stress strain
 

stress strain
this is probably really simple but i'm just not seeing it. what error is made if you measure strain at a point ex and estimate the stress as eex where e is young's modulus?
none, if the measurement is unitless, (inch/inch or %).
that's correct, as long as you are in the elastic range and the material has a straight line up to yield.
depends on the strain field where you put the gage--is the strain in a uniaxial strain field (say the gage section of a dogbone coupon), so that the x-direction is the only direction with non-zero stress? if so, you can use e*ex=sigma-x to estimate normal stress in the x-direction. otherwise, it's more complicated than that.