How contact formulation may impact your results
For a free-free modal analysis where there are no loads or constraints, there will be 6 rigid body modes: three translational (UX, UY, UZ) and three rotational (ROTX, ROTY, ROTZ). This means the body will not undergo any internal deformation but will be able to translate or rotate freely. In a free-free modal analysis, the first three modes correspond to the rigid body modes, which represent the overall translation and rotation of the structure as a whole. These modes have frequencies close to zero.
The Augmented Lagrange contact formulation (which is the "program default" contact formulation) is known to suppress the rigid body modes in a modal analysis. It applies additional constraints and penalties to enforce contact conditions, which can affect the behavior of the rigid body modes. As a result, the Augmented Lagrange formulation may produce only three nonzero modes, excluding the rigid body modes from the analysis.
On the other hand, the MPC contact formulation handles the rigid body modes differently. It preserves the rigid body modes in the analysis and allows them to be captured as part of the solution. This formulation avoids the suppression of the rigid body modes and enables all six rigid body modes to be present in the modal analysis results.
By switching from the Augmented Lagrange contact formulation to the MPC contact formulation, you are essentially changing the treatment of rigid body modes in the analysis. The MPC formulation allows the rigid body modes to be included, resulting in the expected six modes in a free-free modal analysis.
It's important to note that the choice of contact formulation can have an impact on the modal analysis results and should be selected based on the specific requirements and characteristics of the problem at hand.