Spherical lens transducer

We have also received and ultrasonically examined 21 silicon nitride tensile specimens. These samples are cylindrical and have a diameter of 6.4 mm in the gage section, which was the only region inspected. The evaluation was conducted using a 75-MHz. spherically focused transducer. As we have mentioned in previous reports, the sharply curved entry surface of small-diameter cylindrical specimens introduces both severe astigmatism and spherical aberration in the ultrasonic beam distribution inside the solid and limits the depth at which critical flaw sizes can be detected to a few hundred microns. Our work has indicated that this limitation could be greatly reduced by aspherical shaping of the transducer lens, but the cost of such a transducer has thus far proved prohibitive. No indications were observed in any of the specimens although we could not perform a 100% inspection on any of the rods because of equipment limitations. 

This system consists of a symmetrical pair of lens elements connected by a small volume of liquid. Each lens consists of a single spherical interface between the liquid and a lens rod. The lens element is formed by polishing a small concave spherical surface in the end of a sapphire rod. At the opposite end of the rod, a thin film piezoelectric transducer is centered on the axis of the lens surface. 

Fig. 2.1. A lens for high-resolution acoustic microscopy in reflection. The central transparent part is a single crystal of sapphire, with its c-axis accurately parallel to the axis of the cylinder. The sandwich structure at the top is the transducer, with the yellow representing an epitaxially grown layer of zinc oxide between two gold electrodes. The pink shaded areas within the sapphire represent the plane-wavefronts of an acoustic pulse they are refracted at the lens cavity so that they become spherical in the coupling fluid. A lens for use at 2 GHz would have a cavity of radius 40f[Pg.8]

A simplified picture of the transducer, lens surface, and reflecting object is shown in Fig. 7.4. The waves radiated by the transducer are refracted by the lens so as to form a spherical wavefront centred on the focal point of the lens. Each point on this wavefront can be described by its angular coordinates from the focus let these be 6 for the zenithal angle (i.e. the angle to the lens axis, again taken to be normal to the specimen surface) and for the azimuthal angle. Thus the spherical wave emerging from the lens can be described by... 

The need to measure V u) as a complex-valued quantity can be met by using the accurate amplitude and phase measurement system described in Chapter 5. A system like that has been used to demonstrate material characterization by inversion of V(z) (Liang et al. 19851 ). The frequency was about 10 MHz, so that attenuation in the water was negligible and mechanical stability could be readily achieved to a very small fraction of a wavelength. A spherical transducer was used instead of a combination of a planar transducer and a lens this removes the cos 8 term in (8.1), but the principle of the inversion formulation remains the same. Figure 8.2(a) shows the amplitude and phase of an experimental V(z) curve for fused silica. The phase... 

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