FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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This together with the expression of T gives
v = c (At/T + 1)
Thus, from the sole measurement of time delays, and the knowledge of c, one
obtains the value of the velocity of sound in the sample.
Ill - EXPERIMENTAL PROCEDURE AND RESULTS
The samples (35) corresponding to a broad range of densities were furnished
by DuPont Canada and Union Carbide Canada Ltd. They were in the form of press
molded plaques (80 X 50 X 1.9 mm) prepared according to ASTM standards from a
variety of commercial grades of P.E. In the measurements, they were used as
is with no preparation. A number of pieces (2 to 4) were cut from the plaques
and their density measured in a density gradient column following the standard
ASTM procedure. The values are given with an accuracy of +_ .0007 g/cm at a
constant temperature t = 23°C.
The ultrasonic set up was designed to accomodate such samples and placed in
a bath of demineralized water at 23°C. The transducer is of the
commercially available immersion type. In order for the echos to be well
separated in time, the pulses must be short: this calls for a highly
damped transducer operating at high frequency. However, the attenuation of
sound in P.E. rapidly increases with frequency and this sets an upper limit.
We found that 3 MHz constitutes a good compromise between time resolution and
signal amplitude. The reflector, located roughly 15 cm from the transducer,
is a flat piece of glass with an absorbant backing so that no signals (echos)
are issued other that the reflexion from the front face. The set up is rather
simple and the only requirement is that the samples and reflector be
perpendicular to the sound beam.
Concerning the electronics, we used a Metrotec puiser (Mod. MP 203) capable of
delivering short (100 n sec) high voltage (300 volts) pulses with a repetition
rate or 50 to 100 Hz. The receiving amplifier was also Metrotec (Mod. MR 101)
wiht a 60 dB gain factor and a 20 MHz bandwidth. The RF signal is monitored
on an oscilloscope (H.P. Mod. 1743A) which is equipped with a dual time base
that allows time delay measurements with an adequate resolution (3 n sec or
better).
The sound velocity in the water has to measured: this is accomplished by
displacing the reflector by a known distance and noting the arrival times. We
obtain a value of c = (1.483 +_ 0.001) X 10 5 cm/sec at 23°C, in good agreement
with what is found in the literature from which we quote the temperature
dependence: 200 cm/sec°C.
The results for our measurements of sound velocity (at 3 MHz) in
polyethylene samples of different densities are illustrated in Fig. 2. To
a good approximation, the velocity is proportional to density.