Results of a preliminary study investigating a simple method for detecting looseness in bolted fasteners of a steel structure are presented. Extensive research by many investigators demonstrates that the frequency response of a bolted structural member changes when one or more bolts are loosened. A simple and effective method for detecting looseness can be applied to bolted joints, using inexpensive and commonly-available equipment and software. The difference between spectrograms associated with tight and loose bolts is clearly apparent due to the presence of a tell-tale mode when one or more bolts are loosened. Further, a striking difference can be elucidated between audio signals associated with tight and loose bolts, with only minimal post processing. A time-frequency spectrogram was used to identify higher-order modes that are affected by looseness in a horizontal I-beam with bolted connections. One higher-order mode was found to be particularly sensitive to bolt loosening. Results show that the natural frequency associated with a higher-order mode provides a tell-tale acoustic response that can be elicited with an open-output impact test using a common hammer.
Guarino, Joe and Hamilton, Robert. (2015). "Stethoscope-Based Detection of Detorqued Bolts Using Impact-Induced Acoustic Emissions". Proceedings of Meetings on Acoustics, 22(1), 065001-1 - 065001-14.