Axial stress measurement for tightened bolt based on the effect of stress on ultrasonic wave attenuation

Accurate assessment and monitoring of axial stress during the service life of the bolt connection is of great significance in ensuring the stability and reliability of the aerospace structure. The current acoustoelastic method requires both transverse and longitudinal mode waves and is less sensitive to small-size bolts, resulting in relatively larger errors in the evaluation of axial stress. In this paper, an axial stress measurement method with only a single longitudinal/transverse transducer for tightened bolts based on a novel attenuation-based model is introduced. The ultrasonic responses under axial preload in different frequency bands are comprehensively reflected through the energy attenuation coefficient matrix. In addition, an experimental method for selecting the optimal frequency band is given. Furthermore, the semi-permanent coupling method is proposed to provide a stable coupling condition to verify the feasibility of the attenuation-based model. Loading tests and ultrasonic tests were conducted on two kinds of strength-grade materials to obtain the calibration curves of both the longitudinal and transverse attenuation models. The experimental results show that the energy attenuation coefficient is positively correlated with the axial stress in a specific frequency range and the calibrated curve has a high fitting rate with a quadratic fitting function. The average relative error of the prediction results is less than 7.5%, which is better than the conventional acoustoelastic effect model.