On strong ellipticity conditions for piezoelectric materials of the crystal classes 6 mm and 622

The conditions of strong ellipticity for piezoelectric materials of the cry stal symmetry classes 6 mm and 622 are presented. These are derived using p ositivity conditions for quadratic, cubic and quartic polynomials. Moreover , a simplification of the slowness curve for the class 6 mm was carried out , enabling a simplified set of strong ellipticity conditions. Some piezomat erials were analysed demonstrating the feasibility of the proposed simplifi cation. (C) 2000 Elsevier Science B.V. All rights reserved.