JPH0462468A - Acoustic emission sensor - Google Patents

Abstract

PURPOSE:To detect only a longitudinal wave and make a quick response by composing a piezoelectric element of at least three kinds of different-height columnar bodies and polarizing it in its height direction. CONSTITUTION:When a wave receiving plate 1 is brought into contact with the surface of a body 7 to be inspected, an acoustic emission(AE) wave generated in the body 7 to be inspected is transmitted to a synthetic resin-ceramic compound piezoelectric element 2 thorough the wave receiving plate 1 and an electrode 24 to make the columnar ceramic piezoelectric body 21 expand and contract in its height direction. A potential difference is generated across the piezoelectric body 21 by the expansion and contraction and led out as an AE through lug terminals 6. The piezoelectric body 21 has its crystal axis oriented in the height direction and is polarized in the direction, so only the longitudinal vibration component is detected. Further, the element 2 consists of three kinds of different-height piezoelectric body 21, so the detection frequency range is wide. Consequently, variation in AE generation intensity can be detected faithfully with good sensitivity.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to acoustic emission (Acou
This relates to an acoustic emission sensor that detects stic emissions.

[Prior Art] When a solid is destroyed or plastically deformed, the energy that has been stored as internal strain is released as elastic waves (sonic waves and ultrasonic waves), and this phenomenon is called acoustic emission (hereinafter abbreviated as AE). ), and the above elastic wave is called an AE wave. The so-called AE method, which is a method of detecting the occurrence of scratches or destruction in a material by observing this AE wave while applying a load to the material, is described in "Steel Handbook," 3rd edition, Volume 1V. As described on page 468, it is applied to material fatigue tests and materials research.

A piezoelectric element is usually used as an ultrasonic receiving element in an AE sensor that detects AE waves.
As disclosed in Publication No. 20890, the contact portion between the piezoelectric element and the subject is made of an electrically insulating material, and
A balanced AE sensor has been proposed which is resistant to electrical noise by stacking piezoelectric elements in two stages, and which also prevents a phase difference between the AE multiplied signals detected by each piezoelectric element.

[Problem to be solved by the invention]

By the way, when ultrasonic waves propagate through an object, longitudinal waves propagate faster than transverse waves. Therefore, if only the longitudinal waves in the AE waves are detected, it is possible to faithfully detect changes in the AE generation intensity at the AE generation source, but with the conventional configuration described above, there is a problem in that not only longitudinal waves but also transverse waves are detected. Has a point.

Therefore, in order to detect only the longitudinal waves (longitudinal vibration mode) in the AE wave, a prismatic ceramic piezoelectric body was polarized in the height direction and arranged in a synthetic resin matrix. It is conceivable to use the element in an AE sensor. However, in this case as well, the rise time of the AE sensor is slow when detecting the AE wave, so there is a problem that it is not possible to respond quickly to changes in the AE generation intensity.

[Means for solving the problem] The acoustic emission sensor of the present invention has the following features:
In order to solve the above problem, acoustic emissions are detected using a synthetic resin-ceramic composite piezoelectric element in which a plurality of pillar-shaped ceramic piezoelectric bodies are arranged in a synthetic resin matrix so that their height directions are almost parallel to each other. In the acoustic emission sensor, the synthetic resin-ceramic composite piezoelectric element is characterized in that columnar ceramic piezoelectric bodies of at least three different heights are used and are polarized in the height direction. .

[Function] According to the above configuration, since the columnar ceramic piezoelectric body is polarized in the height direction, the synthetic resin-ceramic piezoelectric body is polarized only in the longitudinal vibration mode in the AE wave propagating in the height direction. A potential difference is generated between both end faces of the composite piezoelectric element. As a result, only the longitudinal vibration mode is detected. Also, at least 3
Since column-shaped ceramics and piezoelectric materials with different heights are used, resonance points appear depending on the height, and the detection frequency range of the AE wave is wide.As a result, the rise time of the AE multiplied signal is also shortened. , responsiveness is improved, and reception sensitivity is also improved.

〔Example〕

An embodiment of the present invention will be described below based on FIGS. 1 to 5.

As shown in FIG. 1, the acoustic emission sensor (hereinafter abbreviated as AE sensor) of the present invention is provided on a delivery plate 1 and a wave receiving plate 1 that receive AE waves from a subject 7. It mainly consists of a synthetic resin-ceramic composite piezoelectric element 2 that converts into an electric signal, a case 4 that houses the synthetic resin-ceramic composite piezoelectric element 2, and a pair of lug terminals 6 for taking out the AE multiplied signal.

As shown in FIG. 2, the synthetic resin-ceramic composite piezoelectric element 2 is a columnar ceramic piezoelectric body 2 with a square cross section.
1, 21, etc. are arranged in a synthetic resin matrix 22 so that their height directions are almost parallel, and electrodes 23, 24 are provided on both end surfaces. In the body 21, the crystal axes of the piezoelectric crystal particles are oriented in the height direction (vertical direction in FIG. 1) and polarized in that direction. Furthermore, columnar ceramic piezoelectric bodies 21 having different heights are used on the left side, center, and right side of the synthetic resin-ceramic composite piezoelectric element 2, respectively. Therefore, the lower electrode 24
is flat, but the upper electrode 23 has three stages.

The synthetic resin-ceramic composite piezoelectric element 2 is adhered onto the wave receiving plate 1 with an adhesive layer 3 (FIG. 1). The upper and lower electrodes 23 and 24 are connected to a pair of lead wires 5 and 5.
is connected to the lug terminals 6, 6.

In the above configuration, when the delivery plate 1 is brought into close contact with the surface of the subject 7, the AE waves generated inside the subject 7 are transferred to the subject 7.
It propagates through the inside and reaches the surface, and the receiving wave + anti-1 and the lower electrode 2
4 to the synthetic resin-ceramic composite piezoelectric element 2, causing the columnar ceramic piezoelectric body 21 to expand and contract in its height direction.

Due to this expansion and contraction, the columnar ceramic piezoelectric body 21
A potential difference is generated between both ends of the signal, and this is taken out from the lug terminals 6 as an AE multiplied signal.

In the AE sensor of the present invention, the crystal axis of the columnar ceramic piezoelectric body 21 is oriented in the height direction, and the crystal axis is polarized in that direction. However, even if it is expanded or contracted in other directions, for example, in a direction perpendicular to the height direction, no charge is generated at both ends. That is, in the AE wave reaching the surface of the subject 7 from inside, only the longitudinal vibration component is detected.

As a result, even if longitudinal waves and transverse waves caused by temporally and spatially different AEs have different propagation velocities and reach the AE sensor at the same time, only the longitudinal vibration component will be detected and the inside of the object 7 will be detected. It is possible to faithfully detect the intensity of the AE that occurs and its changes.

By the way, the potential difference generated between the electrodes 23 and 24 of the synthetic resin-ceramic composite piezoelectric element 2 due to the AE wave is unique and determined by the material and shape of the columnar ceramic piezoelectric body 21. Therefore, in a preliminary experiment, when ultrasonic waves of known intensity were received, the electrode 23
・If you find the relationship between the potential difference between electrodes 23 and 24, you can use this to calculate the AE from the measured potential difference between electrodes 23 and 24.
This means that the strength of the waves can be calculated.

In addition, in the AE sensor of this embodiment, the synthetic resin-ceramic composite piezoelectric element 2 is composed of columnar ceramic piezoelectric bodies 21 of three different heights. It has three different resonance frequencies (resonance points) depending on the For this reason, AE
The detection frequency range of the sensor becomes wider, and the characteristics of the ideal AE sensor shown in FIG. This approaches the characteristic that the detection frequency range 28 is wide.

The fact that the detection frequency range of the AE sensor of this embodiment approaches the characteristics of an ideal AE sensor will be explained below with reference to FIG. 4, which shows the frequency dependence of the impedance of the AE sensor.

If the heights of the columnar ceramic piezoelectric bodies 21 (Fig. 1) are all the same, the AE sensor will have one resonant frequency, and this resonant frequency is denoted by f on the low frequency side in the figure. . In this case, in the frequency range lower than the resonance frequency f1, the impedance of the AE sensor decreases as the frequency increases, but at the resonance frequency f
, as shown in the figure, it decreases rapidly and then rises significantly. In a frequency region higher than the resonance frequency f, as the frequency increases, it decreases along a curve 29 shown by a broken line in the figure. Detects AE waves,
In order to obtain a predetermined output level, the impedance must be high to some extent, so the detection frequency range of this AE sensor is limited to the bass range of 25°.

Next, when the columnar ceramic piezoelectric body 21 has two different heights, the AE sensor has two resonance frequencies, and the resonance frequencies are fl and f2, and f2 > f.

In this case, in the frequency range lower than the resonance frequency f2,
The impedance of the AE sensor shows the same frequency dependence as above, but at the resonance frequency f2, it sharply decreases again and then rises significantly. In a frequency region higher than the resonance frequency r2, as the frequency increases, it decreases along a curve 30 shown by a broken line in the figure. At this time, as is clear from the comparison between the curve 29 and the curve 30, in the frequency region higher than the resonance frequency f2, the impedance is higher than in the case of the AE sensor using the columnar ceramic piezoelectric body 21 having the same height. is increasing.

Therefore, the output level can be maintained up to a higher frequency range. In other words, the detection frequency range is expanded, and the low frequency range 25
' and midrange 26' AE waves can be detected.

Furthermore, if columnar ceramic piezoelectric bodies 21 of three different heights are used as in this embodiment, the AE sensor will have three resonance frequencies. Its resonant frequency is f,
Let f2 and f3 be f:l>f2>f. In this case, in the frequency region lower than the resonance frequency 13, A
The impedance of the E sensor shows the same frequency dependence as above, but at the resonance frequency f, it sharply decreases again and then rises significantly. In the frequency range higher than the resonance frequency f, as the frequency increases,
It decreases along a curve 31 shown as a solid line in the figure. At this time, as is clear from the comparison between the curve 30 and the curve vA31, in the frequency range higher than the resonance frequency f3, the AE sensor using the columnar ceramic piezoelectric body 21 of the above two types of heights Impedance is increasing. Therefore, the output level can be maintained up to a higher frequency range, and the detection frequency range 28' is the bass range 25',
This means that a midrange range of 26' and a high range of 27'' can be covered.

In this way, the frequency characteristics of the AE sensor of this example approach the characteristics of an ideal AE sensor (FIG. 3). When the detection frequency range 28' is widened, AE waves in a wide frequency range can be detected, responsiveness is improved, and pulsed AE waves containing many frequency components can also be received. Furthermore, the rise time is shortened, and a large AE multiplied signal can be obtained. That is, the reception sensitivity (delivery sensitivity) of AE is also improved.

Specifically, it is possible to use, for example, a barium titanate sintered body, a lead titanate sintered body, or a PZT (lead zirconate titanate) sintered body as the material of the columnar ceramic piezoelectric body 21. Although it is preferred in terms of detection sensitivity, any material can be used as long as it has piezoelectric properties and is polarized in the height direction. In addition, its shape must be columnar, and the ratio between its height and the length of one side of the bottom must be 1 or more, and it is preferable that it be 2 or more in terms of AE detection sensitivity, and this ratio should be 2 or more. , more preferably in the range of 2 to 6. Further, the elastic modulus of the ceramic piezoelectric body 210 is 600
0 kgf/mm" or more is preferable in terms of AE detection sensitivity.

The synthetic resin used for the synthetic resin matrix 22 may be any synthetic resin as long as it can be combined with and integrated with the columnar ceramic piezoelectric body 21. Specifically, for example, silicone rubber, urethane rubber, butadiene rubber, nitrile rubber, ethylene-propylene rubber, chloroprene rubber, fluororubber, ethylene-acrylic rubber, polyester elastomer, chlorophyll rubber, acrylic rubber, or chlorinated ethylene rubber. However, it is preferable to use silicone rubber, urethane rubber, or butadiene rubber in terms of AE detection sensitivity, and it is also preferable to thicken (attenuate) the transverse vibration mode and detect only the longitudinal vibration mode. The elastic modulus of synthetic resin is 1 to 50
kg f 7 mm2 is preferable in terms of AE detection sensitivity, and it is more preferable to match the acoustic impedance between the AE sensor and the subject 7 (FIG. 1).

In addition, in the synthetic resin-ceramic composite piezoelectric element 2, the ratio of the total volume of the columnar ceramic piezoelectric bodies 21, 21 and the volume of the synthetic resin matrix 22 is 8/92.
A range of 40/60 is preferable in terms of AE detection sensitivity.

Hereinafter, a urethane rubber-PZT (lead zirconate titanate) composite piezoelectric element will be cited as a specific example of the synthetic resin-ceramic composite piezoelectric element 2, and the manufacturing method thereof and the performance of an AE sensor using the same will be explained.

A cylindrical (φ10 mm x 10 mm) polarized PZT sintered body (manufactured by Honda Electronics Co., Ltd., model number HC-50O3) is used as the material for the columnar ceramic piezoelectric body 21, and a groove with a depth of 5 mm is formed on one end surface. Form 5 lines at 1°5mm intervals,
In the direction perpendicular to this, make a groove of 1.5 m with a depth of 5 mm.
After forming 5 prisms at m intervals and processing them so that 32 square prisms of 1 mm x lmm x 5 mm are regularly arranged and standing upright on a PZT disk with a thickness of 5 mm, 10 prisms are formed in two rows at the ends on the PZTH plate. The height of the prisms is 2 mm, the center 2 rows 1
A PZT microfabricated product was obtained by processing the two prisms to have a height of 3 mm and the remaining two rows of 10 prisms to have a height of 4 mm. And each resonance frequency is 860
The height was finely adjusted to 420 KH2350 KH2. In addition, for the above processing,
An ultrasonic machining device (manufactured by JEOL Ltd.) equipped with a tool made of 545C (structural carbon mesh) was used.

The PZT microfabricated product obtained in this way is fitted into a silicon mold, and a synthetic resin matrix 2 is placed in this mold.
Filled with electrically insulating urethane rubber (manufactured by Sanniresin Co., Ltd., trade name 5LJ-2153-9, hardness 52, black) as material No. 2, left at room temperature for one day, and then dried at 60°C in a dryer.
A urethane rubber-PZT composite in which a urethane comb-PZT was formed on a PZT disk was prepared by curing the urethane rubber for 5 hours and removing it from the mold.

Next, the PZT disk portion of the urethane rubber-PZT composite was cut off with a diamond blade (crystal cutter manufactured by Maruto Co., Ltd.), and a total of 32 square columns made of PZT of three different heights were placed in the urethane rubber matrix. However, a regularly arranged urethane rubber-PZT composite piezoelectric material was prepared, and both end surfaces were further polished with sandpaper.

Then, silver paste (manufactured by Deguza Corporation, product name: DEMETRON 62
90-0275) and baking at 120° C. for 30 minutes, a silver electrode as the electrode 23 was formed. In addition, a gold electrode as the electrode 24 was formed on the other end face portion without a step by sputtering. The gold electrode surface has a thickness of 0.2 as the transfer plate 1.
mm alumina thin plate (manufactured by Mitsubishi Mining Cement Co., Ltd., model number MA
10φ) was adhered to B-L201 with adhesive.

Then, connect the lead wire 5 to the top surface of the silver electrode and the side surface of the gold electrode.
were soldered to obtain an AE sensor consisting of a urethane rubber-PZT composite piezoelectric element.

In order to confirm the responsiveness of the A and E sensors, φ0.
A 5 mm pencil refill (hardness H) was crushed to generate a pseudo AE wave, which was received by the AE sensor and its rise time was measured.

In addition, for comparison, the first comparison AE sensor (Comparative Example 1) in which the height of all the 32 PZT prisms was 4 mm, and the AE sensor (Comparative Example 1) in which the height of the 16 prisms in the 32 PZTO mentioned above were 3 mm. A second comparative AE sensor (Comparative Example 2) in which the height of the remaining 16 prisms was 4 mm was fabricated using the same materials and manufacturing method as above, and the rise time of the pseudo AE wave was measured.

For measurement, 400mm x 400mm x 6 as a transmission medium.
Using a 0mm aluminum plate, the pseudo A
E waves were detected using the three types of AE sensors mentioned above. Also, A.E.
For signal detection and recording, a digital storage oscilloscope (manufactured by Hewlett-Paracard, model number HP-54) was used.
201D/Boo 54201Db it/200MHz)
A 1 m long coaxial cable (equivalent to 5D2V) was used to connect to the AE sensor.

The measurement results are shown in Table 1. Further, FIG. 5 shows the waveform of the pseudo AE wave observed by the AE sensor of this example.

In the AE sensor of this embodiment, the time from the start of reception of the pseudo AE wave to the peak, that is, the rise time t
was 800ns. On the other hand, in the first and second comparative AE sensors, the first rising time t is 1290 ns and 882 ns, respectively.
It was s.

That is, columnar ceramic piezoelectric bodies 21 with different heights
In the AE sensor of this example and comparative example 2 using
Compared to Comparative Example 1, which uses columnar ceramic piezoelectric bodies 21 of the same height, the rise time is 2/2.
It only took 3 hours, and the responsiveness was clearly improved. Furthermore, in the AE sensor of this example, the rise time t is approximately 10% shorter than that of Comparative Example 2.

Further, in the AE sensor of this example, the peak level at the rise time t of the pseudo AE wave, that is, the receiving sensitivity S was 84 mV. On the other hand, the first and second
For the comparative AE sensors, the receiving sensitivity S is 50.
mV and 48 mV.

That is, in the AE sensor of this example, the reception sensitivity S is improved by 60% or more compared to those of Comparative Examples 1 and 2.

As described above, in the AE sensor of this embodiment, the detection frequency range 28' is wide, so even high frequency components can be detected (
(see waveform in Figure 5), rise time and reception sensitivity S
is improving.

In the above embodiment, the columnar ceramic piezoelectric bodies 21 have three heights, but any number of heights may be used. For example, the same effect can be obtained even if the heights are four.

Note that if there are too many types, the resonant frequency will increase and the total bandwidth will be widened, but the number of column-shaped ceramic piezoelectric bodies 21 for each band light will decrease, resulting in poor reception sensitivity S.

Further, in this embodiment, the columnar ceramic piezoelectric body 21 having a rectangular cross-sectional shape is used, but the cross-sectional shape is not limited to this, and may be, for example, hexagonal. In addition, it is not limited to polygons (it may also be curved surfaces such as circles or ellipses).

As described above, since the AE sensor of the present invention has a wide detection frequency range of 28°, it can be used for qualitative analysis to detect all kinds of flaws occurring within the object 7, and moreover, it can detect only the longitudinal vibration mode in the AE wave. Since it can be detected, it can also be used for quantitative analysis to detect the progress of the above-mentioned damage generation. Furthermore, since the synthetic resin ceramic composite piezoelectric element 2 with excellent piezoelectric properties is used, it is small and has high detection sensitivity.

The AE sensor of the present invention can be used to detect not only AE waves but also all elastic waves propagating in gases, liquids, and solids. It can also generate waves.

[Effect of the invention] The acoustic emission sensor of the present invention has the following effects:
As described above, since the columnar ceramic piezoelectric body is polarized in the height direction, both end faces of the synthetic resin-ceramic composite piezoelectric element are polarized only in the longitudinal vibration mode in the AE wave propagating in the height direction. generates a potential difference. As a result, only the longitudinal vibration mode is detected. In addition, since we used at least three types of columnar ceramic piezoelectric bodies with different heights,
A resonance point appears depending on the height, and the detection frequency range of the AE wave becomes wider. Therefore, the rise time of the AE multiplied signal is shortened, and responsiveness and receiving sensitivity are improved.

As a result, changes in the AE generation intensity at the AE source can be faithfully and sensitively detected (acoustic
This has the effect of providing an emission sensor.

It is a diagram.

FIG. 2 is a partially cutaway perspective view of a synthetic resin-ceramic Nox composite piezoelectric element.

FIG. 3 is a characteristic diagram showing the relationship between the temperature and frequency of an ideal AE sensor.

FIG. 4 is a characteristic diagram showing the frequency dependence of impedance of the AE sensor.

Figure 5 shows the pseudo A observed with the AE sensor of this example.
It is a waveform diagram of an E wave.

1 is a wave receiving plate, 2 is a synthetic resin-ceramic composite piezoelectric element, 3 is an adhesive layer, 4 is a case, 7 is a subject, 21 is a columnar ceramic piezoelectric body, 22 is a synthetic resin matrix,
23 and 34 are electrodes.

Patent applicant: Tanezu Kaseihin Kogyo Co., Ltd.

[Brief explanation of drawings]

1 to 5 show one embodiment of the present invention. Figure 1 shows a vertical cross-section of the AE sensor according to the present invention.

Claims (1)

[Claims] 1. In an acoustic emission sensor that detects acoustic emissions using a synthetic resin-ceramic composite piezoelectric element in which a plurality of pillar-shaped ceramic piezoelectric bodies are arranged in a synthetic resin matrix so that their height directions are substantially parallel to each other, the above-mentioned synthetic resin - An acoustic emission sensor characterized in that the ceramic composite piezoelectric element uses columnar ceramic piezoelectric bodies of at least three different heights and is polarized in the height direction.