what the strain gauge rosette measures is in status of plane strain or plane stress?
I read some textbooks about mechanics of material. regarding strain measurement with strain gauge rosette, most of them regard it as in the plain strain status. But in my opinion, what the strain gauge rosette located is a fress surface. In this way, the thress stress components related to this plane, that is zz, zx, zy, should be zero, assuming that the out normal of this plane is in the direction of Z axis. then there are sigmaxx, sigma xy, sigmayy to be measured. Is there anything wrong with my understanding?