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 nondestructi ve testing, lithium niobate (LiNbO3) can be used When cut under the sh ape of long bars with proper orientation and width to thickness ratio W/T. However, due to a strong material anisotropy, the modeling Of suc h elementary,bar transducers, for the sake of optimization, is quite d ifficult. In this paper, the use of the finite element method, with th e 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 di fferent WIT ratios have been analyzed and numerical results have been carefully and successfully compared with measurements. Main results co ncem the (YZw)36-degrees cut for which symmetrical directivity pattern s 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 radi ation characteristics is provided in terms of vibration modes. This an alysis is the first application of new damping finite elements develop ed together with a specific extrapolation method and presented in a pr evious, companion paper [Assaad et al, J. Acoust. Soc. Am. 562-573 (19 93)].