Detection of structural bolt detorquing using direct stethoscope measurement

A method for detecting loosened bolts in a structural joint based upon open-loop acoustic measurement is presented. The acoustic measurement is taken directly on the bolt head. The response of the bolt to a proximal hammer impact is evaluated and characterized using wavelet decomposition of the signal measured from the bolt head. Data were also taken from an accelerometer mounted longitudinally and transversely on the bolt head. Results from the stethoscope and the accelerometer are presented from a set of structural bolts in several conditions of preload and looseness. A stethoscope applied to the loose bolt and a proximal bolt during impact recorded a marked difference between tight, finger tight, and loose bolts. The study could enable a quick and simple method for detecting and evaluating detorqued bolts in structural joints.