DIRECT DETERMINATION OF GROUP-VELOCITY SURFACES IN A CUSPIDAL REGION IN ZINC

A direct determination of group velocity surfaces of zinc from the group velocities measured in various directions within and without a cuspidal region is described. A zinc crystal disk oriented in the symmetry direction [0001] is used as a specimen. Quasilongitudinal (QL) and quasitransverse (QT) elastic pulses propagating in a wide range of directions are generated by a broadband pointlike source activated by the fracture of a tiny glass capillary on the surface of the specimen and they are detected at epicentral and off-epicentral positions on the opposite face by a miniature capacitive transducer which senses a displacement normal to the surface. The group velocities associated with QL and QT modes are directly measured from the detected signals. The QL mode arrival is easily identified as the first point from which the signal jumps from the noise level. In the cuspidal region, there are three branches of QT group velocity: fast QT (FQT) ray branch, intermediate speed QT (IQT) ray branch, and slow QT (SQT) ray branch. It is observed that the arrivals of FQT, IQT, and SQT rays correspond to the first zero crossing, to the first negative minimum, and to the second local minimum, respectively, in the displacement signal. In the epicentral signal, both IQT and SQT rays coincide, whose arrivals correspond to single sharp negative minimum, and the FQT ray arrival is found at the first zero crossing after the arrival of a longitudinal wave. Outside the cuspidal region, both FQT and IQT branches disappear and so do their corresponding characteristics, i.e., the zero crossing and sharp negative minimum in the displacement curve. The SQT ray arrival outside the cuspidal region is observed at the first sharp minimum in the displacement signal. Finally, the group velocities of pure transverse waves with shear horizontal polarization propagating in various directions of a [0001] zonal section within and without the cuspidal region are measured with the miniature piezoelectric shear transducer (PZT) source and shear PZT detector.