An elastic-wave ellipsometer for measurement of material property variations

Abstract

Electromagnetic-acoustic transducers (EMAT’s) can excite and detect elastic shear waves with electronically controlled elliptical polarizations. These can be used to construct an ellipsometer for precise measurement of mechanical properties of solids, in analogy to devices presently used in optical studies. The elastic-wave case differs from the optical case in two important ways. Longitudinal as well as transverse waves will, in general, exist, and the propagation medium, as well as the surfaces, play an important role in determining the system response. A device is described which is designed to avoid the former mode conversion effects on thin plates. The results of two simple experiments to demonstrate its performance are then reported. In one, measurement of the texture of a metal plate demonstrates the ability to sense bulk property changes which alter the relative velocities of the two-wave components. In the second, measurements of the effect of a fluid on one side of the plate demonstrates the ability to sense surface changes which alter the relative attenuation of the two-wave components. The technique appears likely to find future application in the high-precision measurement of a number of elastic properties, particularly in view of the fact that it requires no couplant and hence can perform well under various adverse conditions.