Ultrasonic Waves- Lecture notes MODULE IV PDF

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Summary

Production of ultrasonics- Magnetostriction method – Piezoelectric
method- Properties of ultrasonics- Nondestructive Testing- Applications...

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Ultrasonic Waves INTRODUCTION: Ultrasonic or supersonic waves are sound waves with frequencies above the audible range (i.e., frequencies greater than 20 kHz). Infrasonic waves are sound waves with frequencies under the audible range (i.e., 20 Hz). Ultrasonic waves have extremely short wavelengths, which account for many of its fascinating applications, including fault identification, drilling, welding, soldering, cleaning, marine applications, medical diagnostics, non-destructive testing of finished products, and so on.

Production of Ultrasonic waves  Only two main ways for creating ultrasonic waves exist, both of which are based on two separate phenomena: magnetostriction and the piezoelectric effect.  The magnetostriction approach is used to create waves in the 20 kHz to 100 kHz frequency range, whereas the piezoelectric method is used to create waves with frequencies more than 100 kHz.

Magnetostriction Method: Principle, Construction, Working, Advantages, and Limitations Principle: When a ferromagnetic rod, such as nickel, is magnetized. Longitudinally, it changes length by a very little amount. This is referred to as the Magnetostriction effect.

Construction: The magnetostriction ultrasonic generator circuit diagram is depicted in the fig below. Between two knife edges, a short permanently magnetized nickel rod is fastened. On the right-hand section of the rod, a coil L1 is coiled. A variable capacitor is denoted by the letter C. The collector-tuned oscillator's resonant circuit is made up of L1 and C1. The base circuit is connected to coil L2, which is coiled on the rod's LHS. As a feedback loop, coil L2 is used.

Working: The resonant circuit L1C1 sets up an alternating current o frequency when the battery is turned on.

Along the length of the nickel rod, this current flowing around coil L1 produces alternating magnetic fields of frequency f. The Magnetostriction effect causes the rod to vibrate. Ultrasonic waves are created by the rod's oscillations. The coil L2 produces an E.M. due to the rod's longitudinal expansion and contraction. This e.m.f. is applied to the transistor's base. As a result of positive feedback, the amplitude of high-

frequency high oscillations in coil L1 is enhanced. By changing the capacitor, the produced alternating current frequency can be aligned with the rod's inherent frequency. Condition for Resonance:

Frequency of the oscillator circuit = Frequency of the vibrating rod

Where 'l' is the rod's length, 'E' is the rod's Young's modulus, and ‘ρ’ is the density of the rod's material. The rise in collector current seen in the milliammeter indicates the resonance state. Advantages: They can generate a significant amount of acoustic power while maintaining a high level of efficiency. The cost of construction is minimal. Magnetostriction Mechanically, oscillators are tough. Limitations The resonance curve's breadth is quite broad. It is caused by the vibrations of ferromagnetic material's elastic constants as the degree of magnetization increases. As a result, we won't be able to achieve a steady single frequency. It can only generate frequencies up to 3MHz. The temperature affects the frequency of oscillations.

Pierce oscillator: G W. Pierce was the first to develop an ultrasonic oscillator based on the magnetostriction phenomena. Fig. 1.2.2 shows a circuit diagram for a magnetostriction ultrasonic generator employing transistors. It's a Colpitts oscillator in essence. The resistances R1, R2, R3, and R4 are used to bias the transistor T. The tank circuit is made up of the inductance Land capacitors C2 and C3. Oscillations grow up in the tank circuit when the circuit is turned on. Through the feedback capacitor C6, the oscillations are sent back to the transistor base.

The oscillations corresponding to the relevant frequency are enhanced and sustained. The oscillations that emerge at the oscillator circuit's output terminals are supplied to a current amplifier, which amplifies the level of the oscillations. The current amplifier's output is sent to the magnetostriction coil through a coupling capacitor C5• The magnetostriction coil generates ultrasonic waves in response to the high-frequency electrical signal. The tank circuit oscillation frequency can be adjusted by changing the values of capacitors C2 and C3, which in turn changes the frequency of the ultrasonic waves at the output.

Principle, Construction, Working, Advantages, and Disadvantages

Piezo Electric Crystals Piezoelectric crystals are crystals that produce piezoelectric and Piezoelectric effects in the same way. Quartz, Tourmaline, Rochelle Salts, and other minerals are examples. Fig 1.4.1 represents the typical example of a piezoelectric crystal (Quartz). It's hexagonal, with pyramids on each end. It is made up of three axes. Viz., (i) The optical Z-axis, which connects the pyramid's edges. (ii) Electrical axis (X-axis), which connects the hexagon's corners, and (ii) Mechanical axis, which connects the hexagon's center or sides, as illustrated.

X-cut and Y cut crystals X-Cut crystal: An X-cut crystal has been cut perpendicular to the X-axis, as seen in figure 1.4.2. Longitudinal ultrasonic waves are usually generated using X-cut crystals. Y-Cut Crystal: A Y-sliced crystal is one in which the crystal is cut perpendicular to the Y-axis, as seen in figure 1.4.3. Y-Cut crystals emit transverse ultrasonic waves in general. Piezoelectric Effect Definition: A potential difference is created across the electrical axis concerning the optical axis when mechanical stress is applied to the mechanical axis concerning the optical axis. Inverse Piezoelectric Effect: Definition: When an alternating electric field is supplied to the electrical axis about the optical axis, the mechanical axis expands or contracts concerning the optical axis.

Production of Ultrasonic waves – Piezo Electric Effect Principle: The Inverse piezoelectric effect is the basis for this. A quartz crystal is set into elastic vibrations along its mechanical axis when it is subjected to an alternating potential difference along the electric axis. The vibrations will be large in amplitude if the frequency of electric oscillations matches the natural frequency of the crystal. The crystal creates ultrasonic waves if the frequency of the electric field is in the ultrasonic frequency range. Construction: Figure 1.5 illustrates the circuit diagram. The circuit is a base-turned oscillator. A slice of Quartz crystal is sandwiched between metal plates A and B, forming a parallel plate capacitor with the crystal serving as the dielectric. This is connected to the electronic oscillator via the transformer's primary coil L3. The primary transformer is made up of coils L2 and L1 from the oscillator circuit. The base coil L1 is inductively connected to the collector coil L2. The oscillator's tank circuit is made up of coil L1 and variable capacitor C.

Working: The oscillator produces high-frequency oscillations when the battery is turned on. Due to transformer action, an oscillating e.m.f is induced in coil L3. As a result, the crystal is now exposed to the alternating voltage at a high frequency. The capacitance of C1 is changed until the frequency of oscillations produced is in tune with the crystal's inherent frequency. Due to resonance, the crystal now vibrates with a higher amplitude. As a result, high-power ultrasonic waves are generated. Condition for Resonance: Frequency of the oscillator circuit = Frequency of the vibrating crystal

Where 'l' is the rod's length, 'E' is the rod's Young's modulus, and " ρ is the density of the rod's material. For fundamental, first overtone, second overtone, and so on, use ‘P’ = 1,2, 3,…. Advantages:  It is possible to generate ultrasonic frequencies as high as 500MHz.  The power output is high. Temperature and humidity do not affect it.  It outperforms the Magnetostriction oscillator in terms of efficiency.

 The resonance curve's breadth is relatively narrow. As a result, we can obtain ultrasonic waves with a stable and consistent frequency. Disadvantages:  The quartz crystal is quite expensive.  The process of cutting and sculpting the crystal is fairly difficult.

Piezoelectric oscillator: Langevin was the first to construct an ultrasonic wave generator based on piezoelectric phenomena in 1917. Figure 1.2.5 shows a circuit diagram for a piezoelectric ultrasonic generator utilizing a transistor. It's a Hartley oscillator in essence. The network of resistances R1, R2, R3, and R4 is used to bias the transistor T.

The rotating circuit is made up of the coils L1, L2, and the capacitor C4. The coupling capacitor C2 connects the tuning circuit to the amplifier T. The amplifier receives positive feedback from capacitor c3. The tuning circuit's oscillations are sustained, and the electrical signal acquired at the output is applied to the piezoelectric crystal's electrodes via the coupling capacitor C5. The piezoelectric crystal generates ultrasonic waves when a high-frequency electrical pulse is applied to it. The frequency of these ultrasonic waves can be changed by changing the values of the tuning circuit's components.

Ultrasonic Nondestructive Testing Principle: The transmission of Ultrasound through the medium and its reflection or scattering at any surface or internal imperfection in the medium due to a change in acoustic

impedance is the underlying idea underpinning ultrasonic inspection. The term "discontinuity" refers to the presence of a fault, such as a detectable flaw, cracks, or a hole in the material. The reflected or scattered sound waves are picked up and amplified, allowing the faults in the specimen to be properly identified.

Block diagram of the Ultrasonic Flaw detector Principle: When the medium changes, the Ultrasonic waves are reflected back to the source. Ultrasonic defect detectors work on this premise. As a result of the intensity of the reflected echoes, faults can be found without harming the material, making this a nondestructive technology. Working: a. The pulse generator produces high-frequency waves, which are then applied to the Piezo-electric transducer and recorded in the CRO. b. Ultrasonic waves are produced by resonating piezoelectric crystals. c. Ultrasonic waves are passed through the specimen in question. d. These waves pass through the specimen and are reflected at the opposite end. e. The transducer receives the reflected Ultrasonic and converts it to electric impulses. The CRO amplifies the reflected impulses and records them. f. If the reflected pulse is the same as the transmitted pulse, the specimen is free of defects. g. If the specimen has a flaw, such as a small hole or pores, the Ultrasonic will be reflected by the holes (i.e. faults) due to a change in the medium. h. The position of the hole can be determined by measuring the time delay between sent and received pulses. The depth of the hole can also be calculated based on the height of the pulse received.

PROPERTIES OF ULTRASONIC WAVES: 1. As the frequency of ultrasonic waves increases, so does the speed of propagation. 2. The wave's wavelength is extremely short; thus, the diffraction impact is minimal. 3. As a result, they may be transported over long distances with minimal energy loss. 4. They have a lot of energy. Ultrasonic waves can attain intensities of up to 10 KW/m2 due to the high frequencies involved. In most cases, intensities of 1 to 2 KW/m2 are used. 5. When ultrasonic vibrations are transmitted across a liquid medium, alternate areas of rarefaction and compression are created. Negative local pressure at the rarefaction site promotes local boiling of the liquid, as well as the growth and collapse of bubbles. Cavitation is the term for this occurrence. 6. Due to the reflection of ultrasonic waves from the opposite end, a stationary wave pattern is generated when they are propagated into a liquid bath. As a result, the density of the liquid fluctuates from layer to layer along the propagation route. This produces a planar diffraction grating that can diffract light.

DETERMINATION OF WAVELENGTH AND VELOCITY OF ULTRASONIC: Debye and Sears in America were the first to notice light diffraction caused by ultrasonic vibrations flowing through a liquid in 1932. The density of ultrasonic waves propagating in a liquid varies from layer to layer due to periodic pressure variations. The liquid behaves as a diffraction grating when monochromatic light is sent through it at right angles to the waves in this condition. Acoustic grating is the name for this type of grating. This grating functions similarly to a ruled grating. As a result, this method can be used to determine the wavelength and velocity of ultrasonic waves propagating through the liquid. The acoustic diffraction method is the name for this technique.

The experimental setup for determining the wavelength and velocity of ultrasonic waves is shown in Fig. 1.2.6. In a liquid confined in a glass tube, stationary ultrasonic waves are generated. The density of a liquid, and thus its refractive index, is highest at nodal points and lowest at antinodal points. As a result, nodal areas act as opaque regions, and antinodal areas act as light-transparent zones. As a result, the liquid column resembles a ruled grating. When the crystal is at rest, the screen forms a single image of the slit. When a crystal is stimulated, it produces a diffraction pattern.

Application of ultrasonic waves Ultrasonic waves are used in two different fields: a)

Engineering filed

b)

Medical field

The following are examples of ultrasonic wave applications in engineering and industry. 1.

Non-destructive testing (detection of flaws in metals)

2.

Ultrasonic drilling

3.

Welding with ultrasound

4.

Drilling with ultrasonics

5.

Soldering with ultrasonic waves

6.

Ultrasonic cutting and equipment

7.

Cleaning with ultrasonic waves

8.

Sonar

The following are examples of ultrasonic wave applications in medicine. 1. 2. 3. 4. 5.

Sonography for diagnosis Cardiogram with ultrasound Ultrasound of the womb Therapeutic ultrasound Blind people can use ultrasonic guiding.

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