Bolt Axial Stress Measurement Based on the Dual-Mode Electromagnetic Acoustic Transducer

To further simplify the bolt stress measurement process, the feasibility of using a dual-mode electromagnetic acoustic transducer (EMAT) that excites both longitudinal and shear waves is investigated to achieve rapid and accurate measurement. The propagation characteristics of cylindrical guided waves in a rod are studied by a semi-analytical finite element (SAFE) model. Through the combination of the theoretical formula and the finite element simulation, the nonuniform distribution of axial stress is directly used to calculate the time of flight of cylindrical guided waves without building any approximate model. The effect of stress distribution on guided wave modes with different energy concentration regions is analyzed. The propagation characteristics of guided waves and the theoretical values of the group velocity of these two modes are studied by cylindrical guided wave dispersion curve and mode shape analysis. Then, by combining the static stress distribution inside the bolt with the theoretical formula, the influence of the different distributions of guided wave energy in the cross Section of the two modes on the velocity ratio was analyzed. Finally, the feasibility of the theory is verified by the experiment. The result of the experiment in the aluminum bolt verified the effectiveness of the dual-mode EMAT on stress measurement and the novel ultrasonic testing models proposed in this article.