Basic of ultrasonic testing part 5 by rohit sahu -
- 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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