Research On Bolt Axial Stress Measurement Technology Based On Ultrasonic Spectrum Energy Attenuation

Bolts are widely used in various engineering applications as the commonly used couplings in mechanical field.It is very important to accurately evaluate its working state to prevent the failure of key equipment and reduce safety risks.At present,ultrasonic nondestructive measurement technology based on acoustoelastic effect had been widely used in bolt axial stress measurement,but the most commonly used ultrasonic single-wave method and double-wave method had large measurement errors when measuring short bolts.This problem was particularly prominent when measuring ultra-high-strength bolts in some special fields(such as aerospace)and limited hardware equipment,which limited the versatility of ultrasonic testing methods.To solve this problem,a bolt axial stress evaluation method based on ultrasonic spectrum energy ratio was proposed in this paper.Firstly,according to the effective elastic coefficient tensor in non-textured polycrystalline materials and the attenuation coefficient relationship and nine different scattering modes of ultrasonic wave propagating in medium,the stress-dependent attenuation coefficient expressions of ultrasonic longitudinal wave and shear wave propagating in cubic symmetric polycrystalline materials were derived.On this basis,according to the stress state of the bolt,the expressions of stress-dependent longitudinal wave and shear wave attenuation coefficients including the information of bolt material and axial stress were deduced.Secondly,in view of the influence of the cylindrical guided waves and the cluttered Fourier spectrum of the back surface echo in the ultrasonic measured signal of the axial stress of the bolt,a new testing method was proposed for evaluating the axial stress of bolts by using the spectral energy ratio of the primary and secondary back surface echoes instead of the attenuation coefficient.The calculation model of the spectral energy attenuation coefficient related to material inherent parameters and axial stress was deduced.And numerical simulation was performed on the model.Then,in order to solve the aliasing distortion of the Fourier spectrum of the primary and secondary back-wall echoes,empirical wavelet transform algorithm was used to decompose the ultrasonic echo signals.The effectiveness of the separation algorithm is verified by simulating the simulation signal.The normalized weights of the spectral energy attenuation coefficient of each modal component were optimized by the particle swarm optimization algorithm,and a evaluation method of ultrasonic echo spectrum energy attenuation coefficient was proposed.Finally,the calibration and measurement experiments of the axial stress of a batch of bolts were carried out by using the single wave method,the double wave method and the energy attenuation method,respectively,using the built ultrasonic testing system for axial stress of bolts.The experimental results show that,compared with the traditional single wave method and double wave method,the energy attenuation method proposed in this paper has significant advantages in measuring short bolts with high measurement accuracy,and can overcome the limitations of hardware equipment.