5.15.14. Substructure Analysis

Substructuring is a procedure that condenses a group of finite elements into one element represented as a matrix. The single-matrix element is called a superelement. You can use a superelement in an analysis as you would any other element type. By summarizing the mechanical behavior of the bodies, they can be assembled to synthesize the response of a more complex structure, or to model flexible bodies in the Rigid Dynamics solver.

Mechanical provides the options to generate superelements. You use the Condensed Parts object to create superelements on a sub assembly or the Substructure Generation analysis to create a superelement of an entire assembly. When you use the Substructure Generation analysis, you can specify constraints and pre-stress effects and generate load vectors and use the elements in an analysis. When you use the Condensed Parts you can expand their solutions using the Expansion Settings object.

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Supported Analysis Types

Substructuring currently supports the following analysis types:

Substructuring Types

Mechanical enables you to perform:

  • Top-down Substructuring: Using top-down substructuring, the application generates superelements on parts that you specify using Condensed Part objects. Each Condensed Part object enables you to treat a set of bodies as a single superelement consisting of matrices and load vectors with far fewer degrees of freedom than the full finite element mesh. The use pass (described below) is then performed and includes all generated superelements. Using Expansion Settings, the solution expands the superelements for the specified parts. See the Top-Down Substructuring section in the Mechanical APDL Substructuring Analysis Guide for additional technical information.

    Important:  You can use the Substructure Generation analysis to automatically generate superelements that include load vectors from the loading conditions and constraints that you apply. In addition, you can use an upstream Static Structural analysis to create pre-stress effects in the Substructure Generation analysis solution.

    Or…

  • Bottom-up Substructuring: Using bottom-up substructuring, you import superelements using the Imported Condensed Part object that were created in a different Mechanical session. These imported superelements represent subassemblies of the model on which the generation pass is already performed during a different Mechanical session and in the current Mechanical session these superelements are then combined with the rest of the model to perform the use pass on the entire model.

    Important:  Bottom-up Substructuring does not support the Expansion pass.

Condensed Parts Introduction

Condensed Parts are defined by three key pieces of information:

  • A group of bodies whose elements are to be reduced to a superelement.

  • A set of interfaces defining the primary nodes that should be retained in the generated superelement.

  • A list of solution settings that control the reduction process.

Substructuring Process

The top-down substructuring of condensed geometry involves the following operations:

  1. Generation: The preliminary computation, the "Generation Pass," reduces the Condensed Part bodies into a single superelement and its primary nodes, located on the defined interfaces. The remaining interior nodes become hidden from the analyses that use the Condensed Part. For top-down substructuring, the generation pass is done in the same mechanical session as the use pass. For bottom-up substructuring, the generation pass is done in a different mechanical session and superelements are imported in the current session on which no generation pass is required.

  2. Use: Once your Condensed Parts are properly defined and generated and Imported Condensed parts are imported into the Mechanical session, they can be used in the solution (the "Use Pass"). By hiding the interior nodes, flexible bodies can be included in a Rigid Dynamics analysis when they are included in a Condensed Part. They can also be assembled together in Modal analyses to compose the overall vibration properties of a more complex structure.

  3. Expansion (Top-down substructuring only): Following the Use Pass, you can obtain results on elements inside the Condensed Geometry using an "Expansion Pass."

Rigid Dynamics Only

For a Rigid Dynamics analysis, additional results can be produced by the Generation Pass to allow a faster expansion, which does not need to go back to the finite element model to compute stresses and deformation on a condensed part (see Expansion Pass).

Note:  Material and System Damping

  • The solver ignores any Material Dependent Damping (Damping Ratio and Constant Structural Damping Coefficient material properties) specified in the material assigned to the bodies included in Condensed Part(s). Material dependent damping is defined in Engineering Data.

  • If you specify the property Constant Structural Damping Coefficient as global system damping using the Damping Controls of the Analysis Settings, the solver ignores this value during a Modal that includes Condensed Parts or Imported Condensed Parts.

Importing Condensed Parts (Bottom Up Substructuring)

Ansys refers to the substructuring procedure described above as “top-down” substructuring. For this procedure, you use superelements exported from a separate analysis. Given the proper exported file, as listed below, you then use the Imported Condensed Part object to import the file into a new analysis that is processed during the Use Pass when all superelements are assembled.

You produce these superelement files by exporting a Condensed Part from one of the supported analysis systems or using a Substructure Generation analysis.

The Imported Condensed Part object supports the following file types:

  • Exported Condensed Part (.cpa) (default): This file format is for superelements generated in Mechanical using the Condensed Part feature.

  • Generation Pass Output (.sub): This file format is for superelements generated in Mechanical APDL.

  • Super Element Matrix (.dmig): This file format is for superelements generated in NASTRAN.

Note:

  • For a superelement, imported in Exported Condensed Part (.cpa) and Generation Pass Output (.sub) file format, that contains load vectors, you can apply and scale these load vectors using the Load Application object during the Use pass.

  • The application does not import load vectors from Super Element Matrix (.dmig) files. As a result, they cannot be applied to the Use pass.

See the Imported Condensed Part section for the steps to use this option.

Substructure Generation Analysis

As discussed above, you can use a Substructure Generation analysis to create an Exported Condensed Part (.cpa) file. You import this file into a supported analysis using the Imported Condensed Part object in order to perform a "bottom up" substructure analysis.

You can also automatically create an Imported Condensed Part object in a supported analysis using the solution of a Substructure Generation. See the Imported Condensed Parts from Linked Substructure Generation Analysis for the procedure.

Analysis File Management

To improve solution processing times and memory usage when solving the Use Pass and Expansion Pass, the Mechanical APDL Solver (required) refers to the prerequisite files generated by the upstream condensed parts using the entire path to their location. See the SEOPT and MODDIR commands in the Mechanical APDL Command Reference for more information.

Feature Description and Application

See the following sections that describe the use of the substructuring/Condensed Part features.