Using Surface Traction to Evaluate Reaction Forces
You can obtain values of the reaction forces on constrained boundaries by using boundary integration of the relevant components of the surface traction vector.
For 2D and 1D axisymmetric components, multiply the surface traction by the cross-section thickness, and for 1D components multiply the surface traction by the cross-section area before integrating to calculate the total reaction force.
Two different types of surface traction results can be computed in COMSOL Multiphysics:
The first type, contained in the variables interface.Tax, is computed from the stresses. It is always available. Since the surface traction vector is based on computed stress results, this method is less accurate for computing reactions than the other methods.
The second type, contained in the variables interface.Tracx, which are based on the first Piola–Kirchhoff stress and represent a force per undeformed area. These nominal traction variables are computed with a higher accuracy when the Compute boundary fluxes checkbox in the Discretization section is selected for the Solid Mechanics interface.
In case of geometric nonlinearity, the two types of traction variables are interpreted differently. The interface.Tax variables are based on Cauchy stress, and contains a force per current area. If you integrate them you must use the spatial frame. The interface.Tracx variables are based on the first Piola–Kirchhoff stress and contains a force per undeformed area. An integration must then be done on the material frame.
Computing Accurate Fluxes in the COMSOL Multiphysics Reference Manual