Ultrasonic wave propagation coefficient

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. ... 

Elastic Coefficients The elastic stiffness coefficients Cy can be calculated from the measured velocity of propagation of bulk acoustic ultrasonic waves, according to the Papadakis method (quartz transducer with center frequency of 20MHz) (Papadakis, 1967), on differently oriented bar-shaped samples using the equations given by Truell et al. (1969) and corrected for the piezoelectric contributions (Ljamov, 1983 Ikeda, 1990). The samples were oriented in axial directions XYZ, and 45° rotated against the X- and Y-axes, respectively. In order to obtain optimized values for the elastic materials parameters, the elastic stiffness coefficients Cy were used to calculate and critically compare the results of surface acoustic wave (SAW) measurements. 

Since 1970 the main anomalies in the propagation of ultrasonic waves in non-oriented samples of liquid crystals have been discovered these are the phase velocity dispersion and the nonclassical (a/f = constant) behavior of the ultrasound absorption coefficient due to the relaxation phenomenon. The anomalies are manifested in the megahertz Irequency range and are most pronounced in the vicinity of phase transitions. Kapustin [14] was the first to begin extensive investigations on the anomalies, and look at all types of liquid crystals. Most of the work done since 1980 has involved the use of a dc magnetic or electric field to... 

Determination of the viscosity coefficients from the mechanical wave propagation and attenuation in the ordered nonatic phase is probably the closest to the first principles methods. The shear impedance technique is based on measuring the reflection and attenuation of ultrasonic shear waves [90-92]. The conqtlex shear impedance of the nematic sample, Zn = Rn + iXn, is determined from the complex... 

In practice ultrasound is usually propagated through materials in the form of pulses rather than continuous sinusoidal waves. Pulses contain a spectrum of frequencies, and so if they are used to test materials that have frequency dependent properties the measured velocity and attenuation coefficient will be average values. This problem can be overcome by using Fourier Transform analysis of pulses to determine the frequency dependence of the ultrasonic properties. 

Theoretical study of the propagation of the elastic wave in 1-3 composites [8334.147-149] enable one to indicate how electromechanical properties oi piezoelectric composites depend on the properties of the component phases and the volume fraction of tte piezoelectric ceramics. The dependence between electromechanical coupling foctor k) and the acoustic impedance (Z) can be determined theoretically for composites of d erent ceramic contents and to find a compromise between increasing k and Z while increasing ceramic contents. High values of coefficient k and small acoustic impedance Z are required in applications for ultrasonic transducers. 

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