On the propagation of elastic waves through composite media

Summary

With the aid of an ultrasonic pulse technique, the propagation of elastic waves (longitudinal as well as transverse) through polyurethane rubbers filled with different amounts of sodium chloride particles was studied. The velocity of both longitudinal and transverse waves was found to increase with filler content. From the measured wave velocities, the effective modulus for longitudinal waves,L, bulk modulus,K, and shear modulus,G, were calculated according to the relations for a homogeneous isotropic material. All three moduli appear to be monotonously increasing functions of the filler content over the whole experimentally accessible temperature range (−70 °C to + 70 °C forL andK;}-70 °C to about −20 °C forG) and they, moreover, reflect the glassrubber transition of the binder.Poisson's ratio,μ, was found to decrease with increasing filler content and show a rise at the high temperature side of the experimentally accessible temperature range (about −20 °C) as a result of the approach of the glass-rubber transition.

In addition to the velocities, the attenuation of both longitudinal and transverse waves was measured in the temperature ranges mentioned. It was found that in the hard region tan δ_L as well as tan δ_G are independent of the filler content within the accuracy of the measurements. In the rubbery region, however, tan δ_L, increases with increasing filler content.

Finally, the experimental data are compared with a simple macroscopic theory on the elastic properties of composite media.