How does metal fatigue work? Is it reversible?

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If I bend a piece of metal forward and back a bunch of times, eventually it will break even if I couldn’t break it in the beginning. I believe that is due to metal fatigue(?) How does that work? What’s specifically getting “tired”? the chemical bonds? the structural integrity?

Say I have a piece of metal that I bent and unbent thousands of times. If I melt it back and recreate the shape, does the fatigue “go away”? How does it work?

In: Engineering

More of structural integrity. Fatigue is basically a slow process of worsening the already present microscopic tears in the metal.

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If you melt it down, you reset the structure and will therefore have the fatigue gone.

Metal is a crystal structure. All the atoms line up in neat little rows.

But these rows aren’t perfect, especially where two “crystals” meet.

When you bend and unbend the metal, these tiny tiny little flaws in the crystal end up grouping together. The more bending you do, the bigger these “voids” end up being. As this happens, the metal in the area gets harder but more brittle. It can’t bend as easily because of the voids getting in the way of all the. Structure moving.

Eventually it gets so stiff and so weak that it just snaps.

Just bending it back straight does not fix metal fatigue. The only real way to get the fatigue gone, forever, is heat it back up to where the atoms can move freely and then re-cool it. (Re-smelt it).

It’s all to do with the crystal lattice and how the different areas of Crystal lattice interact.

[interesting paper ](https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/strengthening.htm)

If you bend a piece of metal, there is a risk that a really tiny crack somewhere on the bend emerges. It can be microscopic, but it is still there. Every time you bend that piece of metal again, that tiny crack grows a little bit. At some point, the crack will be so big that it reaches a critical size – and then the piece will break in half.

If you melt the piece and recreate the shape, the cracks are gone and therefore there is no fatigue until you bend it again enough times for a crack to emerge and grow.

We need to understand what decide the strength of a material. Solid are a bunch of small particles (called atoms) tied together. Solid have a relatively rigid structure. Depending on the material, some are somewhat malleable other are not. Those that are not malleable are more likely to outright snap under tension. Those that are malleable can instead deform slightly to prevent breaking. A simple concept of bend or break.

We use metals (in particular steel) because they are quite malleable (they will bend rather than breaking) without being soft. This mean we can build lot of stuff from it without fear of it breaking, and yet we can still make stuff that need to not move too much (like bridges or buildings, or car). But nothing is perfect. The more something is forced to bend, the more thee bonds between atoms are damaged. While bending prevent things from completely ripping apart immediately, it weaken the bond and make breaking more likely. Of course, in many cases, we can keep doing it without problem for a very long time. While it does become weaker, it’s not too impactful in short terms.

But for heavy machinery, most often those that work non stop (Factories, reactors, trains, buildings, etc…) the material slowly become weaker and weaker. That process is called “metal fatigue”.

The best way to solve it simply to smelt the metal back and reforge it. By doing so, all the bond will break and form anew when cooling. You’ll have a brand new piece to work with.