Introduction to Elasticity/Plane stress

In a three dimensional state of stress there are 6 unique components of the stress tensor (σxx, σyy, σzz, σxy, σyz, σzx). Under the assumption of plane stress (x-y plane), there are only 3 non-zero components of stress (σxx, σyy, σxy). The assumption of plane stress is often applied for structures in which one dimension is very thin compared to the other two -- the canonical examples being plates and shells.