A New Testing Method for Bolt Loosening with Transmitted Ultrasonic Pulse

Diagnosis of the bolt loosening is an important testing method in many fields. This study aims to develop a new nondestructive testing method for the bolt loosening using ultrasonic pulse projected to the side of the bolt. This method focuses on a transmitted pulse which travels across the bolt. This paper represents a demonstration of the proposed method and discusses its mechanism through a geometrical identification of the transmission paths of the ultrasonic pulses. In the basic experiment with an aluminum block and a stainless steel bolt, tightening torque of the bolt could be evaluated via the amplitude of the transmitted wave which consisted of two pulses. Especially, the second pulse was sensitive to the tightening torque. On the other hand, geometrically, we identified six transmission paths in the block. Especially, two paths of them were relatively long and traveled across two load flanks, whose transmittance was strongly affected by the tightening torque. This result suggests that the two pulses which traveled along these paths corresponded with the second pulse in the experiment. And the proposed method is qualitatively demonstrated.