Abstract

Lamb's waves have been of great interest to many scientists and engineers in the field of material characterization and non-destructive evaluation (NDE) for many decades. The most common technique in generating and detecting Lamb waves has been relying on the oblique incidence principle, which is also called coincidence method. In this method, the transducer is attached to a wedge block which is aligned at an angle with respect to the normal of tested material. However, it is often desired to generate Lamb waves with transducers in the remote position for the applications which involve adverse environment, such as one with elevated temperature or gas emission. There are currently several techniques available for performing such tasks, i.e. laser ultrasound and air coupled transducers. In this paper, we will investigate a novel technique to generate and detect Lamb waves by use of cylindrical ultrasonic waveguides attached to the ends with off-shelf ultrasound transducers.

Due to the impedance mismatch between the transducer and the cylindrical rod waveguide, the energy transmission from an off-shelf transducer to the rod is not very efficient. An unique technique which makes use of solid conical waveguide as an impedance transformer between the transducer and the rod waveguide was investigated to solve the problem. Insertion loss measurements for two different waveguide systems, i.e. transducer-rod and transducer-conical waveguide-rod, were conducted to verify the improvement of the acoustic energy transmission of transducer-conical waveguide-rod combination.

Transient plate waves excited by Nelson-Hsu pencil lead breaks were detected and recorded on an aluminum plate by the as-developed waveguides. Dispersion analysis was conducted to verify the plate modes excited by pencil lead breaks at different locations of the plate. Excitation of Lamb waves was also exercised by use of the waveguides. Experimental measurement of the mode velocity was compared with the theoretical predication.

Keywords: Ultrasonic testing; Lamb waves; plate waves; ultrasonic waveguide.

Source: Conference Proceedings of the 4th International Conference of the Slovenian Society for Nondestructive Testing, 24 - 25 April 1997, Ljubljana, Slovenia.
Pages:387

Publisher:
Slovenian Society for Nondestructive Testing, Faculty of Mechanical Engineering, Askerceva 6, SLO 1000 Ljubljana, Slovenia. Tel +386 61 177 12 03, Fax +386 61 218 567, Email: janez.grum@fs.uni-lj.si.