This lecture is about Test Equipment, Couplant and EMAT

1. NON –DESTRUCTIVE TESTING(RME-080)
BASICS OF ULTRASONIC TESTING-6
Prepared By : Rohit Sahu
Assistant Professor (ME)
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DEPARTMENT OF MECHANICAL ENGINEERING
Disclaimer: The materials provided in this presentation and any comments or information
provided by the presenter are for educational purposes only. Nothing conveyed or provided
should be considered legal, accounting or tax advice.

2. BASICS OF ULTRASONIC TESTING-6
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 The test equipment's, apart from the probe, have the display in which signal, the defect signal and
have a power source to power the probe as soon in image below. All those things are build into a
compact small equipment. this has to be connected to that small box, which will contain all other
elements including electronic circuit and the power source and it will also have amplifiers and to
amplify the signal.
 The probe will be connected to a pulse generator and this has to be connected to the system, to first
amplify the signal, so, it will have an amplifier and then, have the display, which can show the
display in terms of amplitude and the time base and An initial pulse and the back wall and then
have sample over here. And if there is anything in between, defect echo coming in between the
initial pulse and the back wall signal.
*Source- NPTEL Lecture 19 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials
Engineering IIT Madras, Ultrasonic Testing and image is taken from web source.

3. BASICS OF ULTRASONIC TESTING-6
In terms of the incident angle, there are two scenarios, when the incident angle is
zero, then, this goes vertically down, The ultrasonic waves goes into the sample.
in some cases, you need to send the waves at a particular angle, Because there
could be cases, where the defects and flaws might lie at an angle or the part itself
is such that you need to send the waves at some angle.
When send the waves at a small incident angle, so, across an interface, there is a
possibility that, if you are using longitudinal waves that at the interface there will be
some kind of mode conversion and a part of the longitudinal waves will be
converted into a transverse wave, which are also known as shear waves, when the
incident angle is more than zero.
Have an incident angle, encounters an interface between two medium a small
incident angle at this interface, then, there is a possibility that a part of this
longitudinal wave will be converted into shear wave and across this interface, since
now it is entering another medium, there will be a change in the path of this wave,
due to refraction effects,.
Source- NPTEL Lecture 20 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering IIT Madras, Ultrasonic
Testing

4. BASICS OF ULTRASONIC TESTING-6
 A small part of this can also be converted into a shear wave or transverse
wave. Using an angle for doing the test, that means because of this mode
conversion two kinds of waves will enter in to the sample, longitudinal wave
and shear wave.
 That means the reflections from the same interface will arrive at different
time and on the screen on the display as the same signal appearing at
different times, because the velocity is different.
 To exclude one of these waves and to exclude the longitudinal wave and the
reason for that we will see in a moment.
 In case, a particular angle which will ensure that only shear waves enter the
sample and longitudinal waves will be reflected back, they will not be
transmitted to the sample. That will depend on the angle of incidence.
Source- NPTEL Lecture 20 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials
Engineering IIT Madras, Ultrasonic Testing

5. BASICS OF ULTRASONIC TESTING-6
Air gap exists, then there will be decrease in the intensity of the waves or there will
be attenuation, into the air.
The entire energy of the sound waves going into the sample and to prevent
attenuation or decrease in the intensity, then to exclude this air gap before you
could send the sound waves into the sample.
In order to exclude this air gap, need to apply something on this interface, which will
exclude the air. if oil or grease or some gel in the form of a thin layer, that will
ensure that the air gap is excluded and now attenuation or the decrease in the
intensity can be avoided.
The particular thing, which is applied to avoid the air gap is known technically, as
couplant.
Source- NPTEL Lecture 20 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering
IIT Madras, Ultrasonic Testing

6. BASICS OF ULTRASONIC TESTING-6
The couplant should satisfy certain requirements, used effectively and to exclude the air gap.
It should be as thin as possible to avoid any alteration of direction of the beam
If it is a thick layer, then, that itself would act as a different medium, due to the presence of
a different medium, sound waves will change their direction. So, in order to avoid that, to
ensure that is applied as a very thin layer, so that the direction of the beam does not change,
when it is passing through the couplant layer.
It should wet both the probe surface as well as the sample surface. So, it should have good
wetting or good spreading property.
It should also be able to fill the small irregularities that you might have on the surface of the
sample, so that it can provide a smooth surface for the probe to move on.
It should not restrict the movement of the probe. So, it should allow free movement for the
probe
It must be easy to apply and easy to remove.
It must be harmless to the surface on which it is applied.
These are the property requirement from a couplant for it to be used effectively.
*Source- NPTEL Lecture 20 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering IIT Madras, Ultrasonic Testing

7. BASICS OF ULTRASONIC TESTING-6
Use a transducer which operates differently and which does not have to be in contact with the sample,
like how the normal large ultrasonic transducer are.
The ultrasonic waves are generated through electromagnetic induction and that is why, these kinds of
probes are known as electromagnetic acoustic transducers.
In the electromagnetic induction, a conductor carrying an alternating current, it have an alternating
magnetic field or a changing magnetic field around it and now if you bring another conductor close to it,
due to that changing magnetic field, the current will be induced in the second conductor.
The phenomena of electromagnetic induction or simply induction. what is being shown over here, in the
diagram below on the right hand side. So, these are the conductors, which carry an alternating current.
conductors carrying the current close to this sample surface, and if it is an electrically conductive
material, then eddy currents will be induced on the surface.
Now, if another permanent magnet, if another magnetic field around this, due to that magnetic field and induced
current, a mechanical force will be generated which is known as the Lorentz force.
*Source- NPTEL Lecture 21 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering IIT Madras, Ultrasonic Testing and image are taken from the web
sources

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