Ultrasonic measurement of bolt-axial stress by a shear-wave electromagnetic acoustic transducer

摘要

An ultrasonic method is presented to monitor the axial stress of bolts using the noncontacting bulk-wave EMAT. The EMAT generates and detects the shear wave propagating in the axial direction of the bolt. The axial stress is evaluated nom thephase shift of the top-to-bottom echo or the resonant frequency shift; both of these are induced by the velocity change from the acoustoelasticity and the extension of the bolt The received echo is processed through the superheterodyne phase detector, and its phase or amplitude spectrum is extracted for the stress measurements. Since the method does not require any coupling material between the transducer and the bolt head, it allows evaluation of the stresses much more easily and quickly than conventional measurements based on piezoelectric transducers. The noncontacting aspect of the EMAT assures high reproducibility of these determinations, and then eliminates measurement errors associated with the operator, the transducer, the surface condition, andsoon.