Applications of ultrasonic reflection mode conversion transducers in NDE

Linearly polarized ultrasonic shear waves constitute a privileged tool for investigation in the field of mechanical behavior of materials and metallic structures as well as for the determination of acoustical properties of po rous materials, These waves are often generated by a piezoelectric plate vi brating in a direction perpendicular to its thickness. However, this way of production presents some difficulties to obtain a perfectly linear polariz ation, To palliate to this inconvenience, shear waves can be generated by t he mode-conversion of a longitudinal wave, This paper deals with the principle and some applications of two types of m ode conversion transducers which permit transmitting and receiving shear wa ves obtained by reflection mode-conversion. These transducers are made up w ith straight ultrasonic probes coupled, by contact, to acoustic delay lines (or relays) of different geometry. The first type of transducer uses an ac oustic relay or delay line of big length. When it is buried in a recipient containing dry sand, it permits the detection of external mechanical distur bances as vibrations and transient shocks. The second type of transducer, f or which the delay line has a reduced length, is used for the detection of material anisotropy and for the study of the polarization direction of conv entional normal incidence shear wave transducers. (C) 2000 Elsevier Science Ltd. All rights reserved.