ELECTROMECHANICAL COUPLING-COEFFICIENTS AND FAR-FIELD RADIATION-PATTERNS OF LITHIUM-NIOBATE BARS (Y-CUT) USED IN HIGH-FREQUENCY ACOUSTICAL IMAGING AND NONDESTRUCTIVE TESTING

For high-frequency applications in ultrasonic imaging and nondestructive testing, lithium niobate (LiNbO3) can be used When cut under the shape of long bars with proper orientation and width to thickness ratio W/T. However, due to a strong material anisotropy, the modeling Of such elementary,bar transducers, for the sake of optimization, is quite difficult. In this paper, the use of the finite element method, with the help of the ATILA code, is proposed to compute resonance frequencies, coupling coefficients, electrical impedances and in-water far-field directivity patterns. (YZw)36-degrees and (YXl)36-degrees cuts with different WIT ratios have been analyzed and numerical results have been carefully and successfully compared with measurements. Main results concem the (YZw)36-degrees cut for which symmetrical directivity patterns are obtained. In this case, a quite constant value is found for the thickness mode coupling coefficient k(e) in the ranges W/T < 0.8 (k(e) almost-equal-to 50%) and 1.2 < W/T < 1.7 (k(e) almost-equal-to 40%). For the (YXl)36-degrees cut, the directivity pattern is nonsymmetrical. In all the cases, a clear physical interpretation of transducer radiation characteristics is provided in terms of vibration modes. This analysis is the first application of new damping finite elements developed together with a specific extrapolation method and presented in a previous, companion paper [Assaad et al, J. Acoust. Soc. Am. 562-573 (1993)].