What is metal fatigue

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How come you can bend metal back and forth and eventually break it. What is happening at a molecular level. How does movement allow for such weakening of certain metal (all metals? idk).

In: Engineering

As you bend the metal, the outermost region is being stretched while the innermost is being compressed. Metal is not very elastic, so this stretching and compressing breaks some of the molecular bonds. Each time you bend it, more bonds break until the entire thing snaps.

There are two types of “bending” in metal: 1) elastic and 2) inelastic. If you bend a piece of metal elastically, it springs back and it unchanged. This is how the springs on your car hit bumps for years and never lose their springy-ness.

Inelastic bending leaves the metal “bent” afterward. This means the bonds are deformed and the atoms moved around. It also produces an effect called “work hardening”. This means that the atoms are harder to move around after they’ve been moved once. If you do this over and over, the hardened parts of the metal focus the strain on the remaining parts, which causes bonds to fail and cracks for form. As the cracks spread, the metal fails and the item breaks.

Metals are not pure cristalline structures extending from one extremity of the object to the other. Depending on the metal/alloy and the fabrication process, they form “grains” of variable dimensions that interact with each other. When your piece of metal is “fatigued” it means too much internal dislocations have taken place (=the grains are not as strongly bound together as they used to)