透過超音波パルスの振幅に着目したボルト締結体の締結力評価

抄録

Non-destructive testing for bolt loosening which can evaluate fastening force quantitatively is needed to prevent accidents due to the bolt loosening. This study proposes an evaluation method for the fastening force of bolt-nut fasteners quantitatively using ultrasonic pulses. In this method, attaching two ultrasonic transducers on the nut, we observe the ultrasonic pulses which passes through the screw thread of the bolt. The projected ultrasonic pulse from one transducer splits into a transmitted pulse and a reflected pulse on a flank of the screw thread. The transmittance and the reflectance depend on contact stiffness on the flank between the bolt and the nut. The contact stiffness on the flanks is positively correlated with contact pressure on the flanks and fastening force of the bolt. If the contact stiffness on a flank becomes larger, transmittance of ultrasound on the same flank becomes higher. We thus can calculate the contact stiffness on the flank of the bolt-nut fastener from amplitude of the transmitted pulses which passes through the bolt. And the proposed method estimates the contact pressure of the flank and the fastening force of the target bolt-nut fastener using the calculated contact stiffness. In this paper, we demonstrate the proposed method for a sample bolt-nut fastener which consists of an M24 bolt and a nut. The proposed method provides valid results about the contact pressure and the fastening force as compared to the applied fastening torque.