Not So Old Rivet Question

[NYTOM]

I was going through my wing checking every rivet I could with the gauge before I close it up. I found a few that were under driven. I used a hand squeezer and I swear those rivets have hardened up. I could barely get them to flatten out more. They were originally driven or squeezed about three months ago. Am I imagining this or do they actually harden up after a while.

[N941WR]

Tom,

When you squeeze a rivet you "cold work" it. Even if you where to squeeze those rivets a second time w/in minutes of the first squeeze, they would be harder to set. These rivets do harden with time but I doubt three months would make that much of a difference. However, I reserve the right to be wrong (again), so check with an expert. After all, I only had a few metallurgy classes 20 some years ago.

[jsharkey]

The type of rivet alloy used on the RVs probably harden due to initial working when you first squeeze them more than through aging so they set best with a single squeeze or hammer. (I seem to remember something about inherent crystal defects being squeezed out during plastic deformation to leave a harder material.) There are other types of rivet alloy that harden with time after initial heat treatment, with in hours in fact, and you see rivet clocks prominently displayed on production lines showing when to stop using a particular batch and send them back for heat treatment.

(It's been a while since I did Materials 101 at college so feel free to expand upon or correct me.)

Jim Sharkey