Diagnosis of bolt loosening using transmitted ultrasonic pulse with mode conversion

Abstract

Bolt loosening may lead to severe accidents in machine and infrastructure facilities. Hence, it is pertinent to detect bolt loosening early. This study proposes a new non-destructive testing method for bolt loosening using ultrasonic pulses projected onto a screw thread of the bolt. This method focuses on the pulse transmitted through the bolt. The mode conversion of the ultrasonic pulse occurs when the pulse reflects and transmits on the flanks of the screw thread. The amplitude of the mode-converted pulse is affected by the contact condition on the screw thread. The transmitted pulse consists of several pulses with different transmission paths. The proposed method calculates an instantaneous frequency of the superimposed transmitted pulse and detects the bolt loosening as a change in the instantaneous frequency, which is caused by the amplitude change of the mode-converted pulses. A simple experiment using an aluminum block and a stainless steel bolt tightened with several kinds of torque is demonstrated in this study. Upon decreasing the tightening torque, the maximal instantaneous frequency of the transmitted pulse drops and appears earlier. To verify the mechanism of the proposed method, identification of the transmission paths and a numerical simulation were implemented. It is elucidated that the two mode-converted pulses that were transmitted through the pressure flanks and were reflected on the clearance flanks primarily affect the instantaneous frequency of the transmitted pulse. Additionally, their amplitude decreases with the decrease in the contact ratio of the pressure flanks. The contact ratio correlates with the tightening torque. This analytical study shows that the maximal instantaneous frequency and the time of the maximal value vary owing to the tightening torque. Therefore, the proposed method provides a reasonable index for non-destructive testing for bolt loosening.

Keywords

• bolt loosening
• non-destructive test
• diagonosis
• ultrasound
• instantaneous frequency