ULTRASOUND AND ULTRASONIC TESTING
reading this section you will be able to do the following:
the acronym "NDT."
how sound is used in NDT to find flaws.
how sound is used in NDT to measure material thickness.
Why is it important to understand sound?
There are many uses for sound in the
world today. We have already mentioned a few. Musicians can benefit
from a greater understanding of sound, architects must understand
sound to design effective auditoriums, detectives can use sound
to identify people, and many new types of technology apply sound
recognition. Another use of sound is in the area of science called
Nondestructive testing, or NDT.
What is NDT?
Nondestructive testing is a method
of finding defects in an object without harming the object. Often
finding these defects is a very important task. In the aircraft
industry, NDT is used to look for internal changes or signs of
wear on airplanes. Discovering defects will increase the safety
of the passengers. The railroad industry also uses nondestructive
testing to examine railway rails for signs of damage. Internally
cracked rails could fracture and derail a train carrying wheat,
coal, or even people. If an airplane or a rail had to be cut into
pieces to be examined, it would destroy their usefulness. With
NDT, defects may be found before they become dangerous.
How is ultrasound used in NDT?
Sound with high frequencies, or ultrasound,
is one method used in NDT. Basically, ultrasonic waves are emitted
from a transducer into an object and the returning waves are analyzed.
If an impurity or a crack is present, the sound will bounce off
of them and be seen in the returned signal. In order to create
ultrasonic waves, a transducer contains a thin disk made of a
crystalline material with piezoelectric properties, such as quartz.
When electricity is applied to piezoelectric materials, they begin
to vibrate, using the electrical energy to create movement. Remember
that waves travel in every direction from the source. To keep
the waves from going backwards into the transducer and interfering
with its reception of returning waves, an absorptive material
is layered behind the crystal. Thus, the ultrasound waves only
type of ultrasonic testing places the transducer in contact with
the test object. If the transducer is placed flat on a surface
to locate defects, the waves will go straight into the material,
bounce off a flat back wall and return straight to the transducer.
The animation on the right, developed by NDTA, Wellington, New
Zealand, illustrates that sound waves propagate into a object
being tested and reflected waves return from discontinuities along
the sonic path. Some of the energy will be absorbed by the material,
but some of it will return to the transducer.
Ultrasonic measurements can be used
to determine the thickness of materials and determine the location
of a discontinuity within a part or structure by accurately measuring
the time required for a ultrasonic pulse to travel through the
material and reflect from the backsurface or the discontinuity.
When the mechanical sound energy comes
back to the transducer, it is converted into electrical energy.
Just as the piezoelectric crystal converted electrical energy
into sound energy, it can also do the reverse. The mechanical
vibrations in the material couple to the piezoelectric crystal
which, in turn, generates electrical current.
Your Turn - Try this normal beam test
A pulse-echo ultrasonic measurement
can determine the location of a discontinuity with a part or structure
by accurately measuring the time required for a short ultrasonic
pulse generated by a transducer to travel through a thickness
of the material. Then it reflects from the back or surface of
a discontinuity and is returned to the transducer.
The applet below allows you to move
the transducer on the surface of a stainless steel test block
and see the reflected echoes as the would appear on an oscilloscope.
What the graphs
The ultrasonic tester graphs a peak
of energy whenever the transducer receives a reflected wave. As
you recall, sound is reflected any time a wave changes mediums.
Thus, there will be a peak anytime the waves change mediums. Right
when the initial pulse of energy is sent from the tester, some
is reflected as the ultrasonic waves go from the transducer into
the couplant. The first peak is therefore said to record the energy
of the initial pulse. The next peak in a material with no defects
is the backwall peak. This is the reflection from waves changing
between the bottom of the test material and the material behind
it, such as air or the table it is on. The backwall peak will
not have as much energy as the first pulse, because some of the
energy is absorbed by the test object and some into the material
The amount of distance between peaks
on the graph can be used to locate the defects. If the graph has
10 divisions and the test object is 2 inches thick, each division
represents 0.2 inches. If a defect peak occurs at the 8th division,
we know the defect is located 1.6 (0.2 x 8) inches into the test
What if the thickness is unknown?
If the thickness of the object is
unknown, it can be calculated using the amount of time it takes
for the back wall peak to occur. The thickness of the object is
traveled twice in that time, once to the back wall and once returning
to the transducer. If we know the speed of our sound, then we
can calculate the distance it traveled, which is the thickness
of the object times two.
What happens when a defect is present?
If a defect is present, it will reflect
energy sooner also. Another peak would then appear from the defect.
Since it reflected energy sooner than the back wall, the defect's
energy would be received sooner. This causes the defect peak to
appear before the backwall peak. Since some of the energy is absorbed
and reflected by the defect, less will reach the backwall. Thus
the peak of the backwall will be lower than had there been no
defect interrupting the sound wave.
When the wave returns to the transducer,
some of its energy bounces back into the test object and heads
towards the back wall again. This second reflection will produce
peaks similar to the first set of backwall peaks. Some of the
energy, however, has been lost, so the height of all the peaks
will be lower. These reflections, called multiples, will continue
until all the sound energy has been absorbed or lost through transmission
across the interfaces.