FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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"Events on earth consist in the interplay of two opposed forces, the yin and
the yang, the female and male principles respectively . The interaction
of these two forces together produce most of the universal phenomena. One
yin and one yang together equal the Tao (the Great Ultimate)".
- James K. Feibleman [1].
INTRODUCTION
The ultrasonic examination of materials has played a major role in nondestructive
testing. One technique for sizing defects is called the time-of-flight
method [2]. It is based on measuring the travel time of sound waves diffracted
or reflected from a defect, and has usually been used to measure crack
depths in one dimension [33. Recently the method has been extended to twodimensional
[t,5] and three-dimensional [6,7] cases. Another technique which
has flourished is the ultrasonic frequency analysis method [8], This method
makes use of the analysis of ehe frequency spectrum of the signal scattered
from the defect and attempts to extract information about its size and physical
properties. Both techniques have provided valuable information and have
been used in research and development for the last ten years. An experiment
is described below in which both techniques are used simultaneously to measure
the size and orientation of simulated flaws.
EXPERIMENTAL PROCEDURE
Figure 1 is a block diagram of the time-of-flight method and ultrasonic frequency
analysis system. The immersion technique is illustrated schematically
with a single transducer acting as both puiser and receiver (pulse-echo). A
Metrotek high energy pulser MP215 was used to excite the 5 MHz 1.27 cm diam
transducer. The highly damped transducer emits a relatively broadband multifrequency
pulse with strong frequency components from 1 to 9 MHz. A brass rod
placed in water acted as an idealized flaw whose size and orientation could be
easily controlled. The flattened smooth ends of brass rods with various diameters
(Fig. 2) were used to scatter the broadband ultrasonic pulse. The signal
was detected by a Metrotek wideband receiver MR106 and fed into a Metrotek
stepless RF gate MG701 where the relevant signals were gated. Both the signal
and the gated signal were displayed on an oscilloscope. The time-of-flight
was measured by using the delayed sweep magnification feature of the oscilloscope.
The gated signal was also applied to an HP8557A Spectrum analyzer
connected to an HP853A Spectrum analyzer display which produced a display of
the relative amplitude as a function of frequency on a cathode ray tube (CPT).
The instrument had a measurement range of 10 kHz to 350 MHz. Adjustable
center-frequency and bandwidth enabled any portion of a displayed spectrum to
be examined in detail.
A mechanical system provided accurate control of the orientation of the transducer
and allowed its orientation to be measured to *0.1°. The distance
between the transducer and the specimen (the water path ) was ~18 cm. The
specimen was thus located in the far field region of the transducer.
Each circular brass rod was positioned vertically at the bottom of the water
tank. The transducer was set at a specific orientation. It was then X- and
Y-scanned to provide an optimum signal from sound waves diffracted from the