Fundamentals of Ultrasonic waves and applications

ultrasonic waves, generation, properties, types, transducers, propagation in liquids, ultrasonic velocity, us absorption, relaxation in binary mixtures, normal and associated liquids. Measurement techniques: interfero method, optical method, sing around method , pulse echo technique, pulse echo overlap method. Application of us waves: cavitation effect, SONAR, us welding, us cleaning.

1. Fundamentals of ultrasonic Dr. Priyanka Tabhane Department of Physics, RTM Nagpur University

2. Ultrasonic waves Vibrational waves of frequency above the hearing range of the normal ear are referred to as ultrasonic.  Frequency is more than about 20000 cycles per second. In minimum frequency range, length of ultrasonic waves in solid is about 8 inch,  in liquid 2.4 inch, in air 0.63 inch . Frequencies of 10kHz to 1 MHz are used for industrial applications, sound ranging, submarine signalling and communication.  Frequencies 10 kilo hertz to 20000 kilo hertz are used in testing materials for flaws, chemical treatment, medical therapies etc.  All frequencies are suitable for investigation of physical properties of matter. Presence of medium is essential for transmission of Ultrasonic waves.  Any material that has elasticity can propagate Ultrasonic waves. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

3. Types of waves -  1. Longitudinal waves or Compressional or Pressure Waves .  2.  Transverse or shear waves 3.  Surface or Rayleigh waves 4.  Lamb or Flexural or Plate Waves Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

4. Longitudinal waves When the motion of a particle in a medium is parallel to the direction of wave propagation longitudinal wave exists they are often referred as L waves they can travel in liquid, solid & gases and are easily turn treated and detected waves have high velocity of travel in most media wavelength in common materials are usually very short in comparison with the cross section area of transducer that is the material that produces the wave longitudinal wave maybe generated within a medium by  vibration of anyone of its surface in a normal direction at an ultrasonic frequency Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

5. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University figure shows  longitudinal wave travelling through medium

6. Shear or transverse waves -   When shear waves are used, the movement of particles in a medium is at right angles to the direction of wave propagation.  They are referred as S waves The wave movement is in the X direction, the particle displacement is in Y direction.   These waves  can also exist either in Limited area or entirely throughout a body.  Beam does not extend to a surface parallel to the direction of travel Have velocity one half of that of  L-waves. Because of lower velocity wavelength of S waves is much shorter than L waves Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

7.  shorter wavelength make them more sensitive to small inclusions Therefore they more easily scattered within the material  S waves do not travel in  liquid or gases since there is little or no elasticity to shear in such material  Shear waves are generated by applying a sharing force to the face of material that is rocking it back and fourth in the direction parallel to the surface  figure shows particle motion and wave direction of S waves Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

8. Surface or Rayleigh Waves Waves can be propagated over the surface of a path without penetrating below that surface to any extent.  These waves are reflected to water waves which travel over body of water  Their velocity depends upon the material itself  Velocity is about nine-tenth of s wave velocity  They are generated by shaking an area of a surface back and forth in manner similar that by which S waves are generated Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

9. Wavelength is extremely short And the plate on which it travels is at least several wavelengths thick.   Surface wave consist of both l and S types of particle motion These waves are used to detect cracks or flaws on or near the surface of test objects  Figure 3 surface waves travelling over a plate  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

10. Lamb or Flexural or Plate Waves Produced in thin metal, whose thickness is comparable to the wavelength of ultrasonic waves. Particle motion in them is similar to shear and surface wave but extends throughout the medium Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

11. Q.2 Discuss properties of ultrasonic waves. Frequency υ Velocity v Wave  length λ Energy E Mode of propagation Transmission of sound waves Reflection at boundary Angle of reflection and refraction Diffraction Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

12. Frequency υ – All the sound waves oscillate at a specific frequency , or number of vibrations or cycles per second. Human hearing extends to a maximum frequency of 20 kilohertz while the majority of ultrasonic applications utilised frequency between 20 kHz to 500 MHz. At frequencies in the megahertz range sound energy does not travel efficiently through air or other gases but it travels  freely through most liquids and common Engineering Materials. And hence ultrasonic waves are high frequency waves.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

13. Velocity v -  The speed of sound waves , varies depending on the medium through which it is travelling ,  affected by medium density ρ and Elastic properties (Elastic constant E ). Different types of sound waves will travel at the different velocities   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

14. Wave  length λ -  any type of wave will have an associated wavelength,  which is the distance between any two corresponding points in the wave cycle as it travels through a medium. Wavelength is related to frequency and velocity by simple equation  Wavelength is a limiting factor that control the amount of information that can be derived from behaviour of wave .   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

15. Energy E -  E = h υ As frequency of ultrasonic waves  is high. So , ultrasonic waves are high energetic waves .   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

16. Mode of propagation -  sound waves in solids can exist in various modes of propagation that are defined by type of motion  involved. Longitudinal and shear waves are the most common mode employed in ultrasonic testing. Surface and plate waves are also used on occasion.  Sound waves may be converted from one form to another. Most commonly shear waves are generated in a test material by introducing longitudinal waves at a selecting angle. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

17. Transmission of sound waves -  The distance that a wave of a given frequency and energy will travel depends on the material through which it is travelling.  In general, material that are hard and homogeneous will transmit sound waves more efficiently than that are soft and heterogeneous or granular.  Beam spreading, attenuation and scattering will decide the distance of sound wave in a given medium. As a beam Travels the leading edge becomes wider, the energy associated with the wave is spread over a larger area,  and eventually the energy dissipitates . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

18. Attenuation is the energy loss associated with the sound transmission through a medium, essentially the degree to which energy is absorbed as the wave front moves forward. Scattering is a random reflection of sound energy from grain boundaries and similar microstructure.  As frequency goes up, beam spreading increases but the effect of attenuation and scattering are reduced. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

19. Reflection at boundary -  When sound energy travelling through a material, strikes a boundary with another material,  portion of energy will be reflected back and the portion will be transmitted through. The amount of energy reflected  or reflection coefficient, is related to relative acoustic impedance of the two materials. Acoustic impedance is material property defined as density  multiplied by speed of sound in a given material. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

20. For any two materials, the reflection coefficient as percentage of incident energy pressure may be calculated formula where R- reflection coefficient Z 1 -  acoustic impedance of first material Z 2 -   acoustic impedance of second material For metal air boundaries, in ultrasonic wave application like flaw detection,  the reflection Coefficient approaches 100 %. Virtually all the sound energy is reflected from crack or other discontinuity in the path of the wave. This is fundamental principle that makes ultrasonic flaw detection,  detection of any object in sea etc.   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

21. Angle of reflection and refraction -  Sound energy at ultrasonic frequency is highly directional and sound waves used for flaw detection in any metal, object detection in sea are well defined. In situation where sound reflects off the boundary, the angle of reflection equal to angle of incidence.  Sound beam that hits surface at perpendicular incidence angle will reflect straight back. Sound beam that hit a straight surface at angle will reflect forward at the same angle . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

22. = Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

23. Sound energy that is transmitted from one material to another bends in accordance with Snell's law of refraction.  Again a beam that is travelling straight will continue in a straight direction, but a beam that strikes a boundary at angle will be bent according to formula θ 1 = incident angle in first material θ 2 = refracted angle in second material v 1 = sound velocity in first material v 2 = sound velocity in second material    Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

24. Diffraction – Ultrasonic waves do not always propagates in rectilinear manner. Eg . Wave passing near the edge of an object has a tendency to become bent towards or around it. This bending of wave is called diffraction. Ultrasonic signals that would normally be received at certain point may be diverted by diffraction & received at some other point. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

25. Velocity in fluids There are number of different types of velocity that can be discussed. Most significant are referred to as  phase velocity, group ( bulk) velocity and signal velocity. Phase velocity may be defined as speed with which a phase is propagated along a wave.  It refers to a condition existing along a line of propagation that seems to show a change in phase  travelling along with and superimposed on the wave itself.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

26. Group velocity is used to indicate the velocity with which the envelope of a wave is propagated when the wave is amplitude modulated. Group velocity is most often considered in ultrasonic work. Signal velocity is very complex condition existing only when a medium is dispersive. In such cases different signals seems to travel with different velocity and actual speed of travel of a particular signal is its signal velocity. Velocity of the wave and that of individual particle of material are not same. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

27. Velocity of sound in liquid is 5 to 10 times the velocity in its vapour at same temperature, due to close packing of the molecules in liquid. The ultrasonic velocity provide an excellent means of studying various types of change of phase in vicinity of melting point of super cooled liquids and round the critical temperature of individual liquids and mixtures, transition in liquid crystals from their anisotropic region to isotropic region. The formula by which ultrasonic velocity were calculated is where , d = Separation between transducer & reflector t = Traveling time period of ultrasonic wave.   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

28. Velocity measurements in fluids shows, associative or dissociative nature of the molecules. If velocity goes on increasing with concentration or temperature means the chemical bond in the solution is strong and it thus shows associative nature of the solution. If the velocity goes on decreasing with increase in concentration or temperature, shows the dissociative nature or breaking up of molecules in the solution. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

29. Only L waves can transmit in liquid and gases. In such cases it can be assumed that the vibration takes place too rapidly for heat to exchange. The velocity in either a liquid or gases is then   Where K- ratio of specific heats B is - compressibility at a constant temperature B ad - adiabatic compressibility.   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

30. Ultrasonic absorption The study of ultrasonic absorption is to understand how the medium responds to the perturbation and relaxes when the perturbation passes on. Absorption losses are characteristic of the medium through which the waves travel and the evaluation can yield the information about the physical properties of the medium. When a plane progressive wave passes through a system, the amplitude or intensity of a sound wave in a system decreases with the distance travelled by the wave, which is called attenuation . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

31. This attenuation arises from deviation of energy from the plane wave by regular process like reflection, refraction, diffraction, scattering and absorption of energy by the medium, when the mechanical energy is converted into heat by the internal friction. The ultrasonic absorption (α/f 2 , where f = ultrasonic frequency) in the fluid media are attributed mainly due to the processes viz.; absorption due to viscosity, absorption due to thermal (heat) conduction. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

32. Absorption due to Viscosity: The viscosity (fluids resistance to flow) of a fluid corresponds to the shear elasticity in a solid . If the medium has a finite viscosity, the frictionless losses of the energy occur. This gives to the ultrasonic absorption due to shear or classical viscosity and is given by , Where, α = absorption coefficient = ultrasonic frequency η s = coefficient of shear viscosity ρ = density of the medium u = ultrasonic velocity   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

33. Ultrasonic absorption due thermal or heat conduction: Heat conduction  (or  thermal conduction ) is the movement of heat from one object to another one that has different  temperature  when they are touching each other. For example, we can warm our hands by touching hot-water bottles. When the cold hands touch the hot-water bottle, heat flows from the hotter object (hot-water bottle) to the colder one (hand). People make things with different thermal conductivity, for example cookware to heat things or insulated containers to keep hot things hot or cold things cold. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

34. The absorption coefficient of material  can be determined by measuring the heating which occurs as a result of ultrasonic irradiation.  When narrow focused beams, heat a sample over the available volume, material restricted to small dimensions,  then the effect of heat conduction to surrounding unheated regions become significant, complicating the relation between major temperature and acoustic parameters.  The sound energy is also absorbed due to the ability of thermal conduction of the medium. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

35. The heat energy is conducted from higher temperature regions to the lower one and so the compressed region will return lesser work on the expansion than the work required compressing it. This gives rise to the absorption of sound energy ( α / f 2 ) thermal as: K = coefficient of thermal conductivity C T = isothermal heat capacity γ =ratio of isobaric and isochoric heat capacities   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

36. Absorption testing -  Ultrasonic testing has been successfully used in testing of materials by noting the absorption pattern.  Typical pattern is shown in figure. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

37. These results can be correlated to differences in grain structures or the internal condition of material.  One successful application has been the testing of material in plates where the thickness of the plate was too small to permit ordinary reflective thickness measurements.    On a plate that is sound , a series of reflections of considerable Height and spaced at the proper distance will be clearly noted. when the plate is laminated, reflections entirely disappear,  thus giving a very strong and sharp indication of defects. the laminated parts can be located exactly because of directional characteristics of ultrasonics .  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

38. Ultrasonic generators S ound waves are generated or received by Transducer,  that is any device which converts energy of one form to that of another.  In this case transformation of ultrasonic energy to or from electrical, mechanical  or other forms of energy. A reversible transducer is one which will make the conversion in both direction with equal  efficiency.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

39. Different types of transducers are classified as Piezoelectric oscillator-  uses piezoelectric effect -which is reversible- frequency range extends from  20 kilo hertz to 10 gigahertz Magnetostrictive oscillators-  magnetostriction effect- reversible-  not used at frequencies higher than 40 kilo Hertz,  but range can be extended without difficulty to over hundred kilo Hertz Mechanical transducer-  purely mechanical oscillators-  whistle siren- Irreversible- used in high power applications-  frequency beyond 50 kilo Hertz Electromagnetic transducers-  intensity application at low frequency  given in audible range   Electrostatic transducers-  low intensity with a upper frequency of 200 kilo Hertz  Miscellaneous transducers-  includes thermal chemical and optical transducer   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

40. Piezoelectric effect In 1880,  Curie brothers, J Curie and P Curie, discovered that when a crystal having 1 or more polar axis or lacking Axis symmetry is subjected to mechanical stress and potential difference occurs.  Inverse Piezo electric effect- The opposite effect predicted by Lippmann in 1881 and verified experimentally by Curie Brothers in the same year :  when an electric field is applied in the direction of Polar axis causes a mechanical stream in crystal segment. The amount of strain is is directly proportional To  the intensity of applied electric field   PEE occurs in several natural and artificial crystals and defined as a change in the dimensions when an electric charge is supplied to a crystal faces.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

41. Piezoelectric effect - when an AC voltage is applied across Piezo Frequency of Crystal, Which is determined by the physical dimensions and by the way the Crystal is cut . the electric crystal, such as Quartz crystal , vibrates at the frequency of applied voltage. Vibrations of maximum amplitude occur at the natural resonant frequency of the vibration Natural resonant frequency of the vibration is given by Where L is the length of thickness of the Crystal plate, Y is Young's modulus along the appropriate direction And is the density of the Crystal .     Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

42. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Quartz is very commonly applied for ultrasonic generation. a quartz crystal is shown in figure with the hexagonal cross sectional normal to the non polar optical axis denoted by Z axis. the axes joining opposite edges are designated as x axis and associated axis which are perpendicular to these and joining opposite faces are termed as y axis. X and y axis are polar axis and slab cut with their faces perpendicular to them manifest the piezoelectric effect crystal that cut with their faces perpendicular to an x axis and y axis are termed X cut and Y cut crystal respectively. the X cut crystals are utilised to propagate compression waves and Y cut crystal are applied to generate shear waves.

43. Piezoelectric crystal can oscillate in either of two mode - fundamental and overtone. Fundamental frequency of a crystal is the lowest frequency at which it is naturally resonant. Because a slab of Crystal cannot be cut too thin without fracturing, there is an upper limit on the fundamental frequency. For most crystals the upper limit is less than 20 megahertz. For higher frequencies crystal must be operated in the overtone mode.    Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

44. Piezoelectric oscillator - Ultrasonic wave generator was first designed in 1917 by Langevin . It is a triode valve oscillator.  The plate coil L 2 is inductively coupled to grid coil L 1 . When the circuit is switched on the valve starts functioning as oscillator producing oscillations at frequency given by    Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

45. The frequency of oscillations can be controlled by varying the capacitor C. The capacitor is varied till the frequency of oscillations of oscillator matches with natural frequency of the piezo electric crystal . The Crystal subjected to AC voltage produces Ultrasonic waves in the surrounding air.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

46. Measurement techniques The methods of ultrasonic velocity and absorption may be categorized as below: i)  Progressive Continuous Wave method ii) Interferometer method or Standing wave or Resonance method iii)   Mode conversion or Total Reflection method iv)  Optical method v)  Reverberation technique vi )  Impedance method vii)  Pulse technique Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

47. Optical diffraction method When ultrasonic waves are generated in a liquid in a rectangular vessel, the wave can be reflected from the walls of the vessel. These reflected waves are called echoes. The direct and reflected waves are superimposed, forming a standing wave . The  density of the liquid at a node is more than the density at an antinode. Hence , the liquid acts as a diffraction grating to a parallel beam of light passed through the liquid at right angles to the wave. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

48. The diffraction grating formed in this way is analogous to a conventional diffraction grating with lines ruled on a glass plate. The less dense antinodes refract light less and are similar to the transmitting slits of a conventional grating. The denser nodes refract light more and are similar to the opaque part of a conventional grating . optical diffraction method may be used for measurement in transparent liquids and solids.  when sound waves pass through a transparent material, periodic variations takes place in the refractive index,  mostly in regions of compression and minimum in regions of expansion. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

49. these variations produce optical diffraction grating  having a spacing equal to ultrasonic wavelength. When a beam of light,  originating from a monochromatic source ( Mercury discharge tube placed behind the suitable filter or sodium lamp), passing through a narrow slit, is rendered parallel by placing slit at focal point of converging optical lens.  A parallel monochromatic beam passes through the medium And is brought to a focus in the focal plane of microscope .  In absence of ultrasound only a single image of slit is observed, but when propagation takes place, several equally spaced parallel images of the Slit are observed as a result of diffraction.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

50. The distance of separation of neighbouring images can be measured by means of travelling microscope varies in inverse proportion to spacing of grating. i.e. wavelength. The graticule is calibrated either by means of preliminary measurement with liquid in which the velocity of sound is known or by employing optical grating of  known spacing. ( eg 40 lines per millimetre) Velocity of sound is determined by multiplying the measured value of wavelength by the frequency of source.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

51. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Arrangement for optical diffraction method

52. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Stationary wave pattern Image of sodium slit obtained with ODM  

53. The grating element is equal to the  wavelength of the ultrasonic waves, denoted by  d. If λ is the wavelength of the light passed through the grating that is diffracted by an angle  θ, then the n th order of the maximum is given by: d sin θ = n λ or d= n λ/ sin θ now, velocity of ultrasonic wave, v = fd or d=v/f, v/f= n λ/ sin θ or v= n f λ/ sin θ   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

54. Advantage of this method is that velocities can be determined in comparatively small samples of material.  Once the system is setup, measurements can be made on different samples in fairly Rapid succession.  Sensitivity of method depends on fineness of ultrasonic grating.  If the grating is too coarse, the image formed by diffracted light beams  are too close together for proper resolution . For liquids the lowest practical frequency is about 10 megahertz for which Quartz crystal source is necessary . The method  is unsuitable for gases because of the low degree of contrast in refractive index between compressed and rarefied regions of wave. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

55. Interferometer method or Standing wave or Resonance method An acoustic interferometer is an instrument for measuring the physical characteristics of sound waves in a gas or liquid. It may be used to measure velocity, wavelength, absorption, or impedance. In this technique, there is a X-cut quartz crystal in the bottom of the cell which when excited by external r.f . source having same frequency as the fundamental frequency of the X-cut quartz crystal, ultrasonic waves are produced and propagate towards metallic reflector, which is fitted with a micrometer (with least count 1 µm) which can be moved up and down in the sample kept in cell . Ultrasonic waves reflected from metallic reflector superpose with propagating ultrasonic waves and standing wave pattern is formed. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

56. When distance between transmitting crystal at the bottom of the cell and metallic reflector equals integral multiple of λ/ 2 (λ is the wavelength of the ultrasonic waves), maxima in the micrometer fitted with interferometer is observed. By monitoring known number of maxima in the cell and noting down corresponding distance reflector is moved, the wavelength λ of the ultrasonic waves is determined which is used for the determination of the ultrasound velocity, u (u = n λ) at a given frequency. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

57. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

58. Pulse Technique Technique which utilized pulse ultrasound are the most widely used for testing materials because the experimental configuration is simple to design and operate, measurements are rapid, non-invasive and non-intrusive, there are no moving parts and the technique can easily be automated. This technique was introduced by Pellam and Galt and Pinkertan in 1946, from the principle of radar. This is the most popularly used technique to determine the velocity and absorption in liquids and solids in the frequency range of a few MHz to tens of GHz and in gases; the frequency range of tens of KHz to a few hundreds of MHz Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

59. Pulse method consists of generation of short regular pulses of ultrasonic waves in the test sample, the time taken for them to pass through a measured distance be measured. It is mainly based on the principle that the time required for the passage of short duration pulses through a sample of known thickness is measured accurately to determine the ultrasonic velocity. The absorption coefficient (  ) is measured from the height of different echoes formed due to passage of a short duration pulse through the sample of known thickness. The equation used to determine (  ) is given by: Where A & A are the amplitudes or heights of first & second echo respectively &‘t’ is the time.   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

60. More sophisticated pulsed methods have been developed to improve the accuracy of measurements (e.g., Pulse echo overlap, sing around, pulse interferometer) . The simplest and most widely used technique for making ultrasound technique for making ultrasound measurements is called pulse-echo technique . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

61. Sing-Around Technique : This method is an automated method for measuring ultrasonic velocity with high accuracy. It is very easy to handle, convenient and versatile. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Block diagram of Sing-Around Technique

62. In this method a trigger transmitter sends an electrical pulse to the transmitter transducer which generates mechanical waves in the specimen. The pulse waves received by the receiving transducer are amplified and then used to trigger the transmitter. This is pulse transmission device for which short pulses from transmitter T  pass through the material and are picked up by the receiver R. On receipt of each pulse by R activates the pulse generator to cause another pulse to be transmitted by T. The pulse repetition frequency is determined by the time taken for the pulses to pass through the material by the frequency counter.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

63. the velocity is determined if the thickness of the sample is known. This loop runs continuously, the frequency of occurrence is measured. The principle disadvantage of this system is that any change in amplitude of the selected cycle, results in significant change in repetition rate.   Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

64. Pulse Echo Overlap (PEO) Technique: The PEO method utilizes either rf bursts (to measure phase velocity) or broad band pulses (for group velocity). the PEO method is capable of measuring accurately from any cycle of one echo to the corresponding cycle of next echo. In PEO method overlap of echo is accomplished by visual observation by the technician performing the measurements. The main drawback of this method is that it cannot be automated as its echoes are overlapped by the observer in scope time and not in real time. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

65. This technique has the following advantages. Using this method, ultrasonic velocity and attenuation measurement can be made simultaneously. It can be operate either with the transducer bonded directly to sample or with buffer rod interposed between them. It can be operated by broad band pulses or r.f . burst pulses. However the broad band pulses are more adequate for proper overlap and in a non dispersive media . It allows the transmitter to operate at a much lower frequency. It can be used to measure the group velocity and phase velocity. It requires minimum two echoes for overlap and so it can be used for highly absorbing specimen. It does not depend upon the shape of the echoes. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

66. The block diagram of PEO technique is shown in Figure. At regular intervals, the trigger pulses from a trigger generator are used to excite the time base of CRO and the pulse generator simultaneously. The time base is connected to the x-plates of a CRO. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

67. The pulse generator generates the short duration modulated pulses enclosing r.f . signal at desired frequency, and they are used to excite the transducer at its natural or fundamental frequency. At the same time, the signals (pulses) are also applied to the y-plates of the CRO through a narrow band amplifier tuned at the desired frequency. The ultrasonic waves generated by the vibration of transducer are allowed to propagate through the sample under investigation. The waves after traveling through the sample are reflected by reflector and are received back by the transducer which acts as a receiver also. The received ultrasonic pulses are converted into an electrical signal, which appears at the y-plates of CRO after amplification through a tuned amplifier. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

68. The time duration, in which the pulse passes through the sample and received by the transducer after reflection from the reflector, the electron beam in CRO will have moved a given distance towards right and the signal corresponding to received echoes will be at a lower height than the transmitted one. By synchronizing the time base of CRO with the pulse repetition rate, stationary pulses (echoes) on the screen of CRO can be obtained. The traveling time (t) of the waves through the sample can be determined from the time base of CRO or a frequency counter connected with the circuit and by measuring the distance between transducer and reflector, the ultrasonic velocity can be calculated by using formula v= fd = d/t The attenuation (  ) can be determined by noting the values of the heights (amplitudes) of the echoes. Thus ultrasonic absorption (  /f 2 ) can be calculated, where, f = frequency of vibration of the transducer. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

69. Fine echo wave train pattern Selection of two echoes Overlapping of two selected echoes Fine amplitude pattern for measurement of Attenuation Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

70. Pulse Echo Techinque The pulse generator sends an electric pulse to the transmitter probe, which produces an ultrasonic pulse. This ultrasonic wave spreads into the specimen and is reflected to the receiver, which transforms the wave into an electric signal. This signal is then send to the amplifier and from there to the cathode ray (CR) tube, which displays the signal as peaks. The horizontal axis is proportional to the time t. The vertical axis shows the amplitude of the signal. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

71. The time base generator produces a high frequency wave and makes the spot move across the CR tube.  The first peak on the display represents the generated ultrasonic pulse. The ultrasonic wave travels through the specimen until it is reflected or scattered by a surface. The reflected part of the wave can be seen as peak on CR tube. The other part of the wave continues to the back wall of the specimen and will be reflected there. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

72. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Images of ultrasonic probe of 1MHz, 2MHz & 5MHz frequency Connection Details for Viewing Signal on Oscilloscope

73. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University Echo pattern on two channel digital storage oscilloscope Simple Echo pattern

74. Single Relaxation for internal degrees of freedom Relaxation usually means the return of a perturbed system into equilibrium. Each relaxation process can be categorized by a relaxation time τ.  The degrees of freedom refers to the number of ways a molecule in the gas phase may move, rotate, or vibrate in space.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

75. Sound absorption in fluids is many times larger than that resulting from classical causes: viscosity and heat conductivity of fluid. The total energy of molecules in a fluid is distributed partly,  among their translational or external degrees of freedom, and partly among their rotational and vibrational or internal degrees of freedom.  Under equilibrium conditions, the distribution of energy among the various external and internal degrees of freedom will be that characteristic of their common temperature. When the energy of an element of fluid is suddenly increased, there is not only redistribution of energy among the various translational degrees of freedom, but also the exchange of energy between the external and internal degrees of freedom Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

76. The relevant time constant characterizing the processes is of the order of Collision time Tc between the molecule. Establishment of thermal equilibrium between the translational and internal degrees of freedom is in general much slower, the corresponding time constant on the relaxation time t being much larger than Tc . As a consequence there is a lack of thermal equilibrium between the two sets of degrees of freedom.  The consequent irreversible exchange of energy between the two energy modes give rise to an additional absorption of sound in fluids, the absorption per wavelength attaining a maximum value when the period of sound wave is of the same order of magnitude as relaxation time. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

77. Relaxation in binary mixtures Consider a mixture of two gases ‘A’ and ‘B’ at pressure ‘P’. Let the mole fraction of gas ‘A’ be (1 - x)  and that of gas ‘B’ is ‘x’. Let specific heat at constant volume of two gases  in their pure state, per gram mole, be ( C v ) A and ( C v ) B respectively . let us assume that, the pure gas ‘A’   at pressure ‘P’ has a single relaxation time ‘ τ AA ’, internal specific heat C A and the molecule of  gas ‘B’ are spherical i.e. it has no Internal energy. In the mixture, molecule ‘A’ can exchange its internal energy with translational  energy of the molecules not only in collision with another ‘A’ molecule but also in a collision with ‘B’ molecules.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

78. let the relaxation time of single ‘A’ molecule in pure gas of ‘B’ molecules at pressure P be τ AB . As only the binary collisions are effective, ‘A’   molecules in the mixture will relax their internal energy  at rate characterized by relaxation time τ 11 , such that, 1/τ 11 = (1-x )/ τ AA + x/ τ AB Since of total number of  collisions suffered by ‘A’ per second,  a fraction (1-x) are of ‘AA’ collisions and a fraction (x) are of ‘AB’   collision s . Similarly, if the gas ‘B’   is in its pure state is assumed to have a single relaxation time τ BB ,  internal specific heat C B , while ‘A’ molecule assume to be spherical, the mixture will have relaxation time τ 22 1/τ 22 =x / τ BB + (1-x)/ τ BA where  τ BA Is define similar to  τ AB Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

79. Finally if both ‘A’ and ‘B’ molecules in the mixture possess  internal energy, the mixture may be regarded as having two degrees of freedom  excited in parallel with relaxation time τ 11 and τ 22 , provided that, in the ‘AB’ collisions,  no exchange of energy occurs between the internal degrees of freedom of ‘A’ and those of ‘B’. the specific heats associated with the relaxation Time  τ 11 and τ 22 are C 1 = (1-x)C A , C 2 = xC B C v = (1-x)( C v ) A + x (C V ) B Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

80. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University At low frequencies absorption α angular frequency As ω increases absorption increases and rises to a maximum and then decreases to zero proportionately to ω -1 . The velocity curve has a inflation, at ω = ω m . Dispersion and absorption due to thermal relaxation are significant over a wide range on both sides of ω m

81. Normal and associated liquids Attractive interaction between the molecules play a minor role  in liquids. These liquids generally called as normal liquids . Normal liquids form a majority and in particular many organic liquids belong to this group.  T hese liquids obey certain empirical rules,  with regard to various physical properties such as vapour pressure, viscosity etc.  There exist a large minority of liquid  which form an exception. These liquids are known as associated liquids ,  since the deviations from empirical rules arise from the tendency of two or more molecules form a group or a larger molecule.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

82. This associativeness is found to be most predominant amongst polar molecules. Different liquids show different degrees of  association depending upon the strength of intermolecular attractive interaction, temperature etc. In weakly associated liquids, we may have association between just two molecules, eg .   acetic acid where the monomer CH 3 COOH molecules and dimar (A  dimer  is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular) (CH 3 COOH) 2 molecule exist simultaneously. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

83. At the other end of the scale are liquids like water, primary alcohols and viscous liquids like glycerol,  where the association is amongst a large number of molecules so that long range order exist. Distinction between normal and associated liquids is quantitative rather than qualitative, and a liquid which behaves with respect to some physical properties  like associated liquid, at room temperature, behave like a normal liquid at higher temperature, where the thermal energy would tend to break the links between the molecules.   The increase magnitude of the speed of sound in the liquid state  relative to that in vapour state is largely due to the close packing of molecules in liquid.   It is of the same order of magnitude in both normal and associated liquid. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

84. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University We know that, Ultrasonic absorption in liquids is due to their viscosity and thermal conductivity,   and due to thermal relaxation between translational and internal degrees of freedom. the classical causes and the internal thermal relaxation in normal liquids explains the sound absorption and dispersion.  in associated liquid,  existence of other relaxation processes has to be assumed. since the coefficient of volume expansion of water is zero at 4 o C and hence attenuation due to internal thermal relaxation   vanishes at this temperature yet the observed attenuation is about 3 times the classical value throughout the temperature range 0 o C to 90 o C. example; in weakly associating liquid such as acetic acid in equilibrium  their exist certain number of monomer molecules and  short number of  dimer molecule.

85. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University temperature and pressure changes associated with the sound wave will perturb   this equilibrium.   since the equilibrium is restored at a finite rate a typical relaxation process is expected.  similarly the equilibrium long range order existing among the molecules in a highly associated liquid by the sound waves and the tendency of the system to come to equilibrium will give rise to sound absorption. In highly viscous liquids like glycerol existence of large relaxation times has to be assumed to explain absorption and dispersion.  highly associated liquids will have  different properties from normal liquids.

86. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University In liquids  the sound wave may perturb the equilibrium  between i )  the internal and transnational  degree of freedom ii)  isomeric forms of molecules iii)monomers and dimers in weakly associated liquids or between polymers in associated liquids iv) Equilibrium degree of long range order in highly associated liquids . In normal liquids, where the molecules can be considered as having a negligible attractive interaction between them, it is only temperature, and not pressure difference, associated with the sound wave cause a lack of equilibrium between the internal and external degrees of freedom.

87. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University i.e. in normal liquids,  isothermal pressure or volume change could not give rise to relaxation effect unless there exist the small viscous effect . In contrast the processes ii), iii), iv) involves changes in structure either of an individual molecule or of group molecules . If these structural changes involves a change in volume,  then in general both the temperature and pressure fluctuations can induce lack of equilibrium. 

88. Application of ultrasound Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

89. Ultrasonic flaw detection Flaw detection is the oldest and most common industrial application of ultrasound.  Since 1940 the laws of physics that govern the propagation of sound waves through solid material have been used to detect hidden cracks, voids,  porosity  and other discontinuities in metals, composite, plastic and ceramics. High energy sound waves reflect from flaws in predictable way, producing  distinctive  echo patterns that can be displayed and recorded by portable instruments.  Ultrasonic testing is completely non destructive and safe and it is a well established test method in many basic manufacturing processes and services industry especially involving welds and structural metals. Pulse echo method is used for  flow detection. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

90. The sound energy is introduced and propagate through the material in the form of waves. when there is discontinuity such as a crack in a wave path,  part of the energy will be reflected back from the flaw surface.  the reflected wave signal is transformed into electrical signal by the transducer and is displayed on the screen. knowing the velocity of the wave, travel time can be directly related to the distance that the signal travel . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

91. Signaling  Since the frequency of the ultrasonic can be made very high so that wavelength is reduced to small value.  There for ultrasonic can be transmitted in the form of narrow beam of small amplitude and large energy  The ship  captain uses ultrasound next to steer it's path it m when life fails within few metres . Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

92. Sonar Ultrasonic waves sent by the ultrasonic transmitter towards the target through water and are received by the receiver lower into the sea water from the ship. Doing the time interval between sending and receiving of ultrasonic we can find the position of target aur submarine.  We can also find the direction and the velocity of the moving submarine with these apparatus.   Sona is used for the detection of enemy submarines inside water during what time in a senior manual as a retar is used to trace the enemy airplanes in the atmosphere Sonar is used for detecting the   glaciers icebergs, sea rocks and other submerge object Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

93. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

94. Cavitation  When a sound waves travels through a liquid,  during the positive of cycle of wave,  the liquid is compressed and during the negative cycle it expands When the acoustic pressure is greater than the hydrostatic pressure, voids are created.  These voids expand during the negative half cycle and collapse abruptly.  It places energy,  it releases energy, giving rise to shock waves.  The local temperature rises to 10 hundred and thousands degrees for very short periods at the frequency of wave propagation. This is called cavitation.  The cavitation process is used in imulsification of two immiscible liquids.  In the food industry, this method is used in preparation of dairy products sources gravis  salad creams and synthetic creams  It is used in extraction of hops in brewing industries.  The sterilisation of milk and tendering of meat by breaking down its fibres are some other applications. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

95. Ultrasonic cleaning  In the process both agitation and cavitation are involved The cleaning of large components is carried out at low frequency range from 18 kilo h to 40 khz ,   here cavitation is most active  For delegate articles,  higher frequency range 100 kilowatt and one megahertz is used where cavitation is almost absent The process is used in removal of grease from engine components, oil, dust , buffering compounds etc from printed circuit boards jewellery and various instruments and devices.  Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

96. Acoustical holography   The waves used in the reconstruction does not have to be the original,  and this allows the use of ultrasonic waves and subsequent reconstruction of the image using light from a laser.  But size of image changes in proportion to the ratio of wavelength of reconstructing wave to the wavelength of the original eliminating waves.  There are number of methods of making a political holograms  One of the method is liquid levitation,  where the acoustic waves from the below the surface of liquids forms an ultrasonic image at the surface of the liquid that can be rendered visible on photographic plate.   The reference been on to the surface or by generating a separate wave through a second transducer.   The height of bulge of the liquid surface due to the impetus of an acoustic wave is critical,  it should be small as compared to the wavelength of the light. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

97. Medical applications Ultrasound technique have replaced x-rays in almost all areas of medical diagnostic Prenatal corrective surgery has been made possible by the ultrasound studies in vivo babies Ultrasound as many applications in the field of medical diagnostics  It is particularly well known for is used in the following areas  Pregnancy and gynaecological   problems Kidney examination( kidney stones can be pulverised by ultrasound and then flushed out of the system) Examination of scrotum, testes etc Examination of thyroid, gullets, larynx  and allied organs Craniotomy or the study of cranium layer by layer by a safe non invasive method  A craniotomy is a surgical procedure in which a part of the skull is temporarily removed to expose the brain and perform an intracranial procedure Dr Priyanka Tabhane Department of Physics, RTM Nagpur University

98. Dr Priyanka Tabhane Department of Physics, RTM Nagpur University