cobalt expansion on fishplate joints

[Peter]

what is the purpose of cobalt expansion on the holes on a joint.

Cheers

I am guessing you have only heard of this and not seen it written because I think what you are asking about is "Cold Bolt-Hole Expansion". I don't understand the physics of it, but when you drill a metal, you are likely to get tiny cracks (drilling after all is basically chipping away at the metal in tiny stages quite fast). Pulling something slightly bigger through the hole after you have drilled it helps to stop this. I am guessing it compresses the metal around the hole, so when you do it, the metal it being malleable (able to be hammered or pressed permanently out of shape without breaking or cracking), you undo any cracks left in the hole and hence stop them propagating.

In simple answer to the question, it helps stops cracking around the holes.