The Abaqus Interface for MSC.ADAMS uses component modal synthesis to combine the fixed-interface normal modes and the substructure recovery vectors into a basis of modal degrees of freedom that will be used for dynamic analysis in MSC.ADAMS. This modal basis spans a space that includes the rigid body response of the substructure. Typically, for a non-prestressed, unrestrained body in three dimensions, one expects to find six rigid body modes with associated zero eigenvalues.
The situation is, in general, different for prestressed models, where an unrestrained structure may have less than six modes with zero eigenvalues. Prestressing may change the expected zeroes into values that are significantly positive or negative, depending on the sign of the prestress.
By default, the Abaqus Interface for MSC.ADAMS deletes modes with negative eigenvalues and reorthogonalizes the reduced basis. If you want to retain modes with negative eigenvalues, define the environment variable.
On UNIX platforms type the following command:
setenv MDI_MNFWRITE_OPTIONS negative_roots_OK
On Windows platforms type the following command:
set MDI_MNFWRITE_OPTIONS=negative_roots_OK
To determine if a model will have negative eigenvalues when translated by the Abaqus Interface for MSC.ADAMS, you can add a *FREQUENCY step with no boundary conditions to the input file. If this step is added to the run that creates the results file used by the Abaqus Interface for MSC.ADAMS, it must not write anything to the results file.