Elongation Percentage

Surely it means 'it got longer by 12% jsut before it broke'
So if it was 100mm to start with, then at at break it was 112mm...
I mean it's not my field, but it seems pretty obious to me.
Dunno how you use it in a 'design calculation' whatever one of those is .
Del

Agree with first part.

I don't think you would use it in a design calculation, at least not directly. Unless it's something pretty unusual, it wouldn't be designed to break. OP come back if it is. Normally the important thing is the design allowable stress (much lower than breaking stress, whatever the elongation).

Elongation gives an idea of material toughness, 12% sounds about right for a metal like steel. By contrast, a glass rod would break at much smaller elongation. If selecting a material from scratch you might consider elongation, but usually you have a pretty good idea up front what to make something out of.

Cheers..........Codey

In some designs you may want something to elongate before failure. This elongation can be tested for so that it can be replaced before complete failure.

Simple example is head bolts on engine. Torque down the bolts under a lot of stress. Heating and cooling of engine adds more. The mechanic needs to decide when it's necessary to replace the bolts during a repair. He can take the measurements but he is not an engineer. So the engineers need to set some limit and how to test for it.