Physics:ULTRASONIC AND APPLICATION

UNIT-III                    ULTRASONICS AND APPLICATIONS

Introduction-Properties-Production: Magnetostriction effect, magnetostriction generator-  piezoelectric effect, piezoelectric generator – Ultrasonic detection- acoustical grating-Applications:  Cavitation, cleaning, SONAR,– Non destructive testing: Pulse echo system, through transmission,  resonance system- Medical applications: cardiology, neurology, ultrasonic imaging (A, B and TM- Scan).

3.1  INTRODUCTION

The sound waves above the frequency of 20000 Hz (20 KHz) are known as ultrasonic waves. Ultrasonic waves are inaudible to humans, because. their high frequencies are unable to vibrate the human ear drum. The ultrasonic waves are high frequency sound waves smaller wave length. Its wave length at room temperature is 0.0175 m. Since they are sound waves, they require a material medium to travel and have most of the properties of sound waves.

In addition to this, the ultrasonic waves are having high energy and it helps in large number of applications in engineering and medical fields. The ultrasonic waves are usually generated by the application of magnetostriction effect or piezo electric effect. The piezo electric generator is more efficient than the magnetostriction generator.

3.2  PRODUCTION OF ULTRASONICS

1. MAGNETOSTRICTION OSCILLATOR.

Magnetostriction Effect

Principle: When an alternating magnetic field is applied to a rod of ferromagnetic material such as nickel, iron, cobalt, then the rod is thrown into longitudinal vibrations producing ultrasonic waves at resonance.

Construction:

The magnetostriction generator consists of a ferromagnetic rod clamped at the centre. The two ends of the rod are wound by coils l₁ and l₂. The coil l₁ is connected to the collector output of the transistor and the coil l₂ is connected to the base of the transistor as shown. The frequency of the oscillatory circuit can be adjusted by the capacitor C₁ connected across the coil l₁. a battery is connected in the circuit that acts as a source.

Working:

The battery is switched on and oscillating current is produced by the collector of the transistor. This current is passed through the coil L₁ which magnetizes the ferromagnetic rod. The magnetization changes due to the oscillating current from collector and the rod begins to vibrate. This is due to magnetostriction effect produced on the ferromagnetic rod. The magnetization of rod causes an induced emf in the neighboring coil L₂ due to mutual induction and the induced current is given to the base of transistor as positive feedback. Hence the oscillations are sustained without being damped. Now the rod is capable of vibrating with its natural frequency. Resonance condition: when the frequency of oscillatory circuit is adjusted with the help of capacitor to match the natural frequency of the rod, then resonant condition is said to have been achieved and ultrasonic waves are produced from the two ends of the rod.

Condition for resonance:

 Frequency of the oscillatory circuit = frequency of the vibrating rod

  1/2π √L1C1=1/2l√y/p

Where ‘l’ is the length of the rod

‘y’ is the young’s modulus of the rod

‘ρ’ is the density of the rod

Merits:

1. The oscillatory circuit is easy to construct.

2. The experiment is inexpensive

3. It can produce frequency up to 3 MHz

Demerits:

1. It cannot produce frequency above 3 MHz.

2. It cannot produce a stable frequency at the output

3. As frequency is inversely proportional to length of rod, frequency cannot be increased as

length of rod cannot be reduced.

2. PIEZO ELECTRIC OSCILLATOR

Piezoelectric effect:

When crystals like quartz or tourmaline are stressed along any pair of opposite faces, electric charges of opposite polarity are induced in the opposite faces perpendicular to the stress. This is known as piezoelectric effect.

Inverse piezoelectric effect:

When an alternating e.m.f is applied to the opposite faces of a quartz or tourmaline crystal it undergoes contraction and expansion alternatively in the perpendicular direction. This is known as inverse piezoelectric effect. This is made use of in the piezoelectric generator.

Piezoelectric generator:

A slab of piezoelectric crystal is taken and using this parallel plate capacitor is made. Then with other electronic components an electronic oscillator is designed to produce electrical oscillations > 20 kHz. Generally one can generate ultrasonic waves of the order of MHz using piezoelectric generators. Quartz slabs are preferred because it possesses rare physical and chemical properties. a typical circuit diagram is given below.

The tank circuit has a variable capacitor 'c' and an inductor 'l' which decides the frequency of the electrical oscillations. When the circuit is closed current rushes through the tank circuit and the capacitor is charged, after fully charged no current passes through the same. Then the capacitor starts discharging through the inductor and hence the electric energy is in the form of electric and magnetic fields associated with the capacitor and the inductor respectively.

Thus we get electrical oscillations in the tank circuit and with the help of the other electronic components including a transistor, electrical oscillations are produced continuously. This is fed to the secondary circuit and the piezoelectric crystal (in our case a slab of suitably cut quartz crystal) vibrates, as it is continuously subjected to varying (alternating) electric field, and produces sound waves. When the frequency of electrical oscillations is in the ultrasonic range then ultrasonic waves are generated. When the frequency of oscillation is matched with the natural frequency of the piezoelectric slab then it will vibrate with maximum amplitude.

Condition for resonance

Frequency of the oscillatory circuit = frequency of the vibrating crystal

   1/2π √L1C1=p/2l√y/p

Where ‘l’ is the length of the rod

‘y’ is the young’s modulus of the rod

‘ρ’ is the density of the rod

p= 1, 2, 3 ----etc. for fundamental, first overtone, second overtone etc, respectively.

Advantages

1. It is more efficient than magnetostriction oscillator.

2. Ultrasonic frequencies as high as 5x10⁸ hertz (500 mhz) can be obtained with this

ocillator.

Demerits

1. The cost of piezo electric quartz is very high.

2. Cutting and shaping of quartz crystal are very complex.

3. This oscillator is not affected by temperature

and humidity. 

Difference  between  magnetostriction & piezo- electric method:
S. no    Magnetostriction Method	Piezo-electric method
1              We cannot obtain constant frequency of ultrasonic waves.	We can obtain constant frequency ultrasonic waves.
2              It generates law frequency of ultrasonic waves (3 MHz)	It generates law frequency of ultrasonic waves (500 MHz)
3              The peak of resonance curve is broad.	The peak of resonance curve is narrow.
4              Frequency of oscillation depends on temperature.	Frequency of oscillation depends on Properties of ultrasonic waves

3.3  Properties of Ultrasonic waves:

1. The ultrasonic waves are high frequency sound waves.

2. They are having smaller wavelength.

3. They produce heating effect when passes through the medium.

4. They get reflected, refracted and absorbed by the medium similar to the ordinary sound waves.

5. They act as catalytic agents to accelerate chemical reactions.

6. They produce stationary wave pattern in the liquid while passing through it.

3.4  ACOUSTIC GRATING

       VELOCITY OF ULTRASONIC WAVES IN LIQUID MEDIUM

When ultrasonic waves are passed through a liquid, the density of the liquid varies layer by layer due to the variation in pressure and hence the liquid will act as a diffraction grating, so called acoustic grating. Under this condition, when a monochromatic source of light is passed through the acoustical grating, the light gets diffracted. Then, by using the condition for diffraction, the velocity of ultrasonic waves can be determined.

Construction & Working:

The liquid is taken in a glass cell. The Piezo-electric crystal is fixed at one side of the wall inside the cell and ultrasonic waves are generated. The waves travelling from the crystal get reflected by the reflector placed at the opposite wall. The reflected waves get superimposed with the incident waves producing longitudinal standing wave pattern called acoustic grating. If light from a laser source such as He-Ne or diode laser is allowed to pass through the liquid in a direction perpendicular to the grating, diffraction takes place and one can observe the higher order diffraction patterns on the screen. The angle between the direct ray and the diffracted rays of different orders (θn) can be calculated easily.

According to the theory of diffraction,

d sin θ = n λ -----(1)

d – Distance between any two successive nodes or antinodes of stationary waves

n – Order of diffraction

λ – Wavelength of the monochromatic light used

θn – Angle of diffraction for nth order

where n = 0, 1, 2, 3, … is the order of diffraction,

λ is the wavelength of light used and d is the distance between two adjacent nodal or anti-nodal planes. Let λm be the wavelength of ultrasonic waves in the liquid medium, then

2d = λm

d = λm/2 ------- (2)

From equation 1 and 2, we have

(λm/2)sinθn = nλ

λm= 2nλ/ sinθn .......(3)

Knowing the wavelength of monochromatic light and measuring θn the wavelength of ultrasonic waves in a given liquid medium is calculated.

Velocity of ultrasonic waves ν is known, then the velocity of ultrasonic waves in the liquid is found from the relation.

v = νλm --------- (4)

From equation 3 and 4, we have

v = 2nλν/sinθn

Experimental Arrangements

In an ultrasonic cell (Fig.), the liquid under study is taken. Ultrasonic transducer is fixed at one side of wall inside the cell and ultrasonic waves are generated. The sound waves travelling from the transducer get reflected from the opposite wall and standing wave pattern is produced. Thus, acoustical grating is formed. The light from diode laser is passed to pass through the liquid in a direction perpendicular to the acoustic grating. Now, the light gets diffracted and diffracted image can be seen on the screen. The diffraction angles for different orders are measured by using the distance of diffracted beams from the central beam.

Using the given formula, the wavelength and velocity of ultrasonic waves in the liquid are calculated

Wavelength λm = 2nλ/sinθn

Velocity v = 2nλν/sinθn

3.5  DETECTION OF ULTRASONIC WAVES:-

(1) Kundt’s tube method:

Ultrasonic waves can be detected with the help of Kundt’s tube. At the nodes, lycopodium powder collects in

the form of heaps and blown off at the anti nods.  

Demerit:

1. This method cannot be used if the wavelength of ultrasonic waves is very small i.e., less than few mm.

2. In the case of a liquid medium, instead of lycopodium powder, powdered coke is used to detect the position

    of nodes.

(2) Sensitive flame method:

A narrow sensitive flame is moved along the medium. At the positions of antinodes, the flame is steady. At the positions of nodes, the flame flickers because there is a change in pressure. In this way, positions of nodes and antinodes can be found out in the medium.

(3) Thermal detection method:

•         

This is the most commonly used method of detection of ultrasonic waves. In this method, a fine platinum wire is used. This  wire is moved through the medium.  Due to alternate compressions ad rarefactions, adiabatic  changes in temperature takes place.

The resistance of the platinum wire changes with respect to time. This can be detected with the help of  bridge arrangement. At the position of nodes, there is one value for temperature while at the antinodes, it will have some other value. i.e., the variation of temperature exists. This will be indicated by the undisturbed balanced position of the bridge.

(4) Quartz crystal method:

         This method is based on the principle of Piezo-electric effect. When one pair of the opposite faces of a quartz crystal is exposed to the ultrasonic waves, the other pairs of opposite faces developed opposite charges. These charges are amplified and detected using an electronic circuit.

3.6  Applications of ultrasonic waves:

Ultrasonic waves have a large number of practical applications.

1. The ultrasonic waves are used to find the depth of the sea, distance and direction of submarine   and    

     depth of the rocks in the sea.

2. They are used in non-destructive testing to detect cracks in the materials, welding, castings etc.

3. They are used in the ultrasonic microscope to detect concealed objects.

4. They are used to heat the substance. 5. They are used to bore holes in steel and other hard metals.

6. They are used to accelerate chemical reactions.

7. They are used to solder the aluminium. In ordinary soldering method aluminium cannot be soldered.

8. They are used in the mixing of alloys of different compositions.

9. They are broadly used in medical field such as to detect tumours, abnormal growth, cancer, broken

     teeth, relieving neuralgic pains, head ache, body massage action and also used in bloodless surgery.

10. They are used in the sterilization of water and milk.

3.7  ENGINEERING APPLICATIONS:

Ultrasonic Drilling

ü  Ultrasonics are used for making holes in very hard materials like glass, diamond etc.

ü  For this purpose, a suitable drilling tool bit is fixed at the end of a powerful ultrasonic generator.

ü  Some slurry (a thin paste of carborundum powder and water) is made to flow between the bit and the plate in which the hole is to be made.

ü  Ultrasonic generator causes the tool bit to move up and down very quickly and the slurry particles below the bit just remove some material from the plate.

ü   This process continues and a hole is drilled in the plate.

ü 

Ultrasonic welding

•        

ü  The properties of some metals change on heating and  therefore, such metals cannot be welded by electric or gas welding.

ü  In such cases, the metallic sheets are welded together at room temperature by using ultrasonic waves.

ü  For this purpose, a hammer H is attached to a powerful ultrasonic generator as shown in Figure

ü  The metallic sheets to be welded are put together under the tip of hammer H. 

ü  The hammer is made to vibrate ultrasonically.  As a result, it presses the two metal sheets very rapidly and the molecules of one metal diffuse into the molecules of the other. 

ü  Thus, the two sheets get welded without heating.  This process is known as cold welding.

Ultrasonic soldering

ü  Metals like aluminium cannot be directly soldered. However, it is possible to solder such metals by ultrasonic waves.

ü  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. 

ü  The tip of the soldering iron melts solder on the aluminium and the ultrasonic vibrator removes the aluminium oxide layer.The solder thus gets fastened to clear metal without any difficulty.

Ultrasonic cutting and machining

ü  Ultrasonic waves are used for cutting and machining

Ultrasonic cleaning      

ü  It is the most cheap technique employed for cleaning various parts of the machine, electronic assembles, armatures, watches etc., which cannot be easily cleaned by other methods.

3.7  CAVITATION:                               

In general, cavitation is the phenomenon where small and largely empty cavities are generated in a fluid, which expand to large size and then rapidly collapse.  When the cavitation bubbles collapse, they focus liquid energy to very small volume. Thereby, they create spots of  high temperature and emit shock waves. The collapse of cavities involves very high energies. Power ultrasound enhances chemical and physical changes in a liquid medium through the generation and subsequent destruction of cavitation bubbles.

ü  Ultrasound series of compression and rarefaction waves.

ü  At high ultrasonic power rarefaction exceed the attractive forces of the molecules of the liquid cavitation bubbles will form

ü   The bubbles grow due to diffusion i.e. small amounts of vapour (or gas) from the medium enters the bubble during its expansion phase. #The bubbles grow over the period of a few cycles to an equilibrium size for the particular frequency applied.

ü   After crossing their stability, they collapse in succeeding compression cycles which generates the energy for chemical and mechanical effects

ü   At an ultrasonic frequency of 20 KHz, each cavitation bubble collapse actsas a localised "hotspot" generating shock waves with temperatures of about 4,000 K and pressures in excess of 1000 atmospheres

SONAR

ü  SONAR is a technique which stands for Sound Navigation and Ranging.

ü  It uses ultrasonics for the detection and identification of under water objects.

ü  The method consists of sending a powerful beam of ultrasonics in the suspected direction in water.

ü  By noting the time interval between the emission and receipt of beam after reflection, the distance of the object can be easily calculated.

ü  The change in frequency of the echo signal due to the Doppler effect helps to determine the velocity of the body and its direction.

ü  Measuring the time interval (t)  between the transmitted pulses and the received pulse, the distance    

                             d=vt /2                      

             determined using the formula.

ü  where v is the velocity of sound in sea water.           

ü  The same principle is used to find the depth of the sea

ü  Applications of SONAR

ü  Sonar is used in the location of shipwrecks and submarines    on the bottom of the sea.

ü  It is used for fish-finding application .

ü  It is used for seismic survey.

MEDICAL APPLICATIONS:

ULTRASONICS IN CARDIOLOGY :

Cardiology is a branch of medicine dealing with disorders of the heart beat of human or animal. The field includes medical diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease and electrophysiology. Physicians who specialize in this field of medicine are called cardiologists, a specialty of internal medicine. Pediatric cardiologists are pediatricians who specialize in cardiology. Physicians who specialize in cardiac surgery are called cardiothoracic surgeons or cardiac surgeons, a specialty of general surgery. ECHOCARDIOGRAM, often referred to as a cardiac echo or simply an echo, is a sonogram of the heart.

Sonogram

The principle and working of sonogram.

A sonogram is a medical procedure that uses ultrasound waves to create a picture of something that is happening within a person’s body. This is a very common procedure in pregnancy, and is what  produces the black-and-white fatal by using high frequency ultrasonic sound waves.

Working

The heart sounds are converted into electrical signals by means of a heart microphone fastened to the chest wall by an adhesive strip. The electrical signals from microphone are amplified by a phonocardiography preamplifier followed by suitable filters and recorder. Further, the electrodes are also placed on the limps to pick up the electrical activity of the heart and these signals are amplified and recorded.

The Doppler ultrasonic methods provide diagnostic information about the motion of fetal heart, umbilical cord and placenta. The required apparatus consists of a transmitter which emits a continuous ultrasonic wave towards the fetal and a receiver which picks up the reflected waves (fig.) ultrasonic waves of frequency of about 2 MHz is used in this apparatus.

A small amount of gel is placed between the probe and the abdominal wall to obtain good acoustic coupling. When a continuous ultrasonic wave of frequency f is incident upon the fetal heart the reflected wave will have a higher frequency f' if the fetal heart is moving towards the source of sound and a lower frequency f' if the fetal heart moves away from the source of sound. The fetal heart is determined from the variations in the frequency.

The variation of frequency is amplified and can be heard with a loud speaker or displayed in an oscilloscope.  With this method the presence of blood flow in the fetus and mother can be determined. The blood flow in mothers blood vessels can be distinguished from that in the fetus due to differences in pulse rate. Different parts of the fetus can be examined, from the blood stream velocity in different part of the fetus.

ULTRASONICS IN NEUROLOGY :

Neurology  is a branch of medicine dealing with disorders of the nervous system. Neurology deals with the diagnosis and treatment of all categories of conditions and disease involving the central and peripheral nervous system.

3.10  NON DESTRUCTIVE TESTING:

PULSE ECHO SYSTEM, THROUGH TRANSMISSION,  RESONANCE SYSTEM

The process of non-destructive testing of materials using ultrasonic waves by pulse

Echo method.

It is a method of testing a material without destructing or damaging the material. In this method, radiations like X-rays or ultrasonic is passed through the material.

Basic Principle

It is based on the principle of reflection of ultrasound (echo) at the interfaces. The block diagram for the ultrasonic flaw detector is shown in the figure. It consists

i) High frequency generator

ii) Cathode Ray Oscilloscope (CRO)

iii) Transmitting and receiving probes

The ultrasonic flaw detector uses two separate probes, one for transmitting ultrasonic waves and other to receive them after passing through a specimen.

Working

The high frequency generator excites the transmitting probe and it produces ultrasonic sound pulses. These sound pulses are sent into specimen. They strike the upper surface of the specimen and produce a sharp pip (echo) at the left hand side of the CRO screen (Fig.). The ultrasonic sound pulses passes through the different interfaces of the specimen. If the specimen is the good without any defects, this sound wave strikes the bottom surface of the specimen and reflects back. This produces a pip on the right hand side of the CRO screen. Whenever a defect is present between top and bottom surface of the specimen, the most of the ultrasonic sound pulses strike this defect and they are reflected. The reflected sound pulses from the defect reach the receiver probe earlier than back echo.

 This is indicated by a pip on the CRO screen in between left and right.

The time interval (t) between transmission and reception of the sound signal after reflection is measured by using CRO. If the velocity (v) of sound waves in the specimen is known, then the depth of defect from the surface of the specimen (d) is determined by using the relation

                              d=vt/2

The exact size and shape of the defect can be found by examining the specimen from all directions. During the testing both transmitting and receiving probes are moved on the surface of the specimen. The echo pattern appearing on the screen of CRO is studied.

3.11  ULTRASONIC IMAGING (A, B AND TM- SCAN).

Information of pulse echo inspection is displayed on the ultrasonic imaging devices. The type of ultrasonic imaging devices are classified depending on the type of display methods.

They are i). A-Scan display ii). B-Scan display iii). C-Scan display

A-Scan display

This is the simplest form of display. It gives only one dimensional information. A mode

means amplitude modulation. In this mode the reflected echoes are displayed as vertical spikes along a horizontal base line of oscilloscope. The height of the vertical spikes corresponds to the strength of the echo. The position of the spikes along the horizontal axis gives depth of the flaw from the probe.

The depth of the flaw is determined by measuring the time interval between the spikes corresponding to incident pulse and echo pulse from the defect of the specimen.

Advantages:

a) The size and location of the defect can be determined

b) The velocity of ultrasonic wave in materials can be determined of known thickness of the

specimen.

c) Attenuation of sound in the materials can be studied.

d) Thickness of the specimen can be measured by knowing the velocity of sound in the

material

B-Scan

B-scan mode display gives a two-dimensional image. In this technique the transducer is moved on the surface of the specimen. The reflected echoes from various points are displayed as dots on the screen (Fig.)

The brightness and size of the dot depend on the intensity of the reflected echo pulses. Thus B-scan provides exact image of the internal structure of the specimen.

Advantages:

a) The depth and shape of the defect can be found by this method

b) This method gives permanent record of the image of the defect

Disadvantages:

a) It is costlier than A-scan

b) Actual size of the defect is not obtained in B-scan

C-scan (TM SCAN)

In this type of display, the energy of the ultrasonic is adjusted such that the ultrasonic pulse can reach a particular depth from the surface of the specimen. The cross-section of the specimen at that depth is scanned. For this purpose the ultrasonic probe is connected to an X-Y plotter. It is moved over the surface of the specimen in either zigzag fashion or in closely placed parallel lines(Fig.) The intensity of echo received from the section of the specimen is recorded as shading with spaces corresponding to defect regions. The position and cross-sectional area of the defects are obtained. The depth of the defect is not recorded in this method.

Advantages:

a) The position and cross-sectional area of the defect are recorded

b) It is an automatic method

Disadvantages:

a) The depth of the defect information cannot be determined

b) It involves high cost compared to other types of scanning