Materials ultrasound

It is useful to give a brief overview of some of the major advantages and limitations of ultrasound as a tool for characterizing the properties of food materials. Ultrasound is fairly inexpensive to purchase and operate, it is robust and can therefore be used in factories, it is capable of rapid and reliable measurements, in a non-destructive and non-invasive manner. In addition, measurements can easily be automated and so the technique is suitable for on-line measurements as well as an analytical instrument in the laboratory. The major disadvantages... 

Fig. 5.8a,b. Assumed signal intense plaque (a). Source image of TOF-MRA at the level of the carotid bulb displays bright material, ultrasound showed no evidence of a thrombus. On CE-MRA there is a short eccentric stenosis of the left ICA (b)... 

Foods, in general, present online physical measurements with quite a challenge, in particular the need to obtain 100% inspection arising from the highly variable nature of the raw materials. Ultrasound is one technique, which has the potential for 100% inspection using tomographic techniques (Beck et al., 1994), in which the entire sample is penetrated by the inspecting field. 

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. 

In general a thickness measurement using ultrasound is done by measuring the time of flight of the ultrasonic pulse and calculating the thickness of the objeet multiplying the time and the known constant sound velocity in the material. 

Material Veloeity. Defines the velocity, in metres/sec, of ultrasound in the material of the test piece used to convert A-scan signal time to distances within the test piece. 

In this paper we propose a multivariable regression approach for estimating ultrasound attenuation in composite materials by means of pulse-echo measurements, thus overcoming the problems with limited access that is the main drawback of through-transmission testing. 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

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. 

Low-intensity ultrasound uses power levels (typically < 1 W cm 2) that are considered to be so small that the ultrasonic wave causes no physical or chemical alterations in the properties of the material through which the wave passes, i.e. it is nondestructive. However,... 

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]. 

Piezoelectric ceramics, which depend on lead compounds, are used to produce transducers and sensors which make possible ultrasound technologies used in wide-ranging medical and commercial applications, guidance and sensing systems used in defense and commerce, and in addition, new "smart materials" research projects. 

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