Ch-4 Ultrasonics_KB

L. J. Institute of Engineering and Technology Subject: Physics (Group-1) Subject code: 3110011 F.Y. - B.E. Kamaldeep Bhatia

Ultrasonics The sound waves of frequency greater than 20 kHz are called ultrasonic waves. PROPERTIES OF ULTRASONIC WAVES:-  The frequency of ultrasonic waves is greater than 20 kHz.  Their wavelength is small, so they high penetrating power.  They can travel over long distance as a highly directional beam.  They have high energy content.  Their speed of propagation increases with increase in frequency. PRODUCTION OF ULTRASONIC WAVES: 1) Magnetostriction Method:- Principle: When a ferromagnetic material in the form of a rod is subjected to an alternating magnetic field parallel to its length as shown in figure, the rod undergoes alternate contractions & expansions at a frequency equal to the frequency of the applied magnetic field. This effect is known as magnetostriction effect. Construction:- 2

A ferromagnetic rod AB is clamped at the middle X. And the coils ������1 & ������2 are wound at the ends of the rod. The coil ������1 & a variable capacitor ������1 is connected in parallel and this combination forms the tank as resonant circuit. One end of the resonant circuit is connected to the milliammeter and collector of the transistor. The other side of the tank circuit is connected to the battery and emitter. This coil ������2 is connected between base and the emitter and this is used as a feedback loop. Working:- When the battery is switched on, the resonant circuit ������1������1 produces an alternating circuit of frequency. ������ = 1 . 2������√������1������1 As a result, the rods gets magnetised and with change in the magnetisation, the length of the rod changes. This change in length gives rise to a change in flux in coil ������2, and thereby inducing an emf in the coil ������2. This changing emf is applied to the base of the transistor & is fed back to the coil ������1. Capacitor C is applied so that the frequency of oscillator circuit is equal to the natural frequency of rod & thus, resonance takes place. At resonance condition, the rod vibrates longitudinally with maximum amplitude & generates ultrasonic waves of high frequency at the ends of the rod. The frequency of ultrasonic waves produce by this method is given by, ������ = 1 √������ 2������ ������ Merits (Advantages):-  The design of this oscillator is very simple & production cost is low. 3

 At low ultrasonic frequency, large power o/p is possible.  Frequency ranging from 100 Hz to 3,000 kHz can be produced. Demerits (Disadvantages):-  It cannot generate ultrasonic waves of frequency above 3,000 kHz.  The frequency of oscillator depends on temperature.  There will be losses of energy due to hysteresis and eddy current. 2) Piezoelectric effect:- Principle: When pressure is applied to one pair of opposite faces of crystalline (hexagonal crystal) like quartz, tourmaline and rochelle salt, equal & opposite charges appear across its other faces. This phenomenon is known as piezoelectric effect. The inverse of piezo-electric effect is also true. Inverse Piezoelectric effect:- If an alternating voltage is applied to one pair of opposite faces, the corresponding changes in dimensions of other pair of faces of the crystal are produced. This is called inverse piezo-electric effect. 4

Construction:- The quartz crystal Q is placed between two metal plates A & B. The plates A & B are connected to the coil ������3. The coil ������1, ������2 & ������3 are inductively coupled. The coil ������2 is connected to the collector circuit, while coil ������1 is connected with a variable capacitor ������1 & formers the tank circuit. The battery is connected between free end of ������2 & the emitter. Working:- When the battery is switched on, the oscillating circuit produces high frequency alternating voltage given by, 1 ������ = 2������√������1������1 5

The frequency of oscillation controlled by the variable capacitor ������2. Due to the transformer action an emf is produced in coil ������3. This emf are fed on plates A & B. Inverse piezoelectric effect takes place & crystal vibrates. The frequency of vibration is given by, ������ ������ ������ = 2������ √������ Where, P=1,2,3… for fundamental first over tone E= Young’s Modulus ρ = Density of the crystal Merits (Advantages):-  More efficient than magnetostriction oscillator.  Frequency as high as 5 × 108 ������������ can be obtained  Output is very high.  Not affected by temperature & humidity. Demerits (Disadvantages):-  Cost is very high and crystal’s cutting and shaping are very difficult. DETECTION OF ULTRASONIC WAVES Presence of ultrasonic waves can be detected by using any one of the following methods. The methods are. 1. Acoustic Grating Method 2. Quartz crystal method 3. Thermal detection method 4. Sensitive flame method and 5. Kundt's tube method. ACOUSTIC GRATING METHOD: (Detection & Determination of velocity of Ultrasonic waves) Principle:- When ultrasonic waves are passed through transparent liquid medium in a container the waves get reflected. These reflected waves are called echos. 6

The direct & reflected waves superimpose to form the stationery wave pattern. These waves varies the density of the liquid. This change in density of liquids varies the refractive index of the liquid. At the nodes, liquid density is maximum while at anti-nodes it is minimum. This type of liquid-column subjected ultrasonic waves behaves like a grating. If a parallel light is passed through this liquid at right angles to the wave, the liquid acts as a diffraction grating. Such a grating is known as acoustic grating. Experiment:- It consists of a glass vessel containing liquid with a reflector at its top. A quartz crystal Q between two metal plates A & B is placed at the bottom. The metal plates are connected to the oscillator whose frequency is so adjusted that the crystal vibrates in resonance with the frequency of oscillator & produce ultrasonic waves within the liquid. This liquid behaves like a grating. Since the liquid is acting as a grating the monochromatic light beam passing through it gets diffracted & produces a diffraction pattern. The diffraction pattern consists of a central maxima with principle maxima of different orders on either side. If  is the angle of diffraction for the nth order principle maxima then, ������ sin  = ������������ … (1) ������ = ������������������������������������������������������ℎ ������������ ������ℎ������ ������������������������������ℎ������������������������������������������ ������������������ℎ������. 7

d = grating element The grating element ������ = ������������ 2 Where ������������ = ������������������������������������������������������ℎ ������������ ������������������������������������������������������������ ������������������������ ������������ sin  = ������������ 2 2������������ … (2) ������������ = sin  Knowing λ & n and by measuring , the wavelength of ultrasonic waves can be determined. If the resonant frequency of the ultrasonic generator is f, then the velocity of ultrasonic waves is given by, ������ = ������ ������ ������ … (3) Using this acoustic diffraction method, the wavelength & hence the velocity of ultrasonic waves through liquids & gases at various temperatures can be determined. QUARTZ CRYSTAL METHOD This method of detecting ultrasonic waves is based on the principle of piezoelectric effect. When one pair of opposite faces of the quartz crystal is exposed to ultrasonic waves, in the other pair of opposite faces, charges get developed as shown in Figure. These charges are amplified and detected using suitable electronic circuits. Quartz crystal method THERMAL DETECTION METHOD Principle 8

When the ultrasonic waves are propagated through a medium, the temperature of the medium changes due to alternate compressions and rarefactions In the case of stationary waves, there is a change in temperature at nodes and no change in temperature at antinodes. Based on this principle by sensing the change in temperature using suitable components, the ultrasonic waves can be detected. It is the most commonly used method to detect ultrasonic waves. In this method, a fine platinum wire included in Callender and Griffith bridge circuit arrangement, as shown in Figure, is used to detect the ultrasonic waves. This circuit is placed in the region where the presence of ultrasonic waves is to be detected if in the medium there is a presence of ultrasonic waves, then at nodes due to alternate compressions and rarefactions alternate heating and cooling effect gets produced in the platinum wire. Thermal Detection Method Thus, a change in temperature brings about a change in electrical resistance of the platinum wire which can be detected (the balanced position of the circuit gets disturbed) with the help of electrical circuit shown in Figure. At antinodes, the temperature remains constant which is indicated by undisturbed balance position of the bridge. SENSITIVE FLAME METHOD 9

This is a qualitative method to detect ultrasonic waves. When a narrow sensitive flame is moved in a medium of ultrasonic waves, the flame remains steady at the antınodes but flickers at nodes due to the maxımum change in pressure. Thus, by observing the behaviour of the flame, the ultrasonic waves presence can be detected. KUNDT'S TUBE METHOD This method is used to detect ultrasonic waves of low frequency. A Kundt's tube apparatus, shown in Figure, consists of a long glass tube of more than 1 m length and 5 cm in diameter kept horizontally with two supports on a wooden base board. One end of the tube is fitted with an adjustable piston rod with cork. A quartz crystal placed in between the two metal plates is placed at the mouth of the other end of the tube. The glass tube is thoroughly dried by passing through in a hot blast of air. A thin layer of dry lycopodium or cork powder is spread along the entire length of the glass tube and stationary ultrasonic sound waves generated by using piezoelectric method are passed in the air contained in the long glass tube, the lycopodium powder gets collected in the form heaps at the nodes and is blown off at the antinodes. Thus, the formation of heaps at nodes confirms the nature of the transmitted sound wave to be ultrasonic waves. The average distance between two consecutive heaps gives the value of half the wavelength. Thus using the relation v = f λu=2fd, the velocity of the ultrasonic waves in the medium can also be calculated. Kundt's Tube Method APPLICATIONS OF ULTRASONIC WAVES SONAR 10

SONAR is a device which stands for Sound Navigation and Ranging. It is based on the principle of echo sounding. In this technique high frequency ultrasonic waves are used. When ultrasonic waves are transmitted through water, they gets the reflected by the objects under water. In the absence of the object the ultrasonic waves do not get reflected to the receiving transducer. But in the presence of the object the ultrasonic waves get reflected and are picked up by receiving transducer. Knowing the velocity of ultrasonic waves & the elapsed time, the distance of the object can be calculated. Determination of the depth of a sea: The time t taken for the ultrasonic wave to travel to the bottom of the sea & to get reflected back to the top surface is noted using CRO. If the velocity of ultrasonic waves is known, Velocity V = (Distance travelled) = AC+CB = 2CO time taken t t vt CO = depth of the sea (d) = 2 The depth of the sea can be calculated using this formula. NON-DESTRUCTIVE TESTING (NDT) NDT is defined as the process of testing the material without causing any damage or reducing the service life of the component. Objectives:  To increase serviceability  To improve productivity & to increase the profits.  To increase safety. 11

NDT Methods (Ultrasonic Inspection methods): PULSE ECHO SYSTEM Pulse echo system is the most widely used ultrasonic test. A single transducer is used for both sending & receiving the ultrasonic waves. For better contact with surface, couplets like thin oil or glycerine is used. In this method ultrasonic waves are sent at a given time interval from a pulse generator into the sample. As the waves pass through the top surface of sample, there will be a pattern or ‘pip’ on the oscilloscope screen A, & if there is no defect in the sample then ‘pip’ of reflected pulse will appear at ‘C’. If the sample has a defect (flow), then a wave will get reflected back & smaller ‘pip’ will appear on screen as B. 12

ULTRASONIC FLAW DETECTOR (Through Transmission System) Through transmission system requires two transducers, one for transmitting and other for receiving sound waves. The transmitted ultrasonic waves get reflected when they find a defect in the sample. 13

The reflected pulse is received by the receiving transducer in the form of a sound wave & converted into electrical signal & fed to the CRO screen via amplifier, then time taken to come back is noted. By this exact size of detect can be determined. Advantages of Ultrasonic Inspection method:-  It is very cheap & it has a high speed of operation.  Large size sample can be inspected in a very short time.  Location, nature & size of a defect can be accurately determined. Disadvantages (Limitations):-  No permanent record of the defect can be obtained, it can be observed only on the screen.  Only skilled & well-trained technicians can perform this testing. APPLICATIONS OF ULTRASONIC IN INDUSTRY ULTRASONIC DRILLING & CUTTING In drilling technique, a tool driven by as ultrasonic generator is used. Abrasives like boron carbide as silicon carbide are used. A generator makes the tool bit to move up & down very quickly and forces the abrasive particles against the material to be cut thereby removing some material from the plate. This process continuous until the desired depth of a hole is formed over the plate. This same process takes place in ultrasonic cutting. (Note:- Abrasive is a material that is used to sharp or finish a sample by rubbing which leads to part of sample being thrown away.) ULTRASONIC WELDING (COLD WELDING) Ultrasonic welding is process of producing a metallurgical bond between the materials without melting. This welding is also known as a cold welding. 14

Process:- The surface of work-pieces are cleaned & put together. In this welding the bond to the between the welding pieces is produced due to the local application of high frequency vibrations by keeping the work pieces together under pressure. Using an ultrasonic generator the ultrasonic vibration are passed into the work piece through a coupling system or sonotrode which is resting over one of the work piece. The anvil is used to support the welding piece & opposes the clamping forces as shown in figure. A static clamping force is applied parallel to the work pieces. At same time the sonotrode tip oscillates parallel to the interface of the work pieces. Due to the combination of static & oscillating shear forces, an internal stress is created in work piece. After crossing elastic limit a high interfacial slip occurs. Due to this, the breaking & disperse of the surface film takes place which produces metal to metal contact at any point. [Note:- Anvil is a basic tool with a hard surface on which another object is struck.] ULTRASONIC SOLDERING With conventional soldering, aluminium foil condensers, aluminium wires & plates cannot be soldered. Hence to soldering technique can be used. 15

An ultrasonic soldering iron consists of an ultrasonic generator having a tip fixed at its end which can be heated by an electrical heating element. The tip of soldering iron melts the solder on the aluminium & the ultrasound removes the oxides & other contaminants over the surface & allows the solder to fasten. MEDICAL APPLICATIONS OF ULTRASONICS Diagnostic Applications: It is used to detect foetal heart pulses & to measure blood flow rate. The pulse method is normally used in medical field to detect, the defect & location of the defects, as abnormalities in the organs w/o causing any harm to organ. Therapeutic Applications: When ultrasound of low frequency & high power are passed into the tissue, they agitate the tissue to the required level. This level of agitation of the tissue will be almost equal to the level of the thermal heating occurring in the tissue. This type of application has been found to be useful in a number of muscles alignment treatments, also as a pain reliever. APPLICATIONS OF ULTRASONIC IN SCIENCE & ENGINEERING  To detect the defect in metals  To detect ship, submarines in ocean.  Soldering aluminium coil capacitors, wines & plates  Used for cutting & drilling  To accelerates chemical reactions.  To weld metals which cannot be welded by electric or gas welding. Cavitation One of the major application of high power and low frequency ultrasonic sound waves is ultrasonic processor and cleaner. In a processor and cleaner; the principle of cavitation is used. What is cavitation? When high frequency sound waves are passed into a solution,it produces 16

mechanical vibration effect within the liquid. Thus,the violent disturbance in the liquid results in the formation of minute vaccum bubbles(also called microscopic cavities or voids). This process of bubble formation and its subsequent collapse is known as cavitation. As the sound waves continuously propagate through the liquid,each point in the liquid is subjected to alternate negative and positive pressure at the compressions and rarefactions of the sound waves. The bubbles are grown at the microscopic level during rarefaction and it becomes very larger during compression phase. As the bubbles grow larger, they become very unstable and eventually collapse in a violent implosion releasing a shock wave of energy. The resulting shock wave provide the necessary energy used to assist cleaning. The implosion radiate high powered shock waves that dissipate repeatedly at a rate of 25,000~30,000 times per second. The implosions of cavitation bubbles generate temperature of 10,000F and pressure that exceed 10,000 Pa. Ultrasonic cleaning and processor uses this cavitation implosion effect for cleaning. Applications 1.The ultrasonic processor is used for emulsification of immiscible liquids, disruption and killing of micro-organisms etc. 2.The ultrasonic cleaner is used to clean semiconductor components, PCB's, switches, tools, moulds, casting, gears, ball-bearings, jewelleries, coins, precious stones, surgical instruments, camera lenses etc. Ultrasonic Cleaning The ultrasonic cleaning system consists of an ultrasonic generator,a transducer and a tank filled with the required cleaning solution as shown in Figure. 17

The tank is made up of stainless steel. The ultrasonic transducer is mounted at the bottom of the tank. The size and frequency of the transducer selection depend on the capacity of the tank. The tank is filled with the cleaning liquid which may be either aqueous or solvent. When the ultrasonic generator is switched on, it provides a suitable a.c. signal to the transducer and thus, the transducer generates the ultrasonic waves of desired frequency. These sound waves pass into the liquid medium within the tank and the process of cavitation occurs within the liquid. Due to this cavitation process fine particles on objects are pulled into the space releasing them from the object and they become suspended in the liquid by the soap. To increase the effect of the ultrasonic cleaning, the cleaning medium should have low surface tension, low viscosity etc. Advantages of ultrasonic cleaning 1. Environmentally pollution free and less cost. 2. Enhanced cleaning speed for both solvent and aqueous medium. 3. Cleaning is consistency and of higher quality. 4. High safety with less complaints. 5. It occupies less space and reduces labour. 18