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Do you play your car?

(seriously, I've never heard of a car horn with metal fatigue, so perhaps those are very different kinds of vibrations... )

Highly unlikely to impossible. It's a matter of the energy behind the vibrations. I suspect the most accomplished among us isn't developing the equivalent of the hundreds or thousands of horsepower needed to induce the type of mechanical fatigue you're talking about.

At least I hope not.

Fatigue, hell.

I’m ten times the player you are, and my world class sound is so hot, I worry about spontaneous combustion!!

Playing can definitely cause mental fatigue. Happens all the time.

Hup

Brass doesn't precipitation harden like steel, but it does work harden.

I think that given adequate protection from corrosion it would probably soften eventually. A work hardened state is at a higher energy level and thermodynamics will eventually see it return. That time frame is probably in the neighborhood of 10,000 years (based on memories of a metallurgy class back in engineering school.)

I don't see any way vibration could speed the process.

If I may refer to an old thread from 2004, there's some interesting depth here: Hardening Of The Tuba Arteries?.

That discussion is rather focused on hardening, which likely does not happen with age, but I'm pretty sure very old brass instruments get brittle, which is a different thing. Playing is still probably the least of our worries, in the scheme of things that cause stress to the brass, but play enough on a brittle enough instrument and who knows?

yes, it does.