ULTRASONICS

1. ULTRASONICS Introduction- production of ultrasonic waves –non destructive testing of materials - Measurement of velocity of ultrasonic waves in solids and liquids –Elastic constant

2. Molecules in the air vibrate about some average position creating the compressions and rarefactions

3. Categories of Sound Waves • Audible waves • Lay within the normal range of hearing of the human ear • Normally between 20 Hz to 20,000 Hz • Infrasonic waves • Frequencies are below the audible range • Ultrasonicwaves • Frequencies are above the audible range

4. Sound waves of more than 20,000 hertz are known as ULTRASONIC

5. Properties of Ultrasonic waves • They are acoustic/sound waves. • The frequency of ultrasonic wave is greater than 20KHz. • Due to small wavelength the diffraction is negligible. It travels long distance in the medium without any loss. • They are highly energetic. • It travels as well defined sonic beam. • When absorbed by medium, the medium get heated/they produce heating effect. • At room temperature ultrasonic welding is possible. • They drill and cut thin metals. • They act as catalysts in chemical reaction.

6. Production of ultrasonic waves There are different methods for the production of ultrasonics. Magnetostriction method: Piezoelectric method

7. Production of ultrasonic waves N S Magnetostriction Method Principle When a magnetic field is applied parallel to the length of a ferromagnetic rod made of iron or nickel, a small elongation or contraction occurs in its length, This phenomenon is known as magnetostriction. The change in length depends on the intensity of the applied magnetic field, the nature of the ferromagnetic material and the direction of the field applied.

8. If a varying electromagnetic field to a ferromagnetic material, the ferromagnetic rod alternatively expands and contracts, this means the rod is thrown in to vibration. The rod suffers change in length for each half cycle of the alternating current. So the rod vibrates with frequency twice the frequency of the alternating current. The amplitude of vibration is usually small. The rod vibrates violently if the natural frequency of the rod and the a.c current frequency is equal ( Resonance condition ) The longitudinal expansion or contraction in the ferromagnetic rod produces ultrasonic sound waves in the medium surrounding the Ni rod.

9. Construction Positive feed back L-C Circuit As load NPN

10. Working Collector current starts rising and oscillation starts in the L-C circuit. Such oscillation would decay. To maintain sustained ( undamped ) oscillation, The change in current in coil L are feedback to the base emitter input through mutual inductance. The frequency of oscillation of L-C oscillator is ByVarying C the frequency can be adjusted The natural frequency of the rod y- Young’s modulus , l- length , - density a rod of 0.1m length 20KHz

11. Merits • The design of this oscillator is very simple and its production cost is low. • At low frequencies, large power output is possible without the risk of damage to the oscillatory circuit. Demerits • It can produce frequency upto 3MHz only. • It can not withstand at higher temperatures • There will be loss of energy due to hysteresis during the oscillations.

12. Piezoelectric Method • Discovered in 1880 by Pierre Curie in quartz crystals. Piezoelectric Effect When pressure or mechanical force is applied along one pair of opposite faces of quartz crystal then equal and opposite charge are produced along the another pair of opposite faces of the crystal. This phenomenon is called piezoelectric effect. • The electric charges developed by this method are proportional to the amount of pressure. • The converse is also possible . A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)

13. _ + 0V + _ Normal Tension Compression

14. Piezoelectric crystal Quartz Tourmaline Rochelle salt

15. Piezoelectric effect provides a transducer between electrical and mechanical oscillations

16. Inverse Piezo-electric effect • If an E.F. is applied to one pair of opposite faces of the quartz crystal, alternative mechanical expansion and contractions are produced across the other pair of opposite faces of the crystal. This is known as inverse piezo-electric effect.

17. x- axis electric axis which joins the corners of the hexagon Y- axis mechanical axis which joins the centers of the sides of hexagon Z- axis optic axis which joins the edges of the pyramid Quartz crystal Natural quartz crystal Z Y Y x x X- cut crystal ( 5 kHz- 100kHz) Longitudinal waves Y- cut crystal ( 1MHz- 10MHz) Transverse waves

18. Construction Hartley oscillator Crystal is placed between A and B which acts as electrodes L, L1, L2 are inductively coupled Frequency of the oscillator Natural frequency Crystal of the

19. Merits Frequency up to 500MHz can be produced Stable frequency can be generated Using different transducer one can generate wide range of frequency Demerits Piezoelectric crystals are very expensive Cutting and shaping are not easy High energy waves cannot be produced.

20. NON DESTRUCTIVE TEST-NDT quantitatively measure some characteristic of an object. i.e. Inspect or measure without doing harm.

22. Non Destructive Testing Non destructive testing is extracting information on the physical , chemical, mechanical or metallurgical state of materials or structures. The information is obtained through the process of interaction between the information – generating device and the object under test. The process of interaction does not change the test object or impair/spoil it. The information can be generated using X-Rays, gamma rays, neutrons, ultrasonic methods, magnetic and electromagnetic methods, etc..

23. Methods of NDT Thermography Microwave Visual Magnetic Particle Tap Testing X-ray Acoustic Microscopy Acoustic Emission Liquid Penetrant Magnetic Measurements Replication Ultrasonic Eddy Current Laser Interferometry Flux Leakage

24. Principles of Ultrasonic Inspection • Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface. • At surface interfaces some of the wave energy is reflected and some is transmitted. • The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector. • The travel time of the sound can be measured and this provides information about the distance that the sound has traveled.

25. ULTRASONIC NON DESTRUCTIVE TESTING Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic examinations can be conducted on a wide variety of material forms including castings, welds, and composites.

27. Test Techniques – Pulse-Echo (cont.) Digital display showing received sound through material thickness. Digital display showing loss of received signal due to presence of a discontinuity in the sound field.

28. The applications of ultrasonic testing • Flaw detection (cracks, inclusions, porosity, etc.) • Assessment of bond integrity in adhesively joined and brazed components • Estimation of void content in composites and plastics • Measurement of case hardening depth in steels • Estimation of grain size in metals

29. Advantage of Ultrasonic Testing Sensitive to small discontinuities both surface and subsurface. Depth of penetration for flaw detection or measurement is superior to other methods. Only single-sided access is needed when pulse-echo technique is used. High accuracy in determining reflector position and estimating size and shape. Minimal part preparation required. Electronic equipment provides instantaneous results. Has other uses such as thickness measurements, in addition to flaw detection.

30. Limitations of Ultrasonic Testing Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods. Normally requires a coupling medium to promote transfer of sound energy into test specimen. Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise. Linear defects oriented parallel to the sound beam may go undetected. Reference standards are required for both equipment calibration, and characterization of flaws.

31. Measurement of velocity of ultrasonic waves in solids • The velocity of ultrasonic waves in a sold can be measured by pulse echo method. • The setup used will be essentially same as described in the NDT of materials. • The time t between any two echoes is the length of the time required for the pulse to travel through the specimen and back to the transducer. • The amplitude decays exponentially with time. • By knowing the dimensions of the specimen, it is possible to estimate the distance d travelled between the transmitter and the reflecting end which is also equal to the distance between the receiver and the reflecting end. • Knowing d and t, one can evaluate the longitudinal velocity of ultrasonics in the specimen using the relation, • =

32. Measurement of velocity of ultrasonic waves in a liquid

33. Acoustic grating When a standing wave pattern is formed gives fixed nodes and anti nodes. At nodal planes density is minimum, anti nodal planes density is maximum, which causes clear and opaque regions at perfectly regular intervals. Thus the liquid behaves as a diffraction grating called “Acoustic grating”.

35. Determination of elastic constants in solids • The speed of ultrasonic of longitudinal and shear waves are given by, = • E is the Young’s modulus • = Poisson’s ratio = transverse strain / longitudinal strain • Is the density of the solid • n is the rigidity modulus • Thus, knowing CL and CS for a solid, , E and n of the solid can be evaluated.

36. In table Young’s moduli, densities, and sound velocities in rods of typical materials at room temperature are shown.

37. Bulk modulus of liquids • The elastic behaviour of a liquid is characterized by its bulk modulus K. It is related to the velocity vu of the ultrasonic wave through the equation. • Where  is the density of the liquid. • Hence knowing vu & , K can be determined.

38. The velocity of ultrasonic waves in certain liquids is given in table..

39. Calculate the velocity of ultrasonic waves in liquid with frequency 100MHz and wavelength of light used is 600nm. Angle of first order diffraction is 50. • A quartz crystal of thickness 0.001m is vibrating at resonance. Calculate the fundamental frequency. Give : young modulus for quartz is 7.9  1010 N/m2 and density of quarts is 2.65  103. • Calculate the natural frequency of 40mm length of a pure iron rod. Given the density of iron is 7.25 x 103 kg/m3 and Young`s modulus is 115x109Nm-2 .