For two-dimensional problems, there are two possible approximations: plane strain and plane stress. The selection is made in the settings for the Solid Mechanics node.

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 3-9 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 all out-of-plane shear components of both stress are zero.