Detecting loose bolts with ultrasonic waves
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Shibaura Institute of Technology and The University of Edinburgh develop technology to detect loose bolts with ultrasonic waves

by Shun Otsuki
President & Editor in Chief KINSAN FASTENER NEWS


A research group led by Professor Naoki Hosoya of the Department of Functional Engineering at Shibaura Institute of Technology (Tokyo) and Dr. Francesco Giorgio-Serchi of the Faculty of Engineering at The University of Edinburgh (UK) has developed a method that can detect loose bolts by measuring ultrasonic vibrations.

Accidents caused by loose bolts have occurred in all fields such as infrastructure, airplanes, and vehicles, but the technology to detect looseness has not been completely established. There is a human tapping sound test as a conventional method, but it depends largely on the skill level, and it can only be judged by the level of sound that can be heard by humans. There is also a problem that such skilled workers are lacking. The developed method measures and quantifies the slightest vibration when a bolt is hit with a hammer with an accelerometer, and can detect even the slightest looseness that cannot be detected by the tapping sound inspection.

If it can be put into practical use, it will be possible to inspect at low cost with a general-purpose test device without relying on the proficiency level of the inspector. Since there is no new device to be added to the bolted joint, it is also a great feature that the existing joint can be inspected.

According to the research results, it was found that when the object is hit and vibrated, the natural vibration of the vibration mode type at the tip of the bolt increases when the bolt is tightened, and decreases when the bolt loosens.

It was confirmed that there was a clear relationship between the natural frequency of the bolt tip and the axial force, and it was shown that the frequency decreased as the axial force decreased. In past experiments, they have succeeded in associating the relationship between the natural frequency and the axial force by hitting the head side of the bolt.

Professor Hosoya of the research group majoring in vibration and acoustics said, "Technically, it is possible to measure by completely non-contact using a pulse laser, but in terms of cost, this hammering inspection is a highly practical method."
In the future, they are also aiming to realize remote measurement and automation, and they are also starting joint development with companies.