JP2003207487A - Defect inspection method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a defect inspection method that can speedily and conveniently detect corrosion and a crack in an object to be inspected. <P>SOLUTION: The apparatus excites elastic wave motion in the object to be measured, and detects the elastic wave motion propagating in the object at least at one point. The device estimates cross-sectional distribution related to the prescribed direction of the object based on the obtained elastic wave motion, and detects the defect of the object based on cross-sectional distribution related to the prescribed direction to be possessed by the object and the estimated cross-sectional distribution. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting defects such as corrosion and cracks generated in an object, and more particularly, an elastic wave that excites elastic waves in an object to be inspected and propagates in the object to be inspected. And a method for detecting a defect from the signal.

[0002]

2. Description of the Related Art Defects such as cracks and corrosion in structural materials and pipes can lead to serious accidents such as destruction of structures, facilities and equipment, and therefore regular inspection is necessary. For example, in structures such as high-voltage power transmission towers, road signs, and streetlights, many solid or hollow rods are used as structural materials. Since many of these structures are exposed to wind and rain, it is unavoidable that some of the rods will corrode over time and their strength will decrease. Further, a large number of hollow rods or pipes are used for heat exchangers and fluid transportation pipes, and defects in these pipes can lead to major accidents such as pipe breakage. For inspection of such an object, at present, in addition to visual inspection, inspection using ultrasonic waves, inspection using a fiberscope camera in the case of a tube, inspection using electromagnetic induction, and the like are performed. However, since these inspection methods are “point” inspection methods in principle, it is necessary to repeat a large number of inspections along the object. Further, since the inspection is often performed under adverse conditions such as working at a high place, which requires a long inspection time and enormous cost, a simple and high-speed defect inspection method is strongly desired.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a simple defect inspection method capable of inspecting the entire state of an object to be inspected at high speed and easy to carry out even under adverse conditions such as high places.

[0004]

According to the present invention, an elastic wave is excited in an object to be inspected, and the elastic wave propagating in the object to be inspected is detected. Since the elastic wave in the object to be inspected is reflected at the place where the cross-sectional area of the propagating medium changes, the elastic wave signal includes information on the change of the cross-sectional area of the medium. The cross-sectional area distribution along the predetermined direction of the object to be inspected is estimated from the detected elastic wave signal, and the defect is detected from the difference from the sound cross-sectional area distribution. The inspection point does not need to be scanned over the entire length of the object, and the entire cross-sectional area distribution along the predetermined direction is obtained from elastic waves detected at a certain location on the object, so the inspection is fast and simple, and can be used for inspection under adverse conditions. Is also suitable.

FIG. 2 is a schematic diagram showing an elastic body in which elastic waves propagate in one direction. The elastic body is expressed as a chain of short sections of length l. The mth section and the mth section to the right of it
Focus on the boundary surface with the -1 section. If the mth section and the mth
If the cross-sectional areas of the −1 section are exactly the same, the waves traveling in the elastic body pass through this boundary surface without any change. However, when there is a difference in the cross-sectional area, the wave incident on this boundary surface causes reflection. That is, a part of the wave moving from the left to the right in the m-th section is transmitted through the boundary surface to the m-th section, and the rest is reflected to the left by the boundary surface. On the other hand, the waves traveling from the right to the left in the m-1th section are partially transmitted through the boundary surface and proceed from the right to the left in the mth section, and the rest are reflected to the right in the m-1th section. Go back in the direction. The same phenomenon occurs at the boundary surface between the m-th section and the (m + 1) -th section on the left. The wave observed in the elastic body is the sum of all these transmitted and reflected waves.

When there is a change in the cross-sectional area at a certain place in the elastic body, reflection occurs there, and this reflected wave propagates to the entire elastic body. Further, at the end of the elastic body, since the cross-sectional area generally changes, reflection also occurs. The wave observed at one point of the elastic body is a superposition of all transmitted waves and reflected waves. Therefore, the wave motion in the elastic body contains contributions from all the reflections that occur in the entire length of the elastic body, even if they are detected at one place. By the way, how much reflection occurs at the boundary surface where the cross-sectional area changes is
It can be obtained from the particle velocity continuity condition and the force balance condition at the boundary surface.
The reflectance at the boundary surface between the sections is the ratio of the cross-sectional areas of both sections Sm /
It is determined by Sm-1. The vibration detected at a certain point in the elastic body contains information on the cross-sectional area ratio at all locations where cross-sectional area changes exist.By analyzing this vibration waveform, the wave propagation direction over the entire length of the elastic body is analyzed. The cross-sectional area distribution can be estimated.

If the elastic body is cracked, the cross-sectional area at that location is reduced. When the elastic body is corroded, the corroded portion becomes brittle, so that the cross-sectional area when viewed as a medium of elastic wave also decreases. Thus, if the elastic body has some kind of defect, the cross-sectional area at that location changes. Since the cross-sectional area distribution estimated from the vibration waveform is different from the cross-sectional area distribution of a sound elastic body having no defect, the existence and position of the defect can be known. The above applies not only to solid elastic bodies, but also to hollow elastic bodies such as pipes, and it is possible to know the presence and position of defects from the distribution of the pipe wall cross-sectional area, that is, the pipe wall thickness. it can.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention excites elastic waves in an object to be inspected, detects elastic waves propagating in the object to be inspected at at least one point of the object to be inspected, and based on the obtained elastic waves. A cross-sectional area distribution in a predetermined direction of the inspected object is estimated, and a defect of the inspected object is detected based on the cross-sectional area distribution that the inspected object should have in the predetermined direction and the estimated cross-sectional area distribution. To excite elastic waves in the elastic body that is the object to be inspected,
The object to be inspected is vibrated by using a piezoelectric element, a magnetostrictive element, an electrokinetic conversion element, or the like. If the inspection object is a ferromagnetic material,
A magnetic field can be applied to the object to be inspected to expand and contract due to the magnetostriction effect, and the elastic wave can be directly excited.
Further, when the object to be inspected has conductivity, a magnetoacoustic transducer (EMAT) can be used. Furthermore, it is also possible to excite elastic waves by giving a mechanical impact with a hammer or the like. For the detection of elastic waves, an acceleration pickup using a piezoelectric element, an electrodynamic velocity pickup, a method utilizing the magnetostrictive effect, a magnetoacoustic transducer (EM)
Other than AT), a laser Doppler vibrometer, a laser displacement meter, or a strain gauge can be used. The problem of estimating the cross-sectional area distribution in the wave propagation direction from the detected elastic wave is one of so-called inverse problems, and various algorithms have been devised.

[0009]

EXAMPLES (First Example) The details of the present invention will be described below with reference to Examples. FIG. 1 is a first embodiment of the present invention. In FIG. 1, 1 is an iron bar to be inspected. 2 is
It is an exciting coil and is wound around one end of a steel bar.
A pickup coil 3 is wound around the other end of the iron rod. A noise generator 4 generates white noise. An amplifier 5 amplifies noise from the noise generator and supplies a current to the exciting coil. A detection circuit 6 is combined with the pickup coil 3 to detect the vibration of the end of the iron rod. A signal processing device 7 analyzes the vibration waveform detected by the detection circuit to calculate the cross-sectional area distribution of the iron bar. A display device 8 graphically displays the cross-sectional area distribution calculated by the signal processing device.

In this embodiment, since the object to be inspected is an iron rod, which is a ferromagnetic material, the magnetostriction phenomenon is used to excite elastic waves. By driving the excitation coil by amplifying the white noise generated by the noise generator, the iron rod is randomly magnetized, and the magnetostriction that accompanies this is propagated in the iron rod.
The elastic wave propagating to the other end is also detected by utilizing the magnetostriction phenomenon.

The substance of the signal processing device 7 is a digital computer, and internally executes a process as shown in FIG. That is, the detected signal is A / D converted, passed through an inverse filter that compensates for the characteristics of the excitation source and the vibration detection circuit, and then PARCOR (partial autocorrelation) analysis is performed. FIG. 4 shows a lattice type filter (PARCOR analysis filter) showing the flow of processing of the PARCOR analysis unit in FIG. The process of calculating the cross-correlation between the signals above and below the grid and subtracting the mutually correlated components from each signal to obtain the signal of the next stage is performed up to the required number of stages according to the length of the rod. To do.
The m-th stage cross-correlation coefficient km is called the m-th order PARCOR coefficient. km
Is, when the bar is represented by a chain of short intervals as shown in Fig. 2,
It corresponds to the reflection coefficient at the boundary between the m-th section and the (m-1) th section, and the cross-sectional area is km = (Sm-Sm-1) / (S
It is known that there is a relationship of m + Sm-1).
The cross-sectional area distribution calculator of FIG. 3 receives the PARCOR coefficient ki
From this, the cross-sectional area distribution of the iron rod in the axial direction is calculated and output to the display device 8.

It is known that the residual signal whose correlation is sequentially removed up to an appropriate number of stages by the PARCOR analysis filter of FIG. 4 becomes white noise. The output signal of the detection circuit when white noise is input to the excitation coil is white noise as the result of passing it through the PARCOR analysis filter.Therefore, the PARCOR analysis filter uses an excitation coil, a bar, a pickup coil,
It can be seen that it is an inverse filter of the system including the detection circuit. Therefore, if a PARCOR analysis is performed on a healthy rod in advance and a PARCOR analysis filter that uses the PARCOR coefficient is configured, an inverse filter that compensates for the characteristics of the excitation source and vibration detection circuit is automatically created. Figure 3
The inverse filter shown in is constructed in this way.

(Second Embodiment) FIG. 5 shows a second embodiment of the present invention. Reference numeral 9 is a signal generator, and 10 is a signal processing device. In this embodiment as well, the elastic wave is excited by utilizing the magnetostriction phenomenon, but unlike the first embodiment, the exciting coil and the pickup coil are mounted at positions close to each other on the rod. In this embodiment, the pulse generated by the signal generator is amplified to drive the exciting coil to excite a pulse-like elastic wave in the rod. At this time, the synchronization signal is output from the signal generator to the signal processing device (the dotted line in the figure shows the flow of the synchronization signal). The vibration propagated through the rod and reflected is detected by the pickup coil and the detection circuit, and the signal processor 7 obtains the impulse response of the rod. It is known that the inverse problem of estimating the cross-sectional area distribution of a rod from an impulse response is formulated as an initial value problem of simultaneous linear partial differential equations and can be solved using numerical integration. In the signal processing device 10, this method is used. Is used to numerically calculate the cross-sectional area distribution. In obtaining the impulse response, when the S / N ratio is insufficient, the rod is not excited by the pulse, but the rod is excited by the time-delayed pulse or the pseudo-random signal, and the signal processing device 7 The impulse response may be calculated in.

(Other Embodiments) In the two embodiments described above, the object to be inspected has a rod shape, but the shape of the object to be inspected is not limited to a rod or a tube. For example, for a plate-shaped object, a linearly spread excitation means is used as a means for exciting the elastic wave, the wave propagating in one direction is excited, and the elastic wave is detected at a certain point of the object. In this case, from the detected elastic wave, the cross-sectional area per unit width,
Since the distribution in the wave propagation direction, that is, the target plate thickness distribution in the wave propagation direction can be estimated, defects can be detected based on this (not shown).

[0015]

According to the present invention, the wave propagated and reflected in the elastic body is detected at a certain point on the elastic body, and by analyzing this signal, the cross-sectional area distribution over the entire length of the elastic body is estimated to ensure soundness. Defects are detected based on the difference from the cross-sectional area distribution in other cases. Since it is not necessary to repeat a large number of inspections along the object to be inspected and information over the entire length of the object to be inspected can be obtained by a single inspection, it is an advantage of the present invention that defect detection can be performed quickly and easily.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating propagation of elastic waves in an elastic body.

FIG. 3 is a diagram illustrating a configuration of a signal processing device according to a first embodiment.

FIG. 4 is a diagram showing processing contents of a PARCOR analysis unit in FIG.

FIG. 5 is a diagram showing a second embodiment of the present invention.

[Explanation of symbols]

1 stick 2 excitation coil 3 pickup coils 4 noise generator 5 amplifier 6 Detection circuit 7 Signal processing device 8 display devices 9 Signal generator 10 Signal processing device

Claims (2)

[Claims] 1. An elastic wave is excited in an object to be inspected, the elastic wave propagating in the object to be inspected is detected at at least one point of the object to be inspected, and a predetermined direction of the object to be inspected is based on the obtained elastic wave. A defect inspection method, comprising: estimating a cross-sectional area distribution regarding a predetermined direction; and detecting a defect of the inspected object based on the estimated cross-sectional area distribution that the inspected object should have in a predetermined direction. 2. A vibration exciting means for exciting an elastic wave in the object to be inspected, a wave detecting means for detecting an elastic wave propagating in the object to be inspected, and a predetermined object to be inspected based on the detected elastic wave. An estimation means for estimating a cross-sectional area distribution in the direction; and a detection means for detecting a defect of the inspected object based on the estimated cross-sectional area distribution of the inspected object in the predetermined direction and the estimated cross-sectional area distribution. Defect inspection device characterized by.