Ultrasonic measurement of the bending of a bolt in a shear joint

An ultrasonic pulse/echo technique is used to measure preload in bolts used in structural joints. In this paper, the same instrument is used in a different way to measure change in the ultrasonic measurements due to bending in the bolts. A theory that explains the ultrasonic measurements is developed. The bending loads result in a rotation and a translation of the ultrasonic pulse reflecting face. It also creates a stress gradient in the bolt. This results in a phase variation (or gradient) in the received ultrasonic beam across the face of the transducer. It also results in a physical shift in the received beam relative to the ultrasonic transducer. The phase gradient and the shift in the beam results in change in the pulse travel time. A number of experiments were performed on the bolt to study the effect of the bending on the ultrasonic measurements. The experiments and the theory validate a sensitive new method for measuring the bending loads in the bolts.