Measurement of axial stress in high-strength short bolts using ultrasonic attenuation

Based on the scattering attenuation theory of loaded polycrystalline metals, a bolt axial stress measurement method using ultrasonic echo attenuation is proposed for the problem that the conventional time of flight (TOF) method is difficult to measure the stress of high-strength short bolts. Firstly, the attenuation coefficients of body-centred cubic crystalline materials in the Rayleigh scattering range are described, and the mathematical model between the ratio of the peak of the two bottom echoes and the axial stress of the bolt is further derived. Then, the effects of the transducer wafer diameter, mounting eccentricity and bolt thread on the ultrasonic signal were analysed by finite element simulation, to provide a strategy for transducer selection and mounting. In addition, a magnetic ring clamping device was designed to ensure a tight fit between the transducer and the bolt, and to improve the alignment of both. Finally, a bolt axial stress ultrasonic measurement system was built, and a comparison experiment between TOF method and attenuation method was conducted to predict the axial stress of bolts. The experimental results shown that the average percentage measurement error of the attenuation method proposed in this paper is 4.42%-5.76% lower than that of the conventional TOF method, which is more suitable for measuring the axial stress of bolts.