JPH04268449A - Electromagnetic flaw detection - Google Patents

Abstract

PURPOSE:To facilitate the detection of a defect by detecting a disturbance with a magnetic sensor as caused by a detect of an eddy current induced with an exciting coil to reduce effect of conductivity and magnetic permeability based on local unevenness of a metal organization. CONSTITUTION:A coil 11 for excitation is excited with an access of a probe 1 to an object 5 to be inspected to induce an eddy current to the object 5 to be inspected of a metal conductor. A magnetic sensor 12 detects a secondary magnetic field generated by the eddy current to measure disturbance of an eddy current field disturbed by a defect. When a defect such as cracking exists, a part of a concentric eddy current I2 flows detouring a defect F. As a detouring eddy current I2' flows in a direction different from the normal concentric eddy current I2, a magnetic field B2 generated by the eddy current changes. In other words, the eddy current I2' detouring the defect F flows reversely on both sides of the defect and a magnetic field B2' is reversed at the defect. Thus, the detection of the defect F is possible by measuring the magnetic field B2' based on the disturbance of the eddy current.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for non-destructive testing of various metal materials, and more particularly to an electromagnetic flaw detection method capable of reducing the effects of non-uniformity of metal structure.

0003

BACKGROUND OF THE INVENTION As a non-destructive testing method for metal materials, defect detection using an eddy current method has been widely used.

In the conventional flaw detection method using the eddy current method, for example, eddy current flaw detection test A (edited by the Japan Nondestructive Inspection Association)
(1977), when a detection coil carrying an alternating current is brought close to the object to be inspected to generate eddy currents,
Since the impedance of the detection coil increases or decreases due to changes in eddy current at defects or material changes, defects in the inspection object have been detected by measuring changes in the impedance of the coil.

[0005]

[Problem to be solved by the invention] In the above conventional technology,
The impedance change of the detection coil was measured to determine the presence or absence and size of defects in the inspected object, but the impedance of the detection coil is not limited to material defects.
It also varies greatly depending on changes in the material and the positional relationship between the detection coil and the object to be inspected. In particular, when the object to be inspected is made of ferromagnetic material, the impedance of the detection coil changes greatly due to local non-uniformity of the metal structure.
This made it difficult to determine the target defect signal.

[Configuration of the invention]

[0007]

[Means for Solving the Problems] In the electromagnetic flaw detection method of the present invention, an excitation coil through which an alternating current is passed is brought close to an object to be inspected, and defects are detected by eddy current induced in the object to be inspected. In the electromagnetic flaw detection method, the excitation coil excites a magnetic field perpendicular to the object to be inspected to induce an eddy current, and the magnetic sensor detects a secondary magnetic field generated by this eddy current. The method is characterized in that defects in the object to be inspected are detected by detecting disturbances in the current field of eddy currents disturbed by defects in the object to be inspected.

[0008]

[Operation] When an alternating current is passed through the coil and a perpendicular magnetic field is applied close to the surface of the metal conductor to be inspected, an eddy current, which is an induced current, flows in the metal conductor, and this eddy current reduces the magnetic field caused by the coil. A magnetic field in the opposite direction is generated,
An induced current flows through the coil, changing the impedance of the coil.

In the present invention, we focus on the fact that the distribution of the magnetic field that occurs when the current field of an eddy current is disturbed by a defect is different from the distribution of the magnetic field created by a steady eddy current, and we A magnetic sensor is used to detect a magnetic field perpendicular to the central axis of the excitation coil, which is applied to induce eddy currents. Therefore, it hardly detects the magnetic field in the opposite direction to the excitation magnetic field generated by the steady-state eddy current, and acts to selectively detect changes in the magnetic field in a specific direction based on the disturbance of the eddy current caused by the defect. The influence of changes in electrical conductivity and magnetic permeability due to local changes in metal structure can be reduced.

0010

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an example of the configuration of an electromagnetic flaw detector used in the present invention.

In the figure, the electromagnetic flaw detection device includes a probe 1 having an excitation coil and a magnetic sensor (not shown).
an AC power supply 2 that supplies an excitation current to the excitation coil of the probe 1; a detection device 3 that amplifies the output signal of the magnetic sensor and then outputs a signal component synchronized with the excitation current of the excitation coil; It is composed of a display/recording device 4 for displaying and recording output signals.

The AC power supply 2 generates AC at a predetermined frequency using a frequency oscillator 21, and uses a constant current circuit 22 to constantly control the current value to a constant value in response to load fluctuations of the AC power supply 2, for excitation of the probe 1. It supplies a preset constant excitation current to the coil.

The detection device 3 includes an amplifier 31 that amplifies a detection signal of a magnetic field in a specific direction based on a defect detected by a magnetic sensor in the probe 1 to a predetermined level, and an amplifier 31 that amplifies a detection signal of a magnetic field in a specific direction based on a defect detected by a magnetic sensor in the probe 1, and converts the amplified detection signal into an excitation current of an excitation coil. It is composed of a synchronous detection circuit 32 that extracts and outputs synchronized signal components.

As shown in FIGS. 2(a) and 2(b), the probe 1 includes an excitation coil 11 and a magnetic sensor 12 disposed at its center so that its detection direction is perpendicular to the central axis of the excitation coil. It consists of As the magnetic sensor 12, a magnetoelectric conversion element such as a pickup coil or a Hall element is used, for example.

The excitation coil 11 generates an alternating magnetic field perpendicular to the surface of the object 5 to be inspected, and the magnetic sensor 12 generates an alternating magnetic field perpendicular to the surface of the object 5 to be inspected. This detects a magnetic field in a specific direction that is generated when the current field of eddy currents induced in the inspection object 5 is disturbed by a defect.

When detecting defects in an object to be inspected using the flaw detection apparatus having the above configuration, first, the probe 1 is moved to the object to be inspected 5.
The excitation coil 11 is excited by the AC power source 2 at a preset frequency and a constant current value. This excitation induces an eddy current in the metal conductor that is the object 5 to be inspected. The magnetic sensor 12 detects the secondary magnetic field generated by this eddy current and measures the disturbance of the eddy current field caused by the defect. Here, the disturbance of the eddy current field due to defects will be explained using the schematic diagram in Figure 3.
As shown in a), an alternating current I1 is applied to the excitation coil 11.
When a perpendicular magnetic field B1 is applied to the object to be inspected 5, a concentric eddy current I2 is induced, and this eddy current I2 causes a vertical magnetic field B1 generated by the excitation coil 11.
A magnetic field B2 in the opposite direction is generated and acts to change the impedance of the excitation coil 11. On the other hand, if a defect such as a crack exists, a part of the concentric eddy current I2 flows around the defect F, as shown in FIG. 3(b). Since the eddy current I2' flowing around this defect flows in a direction different from that of the normal concentric eddy current I2, a change occurs in the magnetic field B2 created by this eddy current. That is, Figure 3
As shown in (c), since the eddy current I2' that detours around the defect F flows in opposite directions on both sides of the defect, the magnetic field B2' generated around the current also goes in the opposite direction with the defect as a boundary. Therefore, the defect F can be detected by measuring the boundary B2' based on the disturbance of this eddy current.

Next, the boundary B2 based on the disturbance of this eddy current
We will explain how to detect ' with a magnetic sensor.

As described above, the magnetic sensor 12 is arranged with its magnetic detection direction perpendicular to the central axis of the excitation coil 11, so that the magnetic field generated by the excitation coil 11 and the magnetic field generated by the eddy current are It does not respond to the vertical magnetic field component, but generates a detection signal that responds only to the horizontal magnetic field component that is perpendicular to the vertical magnetic field component and parallel to the surface of the object to be inspected.

That is, as shown in FIG. 4, the excitation coil 1
The eddy current I2 induced by 1 flows concentrically with the excitation coil 11 in areas where there is no defect F, and the secondary magnetic field B2 created by the eddy current I2 is also symmetrical with respect to the central axis of the excitation coil 11. It shows a magnetic field distribution. Further, since the center of the detection direction of the magnetic sensor 12 is made to coincide with the center of the excitation coil 11, horizontal component detection signals directed in opposite directions cancel each other out and do not appear as outputs. On the other hand, as the probe 1 moves toward the defect F, the flow direction of some of the eddy current I2 is disturbed by the defect F, and the magnetic field created by the eddy current is thereby directed toward the center of the detection direction of the magnetic sensor 12. This results in an asymmetrical magnetic field distribution, causing an imbalance in magnetic field components in opposite directions, and the difference is obtained as an output signal of the magnetic sensor 12, making it possible to detect disturbances in the eddy current field due to defects.

Since the output signal of the magnetic sensor 12 has an AC waveform corresponding to the excitation current of the excitation coil 11, synchronous detection is performed by the synchronous detection circuit 32 of the detection device 3 to extract signal components synchronized with the excitation current. Then, it is output to the display/recording device 4 as a DC signal proportional to the strength of the horizontal component magnetic field.

In this case, the change in the detection signal obtained by moving the probe 1 shows 0 at the center position of the defect F, and peaks of opposite polarity on both sides of it, as shown in FIG. 4(b). Therefore, defects can be determined accurately.

What is important here is that the effects of electrical conductivity and magnetic permeability caused by local material changes in the metal structure, which are a problem in conventional eddy current testing, appear as changes in the strength of the magnetic field created by eddy currents. However, it does not affect the distribution of the magnetic field. In the method of the present invention, as described above, changes in the horizontal component of the magnetic field based on disturbances in eddy currents are selectively detected, so that disturbances do not occur in eddy currents even in areas with local material changes. Therefore, it is not affected by local material changes in the metal structure.

[0024]

As explained above, according to the electromagnetic flaw detection method of the present invention, the influence of electrical conductivity and magnetic permeability due to local non-uniformity of the metal structure, which has been a problem with the conventional eddy current flaw detection method, can be solved. This makes it possible to easily detect defects even if the object to be inspected is a ferromagnetic material.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram showing an example of an electromagnetic flaw detection device used in the present invention.

2(a) is a configuration diagram schematically showing a probe in the electromagnetic flaw detection apparatus of FIG. 1, and FIG. 2(b) is a sectional view taken along line A-A.

FIGS. 3A, 3B, and 3C are explanatory diagrams for explaining disturbances in an eddy current field in an object to be inspected.

FIGS. 4(a) and 4(b) are explanatory diagrams for explaining a method for detecting disturbances in an eddy current field according to the present invention.

[Explanation of symbols]

1...probe 2...AC power supply 3...Detection device 4...display recording device 5...Object to be inspected 11... Excitation coil 12...Magnetic sensor 21...Frequency oscillator 22... Constant current circuit 31...Amplifier 32...Synchronized detection circuit

Claims (1)

[Claims] 1. An electromagnetic flaw detection method in which an excitation coil through which an alternating current is passed is brought close to an object to be inspected, and defects are detected by eddy currents induced in the object to be inspected. The excitation coil excites a perpendicular magnetic field to induce eddy currents, and the magnetic sensor detects the secondary magnetic field created by this eddy current. An electromagnetic flaw detection method that detects defects in an object to be inspected by detecting disturbances in the field.