This Lecture is about Different type of probes

1. NON –DESTRUCTIVE TESTING(RME-080)
BASICS OF ULTRASONIC TESTING-5
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-5
A piezoelectric or piezoelectricity, when you supply an electrical signal, you get vibrations. On the other hand, if you
have mechanical strength that can also be converted to an electrical signal. So, let us say an electric field, F, is
supplied. So, with respect to this electric field, mechanical stresses will be generated and if the mechanical stress is
σ, then, it will be proportional to the electric field being applied
Fα σ
Similarly, if you have a mechanical vibration or a mechanical strain that will also generate an electrical signal. So, if, ε
be the mechanical strength, then it will generate this electrical signal F. So, these two again will be proportional,
ε α F
So, this is either way. Electrical signal can be converted into mechanical vibrations and vice versa. So, let us write
these equations once again, by introducing these proportionality constants, α and β , are the constants
F= β σ
ε = α F
So, this kind of transducer, wherein you convert analytical signal into mechanical vibrations, these are known as
electro-acoustic transducer and these two constants, that we just now introduced, that is α and β, these are the
acoustic properties of the element which will control this electro-acoustic transducer. So, for having a good
transmission, β should be high. Similarly, to have a good receiver, α should be high.
with in the elastic limit it is related by the hooks law
Y= (σ/ ε) = (1/ α β)
Y is the young’s modulus since young’s modulus is a constant, it is a material constant; you cannot optimize both α
and β simultaneously
So, if you increase α, for example, then, β has to be reduced because this has to be kept constant. So, that is why, it is
not possible to optimize both α and β together simultaneously in one transducer
*Source- NPTEL Lecture 19 Theory and Practice of Non Destructive Testing by Dr. Ranjit Bauri Dept. of Metallurgical & Materials Engineering IIT Madras, Ultrasonic Testing.

3. BASICS OF ULTRASONIC TESTING-5
The other property which is needed for an ultrasonic
transducer is, the damping capacity, which is the ability to
absorb vibration. Because, by the time the echo comes back
to the transducer, it should be ready to receive them, it should
not vibrate when the echo comes back to the transducer. So,
that means, it should also have good damping capacity, so
that these vibrations can be quickly absorbed and the probe
is ready to receive after the transmission is over. So, that is
why, this damping capacity is also important for a ultrasonic
transducer.
So, PZT is a very good damping material. So, apart from the
piezoelectric property, it also has good damping and in most
of the cases, to enhance this damping, a damping
background is provided in the housing, in which this element
is there.
*Source- NPTEL Lecture 19 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-5
Probes are comes in different shapes, but mainly they are either cylindrical or
some kind of rectangular shape and if you take a cross-section for one of these
cylindrical transducers, for example, the main component is this piezoelectric
element. this is the active element, to which you have two electrical leads
connected, like this, to supply that electrical signal.
The back ground should be made of some material which has high damping
capacity. So, a good damping background is provided in this housing, so that
vibrations can be quickly absorbed after the transmission, so that the transducer
is ready for receiving..
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 second image from web.

5. BASICS OF ULTRASONIC TESTING-5
This is a small cylindrical casing in which you have all the support and the electrical leads and gray area
indicate that it has sound absorbing, high damping background and it comes in different kind of shapes,
like, cylindrical shape or rectangular shape and so on.
So, the ultrasonic beam can be sent normally or perpendicular to the surface, to the sample or it can
also be sent at some angle because in many cases, an incidence at a particular angle is also needed
because defects and flaws can lie in different orientations. They have not to be exactly perpendicular or
they may not be parallel to the front face of the sample, they can lie at an angle also.
Welded joints, welded plates, wherein you have a weld to inspect., if you have a V joint, it is always at an
angle. So, in those cases again, you have to use an angle probe. So, in order to provide that angle, as
you could see in image -2, this kind of wedge is provided here. So, the piezoelectric element is now
sitting on this wedge and this wedge is cutout at a particular angle that is needed. So, this will be the
incidence angle for this particular probe.
Source- NPTEL Lecture 19 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-5
There are two elements and for these two elements, there are two separate connectors
and two separate leads. So, this is the duel element probe
In close surface analysis or close surface inspection, because in that case, since the
reflector is very close to the surface or very close to the probe, this dead zone effects
will be prominent. So, in order to avoid that, this kind of duel element probe can be used,
wherein you have two separate elements, one for transmitting and another for receiving.
So, this is what is being shown over here. So, that is how, the construction is, for these
ultrasonic probes.
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 second image from web.

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