19-10-2021, 02:26 PM
Interesting subject - metal fatigue. Plenty of reading out there for those who want to know more.
Apparently. once a crack gets itself together and starts to propagate is might not be long before failure - however, it does depend on what loads the item sees in service. If stresses are above a general threshold , fatigue failure can start from any minor defect that produces a stress concentration, but the initial damage is likely to be down at the microscopic level. However, once there is a sharp crack, the stress can be 100x background at the tip, and so the more it spreads, and the the load it sees is greater - hence a one-way road to failure. Fortunately, steel is a tough material - from its structural imperfections, for one thing - and items can survive a for long time despite their inevitable (sudden) demise in the end.
The following photos show what failures initiated from surface defects and internal defects tend to look like - similar to the photos above?
Fatigue failure - surface v internal defect.jpg (Size: 128.36 KB / Downloads: 284)
Apparently. once a crack gets itself together and starts to propagate is might not be long before failure - however, it does depend on what loads the item sees in service. If stresses are above a general threshold , fatigue failure can start from any minor defect that produces a stress concentration, but the initial damage is likely to be down at the microscopic level. However, once there is a sharp crack, the stress can be 100x background at the tip, and so the more it spreads, and the the load it sees is greater - hence a one-way road to failure. Fortunately, steel is a tough material - from its structural imperfections, for one thing - and items can survive a for long time despite their inevitable (sudden) demise in the end.
The following photos show what failures initiated from surface defects and internal defects tend to look like - similar to the photos above?
Fatigue failure - surface v internal defect.jpg (Size: 128.36 KB / Downloads: 284)