JPH0378579B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an eddy current flaw detection method for tubes using an interpolated coil.

In the eddy current flaw detection method, defects in the material are generally detected as changes in the impedance of the detection coil. The eddy current flaw detection method outputs a vector defect signal consisting of a resistive component and an inductive component.

In tube eddy current testing using an interpolated coil, the amplitude of the defect signal depends not only on the depth of the defect but also on the cross-sectional area of the defect, so even if the defect is shallow, the amplitude will be large when the opening area is large. Therefore,
It is not possible to accurately determine the depth of a defect by displaying a single-channel recording signal, that is, by displaying only the amplitude.

On the other hand, it is known that the depth of a defect corresponds well to the phase of the defect signal. Furthermore, the ASME (American Society of Mechanical Engineers) standards stipulate that the depth of a defect is detected based on the phase of a defect signal. The phase of the defect signal varies greatly depending on the test frequency, but in the above regulations, when the phase of the noise signal due to lift-off etc. is set to 0, that is, the noise signal appears on the x-axis, the The test frequency is to be selected so that the phase is 135 degrees. In this case, the phase of the defect signal varies within a range of 0 to 180 degrees.

By the way, one way to observe the phase is
There is a method in which the two output signals of a two-channel recording eddy current flaw detector are sent to an X-Y recorder or CRT to draw a so-called figure-eight pattern, and the phase of the defect signal is determined from the inclination angle of this figure-eight pattern.
However, with this method, if the phase is small, the figure-8 pattern almost overlaps the x-axis, making it impossible to read the phase. Further, when the phase of the noise signal is set to 0 as described above, it becomes difficult to distinguish between a defective signal and a noise signal.

This invention was made to solve the above-mentioned problems in eddy current flaw detection, and aims to provide an eddy current flaw detection method for tubes using an interpolation coil that allows accurate and easy reading of the phase of a defect signal.

The eddy current flaw detection method for tubes of the present invention involves inserting a detection coil into the tube and moving it along the tube axis, detecting changes in the impedance of the detection coil, and using an output vector of phase θ as a defect signal. Select an integer n from 2 to 5 depending on the depth of the defect and the magnitude of noise, use the x and y components of the output vector as variables, and display the phase nθ by calculation based on the n-times angle formula of trigonometric functions. The X component and Y component of the vector are determined, respectively, and the defect is displayed using the determined X component and Y component of the display vector.

By multiplying the phase of the defect signal by an integer as described above, the defect indicating pattern appears at a large angle with respect to the base line (x-axis) of the pattern display, and the phase can be easily read. In particular, when the depth of the defect is shallow and the defect indication pattern approaches the x-axis,
This invention is effective when it is difficult to read the phase. Further, when the x component of the defective signal indicates a noise signal, it becomes easy to distinguish between the noise signal and the defective signal.

The operation of multiplying the phase by an integer can be performed by an ordinary analog or digital arithmetic circuit. Such a circuit can be incorporated into an eddy current flaw detector, or in the case of an existing eddy current flaw detector, it can be connected to the flaw detector as an auxiliary device. By setting the phase multiple to a well-rounded integer, the arithmetic circuit becomes simple. Further, a phase multiple of about 2 to 5 is appropriate. If the multiple is extremely large, the defect display pattern will rotate more than 360 degrees in the case of a deep defect, making accurate phase reading impossible. From this point of view, it is preferable that the phase multiple can be switched in several stages. For example, when detecting a defect with a normal depth of about 0.3 mm, the phase is magnified twice. Also,
If the defect is minute and the phase between the noise signal such as lift-off and the defect signal is extremely small, the phase is 5.
will be expanded twice.

Hereinafter, embodiments of the present invention will be described using a case where the phase multiple is 2 as an example.

Let x and y be the output (defect) signals of the two-channel eddy current flaw detector. If the amplitude of the output signal is A and the phase is θ, then x=Acosθ y=Asinθ (1) The following calculations are performed for x and y.

X=( ^x2 − ^y2 )/ ^√x2 + ^y2 = ^A2 ( ^cos2θ − ^sin2
θ)/A=Acos2θ X=( ^x2 − ^y2 )/ ^√x2 + ^y2 = ^A2 ( ^cos2θ − ^sin2
θ)/A=Acos2θ Y=2xy/√x ² +y ² =2A ² sinθ・cosθ/A=Asin2θ…
...(2) As is clear from comparing Equations (1) and (2), the obtained X and Y each represent the component of the signal with the amplitude of the defect signal unchanged and only the phase expanded by twice. ing. If vector signals of components X and Y are input to a display device such as a CRT, a defect indicating pattern with twice the phase will be displayed.

FIG. 1 is a block diagram of a signal conversion device that performs the above calculation, and also shows the flow of signals processed in each circuit.

The signal conversion device 2 is connected to the eddy current flaw detector 1 and receives defect signals x and y. The squaring circuits 3 and 4 square the signals x and y from the eddy current flaw detector 1, respectively, and output x ² and y ² . Multiplication circuit 5
finds the product of the signals x and y. Subtraction circuit 6 and addition circuit 7 calculate the difference and sum of x ² and y ² . The doubling circuit 8 converts the signal xy from the multiplication circuit 5 into 2
Multiply. Square root circuit 9 finds the square root of signal X ² +Y ² and outputs it to division circuits 10 and 11. The division circuit 10 calculates X (=(x ² −y ² )/√ ² + ² ), and the division circuit 11 calculates Y (=2xy/√ ² + ² ). The signals X and Y thus obtained are output to an X-Y recorder or CRT, and the defective signal is displayed in a pattern.

Next, an experimental example of the eddy current flaw detection method of the present invention will be explained.

The test product used in the experiment was an aluminum tube with an outer diameter of 32 mm and a wall thickness of 1.5 mm, with artificial defects (flat-bottom drill holes and circumferential grooves) machined according to ASME standards. The detection coil was made by winding 110 turns of 0.1 mmφ enamelled wire in a circumferential groove with an outer diameter of 24 mm and a width of 2 mm, and two detection coils were arranged with a center spacing of 6 mm. The test frequency was 8KHz and the signal phase for the through drill hole was 133 degrees. The detection coil moved inside the tube at a speed of 0.216 m/s.

FIGS. 2 and 3 each show the experimental results.

FIG. 2 shows the variation of the signal with respect to the diameter and depth (% of wall thickness) of a flat-bottom drilled hole in a tube. The phase of the noise signal is 0, and the signal appears on the x-axis. The conventional figure-8 pattern is converted into a figure-0 pattern, which has the same amplitude but twice the phase, and is displayed.

FIG. 3 shows the variation of the signal with respect to the width and depth of the circumferential groove provided in the tube. As in FIG. 2, the 8-shaped pattern is converted into a 0-shaped pattern with twice the phase and displayed. Furthermore, FIG. 3 shows that the phase increases depending on the depth of the defect (circumferential groove).

This invention is not limited to the above embodiments. For example, the phase multiple is not limited to 2, but 3,
It may be 4, 5, etc. In this case, the defect signal from the flaw detector is processed using trigonometric formulas such as triple angle and quadruple angle.

[Brief explanation of drawings]

FIG. 1 shows an example of a signal converting device used to carry out the method of the present invention, and shows a block diagram of the device,
FIGS. 2 and 3 each show an experimental example of the present invention, and are defect display pattern diagrams. 1...Eddy current flaw detector, 2...Signal conversion device, 3,
4...Squaring circuit, 5...Multiplication circuit, 6...Subtraction circuit, 7...Addition circuit, 8...Doubling circuit, 9...Square root circuit, 10, 11...Divide circuit.

Claims (1)

[Claims] 1 In the eddy current flaw detection method, a detection coil is inserted into the pipe and moved along the pipe axis to detect changes in the impedance of the detection coil, and the output vector of phase θ is used as a defect signal. Select an integer n from 2 to 5 according to , use the x component and y component of the output vector as variables, and calculate the An eddy current flaw detection method for pipes using an interpolation coil, characterized in that defects are displayed by the X and Y components of the obtained display vector.