Noncontact measurement of bolt axial force during tightening processes using scattered laser ultrasonic waves

This paper presents a new methodology for noncontact measurement of the axial force of bolts in their tightening processes using laser-generated ultrasound waves. This method employs ultrasound waves scattered in a bolt shaft to detect axial force changes, while most conventional ultrasonic methods use ultrasound waves propagating linearly along the bolt axis. The ultrasound waves in this study are generated by laser irradiation on the top surface of a bolt. Subsequently, they propagate deeply into the shaft and return towards the top of the bolt through complicated paths due to the multiple scattering in the bolt shaft. Finally, they are detected at the top surface using another laser and a speckle knife edge detector. With an examination based on the finite element analysis and verification experiments, it has been shown that the waveform of the scattered ultrasound shifts in time linearly with increasing the axial force. The time shifts were estimated using the cross-correlation analysis between the measured waveforms and the reference waveform with no axial force. This result demonstrates the feasibility of estimating the change in axial force during tightening processes once the relationship between the time shifts and axial force is obtained for the specific type of bolt to be used in products. Furthermore, the proposed technique does not require machining to flatten a bolt's head and the end, while conventional ultrasonic methods need the flattening procedures, enabling fast, cost-effective axial force measurement in mass production manufacturing processes.