Determination of Elastic Constants of Isotropic Materials at Megacycle Frequencies

Abstract

The frequency dependence of the phase and group velocities for longitudinal and flexural waves in a plate provides the basis for several convenient methods of determining elastic constants. In these methods two independent elastic constants are determined in strip-shaped samples from measured values of certain special frequencies and delay times. One set of data is determined by the frequencies at which certain pairs of modes have either equal group or phase velocities. At other frequencies the delay vs frequency characteristics of various modes have extrema, and the frequencies at which these occur along with the measured delays provide another set of data. Theoretical calculations have been carried out which show the dependence of these measured quantities on Poisson's ratio. The shear wave velocity and Poisson's ratio are determined for a sample behaving essentially as an isotropic, elastic solid by selecting values that bring calculated and measured quantities into agreement. The methods to be discussed are capable of detecting anisotropic or anelastic behavior if either or both of these complications are present and have been used successfully at frequencies up to 35 mc.