Joint Decompression Point
Why Bolt Preload is Important!
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 why 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.