Flexible patch unlocks potential of ultrasound for inspecting irregular components

San Diego engineers develop elastomer-based structure that conforms to curved surfaces to direct soundwaves into engineered structures

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The flexible patch, pictured here stuck to a sphere, is patterned with piezoelectric ‘islands’ connected by copper-spring ‘bridges’

Ultrasound is often used in engineering to detect cracks in metal components. But its usefulness is sometimes limited by the shape of the probe used to direct the high-frequency sound waves into the metal. Ultrasound emitters tend to be flat-bottomed, so if the surface to be tested is not perfectly flat as well, the emitter cannot make a good contact on the component and the technique cannot be used.

“Elbows, corners and other structural details happen to be the most critical areas in terms of failure – they are high stress areas,” said Francesco Lanza di Scalea, a structural engineer from University of California San Diego (UC San Diego) and lead author on the study of the new method of inspecting irregularly-shaped objects. “Conventional rigid, flat probes aren’t ideal for imaging internal imperfections inside these areas.”

Although gels, oils or even water can create better contact between the probe base and the surface, using too much of these can dull some of the signals. Moreover, conventional emitters are quite bulky, so difficult to get into areas with convoluted geometry.

The solution designed by di Scalea and his colleagues, described in a paper in Science Advances, is a thin patch of a silicone elastomer patterned with an array of small electronic components known as islands connected by flexible spring shaped copper wires known as bridges. The islands contain electrodes and piezoelectric transducers that produce ultrasonic waves when electricity passes through them. Both the elastomer and the bridges can stretch and bend, so when the patch is placed on a curved surface, the islands make good contact all the way across the patch’s area.

The team tested the device by attaching it to an aluminium block with a wavy surface. The blog contained 2mm-wide holes and other defects located 2 to 6cm below the surface, and the ultrasound device successfully imaged these features.

“It would be neat to be able to stick this ultrasound probe onto an engine, airplane wing or different parts of a bridge to continuously monitor for any cracks,” said Hongjie Hu, a materials science and engineering Ph.D. student at UC San Diego and co-first author of the study.

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The patch can be used on awkward geometries

The team is now finessing their device so that it can produce real-time imaging. They also hope to integrate both power and data processing into the probe so that it can be used to provide wireless, real-time imaging and videoing, said Prof Sheng Xu, a nanoengineering specialist from UC San Diego’s Jacobs School of Engineering, who also worked on the study.