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This paper describes a more sensitive version of the expanding aperture annular array system reported recently. A preliminary clinical evaluation is presented.

An acoustic emission technique for measuring the failure time in thermal shock experiments is described. The technique offers a unique opportunity to measure the heat-transfer coefficient of the test system and hence, to obtain a fully quantitative measure of the peak surface stress generated during the test. Measurements on soda-lime glass have demonstrated that rapid thermal fracture in the material occurs when the surface stress attains a critical value equal to the propagation stress for the most deleterious surface flaw. The effects of slow crack growth on the failure time are also investigated and correlated with recently developed theory.

Techniques and instrumentation for studying high frequency cyclic crack propagation in ceramic materials are described. Tests performed on a silicate glass show that the cyclic slow crack growth mechanism up to 600 Hz is identical to the quasi-static mechanism. Conversely, a strong cyclic effect on the crack growth rate is observed in a tungsten carbide-cobalt material.

A dynamically-focused annular array system for contact B-scanning has been developed. The design is based on a constant F-number approach, whereby, at short focal lengths, the aperture is increased in proportion to the focal length. This approach allows the use of larger area array elements, thus increasing the sensitivity of the system. Other major advantages include a substantial reduction in the time delays and refocusing rates required for the lens synthesis with a corresponding reduction in the electronic complexity of the system. The initial design employs an array operating at 2.25 MHz, with four annuli active at the near focal length of 1.5 cm. As the focal length increases, the array expands to a maximum of twelve rings, with 4.0 cm outer diameter, for focal lengths greater than 12 cm. A single, tapped delay line with 1 μs total duration provides the time delays for focusing on receive. A variable point or line focus is provided on transmit. Experimental measurements of the focusing properties of the system include beam profiles showing mainlobe width and sidelobe levels and B-scans of a standard test object.