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Advantages of phased arrays  

This article is excerpted from the paper, “The promise of ultrasonic phased arrays and the role of modeling in specifying systems” presented by authors Guillaume Neau, Ph.D. and Deborah Hopkins, Ph.D. at the ASNT Fall Conference & Quality Testing Show held in Houston on October 23 - 27th, 2006. You may download the paper here.

Less movement, faster inspection, better reliability

The advantages of phased-array systems include the ability to perform electronic scanning of the ultrasonic beam, which can reduce inspection times by eliminating or reducing the need move the probe. As illustrated in the figure hereafter, electronic scanning is accomplished by firing successive groups of elements in the array. A complete C-scan image can be obtained with a matrix phased array with the probe in a fixed position. The reliability of inspections can also be improved by reducing the need to move the probe.

electronic scanning using a phased-array probe As is well known, good coupling between ultrasonic probes and the part undergoing inspection is crucial for good acoustic measurements. Each time the probe is moved, there is a risk of losing or degrading coupling. Thus, minimizing the number of times the probe is moved helps to maintain uniform conditions for multiple measurements.  

Example of electronic scanning. A subset of the elements in the array are used to generate a focused beam at normal incidence; this beam is then translated across the test specimen by firing subsequent groups of elements without moving the probe.

Real-time imaging, easier interpretation

Phased arrays allow a broad spectrum of inspection strategies that improve performance, for example, sectorial scanning and focalization after reflection off the back surface of the test specimen.

The most advanced phased-array systems include tools such as dynamic-depth focusing. With real-time imaging, inspections are easier to perform and the reliability of the measurements is also greatly improved. Because thousands of signals are captured and displayed at once, the struggle that operators often have in locating and visualizing defects on the screen is greatly reduced. In addition, the number of false alarms is diminished because of reduced operator dependence, and data recording and traceability are improved. Experimental results obtained using a sectorial scan are shown in hereunder.

real-time crack sizing using ultrasonic phased-arrays Measurements were performed using 32 elements of a 64-element linear array with a frequency of 5 MHz. The test specimen was an aluminum reference block containing planar defects.  

Example of sectorial scanning used for crack sizing. The solid rectangular line indicates the geometry of the test specimen under examination, including two parallel saw cuts. The corner echoes (labeled “a”) resulting from the cuts were easily detected using 32 elements of a linear array with a central frequency of 5 MHz. Diffraction from the crack tips (labeled “b”) is only observed when the beam is appropriately focused and directed.
 

Applying delay laws to improve performance and simplify procedures

Phased arrays can also replace an entire tool kit of conventional transducers.  

A single phased array used in conjunction with appropriate delay laws can reproduce the same acoustic beams achieved with numerous conventional probes, while also providing greater functionality. Using a phased-array controller that allows several types of delay laws per inspection, the results of several different sets of measurements that comprise a complete NDT procedure can be visualized simultaneously in real time.