Metallic materials also increase in resistance under strain. They are typically bonded to the substrate, and although they are fairly hardy and cheap, they are somewhat prone to drift.
Piezoresistive materials which increase in resistance while under strain. Namely semiconductors
Piezoelectric materials which generate an electric field when under strain. Can also work the other way (field induces mechanical strain)
Pressure
Generally can use the same principle as strain gauges, plus a few more:
Optical measures deflection in fiber optic cable
Capacitive sensors change their capacitance based on deflection. Similar to metallic.
”C”
Pressure relief
To size relief valves you have to determine your relief rate based on the system. If you are worried about overpressure, what is the maximum rate pressure could be added?
Oversizing is better than under, but creates waste and larger response oscillations