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The display of an NDT ultrasound machine is
very important because there are different variations such as A, B,
C and the hottest topic of the year S-scan displays. I will not go
into each of these other than to say A-scan is the most frequently
used and S-scan is the new craze with UT ultrasonic phased array
inspection . A down side to UT ultrasound is that if the crack
is not visible to the human eye it can be difficult to
correlate to a customer or untrained person an indication on the
screen equals a crack or flaw. This is not as difficult with an
S-scan display because the representation makes it easy for an
untrained person to interpret the screen.
Phased Array explained by Olympus NDT
UT ultrasound testing is good for performing
inspection though the cross section of the material. It is a
sensitive method and stated that the detectable flaw is usually half
the lambda or half the wavelength. The ability of the sound to
travel through the material is due to its elastic properties,
attenuation and frequency. This also
determines the length of the material detectable. 20 feet or .10”
thick aluminum could easily be inspected with ultrasound.
High frequency equals less penetration do to
attenuation and absorption. Higher frequency also represents
higher sensitivity or resolution. Frequency can be to high
and cause noise of the inspection. With an immersion unit at 50Mhz you get grain structure
attenuation of metals , you can actually see the heat treatment
variations of the metal. Low frequency equals more penetration and
less sensitivity. This principle makes instrumentation and
transducer selection paramount for the appropriate application.
The selection should
allow you to provide sensitivity and penetration at the same
time. The selection of frequency is usually specified
in the procedure being used to perform the inspection if not you
should consult a level III.
A common problem to UT ultrasound inspection is
the detection of surface flaws. Most facilities utilize transducers
for subsurface scans and do not have surface wave transducers
readily available. There are also other more appropriate and cost
effective ways of performing surface inspections such as ET Eddy
Current, PT Liquid Penetrant and MT Magnetic Particle to name a
few.
Critical mistakes in UT ultrasound inspection
include not having an appropriate calibration block and not post
calibrating after the inspection. If you do not post calibrate you
have no way of ensuring that the inspection was performed properly.
Remember that when we are talking about missing a crack we are
referencing the possibility of an inspector missing a crack on a
critical part that could lead to loss of life. This is why NDT
ultrasound technicians should be properly qualified and certified to
a Level II when performing examinations. Qualification is
referencing classroom training and experience hours. Certification
is referencing that the individual has passed a program by testing
and demonstration of their capabilities in accordance with a code
such as SNT-TC-1A or NAS-410 and has been reviewed by a Level III.
Facilities are supposed to have a primary Level III on file for code
conformance. If you do not have a Level III on file we can provide
that service for you (shameless plug # 2) and keep you code
compliant.
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