Noncontact measurement of bolt axial force in 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 shaft. Finally, they are detected at the top surface using another laser and a speckle knife edge detector. With an examination based on the acoustoelastic theory and finite element analysis, we demonstrate that the waveform of the scattered ultrasound shifts linearly with axial force by the cross-correlation method. The proposed technique does not require machining to flatten a bolt's head and the end, while conventional ultrasonic methods need flattening procedures. The proposed technique enables fast, cost-effective axial force measurement in mass production manufacturing processes.