ABSTRACT
The idea of bolt elasticity is a critical one and a bolt acts a little like a very strong rubber band as it is being tightened. Looking at the action of the clamping force, one sees that it is absolutely critical to effective performance. There are a numerous groups around the world that have standards concerning bolts and, combining the different bolt strength levels and materials, there are hundreds of possible material combinations. Tests on the demonstrator show that typically an unlubricated stainless or aluminum bolt will only develop about half the clamping force of a lubricated one before the bolt seizes to the nut. The goal of the bolting patterns is to reduce distortion of the bolted members and result in more uniform clamping force and more reliable assembly life. Fasteners fail mechanically either from overload or from fatigue. The photo clearly shows a fatigue failure with several ratchet marks and several prominent progression marks.
