Ultrasonic techniques

The speed of sound in an elastic medium is a function of elastic modulus and density as follows v = ElpY, where p is velocity, E is modulus, and p is density. Because ceramics are stiffer and less dense than metals, the speed of sound in ceramics is greater than in metals. For example, the speed of sound in iron is 5.13 mm/ ps, compared to 9.78 mm/ps in alumina. This means that, at the same frequency, the wavelength is greater in a ceramic and the resolution essentially halved. This difficulty and the fact that critical flaws in ceramics are much smaller than those in metals mean that frequencies between 10 and 100 MHz are required for ultrasonic evaluation of ceramics. 

There are three different methods of displaying ultrasonic data A-scan, B-scan, and C-scan. Because the received signal is displayed as a function of time, A-scan display is essentially one-dimensional. The transducers do not move, and only material which is in the ultrasonic beam is polled. This type of display is useful for measuring absorption as a function of frequency, density, and elastic moduli. 

B-scan display is essentially two-dimensional. The pulse-echo elapsed time is converted to distance beneath the surface. As the transducer is scanned across the surface, internal defects reflect the wave, as does the opposite surface. From these reflections, an image is generated of a slice of the material. Note that the depth resolution is on the order of tens of microns, but the resolution parallel to the surface is equal to the transducer size. Thus, if the precise position of defects in the plane parallel to the 

C-scan ultrasonics, or ultrasonic microscopy, is essentially a three-dimensional technique. The transducer is rastered across the sample surface in the same pattern as the electron beam in a television. Information on the depth of flaws beneath each position on the sample surface is recorded and stored in a computer. The computer then allows projection of the three-dimensional network of flaws onto the screen. The image is usually rotatable to take advantage of the dimensionality of the data. C-scan may be used to characterize and display the complete flaw distribution in a sample. 

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Up-to-day reliability requirements as applied to NPP strength maintenance govern development of new generation of computerized systems for in-service inspection. These systems in parallel with capabilities of ordinary ultrasonic techniques allow to reconstruct high resolution image of inner flaw and increase available amount of information. 

Some discontinuities may be identified by a conventional two-dimensional ultrasonic technique, from which the well-known C-scan image is the most popular. The C-scan technique is relatively easy to implement and the results from several NDE studies have been very encouraging [1]. In the case of cylindrical specimens, a circular C-scan image is convenient to show discontinuity information. The circular C-scan image shows the peak amplitude of a back-scattered pulse received in the circular array. The axial scan direction is shown as a function of transducer position in the circular array. The circular C-scan image serves also as an initial step for choosing circular B-scan profiles. The latter provides a mapping between distance to the discontinuity and transducer position in the circular array. 

Through this study, we have shown that ultrasonic imagery can be an optimal solution to the different problems in Non Destructive Testing. This method, largely used, would have to be introduced in industry by an investment of the NDT users. This also requires a reorientation and supplementary operators trained in ultrasonic techniques. 

In this paper, the performanees of laser-ultrasound are estimated in order to identify lacks of weld penetration. The laser-ultrasonic technique is applied to cylindrical metallic strucmres (few mm thick) in a single-sided control. The results obtained for different materials (gold-nickel alloy and tantalum) are presented by B-sean views for which the control configuration is discussed with regard to the thermal effects at the laser impact. This testing is performed for different lacks of weld penetration (up to 0.5 mm for a thickness of 2 mm) even in the presence of the weld bead, which corresponds to an actual industrial problem. 

Ultrasonic techniques are an obvious choice for measuring the wall thickness. In the pulse-echo method times between echoes from the outer and inner surface of the tube can be measured and the wall thickness may be calculated, when the ultrasonic velocity of the material is known. In the prototype a computer should capture the measuring data as well as calculate and pre.sent the results. First some fundamental questions was considered and verified by experiments concerning ultrasonic technique (Table I), equipment, transducers and demands for guidance of the tube. 

However, with ultrasonic technique the signal level of the reflected echo pattern is also influenced by the geometrical shape, and misalignment between measuring direction and radial direction. Therefore, the practical demand for guidance must be found by experiments. 

Both ultrasonic and radiographic techniques have shown appHcations which ate useful in determining residual stresses (27,28,33,34). Ultrasonic techniques use the acoustoelastic effect where the ultrasonic wave velocity changes with stress. The x-ray diffraction (xrd) method uses Bragg s law of diffraction of crystallographic planes to experimentally determine the strain in a material. The result is used to calculate the stress. As of this writing, whereas xrd equipment has been developed to where the technique may be conveniently appHed in the field, convenient ultrasonic stress measurement equipment has not. This latter technique has shown an abiHty to differentiate between stress reHeved and nonstress reHeved welds in laboratory experiments. 

Unlike vibration monitoring, ultrasonics monitors the higher frequencies, i.e. ultrasound, produced by unique dynamics in process systems or machines. The normal monitoring range for vibration analysis is from less than 1 Hertz to 20,000 Hertz. Ultrasonics techniques monitor the frequency range between 20,000 and 100 kHz. 

Ultrasonic techniques. Wall thickness can be measured to monitor the progress of general corrosion, cracks can be detected and hydrogen blisters identified. Certain construction materials such as cast iron cannot be examined by ultrasound. Skilled operators and specialist equipment is required. Plant can be examined in situ except when it is above 80°C. 

For detecting stress-corrosion cracks and estimating their depth of penetration, the ultrasonic technique and, to a lesser extent, A -radiography, have proved successful. 

Asai, H. (1961). Study of the hydration-dehydration in polyelectrolyte solutions by the ultrasonic technique. Journal of the Physical Society of Japan, 16, 761-6. 

The discussion above that led to Eqs. (4.2.6 and 4.2.7) assumes that the no-slip condition at the wall of the pipe holds. There is no such assumption in the theory for the spatially resolved measurements. We have recently used a different technique for spatially resolved measurements, ultrasonic pulsed Doppler velocimetry, to determine both the viscosity and wall slip velocity in a food suspension [2]. From a rheological standpoint, the theoretical underpinnings of the ultrasonic technique are the same as for the MRI technique. Flence, there is no reason in principle why MRI can not be used for similar measurements. 

Ultrasonically assisted extraction is also widely used for the isolation of effective medical components and bioactive principles from plant material [195]. The most common application of low-intensity ultrasound is as an analytical technique for providing information about the physico-chemical properties of foods, such as in the analysis of edible fats and oils (oil composition, oil content, droplet size of emulsions, and solid fat content) [171,218]. Ultrasonic techniques are also used for fluids characterisation [219]. 

Singh [238] has recently reviewed emerging ultrasonic techniques for process sensing and control in manufacturing (for structure-property relationships), such as... 

M.J.W. Povey, Ultrasonic Techniques for Fluids Characterization, Academic Press, San Diego, CA (1997). 

W. D. Wang, Inspection Of Refinery Vessels For Hydrogen Attack Using Ultrasonic Techniques, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12, D. O. Thompson and D. E. Chement, Plenum Press, New York. 

Interfacial area measurement. Knowledge of the interfacial area is indispensable in modeling two-phase flow (Dejesus and Kawaji, 1990), which determines the interphase transfer of mass, momentum, and energy in steady and transient flow. Ultrasonic techniques are used for such measurements. Since there is no direct relationship between the measurement of ultrasonic transmission and the volumetric interfacial area in bubbly flow, some estimate of the average bubble size is necessary to permit access to the volumetric interfacial area (Delhaye, 1986). In bubbly flows with bubbles several millimeters in diameter and with high void fractions, Stravs and von Stocker (1985) were apparently the first, in 1981, to propose the use of pulsed, 1- to 10-MHz ultrasound for measuring interfacial area. Independently, Amblard et al. (1983) used the same technique but at frequencies lower than 1 MHz. The volumetric interfacial area, T, is defined by (Delhaye, 1986)... 

Ultrasonic relaxation loss, of vitreous silica, 22 429-430 Ultrasonics, for MOCVD, 22 155 Ultrasonic spectroscopy, in particle size measurement, 13 152-153 Ultrasonic techniques, in nondestructive evaluation, 17 421—425 Ultrasonic testing (UT) piping system, 19 486 of plastics, 19 588 Ultrasonic waves, 17 421 Ultrasonic welding, of ethylene— tetrafluoroethylene copolymers,... 

Zimmerman, M. C Meunier, A., Katz, J. L., and Christel, R (1990). The evaluation of cortical bone remodeling with a new ultrasonic technique. IEEE Trans Biomedical Engineering 37, 433-41. [196]... 

A new ultrasonic technique for studying dewetting and cumulative internal damage in solid propints has been reported (Refs 17 20). This technique yields volume-dilatation data on proplnt in tension, and on damage in uniaxial compression and shear strain fields. Estimates of vacuole size and number density arising from dewetting can be made, as well as can the time dependent void growth at constant strain be observed... 

For the 1,2,4,6-tetramethylpiperidine equilibrium 123 124, AH° = 1.0 kcal mol-1, and for the N-methylpiperidine equilibrium, AH° = 0.88 kcal mol 1 and refer to measurements made on pure liquids. These values are much lower than those obtained by most other techniques (Table XI) and it is agreed2 that values of AH0 obtained by spectroscopic methods are more reliable than those from ultrasonic techniques. AH (axial -> transition state) values, however, are in agreement with those from other methods, and for Af-methylpiperidine a value of 5.02 kcal mol-1 has been obtained from Eyring plots and a value of 5.76kcal mol l, using the foregoing equation.171... 

Over the past half a century or so a wide variety of different applications of ultrasound to food materials have been developed, which reflects the complexity and diversity of food materials, as well as the versatility of the ultrasonic technique. In this section, previous applications of ultrasound to foods are discussed, as well as possible future applications. 

The determination of the thickness of the layers of fat and lean tissue in animal flesh is the most popular use of ultrasound in the food industry at present [5,6]. In fact there are over a hundred references pertaining to this application of ultrasound in the Food Science and Technology Abstracts (1969-1993). In contrast to most other applications of ultrasound in the food industry, which have rarely developed further than use in the laboratory, there are a number of commercial instruments available for grading meat quality [6, 30-32]. This application is based on measurement of time intervals between ultrasonic pulses reflected from boundaries between layers of fat, lean tissue and bone. Ultrasonic techniques have the advantage that they are fairly cheap, easy to operate and give predictions of meat quality of live animals. Other examples of thickness determinations include liquid levels in cans or tanks, thickness of coatings on confectioneries, egg shell thickness. 

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