The set-up of a flex body modal solution in NX Nastran for export to ADAMS MNF or Recurdyn RFI files is possible with the MBDEXPORT case control command. See the NX Nastran Quick Reference Guide for input details.
The use of MBDEXPORT requires special considerations for the modal solution. This is because flex bodies will be attached to other components in the multi-body dynamic (MBD) simulation and local flexibility effects at the connection locations are thus important.
In a standard normal mode solution, the solved modes give good representation of the global dynamics of the component, but the local stiffness effects at the connections are typically not captured because of modal truncation. A modal solution method called Component Mode Reduction of Residual Structure (CMR of RS) is recommended for flex body solutions because it includes both global and local effects. The normal modes method could also capture the residual flexibility effects if residual vectors at the connection DOF are computed.
The CMR of RS method is a variation to the standard SOL 103 modal solution in NX Nastran. It has long been employed by dynamic analysts to include the local stiffness effects in critical areas of a model. The CMR of RS method is a two-step modal solution approach and is equivalent to a Craig-Bampton superelement reduction on the residual structure.