The most common
reason why bolted joints fail is due to the bolt failing to
provide sufficient preload to prevent the external applied
forces overcoming the clamp force acting between the joint
faces.
The slide show presented below illustrates the joint
decompression point. This is when the clamp force acting
between the joint faces, that has been provided by the bolt's
preload, has been reduced to zero by the applied forces.
Once the joint faces have separated the bolt will be
subjected to bending forces and the joint faces to fretting.
This will lead to a loss of preload and the bolt subsequently
failing by fatigue or other mechanism. This is way the
decompression point is taken as a design failure criterion.
The importance of having a high preload can be illustrated
by using the decompression point. The image below illustrates
this point:
As can be seen that a higher preload
raises the decompression point. For this reason it is better
to tighten a fastener up to close to its limit rather than
only partially tightening it.
