Non destructive testing (basic of ultrasonic testing 1) by rohit sahu
- 1. NON –DESTRUCTIVE TESTING(RME-080) BASICS OF ULTRASONIC TESTING Prepared By : Rohit Sahu Assistant Professor (ME) 1 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. CONTENT Introduction Nature of Sound Generation of Ultrasound
- 3. INTRODUCTION . Ultrasonic techniques are very widely used for the detection of internal defects in materials, but they can also be used for the detection of small surface cracks. Ultrasonic are used for the quality control inspection of part processed material, such as roiled slabs, as well as for the inspection of finished components. The techniques are also in regular use for the in- service testing of parts and assemblies. Sound is propagated through solid media in several ways and the nature of sound will be considered first. *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
- 4. NATURE OF SOUND Sound waves are elastic waves which can be transmitted through both fluid and solid media. The audible range of frequency is from about 20 Hz to about 20 kHz but it is possible to produce elastic waves of the same nature as sound at frequencies up to 500 MHz. Elastic waves with frequencies higher than the audio range are described as ultrasonic. The waves used for the non-destructive inspection of materials are usually within the frequency range 0.5 MHz to 20 MHz. In fluids, sound waves are of the longitudinal compression type in which particle displacement is in the direction of wave propagation; but in solids, they are shear waves, with particle displacement normal to the direction of wave travel, and elastic surface waves can also occur. These latter are termed Rayleigh waves. Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD 4
- 5. WAVE VELOCITY Table 1 Velocities of longitudinal sound waves Medium Velocity Ve (m/s) Air* 330 Water 1430 Oil 1740 Aluminum 6190 Copper 4600 Magnesium 5770 Steel 5810 Perspex 2730 Polyethylene 2340 5 *At atmospheric pressure and 15°C. The velocity of shear waves, Vs , within a solid is roughly half the longitudinal wave velocity The velocity of Rayleigh waves, Vr, in a solid is about 90 per cent of that of shear waves *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD Velocity of Longitudinal Waves in different medium given in table 1
- 6. WAVE LENGTHS The wavelength, λ, is related to the frequency and wave velocity as; V = λf where f is the frequency. Table 2 gives the wavelengths of sound in various materials at several frequencies. . It will be seen from Table 2 that an ultrasonic beam with a frequency of 10 MHz will be capable of detecting defects in steel of sizes greater than 0.58 mm, but only defects larger than 4.65 mm would be observed if a frequency of 1.25 MHz were used. *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD 6
- 7. WAVE LENGTH OF SOUND 7 *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD Table 2 Wavelengths of sound (compression waves) in some materials Material ;>.. (mm) for frequency (MHz) of 1.25 2.5 5.0 10.0 Air 0.26 0.13 0.066 0.033 Water 1.14 0.57 0.286 0.143 Oil 1.39 0.70 0.35 0.175 Aluminium 4.95 2.48 1.24 0.62 Copper 3.68 1.84 0.92 0.46 Magnesium 4.62 2.31 1.16 0.58 Steel 4.65 2.32 1.16 0.58 P11rspex 2.18 1.09 0.55 Polyethylene 1.87 0.94 0.47
- 8. NATURE OF SOUND CONT ..… The frequency of sound chosen to inspect a component is very important. High-frequency sound, for example 5 MHz for contact testing applications, would provide good sensitivity, which is a term referring to an ability to detect small material imperfections, called discontinuities. This same sound frequency would also have good resolution, which is a term used to describe the ability of a testing system to display closely-spaced discontinuities separately on the instrument screen. *Source 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
- 9. NATURE OF SOUND CONT. The sound at 5 MHz cannot penetrate through the coarse grained material due to a scattering effect called attenuation, or loss of ultrasonic energy. To test this coarse-grained material, low-frequency sound such as 1 MHz would be selected. Sound at 1 MHz would have good penetration but poor sensitivity and resolution. As mentioned above, the objective of many NDT applications is to look for the presence of discontinuities. . *Source 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 9
- 10. NATURE OF SOUND CONT .... A discontinuity can only be called a defect it if exceeds specific acceptance standards. If we have two cracks of the same length but in different components, one may be acceptable but the other may not, therefore only one crack can be called a defect. We now must look in more depth at the nature of ultrasound, which is at the heart of the method *Source 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 10
- 11. GENERATION OF ULTRASOUND The transducer is capable of both transmitting and receiving sound energy. Ultrasound is generated with a transducer. A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa. Source: Web
- 12. GENERATION OF ULTRASOUND Sound generation – the piezoelectric effect The piezoelectric effect is utilized, which is a property of certain crystals to convert electrical energy into mechanical energy and vice versa – as you seen the fig. below. Fig. Probes are often called transducers because they convert energy from one form to another, ie electrical energy to sound energy and sound energy to electrical energy. Certain crystalline materials show the piezo-electric effect, namely, the crystal will dilate or strain if a voltage is applied across the crystal faces. Conversely, an electrical field will be created in such a crystal if it is subjected to a mechanical strain, and the voltage produced will be proportional to the amount of strain. *Source 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 *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD 12
- 13. GENERATION OF ULTRASOUND CONT. Piezo-electric materials form the basis of electro-mechanical transducers. These may be naturally occurring, artificially manufactured or grown in solution. Naturally occurring crystals, such as quartz, are rarely used these days because man-made ceramic materials tend to give much better properties in terms of sound generation, resolution, etc. A typical ceramic material is barium titanate, lead metaniobate and lead zirconate, are used widely. When quartz is used the disc is cut in a particular direction from a natural crystal but the transducer discs made from ceramic materials such as barium titanate are composed of many small crystals fused together, the crystals being permanently polarised to vibrate in one plane only. . 13 *Source 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. *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD.
- 14. GENERATION OF ULTRASOUND CONT. An ultrasonic wave incident on a crystal will cause it to vibrate, producing an alternating current across the crystal faces. In some ultrasonic testing techniques two transducers are used - one to transmit the beam and the other acting as the receiver - but in very many cases only one transducer is necessary. This acts as both transmitter and receiver. Ultrasound is transmitted as a series of pulses of extremely short duration and during the time interval between transmissions the crystal can detect reflected signals. *Source 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. *Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD. 14
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