One can identify the root cause of the vibration shown in the video below thanks to RDI Technologies Inc.'s motion amplification technology combined with Tremor Vibration Services, LLC.'s analysis. The bolt was loose, which was not visible without this technology that detects subtle displacement and then converts that movement to a level visible with the naked eye. Use torque control and adequate bolting tools to avoid those problems, premature fatigue or failure.
Was the bolt improperly assembled, or did it come loose over time, maybe vibratory loosening?
Indeed, this is a good question As a prerequisite, the proper tightening of the bolt is necessary, but vibration is one of the main reasons bolts come loose. There are regular maintenance checks required in the form of a torque verification. In wind turbines, you need to do this every year but also, in steel construction exist several guidelines.
An updated EU standard requires higher joint quality. The previously used torque pre-tightening procedure (according to DIN-18800) was standard for attaining the proper, specified pre-tightening forces as a control value.
The combination of pre-fastening torque and the target value of the assembly torque was a function of the friction. However, the simple torque procedure was insufficient: The new standard is the torque/angle procedure (combined procedure according to DIN EN 1090-2). The process reduces the influence of friction considerably. State-of-the-art torque multipliers allow you to set a target torque and an additional angle, as shown in this illustrative video.