A system developed by researchers at Tohoku University captures 3D images enabling the detection of defects in metallic structures. The technology, called the piezoelectric and laser ultrasonic system (PLUS), combines the strengths of two different devices to produce its high-resolution 3D images of metallic structure defects, and could enhance safety in power plants and airplanes, among other areas and applications.

In addition to leading to accurate evaluation of material strength, the system could additionally be used to determine how material defects first started to form, said Yoshikazu Ohara, associate professor at Tohoku University.

Currently available “ultrasonic phased arrays” are an effective tool for imaging internal defects in solids, but only in two dimensions. Those devices are made of a piezoelectric one-dimensional array transducer with a limited number of individual elements up to 128. Electrical pulses in the piezoelectric elements are converted to a mechanical vibration that emits ultrasonic waves into the material under investigation. Ultrasonic waves are reflected back from internal defects and converted into electric signals that can be translated into a 2D image.

In PLUS, a laser Doppler vibrometer, which moves around the material’s surface to get a high-quality 2D scan of the area, receives the waves generated in a material from a piezoelectric transducer with a single element. The vibrometer then receives the scattered and reflected waves at a much larger number of points than those that a piezoelectric array transducer can receive.  An oscilloscope transmits the information the laser Doppler vibrometer receives to a computer, where an imaging algorithm processes it and converts it to 3D.

“Ultrasonic phased arrays, which are on the cutting-edge of ultrasonic inspection, can only provide 2D images because of their limited number of elements,” Ohara said. “PLUS makes it possible to have thousands of elements as a result of incorporating the 2D scan of a laser Doppler vibrometer in place of a piezoelectric array transducer.”

Though it’s only been tested on defects in metallic materials, Ohara said the technology can be applied to other materials including concrete and rock simply by changing the phased array transmitter to one that emits a different range of ultrasound frequencies.

A drawback is the long data acquisition and processing time of several hours. The acquisition time can be shortened, though, by adopting a high-speed analog-to-digital converter in place of the oscilloscope, using a more sensitive laser Doppler vibrometer, using different imaging algorithms, and/or employing a graphical processing unit.

The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0021282).