On the Measurement of Ultrasonic Shear Fields in Fused Silica

Abstract

Photographs of transducer radiation patterns in solids often exhibit many of the features predicted by diffraction theory for the radiation from a uniformly illuminated aperture in an infinite black plane. A comparison of quantitative measurements of these radiation patterns with those predicted by theory offers the most direct method of determining the applicability of diffraction theory to the radiation from bonded transducers. We shall discuss the experimental considerations and apparatus to measure ultrasonic shear fields in fused silica. Our technique makes use of the stress-induced rotation of the plane of polarization light passing through an ultrasonic field. Fields are generated by transducers bonded to a polished fused silica test sample. The sample is placed between crossed polaroids. A narrow beam of light passing through the sample and polaroids is detected by a photomultiplier. The output of the photomultiplier is displayed on an oscilloscope while the light beam is slowly moved across the ultrasonic field. By driving the transducer from a pulsed rather than a continuous oscillator and suitably gating the photomultiplier signals, it will be possible to extend measurements to very small shear field strengths with an amplitude range of well over 100. A few preliminary measurements of the radiation patterns from thickness shear mode ceramic transducers will be presented.