Vibration-Based Bolt Tension Estimation for Multi-bolt Joints

Critical bolted connections exist in many engineering structures, from pressurized pipelines to wind turbines. Often there are legal demands for maintaining necessary bolt tension in such joints to prevent failure. The available tools for tightening makes it challenging to obtain the correct tension in a bolt, as well as subsequently checking if a bolt is in fact tightened to the correct level. Recent work proposes to use vibrations for estimating tension in a bolt. Estimation is possible by measuring and analysing transverse natural frequencies and damping ratios induced by e.g. a transversal or longitudinal hammer impact on the bolt itself. The work so far has focused on a single bolt. Most bolted joints consist of many bolts, e.g. a flange connection often has a ring of almost identical bolts. Identical bolts, with almost the same tension, will also have very similar boundary conditions and thus almost the same natural frequencies. If there is only very light damping between two adjoining bolts, a frequency response measured on one bolt after an impact might include the vibrational response of both bolts (the coupling might even be so strong that the two bolts cannot be viewed as entities on their own) leading to the question: How to separate which frequencies belong to which bolt?