Why the difference? In short, the newer Gauss point evaluation (available since v4.3) reports the stress values that have been extrapolated from the Gauss points rather than the nodes (the old way). Without getting into too much of the "under-the-hood" details of how the finite element method works (see Note below), suffice it to
say that the Gauss points are used during the solution process for the force calculation, and, for a nonlinear material, the Gauss points are the only locations where the constitutive law is actually satisfied. In contrast, the Lagrange point evaluation (the original version) is an evaluation based on the derivative of the displacements at the nodes.
In general, this evaluation is not exactly consistent with the force calculations that are actually being performed numerically
within the elements. This inconsistency can be especially notable for non-linear models (e.g. plasticity modeling). Furthermore, there is not any situation we are aware of where switching to the Gauss point evaluation has any downside. For these reasons, we recommend that the Gauss point evaluation be used rather than the nodal evaluation (especially for non-linear models). Here at AltaSim, we are now considering the original stress and strain definitions as a "legacy"
form and recommend getting in the habit of using the variables with the "Gp" instead.
Note: For more information on the difference between Gauss points and Lagrange points, consider the COMSOL Knowledge Base article "Stress and Strain results at Gauss Points"
Solution Number 1176.
Until next time...