Ultrasonic Measurement of Bolt Axial Stress Using the Energy Ratio of Multiple Echoes

The reliability and stability of a mechanical system generally counts on the effective detection and control of the stress state of its bolted connections. In this paper, a novel bolt axial stress evaluation method based on the scattering attenuation of polycrystal using the energy ratio of multiple echoes is proposed. Above all, the longitudinal-incident attenuation coefficients for preload cubic crystalline materials in the Rayleigh regime are introduced. In view of the estimation of frequency-dependent attenuation coefficients are prone to suffer from the disturbance of the cylindrical guided waves, the energy ratios of the first two back-end echoes within the signals are applied instead. Subsequently, a stress measurement experiment is conducted to validate the proposed method, in which the accurate energies of the echoes are evaluated using the S transform. The results reveal that, as long as the probe is firmly bonded, the proposed method can achieve better sensitivity and reliability than that of the existing methods using TOF (Time of flight), especially for short or full-threaded bolts.