Originally Posted by
john m flores
The failure mode is progressive cracking via fatigue, not instantaneous exceeding of ultimate shear resistance. Every thread on the bolt is a stress riser that initiates cracking. Here's an explainer of the mechanism of failure:
https://www.boltscience.com/pages/fa...e-of-bolts.pdf
"The crack can start at some existing defect, such as an inclusion in the metal, or at point of high stress, such as a notch. In the vast majority of applications, the most effective way to ensure that the bolt is fatigue resistant is to ensure that it is tightened sufficiently. A preloaded bolt in a typical joint sustains usually only around 5% (or less) of the applied loading (the remaining 95% reduces the clamp force acting on the joint). Due to this, a properly tightened bolt is highly resistant to fatigue loading. (A conn-rod bolt for example would rapidly fail if it were not tightened.) Because the alternating load is small, so is the alternating stress, usually well below the bolt's endurance limit. Fatigue failures, when they do occur, are frequently the result of inadequate tightening or loosening that can expose the bolt to bending stresses that it is not designed to sustain."