significant change in phase according to the height of the defect cannot be obtained.

但し、δ：表皮深さ，μ：透磁率，ρ：導電率，ｆ：試験周波数

Where δ: skin depth, μ: permeability, ρ: conductivity, f: test frequency

Japan Nondestructive Inspection Association 2003 Spring Conference Presentation Summary 49-50 pages May 2003 Issued

  Eddy current flaw detection is a technique that mainly detects surface defects from changes in eddy current flowing on the surface of a subject. Signals related to defects obtained by eddy current flaw detection are a phase angle and an amplitude. In the prior art, the depth height is evaluated from the change in the phase angle. However, for defects formed on the surface of a material having a plate thickness equal to or greater than the penetration depth, it is difficult to evaluate the height of the defects because a phase change corresponding to the height of the defects cannot be obtained.

  An object of the present invention is to provide a method for evaluating a defect height by paying attention to an amplitude in view of the problems of the above-described conventional technology.

  The present invention that achieves the above-described object is achieved by using a plurality of amplitudes obtained by eddy current flaw measurement under two or more different test frequency conditions for the same defect, or two values 1 and values different in the same dimension calculated from them. The ratio of 2 is used to evaluate the defect height. Further, for setting the flaw detection sensitivity at each test frequency at the time of eddy current flaw measurement, an EDM slit or a fatigue defect greater than the skin depth of the magnetic flux at the lowest frequency condition among the test frequencies is used.

  According to the present invention, in the defect height evaluation by eddy current flaw detection, the defect height can be quickly evaluated from the eddy current flaw measurement result, and no special skill is involved. effective.

  Further, since the defect evaluation procedure is simple, the potential for evaluation mistakes is low, which is effective in improving the reliability of inspection.

  Moreover, since no device other than the device used for general eddy current flaw detection is required, capital investment is unnecessary, and there is an effect in terms of cost.

  Embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is an explanatory diagram of an evaluation procedure when the present invention is applied to the height evaluation of a defect having a defect length of 20 mm or more and a defect height of 10 mm or less. Here, the eddy current flaw detection probe used for the measurement is premised on that the amplitude value when measuring EDM slits having the same height and different lengths does not change when the length is 20 mm or more.

In Process 1, an artificial defect (for example, an electric discharge machining EDM slit, hereinafter referred to as an EDM slit) in a defect height range to be evaluated is detected under two or more different frequency conditions, and the amplitude and EDM slit height are determined. Find the relationship curve. Here, in the eddy current flaw measurement, the amplitude value when flaw detection is performed on the EDM slit 1 having a defect height equal to or greater than the maximum height of the defect height evaluation range is set to be the same in all frequency conditions. That is the point. However, if the amplitude value at the time of EDM slit 1 flaw detection is not the same under all frequency conditions,
The amplitude value at the time of EDM slit 1 flaw detection may be normalized. Usually, when evaluating the height of a defect having a height of 10 mm or less, an eddy current flaw detection probe that can change the amplitude according to the defect height in a range of 10 mm or less is used, and the flaw detection sensitivity is 10 mm. The amplitude is set to 5 V at the time of EDM slit flaw detection. However, for example, in the case of using an eddy current flaw detection probe in which a clear amplitude change according to the defect height is 5 mm or less, the flaw detection sensitivity is set to 5 V amplitude at the time of EDM slit flaw detection with a height of 5 mm. To do.
The frequency condition may be the maximum, minimum and intermediate frequency of the eddy current testing probe. Here, the applied frequency is a frequency range in which flaw detection can be performed with sufficient sensitivity.

  In process 2, the average value and standard deviation of the amplitude are obtained for each EDM slit from the amplitude data at two or more different frequencies obtained in process 1, and the average / standard deviation ratio (hereinafter referred to as X value). And create a relationship curve of defect height.

  In process 3, the defect to be height-evaluated is detected under the same flaw detection conditions as in process 1, the X value is obtained, and the result is applied to the relationship curve between the defect height and the X value created in process 2 to obtain the defect height. To evaluate. The explanatory diagram is an example of height evaluation when X = 15 is obtained when a defect having a length of 20 mm or more is detected, and the height is evaluated to be about 4 mm by applying X = 15 to the drawing. .

  FIG. 2 shows a defect having a length of 20 mm or more and a height of 10 mm or less using an eddy current flaw detection probe having the same height and the same amplitude when measuring EDM slits of different lengths of 20 mm or more. It is an example of the test piece for obtaining the relationship curve of X value and defect height for performing height evaluation. The test piece is provided with an EDM slit having a height of 1 to 10 mm so that a relationship curve between the X value and the defect height in a range of 10 mm or less can be obtained. EDM slits ensure that the output of eddy current flaw detection is not affected by adjacent EDM slits, and the test piece design is such that the width of the test piece is such that the output of eddy current flaw detection is not affected by the test piece width. Is the point. In addition, it is considered that the width of the EDM slit is so small that the output of the eddy current flaw detection is negligible so that the influence of the EDM slit width can be ignored. It should be 3 or less.

  FIG. 3 is an explanatory diagram of an evaluation procedure when the present invention is applied to the height evaluation of a defect having a defect length of 20 mm or less and a defect height of 10 mm or less. Here, it is assumed that the amplitude when EDM slits having the same height and different lengths are measured changes with a length of 20 mm or less.

  In process 1, an EDM slit including a height range to be evaluated for height is detected at two different frequencies, and the relationship between amplitude, slit depth, and EDM slit length is obtained. Here, in the eddy current flaw detection measurement, the amplitude value when flaw detection is performed on the EDM slit 1 having a defect height equal to or greater than the maximum height of the defect height evaluation range is set to be the same in all frequency conditions. That is the point. However, when the amplitude value at the time of EDM slit 1 flaw detection is not equal under all frequency conditions, the amplitude value at the time of EDM slit 1 flaw detection may be normalized.

  In process 2, a relationship curve of X value, defect length and defect height is created for each EDM slit from amplitudes at two or more different frequencies.

  In Process 3, the defect for height evaluation is detected under the same flaw detection conditions as in Process 1, the X value is obtained, and the result is applied to the relationship curve of X value, defect length and defect height created in Process 2. Evaluate the defect height. The explanatory diagram is an example of height evaluation when an X value = 10 is obtained by defect inspection with a length of 7 mm. The length 7 mm and the X value 10 are applied to a relational curve, and the defect height is evaluated to be about 3 mm. . In addition, as the defect length, an instruction length by an eddy current inspection method and an instruction length by an infiltration inspection may be used.

  Fig. 4 shows an eddy current flaw detection probe that does not change when the EDM slits with the same height and different lengths are measured. The height is evaluated for a defect with a length of 20 mm or less and a height of 10 mm or less. This is an example of a test piece for obtaining a relationship curve of an X value, a defect length, and a defect height. The test piece is provided with an EDM slit having a height of 1 to 10 mm so that a relationship curve between the X value and the height in a range of 10 mm or less can be obtained. EDM slits ensure that the output of eddy current flaw detection is not affected by adjacent slits, and the width of the test piece is such that the output of eddy current flaw detection is not affected by the width of the test piece. It is a point. In addition, it is considered that the width of the EDM slit is so small that the output of the eddy current flaw detection is negligible so that the influence of the EDM slit width can be ignored. 3 or less is recommended.

  FIG. 5 shows the result of the evaluation of the height of the defect of stress corrosion cracking (hereinafter referred to as SCC) using the relationship curve between the X value and the height of the defect obtained from the measurement result of the EDM slit by the eddy current flaw detection method. The comparison with the defect height measured by the acoustic flaw detection is shown. From this result, the defect height evaluation result according to the present invention and the defect height measurement value by the ultrasonic flaw detection method agree within the range of variation, and the X value obtained from the measurement result of the EDM slit by the eddy current flaw detection method and It can be seen that the defect height of stress corrosion cracking (hereinafter referred to as SCC) can be evaluated using the relationship curve of the defect height.

  The reason why the defect height of the stress corrosion cracking can be evaluated from the relationship curve between the X value obtained from the measurement result of the EDM slit and the defect height will be described below.

  Since the defect surface of the EDM slit is completely open, an eddy current generated by eddy current flaw detection cannot pass through the defect surface and can be regarded as an electrically insulated defect. On the other hand, a partial connection is observed on the defective surface of the SCC, and there is an electrically conductive portion. Due to the difference in electrical continuity of the defect surface, when eddy current flaw measurement is performed under the same conditions of the EDM slit and SCC having the same height and length, the amplitude of the EDM slit tends to be equal to or larger than the amplitude of the SCC. It becomes. However, it has been empirically found that the SCC defect height can be evaluated from the relationship curve between the ratio of the average amplitude and the standard deviation and the defect height when the EDM slit is measured under two or more different frequency conditions. The relationship of the equation [3] is approximately established between the amplitude and the electrical conductivity of the defect surface, and the function in which the ratio of the average and the standard deviation hardly depends on the electrical conductivity of the defect surface as shown in the equation [4]. It is assumed that the ratio between the average and standard deviation of EDM slits and SCCs of the same height and length is almost the same value. In addition to the ratio of the average and standard deviation in the equation [4], the function form that hardly depends on the electrical conductivity of the defect surface includes the ratio between the average value and the maximum value of the amplitude, the average value and the minimum value of the amplitude. It can be seen that there are many ratios.

  In the present embodiment described above, since the EDM slits or fatigue defects having different heights are used to create a relational curve that enables the evaluation of the height of SCC generated in a thick material having a plate thickness of 10 mm or more, In addition to the equipment used for eddy current inspection and the test piece for creating the relation curve, no special equipment or equipment is required. Therefore, the height of the defect can be evaluated at a low cost, and the height of the defect can be evaluated without a special skill.

  Examples of the present invention will be described below.

  As Example 1, an inspection flow diagram using the present invention is shown in FIG.

The inspection flow diagram is a product inspection flow diagram based on the premise that the inspection passes if it is below the allowable dimension even if there is a defect.
First, the product is checked for defects by measuring eddy currents. In the measurement, in order to detect a minute defect, the flaw detection sensitivity may be set with a shallow EDM slit having a height of 1 mm or the like.
If a defect is found in this inspection, the distribution and length of the defect interval, direction, etc. are measured by an eddy current flaw detection method. In the defect distribution and the defect length measurement, since it is necessary to clearly grasp both ends of the defect opening length, the flaw detection sensitivity may be set by a shallow EDM slit having a height of 1 mm or the like.
The defect interval is measured from the distance between the amplitude peaks, and the defect direction is measured from the surface distribution image of the defect called a C scope image. In addition, regarding the defect length, when performing a safety evaluation in the soundness evaluation of the subject, the signal loss length that is overestimated than the actual length is used, and when performing a realistic evaluation, the actual length is used. It is preferable to use the indicated length by the 6 dB down method or the 12 dB down method, which is equivalent to the above.

  Next, the defect height is evaluated according to the present invention. The defect to be evaluated is measured at two or more frequencies, and the average value and standard deviation of the amplitude are obtained. These ratios are applied to a curve diagram of the relationship between the ratio of the average value of the amplitude and the standard deviation acquired in advance and the defect height, and the defect height is evaluated. In eddy current flaw measurement for height evaluation, it is necessary to obtain a large amplitude change in accordance with the frequency in the height evaluation target range. Therefore, assuming that the height evaluation target range is 10 mm or less, the height It is important to measure EDM slits of 10 mm or more with a sensitivity that can be detected with a sufficient S / N ratio.

  All the lengths and heights of defects obtained from these inspections are recorded, and if all the defects are less than the allowable dimensions, the product inspection is deemed acceptable.

Explanatory drawing of the evaluation procedure in the case of applying this invention to height evaluation of the defect of defect length 20mm or more and defect height 10mm or less. The test piece structure figure for obtaining the relationship curve of defect height and X value. The evaluation procedure figure in the case of applying this invention to the height evaluation of a defect whose defect length is 20 mm or less and defect height is 10 mm or less. FIG. 3 is a structural diagram of a specimen for obtaining a relationship curve of defect height, defect length, and X value. The comparison figure of the defect height evaluation result of the stress corrosion cracking by this invention, and the defect height measurement result by an ultrasonic flaw detection method. FIG. 2 is an inspection flow diagram according to the present invention in Example 1;

Claims (6)

  In the defect height evaluation method using the eddy current flaw detection method, the ratio of two different amplitudes 1 and 2 obtained from a plurality of amplitudes obtained by eddy current flaw measurement under two or more different test frequency conditions Or a ratio of two values 1 and 2 that are different in the same dimension calculated from a plurality of amplitudes, or a value 3 that has a different value in the same dimension as the amplitude 3 calculated from a plurality of amplitudes 3 and A defect height evaluation method characterized by performing defect height evaluation using a ratio.   2. The ratio between two values 1 and 2 different in the same dimension, or the ratio between the amplitude 3 and the value 3 having a different value in the same dimension as the amplitude 3 calculated from a plurality of amplitudes is 2. Average value, maximum value, minimum value, deviation, amplitude value of frequency 1, amplitude value of frequency 2 different from frequency 1, and maximum value obtained by eddy current flaw measurement under more than one kind of different frequency conditions And the difference between the maximum value and the minimum value, the difference between the amplitude value at frequency 1 and the amplitude value at frequency 2 different from frequency 1, and the amplitude value at frequency 2 different from the amplitude value at frequency 1 and frequency 1. A defect height evaluation method characterized by a combination ratio by sum.   The ratio of two different amplitudes 1 and 2 obtained from a plurality of amplitudes obtained by eddy current measurement of the same frequency condition as in claim 1 for a plurality of pseudo defects of different known dimensions in claim 1 Relationship between defect size, ratio of two different values 1 and 2 in the same dimension calculated from multiple amplitudes, and relationship between defect size, or same dimension as amplitude 3 calculated from amplitude 3 and multiple amplitudes A defect height evaluation method characterized in that a defect height is evaluated based on a relationship curve between a ratio of a value 3 having a different value to 3 and a defect dimension.   3. The defect height evaluation method according to claim 2, wherein the defect size is a defect height or a defect height and a defect length. 3. The flaw detection sensitivity at each frequency when measuring eddy current flaws under two different test frequency conditions is the same EDM slit having a height equal to or greater than the skin depth defined by the equation [1]. A defect height evaluation method, characterized in that the amplitude value is obtained when eddy current flaw detection is performed.

Where δ: skin depth, μ: permeability, ρ: conductivity,
f: Lowest test frequency when measuring eddy current flaws 6. The defect height evaluation method according to claim 5, wherein when the flaw detection sensitivity is different at each frequency, the flaw detection sensitivity is standardized to make the flaw detection sensitivity the same.

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