JPH06148150A - Method and device for measuring physical characteristic by acoustic image - Google Patents

Abstract

PURPOSE:To provide a means for measuring the anisotropy and scattered wave of a material having various acoustic characteristics by a focused acoustic image. CONSTITUTION:When a target object 3 is irradiated by use of a spherical sound wave transmitted by an acoustic element 1, a sound wave-impermeable acoustic knife edge 6 is arranged within the opening surface of the acoustic element 1 to partly stop the sound wave, the sound wave reflected from the object 3 is detected by the same acoustic element 1 to take the acoustic image of the object 3, or measure the acoustic characteristic of the object 3. To improve the directional sensitivity of the acoustic image or acoustic element 1, by covering a different part of the opening part, the acoustic knife edge 6 is arranged within the opening surface of the acoustic element 1, and the measurement is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring elasticity and related properties of an object by using acoustic imaging and focused sound waves.

[0002]

Focused ultrasound is a non-destructive measurement of optically opaque objects and local mechanical properties (ie, surface acoustic wave velocity and its attenuation, longitudinal and shear wave velocities, etc.). Widely used in. One of the characteristics of such an apparatus or method is that it is possible (or not) to measure the anisotropy of the mechanical properties of an object related to the crystal structure of the object or the processing method (rolling, pulling, etc.) of the material. . Another practically important feature of such devices and methods is their high sensitivity to small voids and flaws in the body that scatter sound waves.

In order to improve the directional sensitivity of an audio-visual device, JA Hildebrand and LKLam, "Directive ultrasonic microscope for observing elastic anisotropy" (Appl.Phys.Lett.4).
2 (5), 1983.5.1, pp.413-415) (Reference 1), it was proposed to use a spherical acoustic lens equipped with a split acoustic transducer, and then DADavis and HLBerto.
ni, “Bowtie transducer for observing elastic anisotropy” (IEEE Ultrason, Symp.p.
roc,: 735 (1986)) (Reference 2), it was proposed to use a bow-tie type piezoelectric transducer. After that, J. Kushibiki et al. "Quantitative measurement of substances by directional ultrasonic microscopy" (Ultrasonics International 89 C
onference Proceedings pp.736-739) (Reference 3), it was proposed to use a rectangular transducer for the quantitative measurement of velocity and attenuation of surface acoustic waves (SAW).

These methods are described in Ishikawa et al., "Application of anisotropic acoustic lens design and characterization of material properties" (Pr.
oc.Spring Meeting of Jap.Soc.Prec.Eng., 1990, pp.1
111-12) (reference 4). Ishikawa et al.
Slits placed near the surface of the acoustic lens were used to form an acoustic beam with a narrow azimuth propagation direction, which is sensitive to the anisotropy of surface acoustic waves. In addition, in order to improve the sensitivity of standard focused acoustic imagers to scattered waves (small reflective voids, flaws, and other inhomogeneities in the object of interest), WLBond et al. .Letters, 27 (5), 1975, pp.270-272)
(Reference 5) proposed the use of a dark-field method using two different acoustic transducers or split-type transducers for transmitting and receiving sound waves. Also, LR
Clarke et al., "Acoustic Measurement of Abrasive Damage on Ceramic Materials".
(IEEE 1985 Ultrasonic Symposium, pp.979-82, IEEE,
(New York) (6) it has been proposed to cover half of the acoustic lens with an acoustically impermeable wax so as to receive sound waves scattered in a single direction from surface cracks.

The measurement of anisotropy by the above method is chosen for the limited kinds of object properties which are the design objectives, and also
It is based on the design of a fixed acoustic lens that is effective only for this property. For example, the surface acoustic wave velocity V _R is about
An acoustic lens designed to measure the anisotropy of LiNbO ₃ or MnZn ferrites such as 3200-3800 m / s has a surface acoustic wave velocity V _R of Al ₂ O ₃ faster than 4500 m / s.
Or it is not optimal for measuring the anisotropy of Si.

Even with an acoustic lens equipped with a split type transducer (references 1 and 3), localization of the acoustic lens is inevitably performed in the near field of the transducer, which results in a complicated irradiation pattern. Moreover, the distribution of the directional sensitivity of the lens becomes complicated. The sound field of the combination of acoustic lens and slit (Reference 4) is better, but in order to achieve this, it is necessary to make a slit with extremely narrow and parallel edges, the width of which is also fixed. is there. The method of obtaining the sensitivity to scattered waves in an audio device is based on the dark field method (References 5 and 6), but this is also an unadjustable fixed that cannot optimize the state of improving the audio-visual contrast. Device is used.

[0007]

Taking into account that audiovisual methods and measuring instruments are often used for the investigation of various objects having basically unknown properties, such a device can be used for this purpose. Being able to adjust to is an important and very valuable feature. In order to adapt such a measuring device to an object, we have installed a new element inside a standard focused acoustic device, an ultrasonically impermeable acoustic knife edge. The acoustic knife edge can cover portions of the opening of the acoustic lens to block some of the sound waves emitted and received by the acoustic lens, or can be repositioned to the boundary of the acoustic lens. . Due to the axial symmetry of the acoustic lens, the knife-edge delimits a narrow symmetric portion of which the width can be adjusted in the azimuthal plane of the lens in specular mode. The width of this part defines the angular spectrum of the sound rays forming the acoustic signal, so that the measuring device configuration for the acoustic properties of the target object can be adjusted gradually. Such characteristics are useful for measuring anisotropy. For objects that scatter sound waves, the knife edge improves the sensitivity of the audiovisual device to different angles of scattering from the slightly deflected ray to the backscattered wave. This allows a particular feature to be selected in the object structure. Therefore, the technical problem of the present invention is
To provide means for performing anisotropy and scattered wave measurements by means of focused acoustic imaging and by means of a measuring device which has the same azimuth resolution as acoustic imaging and which can be applied to the investigation of objects with different acoustic properties. is there.

[0008]

The object of the present invention as described above is to adjust and change the angular sensitivity according to the target by using a positionable acoustic opaque filter, that is, an acoustic knife edge. It is achieved by modifying a commercially available audio-visual device such as an ultrasonic microscope, an NDE focusing transducer, etc.).

The method and apparatus of the present invention for measuring an object with a focused acoustic wave using an acoustic knife edge will be described more specifically. In the present invention, basically, a target object is a surface of an object. Probing is done by using focused sound waves that are reflected or scattered from or from the internal structure of the object. The object is illuminated by the acoustic waves transmitted by the acoustic lens or the spherical acoustic transducer, which is also used as a receiver for the acoustic waves reflected or scattered by the object. Suitable for objects,
Further, a part of the sound wave emitted from the object is stopped by the ultrasonic impermeable acoustic knife edge arranged on the opening surface of the acoustic element. This action makes the acoustic device more sensitive to both the direction of sound wave propagation (object anisotropy) and the sound waves scattered by the acoustic inhomogeneity of the object. Therefore, we examine the object by standard audiovisual or acoustic velocity and attenuation measurement methods, but by covering different parts of the opening in order to improve the audiovisual and directional sensitivity of the target object optimally,
The acoustic knife edge is placed in the open surface of the acoustic element.

According to the present invention as described above, an axisymmetric acoustic lens or a focusing transducer is used for various objects having various acoustic characteristics to measure anisotropy of an object and to measure nonuniformity. Optimal conditions for can be achieved.

[0011]

The above-described method and apparatus of the present invention can be easily realized by using a commercially available ultrasonic microscope or ultrasonic NDE apparatus, or by using a separately produced focused acoustic apparatus. According to FIG. 1, in order to implement this method, a focused acoustic element 1 (which may be a focused transducer) such as an acoustic lens connected to a standard high-frequency transceiver 2 is required. This device emits focused sound waves through the immersion liquid to the object of interest 3 and receives sound waves scattered or reflected therefrom. The three-dimensional mechanical positioning device 4 can scan the acoustic element 1 in the xy directions to obtain an acoustic image, or can be arranged in the z direction to be used for measurement of velocity and attenuation by surface acoustic waves.
(z) The curve can be obtained. These audiovisuals and data are received and visualized by the processor 5 used to obtain audiovisuals and sound velocity measurements.

The inventors have added a new element, an ultrasonically impermeable acoustic knife edge 6, which covers a part of the opening of the acoustic element to this device configuration, which is common for many applications. This stops some of the sound waves emitted by the acoustic element and also stops some of the sound waves reflected and scattered from the object. The position of this acoustic knife edge and the part of the opening that is impermeable to sound waves can be changed by a mechanical positioning device 7 which moves the knife edge along one direction. Further, as shown in FIG. 2A or 2B, the shape of the edge 6a of the acoustic knife edge 6 is selected to be linear (a) or circular (b), so that the shape of the sound wave stop portion and the sound receiving portion can be changed. Can also be changed.

In the measurement conducted by the present inventors at this time, a self-made Si
O ₂ acoustic lens (center frequency 70 MHz, focal length 3000 μ
m, aperture radius 3200 μm) and sapphire high frequency acoustic lens (center frequency 200 MHz, made by Olympus Optical Co., Ltd.,
A focal length of 500 μm and an opening radius of 700 μm) were used. As transmitter and receiver, with HP 8656A high frequency generator as cw source, multi-function GPIB drive made by Honda Electronics Co., Ltd. 30-400 MHz
A transceiver unit was used. In order to realize mechanical positioning and scanning of the object, a positioning table with a stepping motor for low speed operation manufactured by Chuo Seiki Co., Ltd. and a linear electromagnetic scanner for high speed scanning were used. GPI connected to the NEC PC-9800 computer that captures and processes data on all devices
Equipped with B drive interface. The audio image is also a NEXUS 640 512 × 480 pixel, 256 grade halftone monitor connected to the computer by GPIB.
It is displayed at "0".

Materials that can be used as knife-edge materials are those that have a high reflectivity due to impedance mismatch (such as a metal film) or their acoustically high attenuation (such as a plastic film). Either is because it is impermeable. In this experiment, Teflon (PTFE: product name) film was used as a material with high ultrasonic attenuation that is easily available. The knife edge thickness is determined by the frequency of the ultrasonic wave,
It must be thick enough to provide a reduction in acoustic amplitude greater than dB. We used a 0.5 mm Teflon sheet for a 100 MHz operating frequency,
Then, a Teflon film with a thickness of 0.2 mm was sufficient.

Since the acoustic knife edge method has a high tolerance for positioning accuracy and linearity, various knife edge positioning devices 7 can be easily designed. The two types of configurations used by the present inventors appear to be effective. The first method is by a knife edge mounted parallel to the fast scan axis, which allows the positioning device to be fixed on a stable slow scan axis and also allows the knife edge position to be changed directly during fast scan. We were able to. The second method uses a cylindrical knife edge mount that is rotated by a guide post mounted slightly eccentrically to the acoustic axis. As a result, it is possible to realize a lightweight and precise edge position adjusting device that is absolutely necessary for mechanical audiovisual scanning.

To explain the formation of the acoustic signal by this device, the opening of the acoustic element is defined by the three parts (zones) of FIG.
It should be divided into That is, the portion C covered by the knife edge, the portion A that is antisymmetric with respect to the acoustic axis,
A central portion B located between portions A and C. For sound rays reflected from the surface of the object or the inner surface parallel to the object (for example, the back surface of the object) as well as re-emitted surface acoustic waves, an acoustic signal is formed by recording obtained only from the B part of the acoustic element. Is easy to understand.
The width of the portion defined by the opening angle theta _ke knife edge (Figure 1), the focal length f, when the half-opening angle theta _a, the diameter of the device opening is D, wave propagation direction [Phi _az ( =
It is given by 2 sin ^-1 (2 fcos θ _a tan θ _ke / D). This angle is used as a measure of the directional sensitivity of an acoustic device with knife edges. In general, the smaller this angle (while it is positive), the better the directivity sensitivity of the output signal, but the worse the lateral resolution in the direction perpendicular to the edge boundary. Therefore, the knife edge can preserve the required values of the signal amplitude and lateral resolution of the device for surface acoustic waves or Lamb waves and shear waves or longitudinal waves reflected from the back side, and select the optimum directional sensitivity. .

Some acoustic non-uniformity in the object, ie,
When voids, flaws, surface cracks, coarse grains or twin boundaries are included, the scattered sound waves in the A and B parts of the acoustic element opening are received by the A part which reacts only to scattered waves. . The sensitivity to scattered waves can be increased by reducing the opening angle θ _ke of the knife edge (which increases the ratio of the input of the portion A to the signal).
When θ _ke = 0 (ie half of the acoustic lens aperture is covered by the knife edge), only scattered waves form the output signal of the device, but specular sound waves are completely stopped by the knife edge. It's easy to see. When θ _ke becomes negative, the output signal of the device is 2
It is formed only by sound waves that scatter at angles greater than | θ _ke |. This means that the acoustic knife edge allows the sensitivity of the focusing device for scattered waves to be gradually adjusted and the acoustic scattering angle to be measured. Taking into account the fact that part A is asymmetric with respect to the acoustic axis of the device, this device configuration is also sensitive to the direction of sound scattering.

Therefore, by the acoustic knife edge method,
Not only the sensitivity of the measuring device to the acoustic image or the elastic anisotropy or inhomogeneity of the object is obtained, but also the directional sensitivity and the dark field contrast for the needs of a given target object can be optimized.

As the first object of the directional sensitivity test of the above equipment configuration, the test was started with LiNbO ₃ which is a crystal whose acoustic properties have been completely measured and recorded. FIG. 3 shows [01
[V] z obtained by a knife edge placed along two directions of the X-cut plane of LiNbO ₃ having maximum and minimum velocities of surface acoustic waves at about 15 ° and 115 ° respectively with respect to the [0] axis. ) Shows the curve. For these curves, it is easy to see that the oscillation period differs for the surface acoustic wave velocity, which means that the desired directional sensitivity has been achieved.

For a quantitative measurement of directional sensitivity, we determined the surface acoustic wave velocity of sound as a function of knife edge orientation. Figure 4 shows the good agreement between the calculated LiNbO ₃ surface acoustic wave acoustic velocity and the experiment. The error is related to the interference of surface acoustic waves propagating along adjacent directions and is determined by the knife-edge positioning angle.

FIG. 5 is the variation of the V (z) curve as a function of knife edge opening angle θ _ke . When starting with a narrow opening (small θ _ke ), the reflected signal is weak and almost undetectable, and when the opening is intermediate the oscillations remain at the same frequency, but the amplitude is much larger, and finally,
At large spreads, the vibrations will likely be distorted by the sound waves that begin to propagate perpendicular to the edge of the knife edge. By this action, it is possible to find the optimum opening angle of the knife edge with excellent directivity and sufficient signal.

FIG. 6 illustrates the possibility of solving the same problem for a more complex object, a drawn stainless steel sample. In this case, it is completely impossible to detect regular vibrations when the opening is narrow, but directional sensitivity cannot be obtained when the opening is large. We chose an intermediate opening angle θ _ke = 15 °, which allowed us to measure the difference in velocity along and perpendicular to the pull direction.

Furthermore, the inventors of the present invention were able to measure anisotropy with high lateral resolution by using an acoustic image example of MnZn ferrite with knife-edge devices having different edge orientations, which was related to the excitation of surface acoustic waves. It is confirmed that the crystal contrast depends largely on the edge orientation. Even in the xz acoustic image of the same MnZn ferrite sample, the crystal contrast is determined by the directional sensitivity that can be combined with the acoustic lens and the knife edge because the frequency of the fringes in the z direction is different for various edge orientations. I'm sure.

It is interesting to compare the acoustic image example of the above MnZn ferrite with the acoustic image obtained with the same object placed in focus when surface acoustic waves are not excited. In this case, closing the knife edge reveals high contrast details on the surface which was initially specular. This contrast is associated with the sound waves scattered from the grain boundaries. Similarly, a clear contrast enhancement was obtained with a sample of Si ₃ N ₄ fine ceramics containing cracked Vickers indentations. It is easy to see that in a series of audiovisual images with different edge position angles, the small void contrast is gradually increased and the signal reflected from the crack parallel to the edge of the knife edge is significantly increased. When the edge opening angle θ _ke approaches 0 °, the scattered wave signal from the crack and the void becomes larger than the specular reflection component. This kind of audiovisual is referred to as dark-field audiovisual in the above-mentioned documents 5 and 6, and the main contribution to the audiovisual is given by nonuniformity rather than by the base metal.

Further interesting samples were also obtained for dark field acoustic imaging of carbon fiber reinforced plastic (FRP) composites. The indistinct and poor contrast of subsurface carbon fibers at large edge opening angles θ _ke = 30 ° and 15 ° becomes more pronounced when the aperture is small. At θ _ke = 0 °, the acoustic image is virtually only formed by fibers, which appears to be the major nonuniformity in this material.

Regarding the sensitivity of the knife edge to the direction of the scattered wave, the acoustic reflection image of the FRP having the 45 ° direction fibers arranged at various depths was also observed. The results show that it is the upper layer fibers perpendicular to the edge of the knife edge that are primarily responsible for the audiovisual with large edge openings, and thus only these fibers are visible. When the opening of the edge is small, the specular reflection wave from the upper layer becomes smaller because the portion A contracts (Fig. 7 (a)), but the scattered wave from this upper layer (Fig. 7 (b)) is the incident wave. Remains small because the orientation of is parallel to the fiber. On the contrary, the scattered wave from the layer in the 45 ° subsurface direction (FIG. 7 (c)) becomes relatively large due to the inclination with respect to the sound wave direction, and this fiber is visualized (θ _ke = 3 °). At θ _ke = 0 ° and -3 °, the surface layer could be completely eliminated, and only a clear subsurface acoustic image could be recorded.

As described above, according to the present invention, in the measurement of ultrasonic acoustic images or the acoustic measurement using surface acoustic waves, Lamb waves, backside shear waves, or reflected waves of longitudinal waves, the signal amplitude and The sensitivity to anisotropy can be optimized while maintaining the required value of lateral resolution. Further, according to the present invention, the sensitivity of the ultrasonic device to scattered waves can be adjusted, and the angle of visualized sound scattering can be selected. Therefore, the method allows to continuously adjust the directional sensitivity to the characteristics of the target object and the contrast caused by the non-uniformity. Thus, the present invention is very useful in practical applications.

[0028]

As described in detail above, according to the present invention, by maintaining the lateral resolution of the apparatus by the acoustic knife edge method, it is possible to obtain anisotropy and anisotropy in a commercially available focused audio-visual apparatus or measuring apparatus. It is possible to measure the scattering of sound waves and, moreover, with this method and device it is possible to select the optimum conditions for the measurement of anisotropy and scattered waves that are suitable for the acoustic properties of the object of interest, for example a constant Surface acoustic wave velocity or acoustic scattering angle can enhance selected structural elements of the object.

It is also possible to solve the problem between spatial resolution and measurement accuracy or audiovisual contrast by continuously selecting the angular spectrum of the sound ray for audiovisual formation or velocity measurement. In this case, the optimum opening of the acoustic lens opening needs to be changed not only for each type of object, but for each element of the object to be measured. A valuable advantage of the method and apparatus of the present invention is that they are also easily realized for all types of ultrasound focusing devices, from low frequency NDE test equipment to precision ultrasonic microscopes in the GHz range.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a method and an apparatus of the present invention.

2A and 2B are explanatory views illustrating the shape of the edge of an acoustic knife edge.

FIG. 3: X of LiNbO ₃ having maximum and minimum velocities of surface acoustic waves at about 15 ° and 115 ° from the [010] axis, respectively.
₃ is a graph showing a V (z) curve of a LiNbO ₃ crystal obtained by arranging knife edges along two directions of a cut surface.

FIG. 4 is a graph showing the propagation direction dependence of the surface acoustic wave velocity in X-cut LiNbO ₃ measured by an axisymmetric spherical acoustic lens having an acoustic knife edge.

FIG. 5 is a graph showing the dependence of the V (z) curve of X-cut LiNbO ₃ on the opening angle θ _ke of the acoustic knife edge.

FIG. 6 is a graph showing the dependence of the V (z) curve of stainless steel SUS-304 on the opening angle θ _ke of the acoustic knife edge.

7A to 7C are explanatory diagrams showing knife edge setting sensitivity with respect to the direction of scattered waves in FRP.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Focused acoustic element method, 2 High frequency transmission / reception apparatus, 3 Focused sound wave target object, 4 3 dimensional mechanical positioning apparatus, 5 processing apparatus, 6 acoustic knife edge, 6a edge of acoustic knife edge, 7 mechanical positioning apparatus.

Claims (4)

[Claims] 1. When illuminating an object with spherical acoustic waves transmitted by an acoustic element consisting of an acoustic lens or a focusing transducer, a sound-impermeable acoustic knife edge for stopping a part of the acoustic wave is provided on the acoustic element. An acoustic image or an acoustic element that is placed in the opening plane, emits sound waves using the same acoustic element, and detects the acoustic waves reflected from the object to take an acoustic image of the object or measure the acoustic characteristics of the object. In order to achieve an improvement in the directivity sensitivity of the object, an acoustic knife edge is placed in the opening surface of the acoustic element by covering different parts of the opening, and the above measurement is performed. How to measure characteristics. 2. The method for measuring an object characteristic by means of an acoustic image according to claim 1, wherein the shape of the edge of the acoustic knife edge is linear or circular. 3. An acoustic element comprising an acoustic lens or a focusing transducer as means for transmitting and receiving spherical sound waves, a mechanical positioning device as means for positioning and scanning the acoustic lens with respect to a target object, and sound waves. To the acoustic knife edge, a mechanical positioning device for moving the acoustic knife edge that covers the opening at various positions of the acoustic element, and a signal that stimulates the acoustic element and reflects from the object. An apparatus for measuring object characteristics by means of an acoustic image, comprising: a processing apparatus for taking an acoustic image of the object or obtaining an acoustic characteristic of the object. 4. The object characteristic measuring apparatus according to claim 3, wherein the shape of the edge of the acoustic knife edge is linear or circular.