Eddy current testing method and eddy current testing apparatus

Abstract

The present invention provides an eddy current testing method and apparatus that can reliably and easily detect cracks and scratches on metal surfaces. [Solution] The present invention provides an eddy current testing method for detecting damaged areas on the surface of an object to be inspected by supplying an alternating current to a detection coil 13 having resonant characteristics, and by contacting, scanning with, or scanning the detection coil 13 over each part of the surface of an object to be inspected, and comparing the output alternating current voltages generated at both ends of the detection coil 13 at each part of the surface of the object to be inspected, the frequency of the alternating current is set for detecting damaged areas based on the amplitude difference of the output alternating current voltages generated at the detection coil 13 for each part of the surface of the object to be inspected.

Description

Technical Field

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[0001] The present invention relates to an eddy current flaw detection method and an eddy current flaw detection device for non-destructively detecting cracks and scratches formed on a metal surface.

Background Art

[0002] Conventionally, an alternating current of a specific frequency at which the resonance curve of the output alternating voltage generated by the detection coil in free space intersects with the resonance curve of the output alternating voltage generated by the detection coil on the metal surface is supplied to a detection coil and a compensation coil having resonance characteristics, and the detection coil is scanned on the metal surface. An eddy current flaw detection method and device (Patent Document 1) have been proposed in which cracks and scratches formed on the metal surface are detected by comparing the amplitude changes of the voltages at both ends of each of the detection coil and the compensation coil, and the depths of the cracks and scratches are quantitatively measured.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the eddy current flaw detection method and device as described in Patent Document 1, the amplitude change when detecting cracks and scratches formed on the metal surface is small, and an error occurs in the amplitude change due to slight fluctuations. Therefore, it is difficult to easily detect cracks and scratches formed on the metal surface, and the flaw detection method had to be implemented and the flaw detection device had to be operated while paying close attention.

[0005] Therefore, in view of the above-described situation, the present invention provides an eddy current flaw detection method and an eddy current flaw detection device that can stably and easily detect cracks and scratches formed on a metal surface.

Means for Solving the Problems

[0006] In other words, the present invention encompasses the following inventions. (1) An eddy current testing method for detecting damaged areas on the surface of an object to be inspected, comprising supplying an alternating current to a detection coil having resonant characteristics, bringing the detection coil into contact with each part of the surface of the object to be inspected, scanning while in contact with the detection coil, or scanning while maintaining a predetermined distance without contact, and comparing the output alternating voltages generated at both ends of the detection coil at each part of the surface of the object to be inspected, wherein the frequency of the alternating current for detecting damaged areas is set based on the amplitude difference of the output alternating voltages generated at the detection coil for each part of the surface.

[0007] (2) The eddy current testing method according to (1), wherein, among the surfaces to be inspected, the center frequency of the resonance curve of the output AC voltage of the normal portion is higher than the center frequency of the resonance curve of the output AC voltage of the damaged portion, and the frequency at which the amplitude difference between the resonance curve of the normal portion and the resonance curve of the damaged portion is greatest is set as the frequency for detecting the damaged portion of the AC current.

[0008] (3) The eddy current testing method according to (2), wherein the alternating current is set to a frequency for detecting the damaged part of the object to be inspected, which is a magnetic material, from a low-frequency band lower than the center frequency of the output alternating voltage of each part.

[0009] (4) The eddy current testing method according to (3), wherein the alternating current is set to a frequency for detecting the damaged part of the object to be inspected, which is a nonmagnetic material, from a high-frequency band higher than the center frequency of the output alternating voltage of each part.

[0010] (5) Eddy current flaw detection apparatus comprising: an oscillator that adjusts the AC current to the frequency described in (3) or (4) and supplies it to the detection coil; a rectifier that rectifies the output AC voltage generated at both ends of the detection coil by the oscillator into an output DC voltage; a storage unit that stores the voltage value of the output DC voltage of normal parts on the surface of the object to be inspected from the output DC voltage rectified by the rectifier unit; an analysis circuit including a calculation unit that compares the voltage value of the normal parts with the voltage value of other parts to determine damaged parts on the surface of the object to be inspected; and a notification device that notifies information about the surface of the object to be inspected, including the results determined by the calculation unit. [Effects of the Invention]

[0011] According to the eddy current testing method and apparatus of the present invention, cracks and defects occurring on a metal surface can be stably and easily detected using only the output AC voltage of the detection coil, without the need for a compensation coil. [Brief explanation of the drawing]

[0012] [Figure 1] A block diagram showing an eddy current flaw detection apparatus 1 according to an embodiment of the present invention. [Figure 2] A waveform diagram showing the change in the resonance curve of the detection coil 13 as an analog signal when the object being inspected is a magnetic material. [Figure 3] A waveform diagram showing the change in the resonance curve of the detection coil 13, as shown in Figure 2, as a digital signal. [Figure 4] A schematic diagram showing the process of detecting defects in a metal test piece TP using the detection coil 13. [Figure 5] A waveform diagram showing the correlation between the depth of the cut A in the metal test piece TP and the magnitude of the detected signal. [Figure 6] A waveform diagram showing the change in the resonance curve of the detection coil 13 as an analog signal when the object being inspected is a non-magnetic material. [Modes for carrying out the invention]

[0013] Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0014] The eddy current flaw detection apparatus 1 according to the present invention is a non-destructive testing apparatus that detects cracks and defects occurring on the surface or inside of an object to be inspected, as shown in Figure 1. The eddy current flaw detection apparatus 1 includes a signal detection circuit unit 10 which includes an oscillator, a signal processing circuit unit 30 which corresponds to an analysis circuit, and an output unit 50 which corresponds to an alarm.

[0015] The signal detection circuit 10 includes an oscillator 11 whose frequency can be adjusted, a resistor 12 that suppresses the alternating current supplied from the oscillator 11, a detection coil 13 that receives the alternating current from the oscillator 11 to generate a magnetic field, and capacitors 14 and 15. The signal detection circuit 10 supplies the alternating current generated by the oscillator 11 to the detection coil 13 via the resistor 12. At this time, the detection coil 13 is connected to the capacitors 14 and 15 to form a resonant circuit that gives the detection coil 13 resonant characteristics. In this embodiment, the resistance value of the resistor 12, the inductance of the detection coil 13, and the capacitance of the capacitors 14 and 15 are set appropriately so that the resonant frequency is approximately 1 kHz to 100 kHz. These can be set appropriately depending on how the resonant frequency is set. The signal detection circuit 10 outputs the output alternating voltage generated in the detection coil 13 as a detection signal.

[0016] The signal processing circuit unit 30 acquires the detection signals generated at both ends of the detection coil 13, converts them into a form suitable for analysis, and performs a determination process to determine the damage occurring on the surface of the metal being inspected. The signal processing circuit unit 30 includes a first amplifier 31, a rectifier 32, a second amplifier 33, an A / D converter 34, a voltage value storage unit 35, an arithmetic unit 36, and a D / A converter 37. The first amplifier 31 acquires the detection signal from the detection coil 13 and amplifies the detection signal. The rectifier 32 rectifies the detection signal, which is an AC signal, into a DC signal. The second amplifier 33 amplifies the detection signal rectified into a DC signal by the rectifier 32. The A / D converter 34 converts the detection signal, which is an analog signal, into a digital signal. The voltage value storage unit 35 pre-stores a reference voltage value that serves as a reference when determining damage. As the reference voltage value, the voltage value of the output AC voltage when the detection coil 13 is located at a normal part on the surface of the inspection object is preferable. The arithmetic unit 36 compares the reference voltage value stored in the voltage value storage unit 35 with the detection signal to analyze the damage. The D / A converter 37 converts the result analyzed by the arithmetic unit 36 from a digital signal into an analog signal and outputs it to the output unit 50.

[0017] The output unit 50 is preferably capable of visually or audibly transmitting the analysis result in the signal processing circuit unit 30. The output unit 50 may be a speaker that transmits by means of an alarm, voice, etc., may be a light that transmits by means of lighting and extinguishing, etc., or may be a monitor screen that records the analysis result via a data logger or the like and shows its change in the form of a waveform or numerical value. It is only necessary to be able to convey the analysis result of whether the surface of the inspection object is normal or defective.

[0018] Here, the eddy current flaw detection method used in the eddy current flaw detector 1 will be described. While referring to FIGS. 2 and 3, the case of flaw detecting iron (Fe), which is a magnetic material, as the inspection object will be described.

[0019] When the detection coil 13 of the eddy current flaw detector 1 is positioned in free space, a resonance curve h0 can be obtained from the output AC voltage detected at both ends of the detection coil 13. The voltage at both ends of the detection coil 13 becomes maximum at the center frequency f0 of the resonance curve h0.

[0020] While scanning while contacting each part on the surface of the inspection object, scanning while maintaining a predetermined interval without contacting, when the detection coil 13 is located at a normal part on the surface of the inspection object, the detection coil 13 causes eddy currents on the surface of the inspection object that is a magnetic material, and the inductance of the detection coil 13 increases. At this time, a resonance curve h1 is obtained from the output AC voltage detected at both ends of the detection coil 13. The center frequency f1 of the resonance curve h1 is lower than the center frequency of the resonance curve h0, and the output AC voltage also becomes lower. Therefore, the resonance curve h1 has a small sharpness Q and becomes a curve gentler than the resonance curve h0.

[0021] While scanning while contacting each part on the surface of the inspection object, scanning while maintaining a predetermined interval without contacting, when the detection coil 13 is located from a normal part to a damaged part on the surface of the inspection object, a change occurs in the inductance of the detection coil 13. In particular, the change in the sharpness Q is large. At this time, a resonance curve h2 is obtained from the output AC voltage detected at both ends of the detection coil 13. The center frequency f2 of the resonance curve h2 is lower than the center frequency f1 of the resonance curve h1, but the output AC voltage becomes higher than the output AC voltage of the detection coil 13 at the normal part.

[0022] As shown in Figures 2 and 3, there is an amplitude difference between resonance curves h1 and h2, and the amplitude difference is particularly large in the low-frequency band below the center frequency f1 of resonance curve h1 and the center frequency f2 of resonance curve h2. Among these, the amplitude difference V1-V2 between the voltage V1 of resonance curve h1 and the voltage V2 of resonance curve h2 is the largest. Since the detection sensitivity D of the eddy current flaw detection device 1 changes according to the magnitude of the amplitude difference, the frequency of the amplitude difference V1-V2, which has the largest amplitude difference and good detection sensitivity D, is set in the oscillator 11 as the detection frequency fm for detecting damaged areas. At this time, the voltage V0 of resonance curve h0 at detection frequency fm is smaller than the voltage V1 of resonance curve h1. Because the voltage V0 of resonance curve h0 is smaller than the voltage V1 of resonance curve h1, the lift-off problem is solved. As a result, flaw detection inspections can be performed continuously and easily, for example, in automated production lines such as factories.

[0023] A flaw detection test was performed using an eddy current flaw detector 1 set to a detection frequency fm, as shown in Figure 4. In this flaw detection test, SS400, a general structural rolled steel material according to the Japanese Industrial Standard JIS G3101, was used as the metal test piece TP. The metal test piece TP had cuts A at predetermined depths of 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm, and the detection coil 13 of the eddy current flaw detector 1 contacted each cut A to perform detection. The eddy current flaw detector 1 obtained the results shown in Figure 5 as detection results for free space, each cut A of the metal test piece TP, and its edges. As can be seen from Figure 5, the waveforms when positioned at each depth of cut A are shown. From this, it is shown that the eddy current flaw detector 1 set to a detection frequency fm can detect damaged areas on the surface of the object being inspected.

[0024] For the sake of simplicity, Figure 4 only shows the detection coil 13 and the cylindrical ferrite core 131 around which the detection coil 13 is wound; however, in reality, these components are housed within a probe that is not shown. Although the winding of the detection coil 13 is shown as a single-layer winding, any winding method that can induce eddy currents on the surface of the object being inspected to detect damaged areas is acceptable. Although the shape of the ferrite core 131 is shown as a cylinder, any shape that can efficiently concentrate the magnetic flux around the detection coil 13 to improve the sensitivity of eddy current generation and detection on the object being inspected is acceptable. Furthermore, the material of the ferrite core 131 is preferably a magnetic material with flat temperature characteristics, but silicon steel sheet is also acceptable.

[0025] Next, referring to Figure 6, we will explain the case of inspecting copper (Cu), which is a non-magnetic material, for defects.

[0026] When the detection coil 13 of the eddy current flaw detection device 1 is positioned in free space, a resonance curve h0 is obtained from the output AC voltage detected at both ends of the detection coil 13. As the detection coil 13 is brought into contact with or scanned over various parts of the surface of the object to be inspected, when it comes into contact with a normal area on the surface of the object to be inspected, a resonance curve h3 is obtained from the output AC voltage detected at both ends of the detection coil 13. The center frequency f3 of resonance curve h3 is higher than the center frequency of resonance curve h0, while the output AC voltage is lower. Therefore, resonance curve h3 has a smaller sharpness Q and is a gentler curve than resonance curve h0.

[0027] As the detection coil 13 moves from a normal area to a damaged area on the surface of the object being inspected, as it moves from a normal area to a damaged area, the inductance of the detection coil 13 increases, and consequently, Q also increases. In particular, the change in sharpness Q is large. At this time, a resonance curve h4 is obtained from the output AC voltage detected at both ends of the detection coil 13. The center frequency f4 of the resonance curve h4 is lower than the center frequency f3 of the resonance curve h3, and the output AC voltage is also lower than the output AC voltage of the detection coil 13 in the normal area, but the difference is small.

[0028] As shown in Figure 5, there is an amplitude difference between resonance curves h3 and h4, and the amplitude difference is particularly large in the high-frequency band above the center frequency f3 of resonance curve h3 and the center frequency f4 of resonance curve h4. Among these, the amplitude difference V3-V4 between the voltage V3 of resonance curve h3 and the voltage V4 of resonance curve h4 is the largest. Since the detection sensitivity D of the eddy current flaw detection device 1 changes according to the magnitude of the amplitude difference, the frequency of the amplitude difference V3-V4, which has the largest amplitude difference and the best detection sensitivity D, is set in the oscillator 11 as the detection frequency fm for detecting the damaged area. At this time, the voltage V0 of resonance curve h0 at frequency fm is smaller than the voltage V3 of resonance curve h3.

[0029] Therefore, the eddy current testing method of the eddy current testing apparatus 1 according to the present invention can reliably and easily identify damage such as cracks and scratches on a metal surface using only the output voltage of the detection coil 13.

[0030] Furthermore, although the eddy current testing method and the eddy current testing apparatus 1 using the method described above are explained using a pencil-type probe, they are not limited to this. The probe containing the detection coil 13 may be a hollow through-type probe that allows a long object to be inspected, such as a rod or tube, to pass through the detection coil, or it may be an annular type that surrounds and detects the periphery of metal parts fastened with bolts, nuts, rivets, welds, etc.

[0031] Although embodiments of the present invention have been described above, the present invention is not limited in any way to the embodiments described above, and can be implemented in various forms without departing from the spirit of the invention. [Explanation of symbols]

[0032] 1 Eddy current flaw detection equipment 10 Signal detection circuit section 11 Oscillator 12 resistor 13 Detection coil 14,15 Capacitors 30 Signal Processing Circuit Section 31. First Amplifier 32 Rectifier 33. Second Amplifier 35 Voltage Value Storage Unit 34 A / D converters 36 Arithmetic section 37 D / A Converters 50 Output section 131 Ferrite core A cut D Detection Sensitivity T Test subjects TP Metal Test Specimen f0 center frequency f1 Center frequency f2 center frequency f3 center frequency f4 center frequency FM detection frequency h0 resonance curve h1 resonance curve h2 resonance curve h3 resonance curve h4 resonance curve

Claims

1. An eddy current testing method for detecting damaged areas on the surface of an object to be inspected, comprising supplying an alternating current to a detection coil having resonant characteristics, bringing the detection coil into contact with each part of the surface of the object to be inspected, scanning while in contact with the coil, or scanning while maintaining a predetermined distance without contact, and comparing the output alternating current voltages generated at both ends of the detection coil at each part of the surface of the object to be inspected, The eddy current testing method is characterized by setting the frequency of the alternating current for detecting damage to a specific area based on the amplitude difference of the output alternating voltage generated in the detection coil for each part of the surface of the object to be inspected.

2. Of the surfaces of the object to be inspected, the center frequency of the resonance curve of the output AC voltage of the normal portion is higher than the center frequency of the resonance curve of the output AC voltage of the damaged portion. The eddy current testing method according to claim 1, wherein the alternating current is set to the frequency at which the amplitude difference between the resonance curve of the normal portion and the resonance curve of the damaged portion is greatest, and this frequency is used to inspect the damaged portion.

3. The eddy current testing method according to claim 2, wherein the alternating current is set to a frequency for detecting the damaged part of the object to be inspected, which is a magnetic material, from a low-frequency band lower than the center frequency of the output alternating voltage of each part.

4. The eddy current testing method according to claim 3, wherein the alternating current is set to a frequency for detecting the damaged part of the object to be inspected, which is a nonmagnetic material, from a high-frequency band higher than the center frequency of the output alternating voltage of each part.

5. An oscillator that supplies an alternating current to the detection coil, adjusted to the frequency described in claim 3 or 4, An analysis circuit including: a rectifier unit that rectifies the output AC voltage generated across the detection coil by the oscillator into an output DC voltage; a storage unit that stores the voltage value of the output DC voltage of normal areas on the surface of the object to be inspected from the output DC voltage rectified by the rectifier unit; and a calculation unit that compares the voltage value of the normal areas with the voltage values ​​of other areas to determine damaged areas on the surface of the object to be inspected. An eddy current flaw detection apparatus comprising: an alarm that notifies information about the surface of the object to be inspected, including the results determined by the calculation unit; and a notification device that notifies information about the surface of the object to be inspected.

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