Progressive failure under fluctuating stress: a crack starts at a stress raiser, most often the first thread engaged in the nut, then the thread runout or the head fillet, and grows with each cycle until the remaining section snaps. Fracture faces show smooth beach marks around the origin and a rough final zone. Enough preload is the main defense, since a well-tightened bolt sees only a small fraction of the external load swing.
Classic fatigue, look at the beach marks, it started at the first engaged thread and went most of the way through before it let go.
Also heard as
- fatigue failure
- fatigue crack
Related terms
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Thread rolling
Forming threads by squeezing a blank between hardened dies, displacing metal instead of cutting it away.
UNR
Unified National Round root: an external inch thread with a required rounded root, for better fatigue life.
Hydrogen embrittlement
Delayed brittle cracking of hard steel fasteners caused by hydrogen absorbed from plating, cleaning or corrosion.