I heard that aluminum simply gets weaker and weaker as the fatigue builds up, unlike steel which doesn’t get weaker at some point.
Why does this happen?
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It’s thought that steel doesn’t have one either it’s just a ton of cycles haha. Regardless it’s because of the orientation of the molecules in the crystal structure.
Aluminum has something called a face centred cubic structure. Think of this as lots of little stacked cubes. Steel has a body centred cubic structure. Think of this as a bunch of little stacked octahedrons. The cubes have very clear planes which you can slide an entire level of them one way or another. The octahedrons though lack such planes and no matter which direction you push them they’ll lock up.
So aluminum is more ductile but will continually experience slippage along the planes during repetitive loading, and steel is less ductile but stronger and can eventually achieve minimal fatigue once the molecules have become as packed in as they can get.
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