Plane Strain and Plane Stress Cases
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.
In the plane stress formulation in COMSOL Multiphysics, the plane stress conditions
(3-9)
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 the out-of-plane shear components of the stress tensor are zero.