A stress tensor state with all traction components involving one normal direction zero. In an isotropic solid, its two remaining normal stresses obey and . The out-of-plane strain need not vanish: . Constraining the out-of-plane deformation instead would be a different boundary condition.
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Plane stress is a two-dimensional state of stress that occurs in thin structures where the thickness of the material is small compared to the other dimensions. In plane stress conditions, it is assumed that the stress in the thickness direction (usually denoted as the z-direction) is negligible. This is typically applicable to thin plates or shells where one dimension is significantly larger than the other two.