Fatigue Failure in Fasteners
Fatigue is cracking under repeated load, well below the bolt's tensile strength. It is the most common failure mode in machinery bolts, and the usual root cause is insufficient preload.
Fatigue at a glance
| Where it starts | Thread root at the first engaged thread, or the head-to-shank fillet |
|---|---|
| Fracture look | Beach marks spreading from the origin; final rough overload zone |
| Root causes | Low or lost preload, bending, stress raisers, poor thread form |
| Prevention | Correct preload, rolled threads (rolled after heat treat is best), good fillet, stiffer joint, longer grip |
These tables carry the same figures as the printed sheet below.
The printed sheet
See an error on this sheet?
Tell us what is wrong.
Text version of this sheet
Fatigue failure in fasteners
What it is
FAILURE MODE
Fatigue is cracking under repeated load, well below the bolt's tensile strength. It is the most common
failure mode in machinery bolts, and the usual root cause is insufficient preload.
Fatigue: cracks grow under cycling load
Fatigue at a glance
Where it starts
Thread root at the first engaged thread, or the head-to-shank fillet
Fracture look
Beach marks spreading from the origin; final rough overload zone
Root causes
Low or lost preload, bending, stress raisers, poor thread form
Prevention
Correct preload, rolled threads (rolled after heat treat is best), good fillet, stiffer joint, longer grip
Key points
A properly preloaded bolt sees only a fraction of the external cyclic load. Loose bolts carry all of it.
Governing standards: ASTM F606/F606M-26a; VDI 2230 (joint design). Editions shown are the latest confirmed as of September 2026. The drawing and the governing standard
always take precedence over this sheet.
torquing any fastener. Verify the current governing standard, project documents, OEM requirements, size, thread, coating and lot certification. To the fullest extent permitted by law, The Fastener App disclaims liability
arising from reliance on this reference.
Fastener Guide reference FG-B32593 · thefastenerapp.com/fastener-guide/failure-analysis/fatigue-failure-in-fasteners · © The Fastener App. Reference only — verify the current governing standard, drawing and manufacturer data before specifying or installing.
