Plane Stress
For two-dimensional problems, there are tree possible approximations: Plane Strain, Generalized Plane Strain, and plane stress. The selection is made in the settings for the Solid Mechanics node.
In the plane stress formulation in COMSOL Multiphysics, the plane stress conditions
(3-12)
are not enforced through a modified constitutive relation, as is common in many textbooks. Instead, an extra set of degrees of freedom for the out-of-plane strains are introduced, and Equation 3-12 is enforced by solving for the strains.
For a general anisotropic linear elastic material in case of plane stress, COMSOL Multiphysics solves three equations. For isotropy and orthotropy, only one extra degree of freedom is needed since the out-of-plane shear components of the stress tensor are zero.