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ULTRASONICS Production of ultrasonics by magnetostriction and piezoelectric methods - acoustic grating -Non Destructive Testing - pulse echo system through transmission and reflection modes - A,B and C - scan displays, Medical applications — Sonogram INTRODUCTION * The word ultrasonic combines the Latin roots ultra, meaning ‘beyond’ and sonic, or sound. * The sound waves having frequencies above the audible range i.e. above 20000Hz are called ultrasonic waves. * Generally these waves are called as high frequency waves. * The broad sectors of society that regularly apply ultrasonic technology are the medical community, industry and the military. PROPERTIES e They have high energy content. e Just like ordinary sound waves, ultrasonic waves get reflected, refracted and absorbed. e They can be transmitted over large distances with no appreciable loss of energy. e If an arrangement is made to form stationary waves of ultrasonics in a liquid, it serves as a diffraction grating. It is called an acoustic grating. e They produce intense heating effect when passed through a substance. PRODUCTION OF ULTRASONIC WAVE Ultrasonic waves are produced by the following methods. (1) Magneto-striction generator or oscillator (2) Piezo-electric generator or oscillator Dr. P. Venkata Ramana, AUCE(A) (1) MAGNETO-STRICTION GENERATOR Principle: Magnetostriction effect: When a ferromagnetic rod like iron or nickel is placed in a magnetic field parallel to its length, the rod experiences a small change in its length. This is called magnetostriction effect. The change in length (increase or decrease) produced in the rod depends upon the strength of the magnetic field, the nature of the materials and is independent of the direction of the magnetic field applied. Construction * The experimental arrangement is shown in Figure. x v Y Ultrasonic Ultrason! waves he A by id ++ waves Hy * XY is a rod of ferromagnetic materials like iron or nickel. The rod is clamped in the middle * The alternating magnetic field is generated by electronic oscillator. * The coil Li wound on the right hand portion of the rod along with a variable capacitor C. * This forms the resonant circuit of the collector tuned oscillator. The frequency of oscillator is controlled by the variable capacitor. * The coil Lz wound on the left hand portion of the rod is connected to the base circuit. The coil L2 acts as feed —back loop. Working + When High Tension (H.T) battery is switched on, the collector circuit oscillates with a frequency, f= 1 22/L,C Dr. P. Venkata Ramana, AUCE(A) * Ifan 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 effect is known as inverse piezo electric effect. Construction The circuit diagram is shown in Figure Le : sl -L Ls el B [crystal plate Li oC The quartz crystal is placed between two metal plates A and B. The plates are connected to the primary (Ls) of a transformer which is inductively coupled to the electronics oscillator. The electronic oscillator circuit is a base tuned oscillator circuit. The coils Li and Lz of oscillator circuit are taken from the secondary of a transformer T. The collector coil Lz is inductively coupled to base coil Li. The coil Li and variable capacitor C; form the tank circuit of the oscillator. Working When H.T. battery is switched on, the oscillator produces high frequency alternating voltages 1 with a frequency. l= 22 L,C, Due to the transformer action, an oscillatory e.m.f. is induced in the coil L3. This high frequency alternating voltages are fed on the plates A and B. Inverse Piezo-electric effect takes place and the crystal contracts and expands alternatively. The crystal is set into mechanical vibrations. The frequency of the vibration is given by Pp fy rh Dr. P. Venkata Ramana, AUCE(A) where P= 1,2,3,4... etc. for fundamental, first over tone, second over tone etc., Y = Young’s modulus of the crystal and p = density of the crystal. * The variable condenser C; is adjusted such that the frequency of the applied AC voltage is equal to the natural frequency of the quartz crystal, and thus resonance takes place. * The vibrating crystal produces longitudinal ultrasonic waves of large amplitude. 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. * Itis 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. ACOUSTING GRATING Principle: e 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. e 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: e 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. e 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. e 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. Dr. P. Venkata Ramana, AUCE(A) NON DESTRUCTIVE TESTING What is NDT? e Nondestructive testing is a method of finding defects in an object without harming the object. Most Common NDT Methods e Visual Inspection Method e Liquid Penetrant Method ¢ Magnetic Particle Inspection e Ultrasonic Flaw Detection e Eddy Current Testing X-Ray Diffraction Method How is ultrasound used in NDT? Sound with high frequencies, or ultrasound, is one method used in NDT. 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. Basically, ultrasonic waves are emitted from a transducer into an object and the returning waves are analyzed. If an impurity or a crack is present, the sound will bounce off of them and be seen in the returned signal. There are two methods of receiving the ultrasound waveform: G) attenuation (or through-transmission) and (ii) reflection (or pulse-echo) mode THROUGH TRANSMISSION METHOD Through transmission was used in the early days of ultrasonic testing and is still used in plate and bar production. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Two transducers located on opposing sides of the test specimen are used. One transducer acts as a transmitter, the other as a receiver. A probe on one side of a component transmits (T) an ultrasonic pulse to a receptor (R) probe on the other side. The absence of a pulse coming to the receiver indicates a defect. 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. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces. a Advantages 1. Less attenuation of sound energy 2. No probe ringing 3. No dead zone on the screen 4. The orientation of a defect does not matter in the way that it does on the pulse echo display. Disadvantages 1. The defect cannot be located 2. The component surfaces must be parallel 3. Vertical defects do not show 4. Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information 5. There must be access to both sides of the component. 
