Ultrasonic parameters

The changes undergone by US on interacting with a solid, and the information that can thus be obtained from the solid, have been measured mainly through the US velocity and attenuation under resonant conditions. The ultrasonic parameters to be used and their processing are dictated by the final information required. Thus, the resistance to deformation is obtained by calculating the elastic moduli, the number and nature of which are a function of the nature of the solid (e.g. isotropic, anisotropic). However, equations relating the acoustic measurement to sample density, dimensions, material microstructure and thickness can usually be derived from simple parameters such as US velocity 

One common feature of aii moduii is that any strain generated in a soiid is iineariy proportional to the stress. This obviousiy requires that whatever stress is appiied shouid not break or permanentiy bend the sampie and that no significant energy be dissipated when the sample is strained. To revert to a carefui dynamic description of the dispiacements, forces and energy is mandatory for high dissipation materiais. 

Low-symmetry crystais posses some interesting, difficuit to assimiiate properties reiat-ing to stress waves and eiasticity. Such properties reiate to what is caiied internai strain , by which an externai stress produces strains in the unit ceii of the crystai different from the macroscopic strain. This can occur if, for exampie, a iattice has at each iattice point a group of atoms (the basis) with symmetry different from that of the iattice. 

Most methods for caicuiating eiastic moduii are based on resonance uitrasound spectroscopy (RUS), which is appiicabie to specimens iesser than 1 mm. The equations reiating the resonance frequencies of a soiid with its density, dimensions and eiastic moduii aiiow 

Theoretioaiiy a maximum of 21 tensor eiements of eiastio stiffness for the triclinic crystal (the lowest-symmetry crystal) can be determined with one specimen. However, it is diffioult to assimilate properties relating to stress waves and elasticity for such a low-symmetry orystal. In praotioe, RUS oan determine nine tensor elements for orthotropic symmetry as well as for higher symmetry (isotropic, cubic, hexagonal or tetragonal). 

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In the empirical approach the ultrasonic parameters of a range of samples with known properties are measured. Empirical relationships are then established between the property of interest and the measurable ultrasonic parameters. A typical example of this approach is the determination of the sugar content of fruit drinks [18]. A series of sugar solutions of different sugar concentration are prepared and their ultrasonic velocities are measured. This data is then used to make up a calibration curve which relates the sugar content to the... 

There is a good understanding of the interaction between ultrasound and matter for many materials, and mathematical theories are available to relate the measurable ultrasonic parameters to the composition and microstruture of these systems. The most important examples of these types of system in the food industry are homogeneous liquids, emulsions and suspensions. 

For ideal mixtures there is a simple relationship between the measurable ultrasonic parameters and the concentration of the component phases. Thus ultrasound can be used to determine their composition once the properties of the component phases are known. Mixtures of triglyceride oils behave approximately as ideal mixtures and their ultrasonic properties can be modeled by the above equations [19]. Emulsions and suspensions where scattering is not appreciable can also be described using this approach [20]. In these systems the adiabatic compressibility of particles suspended in a liquid can be determined by measuring the ultrasonic velocity and the density. This is particularly useful for materials where it is difficult to determine the adiabatic compressibility directly, e.g., powders, biopolymer or granular materials. Deviations from equations 11 - 13 in non-ideal mixtures can be used to provide information about the non-ideality of a system. 

Some of the simplest ultrasonic measurements involve the detection of the presence/absence of an object or its size from ultrasonic spectrum (Coupland and McClements, 2001). An ultrasonic wave incident on an ensemble of particles is scattered in an amount depending on size and concentration of the particles. As the ultrasonic parameters depend on the degree of the scattering, it can therefore be used to provide information about particle size. 

Recent advances in ultrasonic resonators have facilitated the development of Inexpensive, convenient devices for fast measurement of ultrasonic velocity and attenuation in small volumes (down to 0.1 ml) with resolution better than 10 % for velocity and 0.1% for attenuation. In addition, comprehensive studies on ultrasonic parameters of liquids in a number of laboratories over the last decade have provided a solid background for the interpretation of ultrasonic data. 

Ultrasonic examination is currently the most commonly used NDT method for inspection of composites. It presents desirable features such as providing information about defects situated deeply inside a material, but equally this method has several limitations. Flaws modify ultrasonic parameters such as wave velocity, refraction, reflection, scattering, and intensity, thus affecting the efficiency of ultrasound in defect location. In order to fully understand the concept of ultrasonic testing, it is necessary to use some mathematics, which will be kept to a minimum. The principle, advantages, and limitations of ultrasonic NDT techniques for composite inspection are described next. 

The primary measurable ultrasonic parameters of a material are velocity, c, and attenuation coefficient, a, defined as follows ... 

Dhake KP, Padmini ARKL (1970) Ultrasonic parameters and hydration numbers in aqueous solutions of electrolytes. Indian J Pure AppI Phys 8 311 -315... 

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