Fibre waveguide transducers for Lamb wave NDE

The authors describe and demonstrate a system exploiting the use of cylindrical ultrasonic waveguides for 'remote' Lamb wave generation and detection in thin plates. Ultrasonic energy is introduced into the cylindrical waveguides from a piezoceramic transducer via an epoxy bond, and the nature of this coupling is investigated theoretically using finite element analysis. The results show that lateral modes in the transducer (related to width and length, or diameter) couple into the waveguide more effectively than the more efficient thickness modes, as a result of the epoxy bond converting lateral motion into longitudinal motion. The thin cylindrical waveguides are then used to deliver ultrasonic signals to specific points in plate-type structures. As such, the waveguides may be considered as point sources, a combination of which can behave as a line array. To demonstrate this, it is shown that Lamb waves are generated and detected by bonding the ends of several copper wires, at half-wavelength spacing, to the surface of a sheet of aluminium. The fundamental symmetric and asymmetric modes are then used to explore possible non-destructive evaluation measurements. Following this, a form of portable 'wire' transducer, comprising an array of waveguide sources, is presented that allows the detection of synthetic flaws in an aluminium plate.