Fundamentals And Applications Of Ultrasonic Waves Pdf
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Although ultrasound competes with other forms of medical imaging , such as X-ray techniques and magnetic resonance imaging , it has certain desirable features—for example, Doppler motion study—that the other techniques cannot provide. In addition, among the various modern techniques for the imaging of internal organs, ultrasonic devices are by far the least expensive. Ultrasound is also used for treating joint pains and for treating certain types of tumours for which it is desirable to produce localized heating.
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- Fundamentals And Applications Of Ultrasonic Waves
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It also includes some advanced techniques used for non-destructive evaluations. At first, basic characteristics of ultrasonic waves propagating in media are described briefly. Secondly, basic concepts for measuring ul- trasonic waves are described with introductory subjects of ultrasonic trans- ducers that generate and receive ultrasonic waves.
Finally, specialized re- sults demonstrating the capabilities of using a buffer rod sensor for ultrasonic monitoring at high temperatures are presented. Keywords Ultrasonic Sensing, Transducers, Nondestructive Evaluation 1 Introduction Ultrasonic sensing techniques have become mature and are widely used in the various fields of engineering and basic science. Actually, many types of conventional ultrasonic instruments, devices and sophisticated software are commercialized and used for both industrial and medical applications.
One of advantages of ultrasonic sensing is its outstanding capability to probe inside objectives nondestructively because ultrasound can propagate through any kinds of media including solids, liquids and gases except vacua. Such ultrasonic data provides the fundamental basis for describing the outputs of ultrasonic sensing and evaluating systems. In this chapter the fundamentals of ultrasonic sensing techniques are de- scribed.
What is ultrasound, how to produce and capture ultrasound, what kinds of methods and equipments can be used to measure ultrasound, and what kinds of information can be obtained from ultrasonic measurements?
These questions are addressed in the following sections and the answers to the questions are briefly explained from the viewpoint of industrial appli- cations. In addition, some specialized results using a buffer rod sensor that is an effective means for high temperature ultrasonic measurements are in- troduced to demonstrate its applicability for nondestrucive evaluations and monitoring.
For further studies on ultrasonic sensing, it is recommended to refer to some books, - for basic theories of ultrasound propagations, - for transducers and instruments, and - for ultrasonic measurements, evaluations, applications and others.
Ultrasound is simply sound that are above the frequency range of human hearing. When a disturbance occurs at a portion in an elastic medium, it propagates through the medium in a finite time as a mechanical sound wave by the vibrations of molecules, atoms or any particles present.
Such mechanical waves are also called elas- tic waves. Ultrasound waves or ultrasonic waves are the terms used to de- scribe elastic waves with frequency greater than 20, Hz and normally exist in solids, liquids, and gases.
A simple illustration of the ultrasonic waves produced in a solid is shown in Fig. The energy of the wave is also carried with it. In a continuous medium, the behaviour of ultrasonic waves is closely re- lated to a balance between the forces of inertia and of elastic deformation. An ultrasonic wave moves at a velocity the wave velocity that is deter- mined by the material properties and shape of the medium, and occasion- ally the frequency.
The ultrasonic wave imparts motion to the material when it propagates. This particle motion is usually specified as a parti- cle velocity v. It is noted in ultrasonic measurements that the particle ve- locity is much smaller than wave velocity. Also, one can understand that no ultrasonic wave propagates in vacua because there are no particles that can vibrate in vacua.
Undisturbed a Forced parallel to surface b Forced normal to surface c Fig. Schematics of ultrasonic waves in a bulk specimen: a equilibrium state with no disturbance, b waves relating to shear transverse vibrations, c waves relating to longitudinal vibrations. The acoustic impedance characterizes the ability of a material to vibrate under an applied force and can be considered as the resistance of the material to the passage of ultra- sonic waves.
There is an analogy between impedance in electrical circuits and the acoustic impedance. The acoustic impedance is useful for treating the transmission of ultrasonic waves between two media, just like that the electrical impedance is effective to characterize a resistance in an alternat- ing electric current circuit. For example, the transmission of an ultrasonic wave from one medium to another becomes maximum when the acoustic impedances of the two media are equal.
The concept of using the acoustic impedance plays an important role in determining of acoustic transmission and reflection at a boundary of two media having different material proper- ties and therefore, the acoustic impedance is an important parameter in de- signing ultrasonic sensors and sensing systems. At a fixed position in the material, the dis- placement changes sinusoidally with time t, where the time required for the wave to propagate the distance between successive maxima is the pe- riod T.
At any time, the amplitude of the displacement decreases periodi- cally with increasing propagation distance because of its attenuation by the material. Some basic features of ultrasonic waves are introduced here. Types of Wave Modes of Propagation What types of ultrasonic waves can exist? The answer to this question can basically be given from solutions of the wave equations that predict wave behaviours by showing that material properties and body shape dictate the vibrational response to the applied forces that drive the wave motion.
De- tails of wave types obtained by solving wave equations and their character- istics are shown in -. In short, there are two types of ultrasonic waves: bulk fundamental waves that propagate inside of an object, and guided waves that propagate near the surface or along the interface of an object -.
Waves that propagate wholly inside an object, independent of its bound- ary and shape, are called bulk waves. Two types of bulk waves can exist in an isotropic medium: longitudinal or dilatational, compression, primary , and shear or distortional, transverse, secondary waves as shown sche- matically in Fig. As mentioned in Section 2. The longitudinal waves can be defined on this basis as waves in which the particle motion is parallel to the direction of the wave propagation.
The shear waves are de- fined as waves in which the particle motion is perpendicular to the direc- tion of the propagation. Both waves can exist in solids because solids, unlike liquids and gasses, have rigidity that is a resistance to shear as well as compressive loads. However, the shear waves cannot exist in liquids and gasses because of no resistance to shear roads in such media.
When the influences of the boundaries or shape of an object are consid- ered, other types of waves called the guided waves are produced. There are three types of guided waves depending on geometry of an object: surface acoustic waves SAWs , plate waves, and rod waves. There are many kinds of SAWs such as Rayleigh, Scholte, Stoneley, and Love waves and the wave propagation characteristics of SAWs strongly depend on material properties, surface structure, and nature at the interface of the object.
When an SAW propagates along a boundary between a semi- infinite solid and air, the wave is often called Rayleigh wave in which the particle motion is elliptical and the effective penetration depth is of the or- der of one wavelength. When an ultrasonic wave propagates in a finite medium like a plate , the wave is bounded within the medium and may resonate.
Such waves in an object of finite size are called plate waves if the object has a multilayer structure, and called Lamb waves if it has a single layer.
Also, when a force is applied to the end of a slender rod, an ultrasonic wave propagates axially along it. Wave propagations in rodlike structures such as a thin rod and hollow cylinders have been studied extensively.
Further information on the guided waves and their characteristics can be obtained in Refs. In general, the wave propagation characteristics of guided waves strongly depend on not only material properties but also the plate thick- ness, the rod diameter, and the frequency. The frequency dependence of the wave velocity of guided waves is called frequency dispersion. While the frequency dispersion often makes wave propagation behaviour compli- cated, it also provides unique materials evaluations using guided waves.
It is noted that similar types of bulk and guided waves can exist for anisot- ropic materials and in general, their behaviours become much more com- plicated than those for isotropic materials -. Velocity Ultrasonic velocity is probably the most important and widely used pa- rameter in ultrasonic sensing applications. Each medium has its own value of the velocity that usually depends on not only propagation medium but also its geometrical shape and structure.
The theoretical values can be ob- tained from wave equations and typically determined by the elastic proper- ties and density of the medium. As a rule of thumb, the velocity of the shear wave is roughly half the longitudinal wave. Although the velocities can be determined theoretically if material properties such as the elastic moduli and density are known precisely, these material properties are not always available for the determination because they change depending on mechanical processing and heat treatments.
Therefore, it is important and necessary to make a calibration measurement for the velocities when one wants to know the correct values for velocities. Attenuation When an ultrasonic wave propagates through a medium, ultrasonic at- tenuation is caused by a loss of energy in the ultrasonic wave and other reasons.
The attenuation can be seen as a reduction of amplitude of the wave. There are some factors affecting the amplitude and waveform of the ultrasonic wave, such as ultrasonic beam spreading, energy absorption, dispersion, nonlinearity, transmission at interfaces, scattering by inclusions and defects, Doppler effect and so on.
In general, the attenuation coefficient highly depends on frequency. Since this frequency dependence reflects microstructures of materials, it can be used for characterizing microscopic material properties relating to chemical reactions and mechanical processes. Further informa- tion on the attenuation can be obtained in Refs. Wavelength is a useful parameter in ultrasonic sensing and evaluations. In ultrasonic detec- tion of a small object, the smallest size that can clearly be detected must be larger than half a wavelength at the operating frequency.
Reflection and Transmission When an ultrasonic wave perpendicularly impinges on an interface be- tween two media as shown in Fig. The ratio of the amplitude of the reflected wave AR to that of the incident wave AI is called reflection coefficient R, and the ratio of the amplitude of the trans- mitted wave AT to that of the AI is called transmission coefficient T.
It can be seen from these equa- tions that the maximum transmission of ultrasonic wave occurs when the impedances of the two media are identical, and most of ultrasonic wave is reflected when the two media have very different impedances.
The reflec- tion and transmission at interface play an important role in designing ultra- sonic sensing systems and understanding experimental results with the ul- trasonic systems. Normal reflection and transmission at an interface between two media.
Refraction and Mode Conversion When an ultrasonic wave obliquely impinges on an interface between two media as shown in Fig. One of important things is refraction in which a transmitted wave has a differ- ent angle from the incident.
Another important phenomenon is mode conversion that is a generation of one type of wave from another type in refraction as shown in Fig. For example, a longitudinal wave incident on an interface between liquid and solid is transmitted partially as a refracted longitudinal wave and partially as a mode converted shear wave in the solid.
Mode conversion can also take place on reflection if the liquid shown in Fig. It is noted that any types of waves can be converted to another type, e. Figure 4 shows a simulation result for refraction and mode conversion, calculated by a finite difference method.
Schematics of reflection, refraction and mode conversion at an oblique interface. A simulation result for refraction and mode conversion. The function of the trans- ducers is to convert electrical energy into mechanical energy which di- rectly corresponds to ultrasonic vibration, and vice versa.
The most com- mon way of generating and detecting ultrasonic waves utilizes the piezoelectric effect of a certain crystalline material such as quartz.
Fundamentals And Applications Of Ultrasonic Waves
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Fundamentals And Applications Of Ultrasonic Waves
Medical ultrasound also known as diagnostic sonography or ultrasonography is a diagnostic imaging technique, or therapeutic application of ultrasound. It is used to create an image of internal body structures such as tendons , muscles , joints, blood vessels, and internal organs. Its aim is often to find a source of a disease or to exclude pathology. The practice of examining pregnant women using ultrasound is called obstetric ultrasound , and was an early development and application of clinical ultrasonography.
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