Originally Posted by AtomHeart
Once metal has bent, the glass-brittle bonds break on a molecular level. You can then push the metal back to exactly the shape it was in before and it will look great. But if you try to bend it again...guess what...it bends easier, and in the exact same spot as before, because all of those broken bonds are still there in the metal. It's called fatigue stress failure. That's why if you bend a paper clip four or five times, it will break.
Your car's collision damage is not a fatigue stress failure, but is instead a combination of tensile failure of the material in tension mode and impact failure. A repair involving "stretching" the metal back into place means that the metal is significantly weaker than the original sheet metal. This is technically not a fatigue issue. The strength of the resulting repaired metal does not lie on the S-N curve of the material. Instead, the material's strength would be compromised as per stress risers in the ruined metal grain structure. The resulting strength would probably be near the material's notched strength.
The paper clip example you gave is an example of fatigue stress failure however.