I am using VISHAY Strain Gauges (specifically EA-06-062AP-120) 120 ohm. I have a setup with the strain gauge output fed into an amplification circuit and thus fed into an oscilloscope. Can anyone tell me what output I am seeing on the oscilloscope? Is the strain gauge outputing voltage? How can I relate what is on the oscilloscope to strain (epsilon) itself? I am not really sure how strain gauges work, any help is much appreciated.
Strain gauges, at the most fundamental level, are resistors. The exact value of the resistance changes with the amount of deformation ('strain') they are subjected to. The magnitude of the change is quite small. Since the materials used also change resistance with temperature, it is necessary to temperature compensate. This is often done by putting two in the same casing, mechanically arranged so that only one will deform. By putting one in each leg of a Wheatstone bridge, resistance changes due to temperature are cancelled out, and resistance changes due to strain will change the bridge output voltage.
Often arranged in opposite legs of a Wheatstone bridge, but other circuits have been used as well. So, depending how you've hooked them up, and how you've hooked up your scope, you could be seeing almost anything. The shift in bridge output is still pretty small, so feeding that into an amplifier is the next step. Preferably something with high impedance so that self-heating in the gauge is not a problem.
Relating the change in voltage output of a wheatstone bridge, knowing the parameters of the bridge, the change in resistance can be determined. From that, the amount of deformation of the gauge is then figured.
If you know the dimensions of the gauge, and the dimensions of the 'thing' it's attached to, you can figure out the stress in the 'thing'. Often, rather than estimate it all with a lot of calculations, the gauge installation is calibrated by exerting a known stress and measuring the gauge/bridge output response. For materials that operate the elastic region of their stress-strain diagram, the relationship between a given amount of stress and the strain it generates is linear. So from just one or two test calibrations, one can interpolate/extrapolate to measure the stress on the 'thing' your gauge is attached.