Abstract: Bolts are often subjected to axial stress in the working process. As one of the most common fasteners, the service quality of bolts is critical to safe production. Nonlinear ultrasonic technology is sensitive to the changes in the internal microstructure of materials, which has great potential in detecting the axial stress of the bolt. According to the propagation law of ultrasonic waves in isotropic materials, the relationship between the nonlinear coefficient and the amplitude of the fundamental wave, the amplitude of the higher harmonic is analyzed. A nonlinear ultrasonic testing system is built. Taking 45-steel and 304 stainless steel bolts as the research object, the nonlinear ultrasonic testing of bolts under different axial stresses is carried out, and the relationship curve between nonlinear coefficient and bolt axial stress is obtained. The results show that the ultrasonic nonlinear coefficient of the bolt increases monotonically with the increase of the axial stress. When the axial stress exceeds a certain value, the nonlinear coefficient increases significantly, which proves that the nonlinear coefficient is sensitive to stress changes and can be used to characterize the axial stress of the bolt. Through normalization and polynomial fitting of the relative nonlinear coefficients, the functional relationship between the bolt axial stress and the normalized relative nonlinear coefficients is obtained. Then two methods of nonlinear ultrasonic and body wave detection are used to test and verify the axial stress of the bolt applied by the electronic tensile testing machine. The error of the axial stress above 200 MPa tested by nonlinear ultrasonic is less than 6%, which is significantly better than linear ultrasonic body wave testing. The research results provide effective support for the axial stress detection of bolts under service conditions.

Key words: bolt, nonlinear ultrasonic measurement, axial stress, nonlinear coefficient