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The stiffness matrix from Femap

N/A
Hello! How can I get stiffness matrix, damping matrix and inertia matrix from Femap?
3 REPLIES

Re: The stiffness matrix from Femap

Innovator
Innovator
Hello!,
Well, not from FEMAP but from NX NASTRAN, that is the Finite Element solver, FEMAP is only the pre- and postprocessor, who perform the calculations is NX NASTRAN.
In FEMAP go to HELP > NX NASTRAN > NX NASTRAN USER's GUIDE > DIRECT MATRIX INPUT, and read all over there:
Overview of Direct Matrix Input

You can use the Bulk Data entry DMIG to input a stiffness (or mass) matrix which connects specified degrees-of-freedom. The matrix so defined will be added to the stiffness (or mass) matrix computed from finite element properties.
The DMIG entry includes provisions for unsymmetrical terms and complex values, both of which are useful in dynamic analysis. These provisions should not be used in static or normal modes. Note that an entry in the Case Control Section is required (K2GG = NAME for a stiffness matrix or M2GG= NAME for a mass matrix).
The primary application of the DMIG Bulk Data entry is to enter stiffness and mass data for parts of the structure which are obtained from another computer run. The format is cumbersome (two matrix terms per continuation entry) and the matrix should be input to high precision. For stiffness matrices only, the GENEL Bulk Data entry is an alternative for manually inputting data.
The finite element approach simulates the structural properties with mathematical equations written in matrix format. Once you provide the grid point locations, element connectivities, cross-sectional properties, material properties, applied loads, and boundary conditions, NX Nastran then automatically generates the appropriate structural matrices. The structural behavior is then obtained by solving these equations.
If these structural matrices are available externally, you can input these matrices directly without providing all the modeling information. Normally this is not a recommended procedure since it requires additional effort. However, there are occasions where the availability of this feature is very useful and in some cases is extremely crucial. Some possible applications are listed below:

    Suppose you are a subcontractor to a classified project. The substructure that you are analyzing is attached to the main structure built by the primary contractor. The flexibility of this main structure is crucial to the response of your component, but the geometry of the main structure is classified. The main contractor, however, can provide you with the stiffness matrix of the classified structure. By reading in this stiffness matrix and adding it to your NX Nastran model, you can account for the flexibility of the attached structure without compromising the security. The stiffness matrix is the inverse of the flexibility matrix.
    Suppose you are investigating a series of design options on a component attached to an aircraft bulkhead. Your component consists of 500 DOFs and the aircraft model consists of 100,000 DOFs. The flexibility of the backup structure is somewhat important. You can certainly analyze your component by including the full aircraft model (100,500 DOFs). On the other hand, if the flexibility at the attachment points on the aircraft can be measured experimentally, then you can add the experimental backup structure stiffness to your component without including the whole aircraft model. The experimental backup structure stiffness matrix is the inverse of the measured flexibility matrix. This way your model size remains at 500 DOFs, and you still have a good approximation of the backup structure stiffness.
    The same concept can be applied to a component attached to a test fixture. The stiffness of the fixture at the attachment locations can be read in as a stiffness matrix. Once again, the experimental test fixture stiffness matrix at the attachment points is the inverse of the measured flexibility at these points.

There are several ways that these matrices can be read in, such as DMIG, GENEL, and INPUTT4. Only DMIG is covered here. The DMIG and the INPUTT4 options offer alternate methods for inputting large matrices. Note that INPUTT4 provides more precision than the DMIG input; the DMIG yields more precision than the GENEL on a short word machine.
Best regards,
Blas.

Re: The stiffness matrix from Femap

N/A
Thanks for the answer! But it is necessary for me the MATRIX OUTPUT from NX NASTRAN (stiffness matrix, damping matrix and mass matrix).

Re: The stiffness matrix from Femap

Innovator
Innovator
Hello!,
You can use EXTOUT with option MATRIXPCH, this will write an ASCII structural stiffness matrix output on the punch file (.pch) in DMIG format:
param,extout,matrixpch
It should be put in the NX NASTRAN Case Control section above subcase statements (in FEMAP under ANALYSIS select PREVIEW INPUT and edit your NX NASTRAN input deck).
If you also need to write loads information, use DMIGPCH option:
param,extout,dmigpch
Best regards,
Blas.