FIBER WAVE-GUIDE TRANSDUCERS FOR LAMB WAVE NDE

The authors describe and demonstrate a system exploiting the use of cy lindrical ultrasonic waveguides for 'remote' Lamb wave generation and detection in thin plates. Ultrasonic energy is introduced into the cyl indrical waveguides from a piezoceramic transducer via an epoxy bond, and the nature of this coupling is investigated theoretically using fi nite element analysis. The results show that lateral modes in the tran sducer (related to width and length, or diameter) couple into the wave guide more effectively than the more efficient thickness modes, as a r esult of the epoxy bond converting lateral motion into longitudinal mo tion. The thin cylindrical waveguides are then used to deliver ultraso nic 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 coppe r wires, at half-wavelength spacing, to the surface of a sheet of alum inium. The fundamental symmetric and asymmetric modes are then used to explore possible non-destructive evaluation measurements. Following t his, a form of portable 'wire' transducer, comprising an array of wave guide sources, is presented that allows the detection of synthetic fla ws in an aluminium plate.