Can a tuning fork go flat? If you would have asked me this question years ago I would have a different answer than now. When I was in college, I carried an A-440 tuning fork with me wherever I went. I had hoped to memorize an A-440 if I kept sounding it frequently. I felt that the fork was calibrated perfectly and was always at 440. In theory, I considered that metal can heat up and cool down and that may affect pitch. I owned and trusted same tuning fork for 25-30 years.
These days we use tuners and we now have apps on our cell phones. When I was in college tuners were more expensive than today. At a gig a few years ago (maybe more years than I think) a colleague said my tuning fork was flat. I though, “Huh? How could that be?” She pulled out her fancy new cell phone with its new tuner app on it. Sure enough, it said the tuning fork was flat. Later I went home and checked it against a couple other tuners. Sure enough, it was flat. Carefully I sanded the ends of the fork until it matched 440. Yay.
Six months later. Someone again said my tuning fork was flat. You must be kidding! How could that be? Once again I checked against the same tuners. Flat again. I looked at the tines. It appeared as though they had been slightly bent toward each other. I tried to bend them back outward. Snap! The tuning fork broke. But now I know, yes, a small tuning fork can go flat.
I did buy another to keep in my cello case for when batteries go dead on my tuner or cell phone, but I no longer trust it like I did.