EK698
Plane stress is defined to be a state of stress in which the normal stress and the shear stresses directed perpendicular to the plane are assumed to be zero.
EK345
Plane stress generally exists when the loaded element is relatively thin in the out-of-plane direction.
EK344
When the stresses on an element act only in one plane, the state of stress is called plane stress.
EK55
In plane stress problem the only nonzero stresses are those in the plane of the problem and normal to the lamina we have no stresses.