How does metal fatigue work on a molecular level?
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Fatigue is a property of the material as a whole. The individual atoms (there aren’t molecules in the conventional sense in a piece of metal) stay the same, but their arrangement changes.
Specifically, fatigue is the result of *crystal defects* – ways in which the position of the atoms in the material differ from a completely idealized crystal. All normal materials contain such defects, and stress in the material concentrates at those defects. This causes the atoms to slip further and further out of place.
You can imagine, say, a stack of books. The stack is quite stable *as long as* all the books are aligned properly. But the alignment is never perfect (i.e., there are slight defects in its “crystal” structure). Over time, small disturbances (specifically, *shear* forces, which operate “side to side” at a right angle to the “up and down” alignment of the stack of books) tend to jostle books in the stack out of place. This manifests as increasingly wide gaps (“dislocations”) between where each book “should” be and where it is. This makes the book stack progressively weaker against those forces.
But for the moment, the stack is still stable against putting another book on top – it’s fatiguing, but it hasn’t broken yet. In particular, *several* such stacks can support a heavy load.
Once the dislocations are strong enough, though, the crystal can start to break down in the other direction, too. In our stack of books, this is like the book stack falling down once the books are sufficiently far out of alignment. This reduces the strength of the material, since the books can no longer “support” the weight they’re holding up. The defect now runs in both directions, and there’s effectively a “gap” in the material. (You’re effectively playing Jenga with the crystal structure.) Once this reaches a critical level, these failures produce a chain reaction as each one adds load to the next, causing a cascading failure, and the material fails quickly and catastrophically.