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2.PiezoelectricSourceTransducerOurunusualpiezoelectrictransducersourcewasdevelopedatt...
2. Piezoelectric Source Transducer
Our unusual piezoelectric transducer source was developed
at the National Bureau of Standards (NBS)
[now National Institute of Standards and Technology
(NIST)] for use as an acoustic emission sensor [9]. Typical
commercial transducers have sensitive areas 10 mm
to 25 mm or more in diameter, a scale useful for detecting
or generating plane acoustic waves. The NBS design,
on the other hand, is optimized for detection of the
highly curved wave fronts characteristic of small buried
acoustic emission sources. For this purpose, its sensing
area is very small—only 0.7 mm in diameter. It can be
used effectively as a point receiver because the diameter
of the sensing area is small compared to most of the
wavelengths to be measured.
The design, illustrated in Fig. 1, incorporates a truncated,
conical, lead-zirconate-titanate (PZT) piezoelectric
element mounted directly on a large brass block
using hard solder. The tip of the element is equipped
with a nickel-plated electrode. The specimen itself, if
metallic, is used as one of the electrodes of the transducer.
With nonmetallic specimens, a thin strip of aluminum
foil is interposed between the specimen and the
transducer element to provide electrical contact. In either
case, the effect of the grounded electrode on the
incident elastic wave is much less than that of the wear
plate which covers and protects the grounded electrode
in transducers of conventional design. The brass block,
which substitutes for the usual backing material, is provided
with two nylon feet, which, together with the
piezoelectric element, provide three-point kinematic
support. The weight of the block ensures good contact
with the specimen.
The overall design clearly makes the transducer rather
delicate and hence not very useful for most commercial
applications. However, for acoustic emission sensing in
the laboratory and for ultrasonic wave generation, the
device has been found to be quite useful.
For the experiments reported here, the transducer was
used as a source excited by the 750 V exponential pulse
waveform shown in Fig. l. The excitation pulse was
generated by a vacuum tube amplifier driven by a simple
exponential pulse generator circuit. 展开
Our unusual piezoelectric transducer source was developed
at the National Bureau of Standards (NBS)
[now National Institute of Standards and Technology
(NIST)] for use as an acoustic emission sensor [9]. Typical
commercial transducers have sensitive areas 10 mm
to 25 mm or more in diameter, a scale useful for detecting
or generating plane acoustic waves. The NBS design,
on the other hand, is optimized for detection of the
highly curved wave fronts characteristic of small buried
acoustic emission sources. For this purpose, its sensing
area is very small—only 0.7 mm in diameter. It can be
used effectively as a point receiver because the diameter
of the sensing area is small compared to most of the
wavelengths to be measured.
The design, illustrated in Fig. 1, incorporates a truncated,
conical, lead-zirconate-titanate (PZT) piezoelectric
element mounted directly on a large brass block
using hard solder. The tip of the element is equipped
with a nickel-plated electrode. The specimen itself, if
metallic, is used as one of the electrodes of the transducer.
With nonmetallic specimens, a thin strip of aluminum
foil is interposed between the specimen and the
transducer element to provide electrical contact. In either
case, the effect of the grounded electrode on the
incident elastic wave is much less than that of the wear
plate which covers and protects the grounded electrode
in transducers of conventional design. The brass block,
which substitutes for the usual backing material, is provided
with two nylon feet, which, together with the
piezoelectric element, provide three-point kinematic
support. The weight of the block ensures good contact
with the specimen.
The overall design clearly makes the transducer rather
delicate and hence not very useful for most commercial
applications. However, for acoustic emission sensing in
the laboratory and for ultrasonic wave generation, the
device has been found to be quite useful.
For the experiments reported here, the transducer was
used as a source excited by the 750 V exponential pulse
waveform shown in Fig. l. The excitation pulse was
generated by a vacuum tube amplifier driven by a simple
exponential pulse generator circuit. 展开
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