Ultrasonic Determination of Stiffness Properties of an Orthotropic Viscoelastic Material

Ultrasonic velocity measurements were used to populate the real portion of the stiffness matrix of an orthotropic viscoelastic material, as a function of frequency. The orthotropy originated from short cellulose reinforcing fibers cast into the polypropylene matrix. The results were consistent with a Zener relaxation model for the material, with a time constant of the order of a few microseconds. It was found that the shape of the dispersion curve varied markedly with orientation: The frequency dependence of the phase velocity was at a minimum along the primary axis of the reinforcing fibers; the fibers were shown to inhibit viscoelastic behavior in this direction. The attenuation coefficient was measured along one primary axis of the material, and found to be consistent with the dispersion curves and localized form of the Kramers-Kronig relationships linking the real and imaginary components of the stiffness matrix.