Combination of Longitudinal and Transverse Ultrasonic Waves for In Situ Control of the Tightening of Bolts

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 (bi-wave method) to remove the time-of-flight measurement of waves in the unstressed state. To avoid the systematic calibration tests (generally carried out before tightening control), an experimental method has been applied. This method allows determination of the effective length under stress and the difference between longitudinal and transverse acoustoelastic coefficients of a tightened bolt. The results show that the proposed method can provide absolute measurement of the tightening force without either loosening the bolt or performing systematically calibration tests with an uncertainty estimated on the order of ±10% at 95% reliability.