Abstract:

The dominating shear motion of a contact-mode shear wave transducer is often accompanied by a weak parasitic longitudinal mode due to its finite size and constrained boundary conditions. The amplitude and phase of this out-of-plane vibration may be exploited for determining the shear polarization direction and for detecting any asymmetry or distortion of the shear transducer's vibration pattern. In practice, the contact-mode shear probe is immersed in water and energized by an rf pulser to serve as the transmitter. A C-scan is then made of the face of the shear transducer using a focused longitudinal wave transducer as the receiver. The resulting image shows the out-of-plane longitudinal vibration of the shear transducer. For commercial contact-mode shear probes, this pattern usually consists of two crescent-shaped regions of higher vibration amplitude but of opposite phase. A line bisecting the two crescents and passing through their centers defines the shear polarization direction. Finite-element modeling of the shear displacement of a circular disk with constrained boundary confirmed the experimentally observed behavior. This approach provides a simple method for characterizing commercial shear wave transducers. [Work supported by Center for NDE, Iowa State University.]