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During their first couple decades, commercial ultrasonic instruments relied entirely on single-element transducers that used one piezoelectric crystal to generate and receive sound waves, dual element transducers that had separate transmitting and receiving crystals, and pitch/catch or through transmission systems that used a pair of single-element transducers in tandem. These approaches are still used by the majority of current commercial ultrasonic instruments designed for industrial flaw detection and thickness gaging, however instruments using phased arrays are steadily becoming more important in the ultrasonic NDT field.
The principle of constructive and destructive interaction of waves was demonstrated by English scientist Thomas Young in 1801 in a notable experiment that utilized two point sources of light to create interference patterns. Waves that combine in phase reinforce each other, while waves that combine out-of-phase will cancel each other.
Phase shifting, or phasing, is in turn a way of controlling these interactions by time-shifting wave fronts that originate from two or more sources. It can be used to bend, steer, or focus the energy of a wave front. In the 1960s, researchers began developing ultrasonic phased array systems that utilized multiple point source transducers that were pulsed so as to direct sound beams by means of these controlled interference patterns. In the early 1970s, commercial phased array systems for medical diagnostic use first appeared, using steered beams to create cross-sectional images of the human body.
Initially, the use of ultrasonic phased array systems was largely confined to the medical field, aided by the fact that the predictable composition and structure of the human body make instrument design and image interpretation relatively straightforward. Industrial applications, on the other hand, represent a much greater challenge because of the widely varying acoustic properties of metals, composites, ceramics, plastics, and fiberglass, as well as the enormous variety of thicknesses and geometries encountered across the scope of industrial testing. The first industrial phased array system, introduced in the 1980s, was extremely large, and required data transfer to a computer in order to do the processing and image presentation. These systems were most typically used for in-service power generation inspections. In large part, this technology was pushed heavily in the nuclear market, where critical assessment more greatly allows use of cutting edge technology for improving probability of detection. Other early applications involved large forged shafts and low pressure turbine components.
Portable, battery-powered phased array instruments for industrial use appeared in the 1990s. Analog designs had required power and space to create the multi-channel configurations necessary for beam steering, but the transition into the digital world and the rapid development of inexpensive embedded microprocessors enabled more rapid development of the next generation phased array equipment. In addition, the availability of low power electronic components, better power-saving architectures, and industry-wide use surface mount board design led to miniaturization of this advanced technology. This resulted in phased array tools which allowed electronic setup, data processing, display and analysis all within a portable device, and so the doors were opened to more widespread use across the industrial sector. This in turn drove the ability to specify standard phased array probes for common applications.
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