Tohoku University develops auto-frequency-adaptive ultrasonic technology of 3D visualization of internal defects in concrete

It is difficult to accurately examine internal defects and cracks in aging concrete structures using non-destructive methods. Professor Yoshikazu Ohara and Graduate Student Yuto Fujikawa from Tohoku University Graduate School of Engineering, have successfully developed a device that can visualize various internal defects in concrete in three dimensions through international collaborative research with Los Alamos National Laboratory in the United States. This will greatly contribute to identifying hazardous locations in aging concrete infrastructure. Their research was published in Applied Physics Letters.

Accidents are beginning to occur frequently in concrete structures such as tunnels, bridges, and highways due to aging, both domestically and internationally. Although internal delamination and cavities that are not visible from the outside are often the cause of issues, visual inspections can only evaluate the surface, and acoustic testing can only assess shallow areas. On the other hand, the industrial application of ultrasonic phased arrays developed in the medical field has progressed as a cutting-edge non-destructive ultrasonic testing method, and technologies for imaging internal defects have also become widespread. However, because concrete has an extremely high ultrasonic attenuation rate, it has been difficult to image the interior of concrete structures using ultrasonic phased arrays designed for metallic materials. This has been one of the most challenging issues in the field of non-destructive testing.

The research group has been working to expand applications to highly attenuative materials, having previously developed PLUS, a three-dimensional ultrasonic imaging method that combines a piezoelectric transducer and a laser Doppler vibrometer, and demonstrated it with metallic materials.

In this study, they advanced new efforts to integrate broadband ultrasonic transmission and reception with automatic frequency optimization functionality based on the PLUS technology (three-dimensional imaging method using piezoelectric transducer transmission and ultra-multiple laser reception) that they have been developing. Because piezoelectric transducers can transmit powerful broadband ultrasonic waves and laser Doppler vibrometers can acquire a wide range of frequencies in a non-contact manner, the system can automatically select and measure the optimal frequency band of ultrasonic waves affected by attenuation. This has realized an auto-frequency-adaptive PLUS that can flexibly handle highly attenuative concrete with different attenuation characteristics for each material.

Furthermore, by taking advantage of PLUS's capability to freely increase the number of points of scanning received laser, it is possible to construct a two-dimensional matrix array on the scale of thousands of elements, far exceeding the limitations of conventional two-dimensional piezoelectric matrix array transducers (approximately 256 elements). This has enabled high-resolution three-dimensional imaging through the fusion of high-density reception and broadband transmission and reception. In fact, using this method, the researchers have successfully achieved three-dimensional visualization of delamination and crack-like defects (slits) in mortar and carbon fiber-reinforced concrete.

With the newly developed auto-frequency-adaptive PLUS, it becomes possible to understand defects inside aging concrete infrastructure in three dimensions, enabling the identification of hazardous locations that were difficult to spot with conventional visual inspections. In situations where updating all structures is not realistic due to economic and environmental constraints, hazardous locations can be prioritized for repair, greatly contributing to the efficiency of maintenance management.

Journal Information
Publication: Applied Physics Letters
Title: Auto-frequency-adaptive 3D ultrasonic phased-array imaging system for highly attenuative materials
DOI: 10.1063/5.0291949