strain gages vs average strain from FEA

1) Watch out. Keep in mind that an average will be always within certain limits. Check out exactly where you are locating the strain gages, and the size of the strain gage compared to your loaded body. Also, make the FEA model mesh size compatible, or you'll have the same problem in the virtual world. If your model is adequately refined to calculate and display a reasonably smooth stress profile, probably you'll have a good approximation from reading the surface nodes that match your physical model strain/stress directly, without having to make extra calculations. 2) Yes, definitely. That's why the strain gage cannot be that big compared to the body, and also that's why the strain gage direction is important. If its correctly positioned, its alignment with the strain field will make sure it will be correctly strained and allow a good measurement.

When comparing strain gauge data with analytic results, beware of high strain gradients and inflections. You don't need to be a genius to understand that a gauge centered on a high gradient inflection point could read "0" strain, and the part could fail at the center of the gauge. The reason is that while the strain gauge accurately provided the average strain, a major peak was missed in the averaging process. Understanding this principle should provide insight to answer your second question.