Ultrasonic Birefringence

The speed of sound through a medium is a function of applied strain. The relationship is governed by the acoustoelastic coefficient. Stress states with unequal principle stresses (i.e., nonhydrostatic) have the effect of introducing anisotropic acoustic behavior to otherwise isotropic materials. This effect is similar to strain-induced optical birefringence (covered in the next section). The advantage is that ultrasonic birefringence can be measured in optically opaque materials. 

There are three ways of measuring ultrasonic birefringence. To measure stress in an entire specimen, one may measure the time of flight of ultrasonic waves as a function of propagation direction. For smaller sections of sample, shear wave velocities are measured as a function of the orientation of the plane of oscillation of the shear wave. Surface stress can be ultrasonically measured using the velocity of Rayleigh waves as a function of direction. Any of these methods will yield the direction of principle stress and relative stress intensities between samples of identical materials. To find actual values of stress, one must know the value of the acoustoelastic coefficient of the material. An experimental setup for measuring bulk acoustoelastic coefficients has been reported by Koshti.  

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Tokuoka T., Iwashitnizu Yu., Acoustical birefringence of ultrasonic waves in deformed isotropic elastic materials, Int. J. Solids Structures, 4 (1968), 383—389. 

TF Systems A TF is a device whose spectral transmission can be controlled by applying a voltage or acoustic signal. There are two main TF devices acousto-optical TF (AOTF), based on diffraction, and liquid crystal TF (LCTF), based on birefringence. An AOTF is a transparent crystal in which an ultrasonic wave field is created,... 

The behavior of water in oil microemulsions has been studied using different techniques light scattering, electrical conductivity, viscosity, transient electrical birefringence, ultrasonic absorption. All these experiments lead us to propose a picture of the microemulsions structure which assignes an important role to the fluidity of the interfacial region. 

Nomura H, Matsuoka T, Koda S (2004) Ultrasonically induced birefringence in polymer solution. Pure Appl Chem 76 97-104... 

Recently Wright et al. have obtained room temperature values of all five elastic constants for uniaxially drawn polymethylmethacrylate (Perspex) and polystyrene (Carinex) by measuring the critical angle for total reflection of an ultrasonic beam incident on immersed samples. The specimens were stretched by similar extents at different temperatures, with orientation assessed by measurements of optical birefringence. 

Most techniques for the nondestructive evaluation of ceramic materials fall into two categories high-energy penetrating radiation (for example. X-ray) and high-frequency elastic waves (ultrasonics). Some NDE techniques have been developed for specialized applications, such as optical birefringence for transparent specimens and shearography for laminar composites. 

Ultrasonic sensors have also been applied in the study of physical foaming agents for foam extrusion [252]. For on-line monitoring of orientation processes birefringence, FTIR spectroscopy, fluorescence and ultrasonics are most suitable. 

A more comprehensive investigation on uniaxially oriented sheets of polymethylmethacrylate and polystyrene was undertaken by Wright et al. [94] using ultrasonic measurements. The results are summarised in Figure 8.29 (a) and (b), where the stiffness constants are shown as a function of the birefringence. Rawson and Rider [95] have also reported ultrasonic data for oriented polyvinyl chloride and observed a similar degree of anisotropy to that seen in Table 8.8 from Hennig s work. 

Ultrasonic longitudinal and surface waves applied to a nematic sample change the birefringence properties of the fluid in the reflective and transmission modes when the ultrasound intensity J exceeds a certain minimum value [9,10,22, 26, 27]. 

This is identical with the Moseley equation (17), if the second term is negligible. The principle limitation of this method is that it measures the average orientation of the total sample. It has, however, been found that a good correlation exists between the orientation data derived from ultrasonic measurements and those obtained from birefringence measurements. ... 

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