How to Assemble Finite Element Models in APDL: CDWRITE and CDREAD Commands Explained

When facing complex models, especially those with complicated part relationships, APDL can be limiting and far less convenient and efficient than Workbench operations. However, if you make good use of the assembly operations in APDL, you can improve operational efficiency to a certain extent.

The main model assembly commands in APDL are:

1. CDWRITE: Responsible for writing current model data for reading;

2. CDREAD: Responsible for reading model data from specified paths;

Other supporting commands include CM, CMSEL, NGEN, etc.

For complex assembly relationships, the general operation steps are as follows:

1. Create finite element models for each component separately, including geometric models and mesh generation, and create component information at key assembly locations. For example, if the final assembly is achieved through contact, you should first create a node at the contact location using the CM command. After creation, use the CMWRITE command to write out each component’s model.

It is recommended that when creating each component, build them according to their positional relationships in the overall structure to facilitate direct reading during later assembly without needing to perform move operations.

2. Create a new model, then use the CDREAD command to read each model;

3. Use the CMLIST command to verify whether each component’s component information has been read correctly. Additionally, verify element types, real constant data, material information, etc., to ensure all information defined in the previous components has been correctly read.

4. In the overall structure, select relevant components using the CMSEL command and establish contact and other relationships to assemble the overall structure.

5. Apply loads and boundary conditions to the overall structure, then solve.

The above operation process, when requiring parameterization, can greatly improve operational efficiency for complex assemblies to a certain extent.

The following specifically explains the CDWRITE command. The command format is:

CDWRITE, Option, Fname, Ext, –, Fnamei, Exti, Fmat, –, Key

Parameter explanations:

Option: Specifies what model content to write out, including All, Comb, DB, Solid, Geom, CM, MAT, Load, SECT. Unless there are special circumstances, most of the time it is ALL, which means writing out all model information.

Fname: Model name, can include path, maximum 248 characters.

Ext: Model file extension, default is cdb

Fnamei: Geometric model file name, can include path, default is same as previous Fname.

Exti: Geometric model file extension, default is iges

Fmat: Output model format, default is Blocked. This format can be considered a decimal block format and can be opened with text software. If the output file is too large, consider specifying Binary to output binary files and reduce file size.

Notes when using CDWRITE:

1. If loads are defined in a local model, only loads in the current load step will be exported;

2. If loads are defined using geometric methods, such as SFA, they will be automatically converted to finite element model loads during the output process;

3. If the local model already has assembly relationships, such as created contacts, these assembly relationships will not be exported during model writing—only component definitions are retained, not assembly relationships;

4. To prevent numbering conflicts when reading models with existing models, the model undergoes numbering reorganization. Element, real constant, and material numbers in the written model may differ from the final read values, requiring manual verification by users;

5. The exported geometric model defaults to IGES. For beam systems, this format does not save geometric properties such as section properties and direction point property specifications. If retention is needed, use the CDOPT command to specify ANF format output to preserve geometric properties.

CDREAD command usage format:

CDREAD, Option, Fname, Ext, –, Fnamei, Exti

Option: Reading model content settings, including ALL, DB, SOLID, COMB

Other parameters are the same as above.

Notes when using CDREAD:

1. When reading, the coordinate system is automatically set to Cartesian coordinates;

2. NUMOFF command is automatically used by default to offset existing data numbers to avoid duplication. NOFFSET can be used to turn off offset—new data will overwrite existing data (use with caution), and errors easily occur.

3. If multiple CDB files exist where one uses real constants and another uses section definitions, the latter will overwrite the former. It is recommended to uniformly use real constants or section definitions to avoid conflicts.

The above is the explanation of model assembly operation steps and related commands in APDL. Interested students with complex model operations might want to give it a try!

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ANSYS Structural Institute

November 3, 2025