Ultrasonic Sensor Eliminates Inspection Blind Spots in Extreme Environments

Title image: Researchers from the Non-Destructive Metrology Group at the Korea Research Institute of Standards and Science (KRISS), from left to right: Kim Seung, Jeong Ju Young, Dr. Seung Hong Min, Dr. Park Chan Wook, and Dr. Park Choon-Su. Credit: KRISS

The Korea Research Institute of Standards and Science (KRISS) has developed an ultrasonic sensor that uses a waveguide to detect defects in all directions without attaching the transducer directly to the inspection target. By allowing ultrasonic excitation and reception at a distance from the surface, the technology is intended for high-risk industrial facilities where heat, toxic gases, or other hazardous conditions have made direct sensor installation difficult.

A waveguide is a tubular structure that guides waves or physical energy along a defined path. Depending on its geometry, it can control the direction of ultrasonic propagation, manage reflection and refraction, and influence the directionality and energy efficiency of the transmitted waves.

Ultrasonic testing (UT) is widely used to verify structural reliability in aerospace, nuclear power, and large industrial plants. Conventional ultrasonic sensors have been difficult to deploy in extreme environments such as high-temperature piping in nuclear power plants or storage facilities holding hazardous chemicals. Because these sensors must be attached to the surface of the inspection target, they are frequently damaged by heat or corrosive substances. In restricted areas where human access is limited, installation may not be possible at all.

Transducers designed for full 360° coverage have typically used segmented configurations, with multiple sensing elements arranged circumferentially. Combining the wave signals from these separate sources can produce interference and distortion, reducing inspection accuracy.

The KRISS Non-Destructive Metrology Group addressed this by introducing a waveguide as an intermediate medium. Ultrasonic waves travel through the waveguide, so the transducer can be positioned away from the hot or hazardous surface while the waveguide carries the signal to and from it.

The sensor generates torsional vibration within a cylindrical waveguide—much like twisting a towel—and transfers that vibration uniformly to the surface under inspection. Ultrasonic waves then propagate evenly in all directions, allowing defect detection with the transducer located at a distance from the target structure. The waveguide can be fabricated from various materials and geometries, and its end can be machined to conform to curved surfaces.

In testing, the sensor achieved approximately 95% directional uniformity and high mode purity, giving consistent signal capture across the inspection range. Signal amplitude was 13.7× greater than that of conventional segmented configurations, supporting rapid scanning of wide areas and higher-resolution imaging.

KRISS reports that the approach makes real-time monitoring feasible in environments where conventional nondestructive testing has been impractical, and that replacing multiple sensors with a single transducer and a low-cost waveguide reduces the cost of inspecting large areas.

Dr. Seung Hong Min of the Non-Destructive Metrology Group at KRISS said the sensor also performs well in liquid environments, allowing inspection of large submerged structures. “Based on its high reliability and versatility, the technology can be readily applied to accident prevention systems across various industrial facilities. By detecting previously inaccessible inspection blind spots, it has the potential to significantly contribute to the prevention of large-scale industrial disasters,” he added.

The research was supported by the KRISS Basic Research Program and published last December in Mechanical Systems and Signal Processing. It is available open-access.