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Ultrasonic waves are electromagnetic waves. Ultrasonic waves behave similar to audible sound waves. They can propagate in an elastic medium, which can be a solid, liquid or gas, but not in a vacuum. An ultrasound beam obey a general wave equation. Each travels at a characteristic velocity in a given homogeneous medium, the velocity depends on the medium properties, not on the wave properties. Like beams of light, ultrasonic beams are reflected and refracted when they cross medium (substances) that have different characteristic sound velocities, and diffracted at edges or around obstacles. Scattering by rough surfaces or particles reduces the energy of an ultrasonic beam.
2.2 Wave Propagation
To understand propagation of ultrasonic wave, it is necessary to understand the elementary characteristics of waves primarily. The main features of a simple harmonic wave, is shown in Figure 2.1
where A denotes the amplitude, w = 2pf is the angular frequency, where f is the cyclic frequency, j is the phase angle at x = t = 0, and c denotes the propagation (phase) velocity. Here, u could represent longitudinal or transverse displacement of a string, particle velocity in a solid, pressure wave amplitude in a gas, or a number other physical quantities. The basic wave parameters of propagation, the wavelength l and the period of vibration T are related through c / Î» = 1/ T = f .
Fig 2.1. Simple Harmonic wave
In order to better facilitate algebraic manipulations needed to solve ultrasonic wave propagation problems,. the above equation can be written as follows
where U is a complex amplitude that includes the phase term, and k is wave number.
Three basic types of waves, dilatational, shear and surface, may exist in a material, depending on whether it is solid or fluid, on the nature of its boundaries and mode of displacement. Propagation velocities will depend on the material and may range from 102 m/s to104 m/s.The basic nature of the waves are shown in Fig 2.2
Fig. 2.2 Different wave modes in solid material
Longitudinal waves, sometimes called compression waves, are the type of ultrasonic waves most widely used in the inspection of metals. They travel through metal as a series of alternate compressions and rarefactions in which the particles transmitting the wave vibrate back and forth in the direction of travel of the waves.
Longitudinal ultrasonic waves are readily propagated in liquids and gases as well as in elastic solids. The velocity of longitudinal ultrasonic waves is about 6000 m per sec in steel, 1500 m per sec in water and 330 m per sec in air.
Transverse waves (shear waves) also are used extensively in the ultrasonic inspection of metals. Transverse waves are visualized readily in terms of vibrations of a rope that is shaken rhythmically. Each particle vibrates up and down in a plane perpendicular to the direction of propagation, rather than vibrating parallel to the direction of wave motion as in the longitudinal wave. A transverse wave is represented schematically in Fig. 13-3, which shows particle oscillation, wave front, direction of wave travel and the wave length (Î» ) corresponding to one cycle.
For the propagation of transverse waves, it is necessary that each particle exhibit a strong force of attraction to its neighbours so that as a particle moves back and forth it pulls its neighbour with it. This causes the sound to move through the material with the velocity associated with transverse waves, which is about 50% of the longitudinal-wave velocity for the same material.
Air and water will not support transverse waves. In gases, the forces of attraction between molecules are so small that shear waves cannot be transmitted. The same is true of a liquid, unless it is particularly viscous or is present as a very thin layer.
Surface waves (Rayleigh waves) travel along the flat or curved surface of relatively thick solid parts. For the propagation of waves of this type, the waves must be traveling along an interface bounded on one side by a solid and on the other side by gas molecules. Surface waves, therefore, are essentially nonexistent in a solid immersed in a liquid, unless the liquid covers the solid surface only as a very thin film.
Surface waves are subject to less attenuation in a given material than longitudinal or transverse waves. They have a velocity approximately 90% of the transverse-wave velocity in the same material. The region in which these waves propagate with effective energy is not much thicker than about one wave length beneath the surface of the metal. At this depth, wave energy is about 4% of the wave energy at the surface, and the amplitude of oscillation decreases sharply to a negligible value at greater depths.
Rayleigh waves follow contoured surfaces. For instance, Rayleigh waves traveling on the top surface of a metal block are reflected from a sharp edge, but if the edge is rounded off, the waves continue down the side face and are reflected at the lower edge, returning to the sending point. Surface waves will travel the entire way around a cube if all edges of the cube are rounded off. Surface waves can be used to inspect parts that have complex contours.
Lamb waves, also known as plate waves, are propagated in metal that is only a few wave lengths thick. A Lamb wave consists of a complex vibration that occurs throughout the thickness of the material. The propagation characteristics of Lamb waves depend on the density, elastic properties and structure of the metal, and also are influenced by the thickness of the metal test piece and the cyclic frequency.
The behavior of waves upon encountering surfaces and boundaries is another fundamental aspect of wave propagation. The simplest situation is depicted in Figure 2.3a, where a wave encounters a boundary at right angle or normal incidence. The interaction only involves reflection of some of the wave and transmission of a portion, with the amount of energy in each part depending on the material characteristics. A more complicated situation may arise, particularly in solids, when the wave strikes at an angle, or at oblique incidence. What may occur, as shown in Fig. 2.3b, is that two types of waves are reflected for a single incident wave. This phenomenon is known as mode conversion, and is illustrated for the case of a pressure wave generating both pressure and shear waves. Yet another aspect is involved when waves encounter
edges. Complex scattering and diffraction of the waves may occur, similar to optics. This is meant to be illustrated by Figure 2.3c.
(a) reflection and transmission , (b) refraction and mode conversion , (c) diffraction and scattering
Fig 2.3 Different types of acoustic wave interaction with material discontinuities
2.2.1 Mathematical Equations in wave propagation
Ultrasonics involves the propagation of acoustic waves. Therefore, it is necessary to understand the basic features of propagating waves and some of the mathematical equations governing simple cases of wave propagation. Simple harmonic propagating wave can be described as follows
where k is the so-called wave number which is related to the reciprocal of wavelength k = 2Ï€/Î» and is introduced mainly for convenience in writing wave propagation expressions. The phase velocity c is meant to rigorously define the velocity of the wave as the speed with which two successive points of constant phase move past a certain point. This serves to distinguish it from other types of velocities associated with waves, such as the group velocity. A propagating wave may be described by several equivalent expressions. Thus,
These all may be considered expressions for a rightward propagating harmonic wave. A leftward wave would simply be given by a sign change, as . The relationship between wave propagation and standing wave vibrations in a system may be shown by superimposing two oppositely propagating waves. Thus, using simple trigonometric identities, it may be shown that
This latter expression describes the vibrations of a system with well-defined nodes and antinodes, (node means a point, line or surface of a vibrating body that is free from vibratory motion).
The propagation of pulse of arbitrary shape may be described mathematically as
where f(x) is an arbitrary function describing the pulse shape. An oscillatory wave packet, may be described by
In a situation involving attenuation of a harmonic wave with a distance, due to internal frictional losses, can be described as
where Î± is the attenuation coefficient.
2.2.2. Longitudinal wave Propagation in thin rods
Consider a rod with coordinate x and displacement u as shown in figure below, to derive a equation for propagation of waves in many mechanical systems.
Figure 2.4 A thin rod (a) with coordinate x and displacement u of a section and
(b) the stress acting on a differential element of the rod.
The equation of motion of an element of the rod as shown in figure is
where A is the cross sectional area and Ï the mass density. This can be reduced as
For an elastic material
where Îµ is the strain in the material. This quantity can be defined as
Substitution of the value of Îµ in equations 2.10 and 2.11, gives the equation for waves in thin rod.
where E denotes Young's Modulus
The other way of representing this equation is
This equation governs the one-dimensional propagation of longitudinal elastic waves in a thin rod. It is usually termed simply the wave equation because it represents the situation for so many problems in mechanical and electrical media. It may be shown that a propagating harmonic wave represents a solution of the wave equation. Thus, substitution of
into (2.13) leads to c = co. In other words, the propagation velocity of the wave must be co. It may also be shown that the arbitrary pulse from also satisfies the wave equation
2.3. Ultrasonic Testing
Ultrasonic testing is one of the widely used methods of nondestructive testing. Its primary application is internal flaw detection and characterization ; it is also used to detect surface flaws, to measure thickness and extent of corrosion, and determination of physical properties, structure, grain size and elastic constants.
When sound waves propagate from one medium to another, at the interface separating the two media some energy is partially reflected and the rest is transmitted. This property is made use to detect flaws because not only interfaces also the flaws can reflect the ultrasonic sound energy. The interaction of the sound energy is stronger for higher frequencies. Hence high frequency ultrasound in the frequency range 0.5 MHz to 25MHz is found suitable for the testing. The waves are generated by using either a Piezo-electric energized crystal cut in a particular fashion to generate the desired wave mode or an Electromagnetic
acoustic transducer. The relation among the intensities of the incident and reflected sound energy is given by
The intensity of the sound wave reflected from the interface generally depends upon the difference in the densities of the pair of media for the given incident wave intensity. Here Ï1 and Ï2 are the densities of the two media 1 and 2 respectively through which the sound wave is propagating. Thus, if the ultrasonic wave propagates from a medium of higher density into a medium of lower density then maximum reflection of intensity takes place at the interface separating the two media. The flaw in the medium results in the reflection of sound energy due to the variation of density and hence their detection is made possible.
Reflections are analyzed electrically and the reflection is called echo.
Testing can be performed by measuring
a) resonant frequencies
b) transit time only
c) ultrasonic wave intensity(acoustic pressure)
d) both intensity and transit time.
2.3.1. Ultrasonic Testing Methods
Ultrasound can be used to detect flaws by measuring (a) intensity and time of flight of reflected sound waves having a single frequency, (b) intensity either transmitted or reflected sound waves having a single frequency or (c) intensity and Time of flight of reflected sound waves having varying frequency. Pulse echo method, widely used method, is based on the measurement of intensity and time of flight of reflected sound waves. Flaws are detected and their sizes estimated by comparing the intensity of reflected sound from an interface (either within the test piece or at the back surface) with the intensity of sound reflected from a reference interface of known size or from the back surface of a test piece having no flaws. The echo from the back surface (back reflection) serves as a reference point for time-of-flight measurements that enable the depth "of some Internal flaws to be measured. It is necessary that an internal flaw reflect at least part of the sound energy onto the receiving transducer for depth measurements to be made. However, echoes from flaws are not essential to their detection. Merely the fact that the intensity of the back reflection from a test piece is lower than that from an identical workpiece known to be free of flaws implies that the test piece contains one or more flaws. This second method of detecting the presence of flaws is, by sound attenuation is used in transmission methods as well as in the pulse-echo method. The main disadvantage of attenuation methods is that flaw depth cannot be measured.
2.3.2. Pulse-Echo Method
In pulse-echo inspection, ultrasonic energy is introduced into a test piece, in short spells at regular intervals of time. On meeting a reflecting surface, some energy is reflected back. The reflected energy is proportional to the size of the reflecting surface. Reflected energy is monitored; both the amount of energy reflected in a specific direction and the time delay between transmission of the initial pulse and receipt of the echo are measured.
Most pulse-echo systems consist of (a) an electronic clock; (b) an electronic, signal generator or pulser ; (c) a sending transducer: (d) a receiving transducer (e)an echo signal amplifier; and (f) a display device. A single transducer acts both as a sending and receiving transducer, In the most widely used version of "pulse echo" systems,. The clock and signal generator usually are combined in a signal electronic unit. Frequently, circuits that amplify and demodulate echo signals from the transducer are housed in the same unit. A block diagram of a pulse-echo flaw detector is shown in Fig.
Fig. 2.5. Schematic diagram of a pulse-echo flaw detector
Pulse-echo inspection can be accomplished with longitudinal, shear, surface or Lamb- waves. Straight-beam or angle-beam techniques can be used, depending on test-piece shape and inspection objectives. Data can be analysed in terms of type, size, location and orientation of flaws, or any combination of these factors. It should be noted, however, that some forms of data presentation are unable to pinpoint the location of flaws unless the flaws are favorably oriented with respect to the initial ultrasonic beam. Similarly, type, location and orientation of flaws often influence the sound-beam characteristics that are used to estimate flaw size.
Most of the pulse echo inspections uses separate sending and receiving transducers. Depending mainly on geometric considerations, these separate transducers may be housed in a single search unit or in two separate search units. "Pitch-catch" is the term used in relation to the separate sending and receiving transducers, regardless of whether reflection methods or transmission methods are involved.
Data Presentation: Information from pulse-echo inspection can be displayed in one of three forms:
(a) A-scan, display of intensity and time-of-flight data
(b) B-scan, display of time-of-flight data or
(c) C-scan, which is a semi quantitative display of echo intensity
The details of the scanning techniques are dealt in detail in Chapter 4.
2.3.3. Immersion Testing Method: Scanning methods that utilize immersion-type search units are classified broadly into three categories: (i) conventional immersion methods in which both the search unit and the test piece are immersed in liquid, (ii) squirter and bubbler methods in which the sound is transmitted in a column of flowing water and (iii) scanning with a wheel type search unit.
In the first type the test piece and transducer are immersed in water and sound (longitudinal) is directed by straight beam technique. Search units are straight beam and accomplish inspection through control and direction of sound beam.
In the second type, water path is generally adjusted to require a longer transit time than the depth of scan. The first multiple of the front reflection will appear farther than the first back reflection on the oscilloscope. The water path is made equal to the ¼ th of the thickness of the testpiece plus 6mm, is the rule of the thumb used.
Air, being a poor transmitter of sound , it has to be eliminated, between the transducer and test piece. For the elimination of air, couplants are used. Water is used as coupulant for smooth surface. Glycerin is added to increase viscosity.
Fig. 2.6 Typical Immersion testing arrangement
( Reproduced by courtesy of Krautkrämer GMBH & Co.
2.4. Advantages and Disadvantages
2.4.1 Advantages. As compared to other methods for nondestructive inspection of metal parts,the principal advantages of ultrasonic inspection are
1. Due to more penetrating power, detection of flaws deep in the part is made possible.
2. The detection of extremely small flaws is permitted due to high sensitivity.
3. Accurate results in determining the position, size, shape, nature and characterization of orientation of internal flaws can be achieved.
4. Only one surface need be accessible.
5. Immediate interpretation, rapid scanning, in line production monitoring and process control are made possible since the operations are electronic and automated. A permanent record of inspection results can be made for future reference.
6. Scanning ability enables inspection of a volume of metal extending from front surface to back surface of a part.
7. Non hazardous nature
1. Requires careful attention by technicians on manual operations
2. Technical knowledge is required to develop inspection procedures.
3. It is difficult to inspect rough parts, parts that are irregular in shape, parts which are very small or thin, or inhomogeneous.
4. Shallow layer discontinuities that are present immediately beneath the surface may not be detectable.
5. Effective energy transfer between transducers and parts being inspected, requires couplants.
6. Both calibration of the equipment and characterization of flaws , needs reference standards.
2.5.Ultrasonic Scanning Equipment
2.5.1. Phased Array System
Ultrasonic phased arrays are a novel method of generating and receiving ultrasound. They create beams by multiple ultrasonic elements and electronic time delays by constructive and destructive interference. The phased array beams can be steered, scanned, swept and focused electronically.
· Electronic scanning permits very more coverage of the components, than a single probe mechanical system.
· Components can be mapped at appropriate angles to optimize probability of detection, by sectorial scanning.
· Due to electronic focusing ,optimization of the beam shape and size at the expected defect location,is possible.
On a whole , the phased arrays permits optimizing defect detection while minimizing inspection time.
Phased arrays offer significant advantages over other traditional methods:
No safety hazards
Inspection as soon as weld is cool
Better defect detection and sizing
Great flexibility in parameter range
Compliant with all known codes
Many special techniques are possible.
Phased arrays use an array of elements, individually wired, pulsed and time-shifted, the array elements can be a linear array, a 2D matrix array, a circular array or some more complex form. Linear arrays are used in most of the applications due to their cost effectiveness.
Fig. 2.7 Schematic of ID linear array
The elements are isolated from each other ultrasonically, and packed in normal probe housings. The cabling consists of a bundle of well-shielded micro co-axial cables. Elements are typically pulsed in groups from 4 to 32, (16 elements for welds) The computer and software calculate the time-delays for a set-up from operator-input on inspection angle, focal distance, scan pattern etc, or use a pre-defined file, making the system user friendly. Based on the time-of-flight from the focal spot, and the scan assembled from individual "Focal Laws", the time delays are back calculated. To provide the phasing accuracy time delay circuits must be accurate to around 2 nanoseconds .
Fig 2.8. Schematic showing generation of linear and sectorial scans using phased arrays
A beam is generated by each element when pulsed; and these beams forms a wavefront by constructive and destructive interference . The individual channels with time delays as specified are pulsed by the phased array instrumentation , so as to form a pre-calculated wavefront. For receiving, instrumentation receives with pre-calculated time delays, then sums the time-shifted signal and displays it.
2.5.2. Portable Phased Arrays
This system uses 16/128 format, i.e. 16 pulsers and a total of 128 elements. OmniScan is the entry system for
phased arrays, and can perform electronic and sectorial scans, to comply with ASME CC 2235 and other codes. OmniScan can operate fully automated scanners and encoders, record all waveform data. It can perform pulse-echo and display A-scans, B-scans, S-scans, and combinations. OmniScan is limited in that it can only perform one type of scan per pass, unless controlled by TomoView. The instrument is shown in Figure 2.9 below. OmniScan uses highly user-friendly software