超音波の瞬時振動数を用いたボルト緩み診断における透過経路の 3 次元的考察

Abstract

Diagnosis of the bolt loosening is important in the industrial field and the medical field such as dental implants. We have proposed a non-destructive testing method for the bolt loosening using an ultrasonic pulse that is projected to the side of the bolt. In the basic experiment, the tightening torque of the bolt can be evaluated via the instantaneous frequency, which varies from hour to hour in the pulse, of the reflected wave or the transmitted wave. The mechanism of the proposed method have been tried to verify through an identification of the transmission paths and a numerical simulation. Our conventional study have assumed that the transmission paths spread on the two-dimensional surface. However, the two-dimensional study have not been able to explain some experimental results perfectly. In this paper, we identified the transmission paths in the three-dimensional space, and tried to simulate the experimental results. For this identification of the three-dimensional paths, surface of the screw thread was divided into many planar rectangular elements. Calculating reflection and refraction of the ultrasonic pulse on each element, many transmission paths were identified. Some transmission paths that reached at the modelled receiving surface were superposed to simulate the experimental result. This simulated result had some reasonable aspects in compared with the result of the two-dimensional study however a few differences with the experimental result remained. Additionally, the three-dimensional simulation is expected to be a supplementary tool of the proposed diagnosis. Especially when the target shape is complex, this technique is helpful to predict the transmission paths and the optimum position of the transducers.