Contactless measurement of bolt axial stress using a shear-wave electromagnetic acoustic transducer

An ultrasonic method is presented to monitor holt axial stress using a noncontacting shear-wave electromagnetic acoustic transducer (EMAT). The EMAT generates and detects the shear wave propagating in the axial direction of the bolts. We evaluated the axial stress from the phase shift of the top-to-bottom echo and from the resonance frequency shift; both are induced by the acoustoelastic velocity change and the bolt extension under the tensile load. The received echo was processed through a superheterodyne phase-sensitive detector, and its phase or amplitude spectrum is obtained for the stress measurements. The contactless aspect of the EMAT allows a much easier and quicker stress evaluation, and also leads to high reproducibility by eliminating error sources associated with the use of contacting piezoelectric transducers. (C) 2001 Elsevier Science Ltd. All rights reserved.