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That is what UT ultrasonic testing is usually
doing. You are measuring to see how far out the sound waves travel
before they are reflected within the material and return to the
transducer. Like a speaker and a microphone (instrument capable of
transforming sound waves into changes in electric currents), a
transducer can send and receive sound. The material that usually
allows this to take place is a piezoelectric (capable of sending and
receiving) material such as quartz and composites.
The next logical question is how can the
instrument measure waves? The answer is actually quite
complicated and simple at the same time. Let’s take you back
to the days of being a youth playing Marco Polo in a pool . To get
the upper hand on things, most kids would put their heads in the
water and every one would scream cheater. The really geeky kid
would do everything to put his or her ears in the water because the
slightest movement under water would be louder and easily detectable. That is because
water has a very good acoustic velocity. Knowing the specific
acoustic velocity of a material allows you to calculate the time it
will take the sound to travel through a material. Usually the
acoustic velocity is based on the specific material that you are
inspecting and does not vary making the calculation
easy. Other times you have complications and reflections to deal
with which is what makes shear wave testing so complicated.
Acoustic velocity of a material and frequency
are not the same thing. When you hear a song playing, the
instruments play at the same frequency so they are recognizable to
you as being a specific instrument. If you are at a pool with
speakers under water you could hear the music very clear
underwater. When you get your head out of the water the music
would be dampened because the velocity of sound in air is not very
good. You can also think of it if you are trying to rest in a tub
with your ears under water, you seem to always hear the tub
draining just a little bit. You would never hear the slight
draining of water out of the tub it would just appear to drain
itself magically but the water magnifies the sound. For you
geeky ones, you are probably thinking that you would see air
bubbles and you are correct.
If you want the complicated version of sound measurement explained
Click Here.
Ultrasound is dependent on a calibration block
or standard: a piece of material of the same composition as the part
being inspected with known indications or crack examples. Sometimes
the standard has no flaws and is used to verify sound path within a
material to calibrate the distance sound is traveling verses time.
With other methods, such as liquid penetrant and magnetic particle,
these blocks are not required making those processes more appealing
for certain inspections. Not having the correct standart can represent a problem when
trying to perform an inspection. If you do not have an appropriate
calibration block you cannot perform the test in conformance to a
governing code. This becomes of the utmost importance when
performing any NDT ultrasonic inspection.
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