Ultrasonic Tightening Assessment of Preloaded Bolted Joints: Comparative Study of Relative Tension Monitoring and Absolute Measurement via EMAT/Piezoelectric Coupling

This article presents a comparison of two ultrasonic tightening control methods for evaluating the tightening state of preloaded bolted assemblies. The first method enables relative monitoring of clamping force evolution over time. The second method is based on coupling an Electromagnetic Acoustic Transducer (EMAT) with a piezoelectric sensor. This hybrid configuration combines the non-contact generation capability of the EMAT with the high sensitivity of the piezoelectric sensor. It relies on measuring the variation in ultrasonic wave propagation time to estimate bolt elongation and preload in an absolute manner. A major challenge addressed is the effect of temperature, which influences ultrasonic velocity. This approach enhances inspection flexibility and allows reliable monitoring without disassembling the joint. A compensation strategy will be developed to correct these temperature-induced variations and ensure an accurate absolute assessment of the tightening force.