Department of Physics. Course Coordinator - Dr.G.Sunita Sundari

Transcription

1 Department of Physics Course Coordinator - Dr.G.Sunita Sundari gunturisunita@kluniversity.in 1

2 Introductio to Ultrasonics Properties of Ultrasonic waves Production Ultrasonic waves Magnetostriction (Definition only) and Piezo Electric Method (Complete) Detection Methods Applications of Ultrasonics Worked Problems 2

3 Introduction Classification of Sound Waves: Depending on Frequency Description 1) Infrasound (Infrasonic) 2) Audible sound Frequency range 0 20 Hz 20 20,000 Hz Example Earth quake Speech, music 3) Ultrasound (Ultrasonic or Supersonic) > 20,000 Hz Bats, Moths, Rodents, Insects, etc. 3

4 (1) Ultrosonics have high energy content/high frequency or small wavelength. (2) Speed of ultrasonic waves depends on frequency. (3) A medium is essential for propagation of ultrasonic waves. (4) ultrasonic waves get reflected, refracted and absorbed. (5) They produce heat when they got absorbed by a medium. (6) Shows negligible diffraction owing to small wavelengths. (7) They can be transmitted over large distances with no appreciable loss of energy. (8) When ultrasonic waves are propagated in a liquid bath diffraction grating (acoustic grating) is formed which can diffract light. (9) When passed through a medium, the ultrasonic waves are partially reflected at discontinuities and this property is used in NDT. 4

5 Ultrasonic waves are produced by the following methods. (1)Magneto-striction generator or oscillator To produce low frequency ultrasonics waves upto few hundred KHz only. (2)Piezo-electric generator or oscillator To produce frequencies as greater as few hundred MHz. 5

6 To produce low frequency ultrasonics (upto 300 KHz) Principle: Magnetostriction effect When a magnetic field is applied parallel to the length of a rod made of ferromagnetic materials such as iron or nickel, a small elongation or contraction occurs in its length. 6

7 The change in length (increase or decrease) produced in the rod depends upon i) the strength of the magnetic field, ii) the nature of the ferromagnetic materials iii) not on the direction of the field. 7

8 Advantages 1. The design of this oscillator is very simple and its production cost is low 2. At low ultrasonic frequencies, the large power output can be produced without the risk of damage of the oscillatory circuit. Disadvantages 1.It has low upper frequency limit and cannot generate ultrasonic frequency above 3000 khz (ie. 3MHz). 2.The frequency of oscillations depends on temperature. 3.There will be losses of energy due to hysteresis and eddy current. 8

9 Principle : Inverse piezo electric effect If mechanical pressure is applied to one pair of opposite faces of certain crystals like quartz, equal and opposite electrical charges appear across its other faces. This is called as piezoelectric effect. The converse of piezo electric effect is also true. If an electric field is applied to one pair of faces, the corresponding changes in the dimensions of the other pair of faces of the crystal are produced. This is known as inverse piezo electric effect or electrostriction. 9

10 X-cut and y-cut quartz crystals: 10

11 Quartz Crystal 11

12 Q = Quartz crystal A,B = metal plate electrodes L, L 1, L 2 = Inductively coupled coils C = Variable capacitor S = Switch f 2 1 LC f p Y pv or, 2l 2l 12

13 Advantages Ultrasonic frequencies as high as 5 x 108Hz or 500 MHz can be obtained with this arrangement. The output of this oscillator is very high. It is not affected by temperature and humidity. Disadvantages The cost of piezo electric quartz is very high The cutting and shaping of quartz crystal are very complex. 13

14 a) Kundt's tube method b)sensitive flame method c) Piezo-electric detector d)thermal detection method e) By acoustic grating method 14

15 a)kundt's tube method Lycopodium Powder is used. Heaps at the Nodes and blown off at Antinodes. Measurement of Wavelength and Velocity of ultrasonic sound can be done using this method. 15

16 Wavelength: The average distance between two successive nodes or heaps is taken as d. It should be equal to the half of the wavelength of ultrasonic waves. This method is suitable for measuring velocity of low frequency ultrasonic waves. It can not be used for high frequency ultrasonic waves. Velocity: Velocity of ultrasonic wave is V ϑ freq of ultrasonic wave 16

17 b) Sensitive flame method When a narrow sensitive flame is moved in a medium of ultrasonic waves. Flame remains stationary at antinodes and flickers at nodes. c) Piezo-electric detector Quartz crystal for detection of ultrasonic One pair of faces of quartz subjected to Ultrasonics. Varying electric charges are produced. These charges are very small and they can be amplified. 17

18 d) Thermal detection method Probe made of thin platinum wire. Temperature of the medium changes due to alternate compressions and rarefactions in a progressive wave. Resistances of the platinum wire changes at node (owing to pressure and hence temperature variations) and remains constant at antinodes (owing to constant pressure and temperature) in a standing wave. Changes in the resistance can be detected by sensitive resistance bridge arrangement. e) Acoustic grating method Principle : Ultrasonic waves are propagating through a liquid medium (stationary waves formed) Density of liquid varies from layer to layer. Monochromatic light is passed - perpendicular direction. Liquid behaves as diffraction grating. 18

19 Reflected waves are called echos. Superposition of the direct and reflected waves - Longitudinal stationary waves produced. Nodes and Antinodes are formed - refractive index changed 19

20 Working: Monochromatic light Diffraction pattern consists of central maxima, first order maxima, etc. d - distance between two nodes or antinode planes. Wavelength of light is given by d sinθ = n λ λₒ is the wavelength of ultrasonic wave d = λₒ /2 Velocity of ultrasonic wave 20

21 Ultrasound Generation Ultrasound is generated with a transducer. A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa. The transducer is capable of both transmitting and receiving sound energy. 21

22 Detection of flaws in metals (Non Destructive Testing NDT) Principle Ultrasonic waves are used to detect the presence of flaws or defects in the form of cracks, blowholes porosity etc., in the internal structure of a material By sending out ultrasonic beam and by measuring the time interval of the reflected beam, flaws in the metal block can be determined. 22

23 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 on the distance that the sound has traveled. 23

24 NDT Pulse echo systems Transmission testing systems Resonance Systems 24

25 Experimental setup Non Destructive Testing NDT Master timer Time base amplifier Transducer Signal pulse generator Echo signal amplifier C R O Metal under Test It consists of an ultrasonic frequency generator and a cathode ray oscilloscope(cro), transmitting transducer(a), receiving transducer(b) and an amplifier. 25

26 Pulse echo systems: NDT Working Process IP BE F Probe plate s delamination Sound travel path Flaw IP = Initial pulse F = Flaw BE = Backwall echo Work piece 26

27 Test Techniques - Pulse-Echo In pulse-echo testing, a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy (an echo). Reflections occur due to the presence of discontinuities and the surfaces of the test article. The amount of reflected sound energy is displayed versus time, which provides the inspector information about the size and the location of features that reflect the sound. f initial pulse back surface echo crack echo crack UT Instrument Screen plate 27

28 Test Techniques Pulse-Echo (cont.) Digital display showing signal generated from sound reflecting off back surface. Digital display showing the presence of a reflector midway through material, with lower amplitude back surface reflector. The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined. 28

29 Working In flaws, there is a change of medium and this produces reflection of ultrasonic at the cavities or cracks. The reflected beam (echoes) is recorded by using cathode ray oscilloscope. The time interval between initial and flaw echoes depends on the range of flaw. By examining echoes on CRO, flaws can be detected and their sizes can be estimated. 29

30 NDT Working Process Transmission testing systems: Through transmission signal 1 T R 1 2 T R Flaw 30

31 Test Techniques - Through-Transmission Two transducers located on opposing sides of the test specimen are used. One transducer acts as a transmitter, the other as a receiver. Discontinuities in the sound path will result in a partial or total loss of sound being transmitted and be indicated by a decrease in the received signal amplitude. Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information. T T 1 2 R R

32 Test Techniques Through-Transmission 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. 32

33 NDT Working Process Resonance Systems: Ultrasonic standing waves are setup with in the specimen causing the specimen to vibrate at greater amplitude. Resonance is then sensed by CRT (cathode ray tube), and that frequency is useful to detect the discontinuity of material. 33

34 Resonant Nondestructive Testing Every part has its unique vibration signature (its resonant frequency). This resonance will be almost exactly the same from good part to part. However it will change when there is an internal or external change or imperfection. 34

35 Resonant Nondestructive Testing Can Detect Cracks, chips, and holes Porosity Residual stress Out-of-tolerance dimensions Variations in hardness or density Bonding, welding, or brazing failures Machining or heat-treating processes 35

36 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. Detailed images can be produced with automated systems. Has other uses such as thickness measurements, in addition to flaw detection. 36

37 Automated Testing Estimated 1 tie per sec NDT CORPORATION 37

38 Inspection of Raw Products Forgings, Castings, Extrusions, etc. 38

39 Machining Welding Grinding Heat treating Plating etc. Inspection Following Secondary Processing 39

40 Inspection For In-Service Damage Cracking Corrosion Erosion/Wear Heat Damage etc. 40

41 Power Plant Inspection Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage. Pipe with damage Probe Signals produced by various amounts of corrosion thinning. 41

42 Applications 42

43 (1) SONAR (Sound Navigation and Ranging ) Depth of Sea: (2) Ultrasonic Drilling (3) Ultrasonic welding (4) Ultrasonic soldering (5) Ultrasonic cutting and machining (6) Ultrasonic cleaning 43

44 Applications of Ultrasonics in Medicine (1)Diagnostic sonography Medical sonography (ultrasonography) is an ultrasoundbased diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, their size, structure and any pathological lesions. They are also used to visualize the foetus during routine and emergency prenatal care. Ultrasound scans are performed by medical health care professionals called sonographers. Obstetric sonography is commonly used during pregnancy. 44

45 Obstetric ultrasound is primarily used to: Date the pregnancy Check the location of the placenta Check for the number of fetuses Check for physical abnormities Check the sex of the baby Check for fetal movement, breathing, and heartbeat. Medical applications: To obtaining information about flow of blood through the heart and the about the condition of heart valves. Its used in blood less surgery Also used for detecting tumors and other defects in human body. To view the Fetus in its mother's womb, viewed in a sonogram. 45

46 (2)Ultrasound therapeutic applications Treating malignant tumors and other disorders, via a process known as Focused Ultrasound Surgery (FUS) or HIFU, High Intensity Focused Ultrasound. These procedures generally use lower frequencies than medical diagnostic ultrasound (from 250kHz to 2000kHz), but significantly higher time-averaged intensities. 46

47 More power ultrasound sources may be used to clean teeth in dental hygiene or generate local heating in biological tissue, e.g. in occupational therapy, physical therapy and cancer treatment. Extracorporeal shock wave lithotripsy uses a powerful focused ultrasound source to break up kidney stones. Focused ultrasound sources may be used for cataract treatment by phacoemulsification. 47

48 (3)Ultrasonic blood Flow meter Ultrasonic waves are used for studying the blood flow by measuring the change in their frequency produced due to Doppler s effect. Note : Physiological effects of ultrasound energy Ultrasound energy has two physiological effects : 1. Enhance inflammatory response 2. Heats soft tissue. 48

49 Some Other Applications of Ultrasonics (1) Ultrasonic guidance for the blind Ultrasonic waves are used for guiding the blind who carries a walking stick containing an ultrasonic transmitter and receiver. Ultrasonic signals reflected from any obstacles are fed to the head phones through a suitable electronic circuit which enables the blind person to detect and estimate the distance of the obstacle. 49

50 (2)Ultrasound in research Scientists often use in research, for instant to break up high molecular weight polymers, thus creating new plastic materials. Indeed, ultrasound also makes it possible to determine the molecular weight of liquid polymers, and to conduct other forms of investigation on the physical properties of materials. Ultrasonic can also speed up certain chemical reactions. Hence it has gained application in agriculture, that seeds subjected to ultrasound may germinate more rapidly and produce higher yields. 50

51 Problems on Ultrasonic's: 1. For a quartz crystal of length 0.05cm, calculate the fundamental frequency of oscillation. In a piezoelectric oscillation oscillator if the velocity of longitudinal waves in the crystal is 5.5 x 10 3 m/sec. 2. A boat sends out ultrasonic pulse to determine the depth of the sea, if the echo is received after 80 msec. What is the depth of sea given that speed of sound in water is 1500m/sec. 3. A Quartz crystal of thickness 0.001m is vibrating at resonance. Calculate the fundamental frequency. Given (Y = 7.9 x N/m 2, ρ = 2.65 x 10 3 kg/m 3 ). 4. To design a piezoelectric oscillator which produces ultrasonic waves of frequency 10 6 Hz with an inductance of 1 Henry and what is a capacitance? 5. A particle crystal in an ultrasonic interference produces stationary waves of frequency 1.5 MHz. If the distance between 6 consecutive nodes is 2.75mm. Find the velocity of ultrasonic waves. 6. A piezo-electric crystal has thickness m. If the velocity of sound wave in crystal is 5750 m/s. Calculate the fundamental frequency of the crystal. 7. Find the fundamental frequency of a quartz crystal plate of thickness is 30 mm. (Given E= 8x 10 9 pascal, density of material = 2.7x10 3 kg/m 3 ). 51