I'm creating a Nerf gun target with Arduino for my little brother. To detect the impact I was thinking about using piezo's. They would be located on the back of the targets (alimunium plates that do not bend). I'd like to know if it's possible to detect an impact, despite the fact that the material doesn't look like it's vibrating (and assuming that the target can't move freely on impact).
A) Yes, it's perfectly possible to detect impacts with piezo's.
B) No, you're not going to use them to detect nerf gun hits.
For piezo's, the problem of distinguishing between noise (vibration, etc) and an impact is (for relatively gentle impacts) a real issue. What you need is a very hard, short impact process, and I'm afraid nerfs just don't fit the bill. Add to this the extra impact-like signals you'll get from things like footsteps, and you're looking at a pretty hopeless situation.
You're free to try for yourself, but be prepared for frustration.