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Learn how to seamlessly convert Ansys substructure models to Nastran for payload dynamic model requirements

Benefits of Ansys Substructure Models

Ansys substructure models provide several benefits for engineers and analysts working on large assembly structural analysis and simulation.

These models allow for the efficient representation of complex structures by dividing them into smaller, more manageable substructures.

The use of substructure models reduces computational resources, such as memory and processing power, required for analysis. This makes it possible to perform true system level modal, random vibrations and shock simulations without compromising on the accuracy in areas of interest.

With Ansys substructure models, engineers can easily update and modify specific components (or substructures) without having to re-analyze the entire structure.

This flexibility saves time and effort in design iterations and allows for quick evaluation of design changes.

Additionally, Ansys substructure models enable the reuse of validated substructures in different analyses, improving efficiency and consistency across projects.

Payload Craig-Bampton reduced model requirements

When creating payload dynamic models, the Craig-Bampton reduced model approach is commonly used.

This approach involves separating the structure into a flexible component, known as the payload, and a rigid component, known as the host structure.

The payload component represents the parts of the structure that undergo significant deformations, while the host structure represents the remaining portions that remain rigid.

Ansys has long had the capability for creating Craig-Bampton based substructures. However some launch firms use other FEA software to integrate payload and host structures for analysis. This has lead to the requirement by some launch firms to require the creation of substructures in Nastran format.


In order to facilitate and improve the interoperability of FEA tools, we will describe the process of creating the a Craig-Bampton reduced model in Nastran format from within Ansys Mechanical.


Creating Craig-Bampton reduced models in Ansys using bottom up substructure generation

Ansys provides a bottom-up substructure generation approach for creating Craig-Bampton reduced models.

This approach involves building the substructure model from the bottom up, starting with the individual components and gradually assembling them into the complete structure.

To create a Craig-Bampton reduced model in Ansys using bottom-up substructure generation, engineers can review this blog and video

A few key points to remember

Remote Points

Reduced substructure models need to connect to the rest of the assembly. We typically want to reduce the size of the model by reducing the number of nodes on the interface. Therefore you should create remote points as interface points. This means instead of selecting a surface with 1000 nodes, you can reduce the model to 1 or in this case 3 interface points.

Substructure Definitions

Use a "Substructure Generation" Analysis system to create the Craig-Bampton reduced model

There are very few settings required. However you will need to identify the Remote Points created earlier and define the interface method.

If you are using these substructure for Ansys analysis, you are ready to start analyzing. However if we want to export this substructure to Nastran format, we will need to use some command snippets.

APDL command snippets for Nastran Export

You need only a few lines of code to export the mass and stiffness matrix to a Nastran DMIG format.

Many years ago, Ansys added the capability to work directly with internal matrices.

The first 2 lines reads in the stiffness and mass matrix from the substructure file and saves them to the CMSKF and CMSMF variables. We then convert these matrices into symmetric full matrices. 

Finally we write out the matrices in DMIG format. 2 files, CMSKS.dmig and CMSMS.dmig will be written to the output directory.

That's it, now we have a couple of Nastran formatted DMIG files ready to be integrated into the rocket. 


Validation and Verification

As an Elite Ansys Channel partner, we have extensive access to Ansys tools. Unfortunately we don't have access to Nastran.

However we are able to work partners to do some basic verification.

Ansys frequency results

Nastran modal values

The frequencies match up exactly! 


Post by MingYao Ding
May 10, 2024