This lecture is about the selection of frequency, detectability, Calibration and Different type of Display.

1. NON –DESTRUCTIVE TESTING(RME-080)
BASICS OF ULTRASONIC TESTING-7
Prepared By : Rohit Sahu
Assistant Professor (ME)
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-7
Low frequency, the penetration depth is high, but the resolution is low.
Resolution is, nothing but, ability of the probe to detect two flaws, which are close to each other. So, if
you have two defects in very close proximity to each other, so then, you should have two different, two
separated defect signals and this would appear as two distinct signal on the display and that would
happen only when the resolution is good. If the resolution is not good, then, these two signals coming
out from two defects lying very close to each other may just merge into one and will come as a broad
peak instead of two distinct separate peaks.
So, this is low resolution, when you have low frequency but penetration is high. So, when penetration is
high that means the attenuation will also be lower and as a result, you will get high intensity peaks, but
you will compromise with the resolution.
*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

3. BASICS OF ULTRASONIC TESTING-7
High frequency, the penetration depth is Low but the resolution is High.
In case of high frequency, the penetration is low, so, the back wall signals which are coming
out, they will be lower in intensity compared to a low frequency wave, because in this case,
due to lower penetration, the attenuation is also high, but the resolution in this case is much
better. So, like in the previous case, if you have the same scenario, two or three defects lying
very close to each other, then, in this case, you will get two distinct or three distinct echoes
corresponding to two or three different defects, which lie close to each other.
So, when you want to do a close surface analysis by ultrasonic testing, then, you should go
for high frequency, because in this case the penetration will be lower.
So, waves will be very close to the surface and on the other hand, if you want to go much
below the surface, if you want to go to higher depth kind of inspection, then, you should
select a lower frequency because in that case the penetration is higher. So, it would be able
to go to the depth that you are looking for. So, that is how, depending on what exactly you
want to do and what kind of defects you are expecting, if you have some idea before hand,
then, based upon this penetration and the resolution aspects you can select either a high
frequency or a low frequency transducer.
*Source- NPTEL Lecture 21 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-7
when we talk about the defect signal or the detectability of a defect, then, one parameter comes into
picture, which is known as signal-to-noise ratio. This is written as s/N and the idea is, to minimize the
noise, so that the signal intensity from a defect is increased and often a signal to-noise ratio of 3:1 is
needed as a minimum requirement.
Particular parameter, it depends on several factors related to probe and the material.
W is the lateral beam width, WX and WY at the flaw depth, Δt is the pulse duration, is the density of the
sample or the medium and V is the velocity of sound wave in the material that the sample is made of.
these are the parameters which control the signal-to-noise ratio or the detectability of a defect.
if decrease the lateral beam width? So, that means, if have a focused beam, then, the detectability of
the defects or the signal-to-noise ratio will be more
if decrease Δt, that means, if the pulse duration is less, which means, if it is a short length or shorter
pulse, then again, the detectability would be better. So, if have a shorter pulse, the detectability is better
and it is the signal-to-noise ratio or the detectability is directly proportional to the size of the flaw, which
is obvious, larger the flaw better would be its detectability.
if the density of the material is higher or the velocity of sound wave in the material is higher, then, the
detectability goes down.
*Source- NPTEL Lecture 21 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-7
A very important aspect of ultrasonic testing, which is about calibration.
Ultrasonic testing is concerned, two types of calibration one is for the distance and other is for the area.
Why area, because the intensity of the echo depends on the scattering area or the area of the interface
or the area of the defect, which is scattering it. So, therefore, the area also has to be calibrated, so that
you would be able to get some idea as to how big is the defect, whether it is a big defect or it is a small
defect.
Distance calibration and another is area, so, these are known as distance amplitude calibration or
distance amplitude correction and area amplitude calibrations. So, these are the two primary types of
calibrations that you do for normal probes.
In case of distance calibration, the metal distance was varied and in case of area calibration, the area of
the hole was varied. So, that get different distances in one case, when vary the metal distance to
calibrate the distance and in the case of area calibration, you vary the size or the area of the hole and
get different areas to calibrate in terms of area. So, that was about calibrating normal probes. and
In case of angle probes, use a block, which is known as IIW block. and in this block have several types
of features, which can be used to calibrate the angle for a particular angular transducer and on the
block, we also had this angle scale graduated and with the help of that, we could calibrate the angle and
apart from this, IIW block is quite versatile, in the sense that apart from calibrating the instrument or the
probe for the angle ..
it can also be used for getting this important parameters, like, the resolution, dead zone and in fact,
able to get some idea about the sensitivity. So, these are the different things that can be done with the
help of the IIW block.
*Source- NPTEL Lecture 22 23 & 24 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

6. BASICS OF ULTRASONIC TESTING-7
The most commonly used display is in terms of the amplitude and the time that have been
seeing so far and that particular display or that particular scan is known as A-scan.
A-scan is a display (as shown in both image) amplitude plotted against time or depth and the
signal in terms of a deflection of the time base or a peak .
A-scan is one dimensional view of the defects.
.
*Source- NPTEL Lecture 25 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials
Engineering IIT Madras, Ultrasonic Testing and image 2 Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT
Common Methods, Professor Robert A Smith published by the British Institute of Non-Destructive Testing

7. BASICS OF ULTRASONIC TESTING-7
B-scan and in this case, A 2D profile of the defects and display is basically an imaginary section through
the sample, can see both the back and front surface and will see at what depth the defects are lying,
when the defect echo appears on the display. (as shown in both image)
How, it generates that 2D profile of the defect.
if ,a sample as shown in image 1 and have two defects at two different depths and moving the probe on
the surface, so, the moment it goes over these defects, then, will see them coming in this display at that
particular depth because as this is an imaginary section through the sample, where we can see the
depth and both the front face & the back face So, this will come as fluorescent points on a phosphor
screen. So, instead of the oscilloscope screen that in case of A-scan, in this case, a phosphor screen,
where these defects signals will come and florescent at the particular depth, where the defect is lying.
So, this is how the displays in the case of B-scan.
*Source- NPTEL Lecture 25 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials
Engineering IIT Madras, Ultrasonic Testing and image 2 Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT
Common Methods, Professor Robert A Smith published by the British Institute of Non-Destructive Testing

8. BASICS OF ULTRASONIC TESTING-7
C-scan and in this case, have plan type, 2D view of the defects and in order to generate that, this has to be
plotted either on an XY plotter. So, the data is recorded by an X-Y plotter or it can be also shown on a computer
screen, which is superimposed on the plan view of the sample. So, you record the data and then, either you can
plot it on X-Y plotter or you can show it on a screen, which will be superimposed on the plan view and that is
how, this plan type view will be generated.
So, let us say, there is a defect on the sample. the side view and the top view of the sample as shown in image
1. So, in order to generate that plan type view of the defects, need to collect the data as a function of the
position of each of the reflecting interfaces and in order to do that, the scan following a particular pattern, as
shown in image 1 in a zigzag manner and so on.
So, collected the echoes as a function of each of these reflecting points, will see a 2D type of view in the
display, when it is finally displayed on the system. So, this is the C-scan view. how it will look like. So, it will show
that top view of the plan view and a profile of this defect at that particular location.
So, in this case the echoes are recorded as a function of position of each reflecting interface and with that will
get a scan and a view . So, these are the different types of scans or views, displays in case of ultrasonic testing
*Source- NPTEL Lecture 25 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering IIT Madras, Ultrasonic Testing and image 2
Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods, Professor Robert A Smith published by the British Institute of Non-Destructive Testing

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