MEASUREMENTS OF ELASTIC-CONSTANTS OF VERY THIN ANISOTROPIC PLATES

This paper describes an ultrasonic technique for the determination of elastic constants of anisotropic plates with thickness less than the ultrasonic wavelength. The technique is based on measuring transmission zeros and maxima for an ultrasonic wave obliquely incident upon a plate immersed in water. The transmission zeros result from interaction between longitudinal (symmetric) and flexural (antisymmetric) guided waves in a thin plate. It is shown that the angle positions of the transmission zeros are strongly dependent on the plate's longitudinal stiffness. The technique is demonstrated for the determination of elastic constants of porous aluminum oxide membranes with parallel through-thickness pores. Such membranes are transversely isotropic with the isotropy plane parallel to the median plane. A nonlinear least-squares algorithm, which takes the elastic constants as free parameters and minimizes the deviation between the measured and calculated angles of the transmission zeros and maxima, was used to determine the plate stiffness and shear modulus. In addition, the shear modulus in the plane of the plate is measured from wave resonances of the fluid-filled pores. A comparison with theoretical predictions using different micromechanical models to estimate the elastic constants of the porous Al2O3 layer is given.