Nondestructive control of bolt tightening : Absolute and differential evaluation

The feasibility of an absolute ultrasonic measurement of the tightening force in bolts was investigated. For this purpose, we used both longitudinal and transverse ultrasonic waves (a biwave approach) to remove the time of flight measurement of waves in the unstressed state. First, we developed an equation based on the acoustoelasticity theory, which relates the ratio of the time of flight of transverse waves and longitudinal waves to the tightening force. Finally, we achieved experimental measurements on different tightening configurations of the tested bolts and compared the suitability and precision of this absolute technique with the classical differential technique. The results show that the absolute technique provides measurement of the tightening force without loosening the bolt with an uncertainty estimated on the order of ±12% at 95% reliability.