The megahertz frequency range ultrasonic methods

Measurements of the velocity and attenuation of elastic waves at ultrasonic frequencies are important, especially for oriented polymers and composites. Compact solid specimens with dimensions of the order of 10 mm are required. 

An alternative approach [19, 20] is to immerse a specimen, thickness d, in a water-filled tank fitted with both a transmitter and a receiver of ultrasonic waves, and measure the change (t) in transit time with and without the specimen in the beam. If v is the velocity in the polymer and that in the water we have 

The various wave velocities, which can be derived from measurements made over a range of incident angles, are related to the elastic stiffness constants. 

In a variation of the above method Wright et al. [21] detected the component of the incident beam that was reflected from the immersed specimen and hence measured the critical angle of incidence. 

More recently this technique has been developed to measure the anisotropy of uniaxial composites [22]. A specimen of uniform thickness, placed between the 

---