piezoelectric materials produce voltages in response to mechanical stress. But if you a constant stress, will it produces constant voltages?

Oh, you mean if I put a rock on it can it generate infinite electricity? I wish!

yea that's kinda what I'm asking. How does it produces voltages in response to mechanical stress?

mechanical stresswork

Well it converts work into electrical energy. I don't really know the details. But no, it won't just keep generating electricity with a rock on it, it has to keep being disturbed (ie you need movement).

You can compare the piezoelectric material to a capacitor where, instead of charging it with a voltage, it is charged by mechanical stress.

So applying stress to the capacitor creates a voltage across the material. If you were to use this voltage (by allowing charge to flow) the voltage would drop until equilibrium was reached. Removing the rock would re-create a potential difference although reversing the sign.

 

As to how it produces the voltage, I suspect that there would have to be some electric dipole moment of the material that is altered by the application of stress.

Mr skeptic, in principle it does generate a voltage as long as there's a rock on it. The voltage will drop as the charge leaks away but there's no violation of the conservation of energy there. The better insulated it is the longer it will keep the voltage.