7/30/2019 A-Scan Principle
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Pulse echo A- scan test method :
K. Chatterjee, 75643 Center for NDT P 00 Rv 02 Self study
material.
Pulse echo test method uses reflected ultrasound as a means of
collectingtest information. A single crystal probe is normally used
for ultrasoundgeneration as well as reception.The transmitter
circuit of the flaw detector supplies short excitation pulses
of few hundred volts at regular interval to the probe crystal.
The excitation
pulse oscillates the crystal to generate short burst of
ultrasound such thatthe arrival of each returning echo may be
identifiable as a discrete event.During the interval between two
successive pulses, the crystal is at rest anddetects any return
echo such as from the back wall. A large percentage ofthe sound is
reflected from the front surface of the test part and theremainder
is reflected by the back surface or discontinuities. The
flawdetectors CRT screen displays the whole operation by producing
separatedsignals of transmission and the time of arrival of defect
echo and the backwall echo. The transmission pulse and subsequent
echoes appear as peaksrising out of the CRTs base line. The
distance between the peaks is ameasure of the defects location or
the parts thickness.Transmission of high frequency ultrasound
cannot takes place in air. It iscarried out through an intermediate
liquid, in bulk or as a thin layer. Oilysubstances or water are
generally used. They are called couplants.
The initial or transmitter pulse appears first in time and
represents theelectrical zero. This is the exact start time of
crystal excitation. The exactpoint in time when ultrasound enters
the test material is called acousticalzero. Acoustical zero is
superimposed within the initial pulse and is notdistinguishable.
The next pulse represents the total elapsed time for soundto travel
from the entry surface to the reflector and back to the
entrysurface again.At the instant the electrical pulse is removed
the oscillations of the crystaldo not cease immediately but
decreases in an exponential manner until theyreach zero. A dead
zone is produced, starting immediately after entry into thetest
surface, in which echoes can not be detected.One single test cycle
is so fast that it is not physically visible in the
detectors screen. Hence the flaw detector repeats the test cycle
severaltimes per second by supplying successive excitation pulses
to the crystaland make the event appear as constant due to
persistence of vision. Thenumber of times, the crystal is
electrically pulsed per second is called thepulse repetition
rate.
A sufficient amount of time betweensuccessive pulses is
necessary to allowultrasound to travel through the materialunder
examination. Higher pulse repetitionrate produces brighter screen
display. Very
high pulse repetition rate producesspurious signals [ ghost
echoes ] on theCRT screen.The ultrasonic pulses used by the
flaw
detector are radio frequency type andhave a serrated look. The
pulses arefiltered and rectified to smooth lookingshapes by the
flaw detector beforedisplay.Pulse echo A-scan method displays
distance along the horizontal scale calledthe baseline and
amplitude of thereflection along the vertical scale. Becauseof
similar return path, the screen is
calibrated to display one way travel only.A scan test method can
accurately locatea discontinuity. The amplitude of the returnsignal
is a relative measure of theamount of reflected energy and
dependson the area and orientation of thereflecting surface.
Amplitude of the signalcan be used for accept / reject
decision.
