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 u ltrasonic wavelength. The technique is based on measuring transmission zeros and maxima for an ultrasonic wave obliquely incident upon a pla te immersed in water. The transmission zeros result from interaction b etween longitudinal (symmetric) and flexural (antisymmetric) guided wa ves in a thin plate. It is shown that the angle positions of the trans mission zeros are strongly dependent on the plate's longitudinal stiff ness. The technique is demonstrated for the determination of elastic c onstants of porous aluminum oxide membranes with parallel through-thic kness pores. Such membranes are transversely isotropic with the isotro py plane parallel to the median plane. A nonlinear least-squares algor ithm, which takes the elastic constants as free parameters and minimiz es the deviation between the measured and calculated angles of the tra nsmission 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 co mparison with theoretical predictions using different micromechanical models to estimate the elastic constants of the porous Al2O3 layer is given.