Module 6 Submodeling

6. Submodeling Submodeling is a finite element technique used to get more accurate results in a region of the model. It is a way to “zoom in” on specific regions of a previously- analyzed model, create a fine mesh, and obtain highly accurate results just for that region. October 30, 2001 Inventory #001571 6-2

...Submodeling The submodeling technique has been successfully used not just for stress analyses, but also for thermal, electromagnetic and CFD analyses. In this chapter, we will describe the submodeling procedure for a stress analysis. The following topics will be covered: A. General Procedure B. Workshop October 30, 2001 Inventory #001571 6-3

Submodeling A. General Procedure Five main steps: 1. Create and analyze the initial model 2. Create the submodel 3. Perform cut boundary interpolation 4. Analyze the submodel 5. Verify that the distance between the cut boundaries and the stress concentration is adequate October 30, 2001 Inventory #001571 6-4

Submodeling ...General Procedure 1. Create and analyze the initial (coarse) model Follows the same steps as in a normal static analysis. The geometry need not include local details such as fillet radii. The mesh must be fine enough for accurate displacement results since submodeling theory is based on displacements interpolated from the coarse model to the submodel. October 30, 2001 Inventory #001571 6-5

Submodeling ...General Procedure 2. Create the submodel The submodel is an independent, more finely meshed model of a region of interest within the initial model. Typically, the submodel will include details — such as a fillet radius — that were omitted in the initial model. Some rules for the submodel: It must be in the same location with respect to the global origin as the corresponding portion in the initial model. It must have the same loading conditions as the initial model: gravity, symmetry boundaries, temperatures, etc. October 30, 2001 Inventory #001571 6-6

Submodeling ...General Procedure Steps to create the submodel: a) Start by changing the jobname (since files from the initial analysis will be needed later). Utility Menu > File > Change Jobname Or /FILNAM command b) Build the submodel by: Clearing the database and starting a new model Or simply clearing the mesh and modifying the initial model Which method you choose depends on how much detail you want to include in the submodel geometry. October 30, 2001 Inventory #001571 6-7

Submodeling ...General Procedure c) Specify desired mesh controls and mesh the submodel. d) Apply all loads and boundary conditions that need to be duplicated from the initial model. Examples: symmetry conditions gravity temperatures October 30, 2001 Inventory #001571 6-8

Submodeling ...General Procedure 3. Perform cut-boundary interpolation This is the key step in submodeling, where displacements from the initial model are mapped to the cut boundaries of the submodel. Steps involved: a) First select the nodes on the cut boundary of the submodel. b) Write those nodes to a file. Preprocessor > Create > Nodes > Write Node File Or use the NWRITE command The file name defaults to jobname.node. October 30, 2001 Inventory #001571 6-9

Submodeling ...General Procedure c) Restore the full set of nodes, save the database, and exit PREP7. Utility Menu > Select > Everything (or ALLSEL command) Toolbar > SAVE_DB (or SAVE command) Main Menu > Finish (or FINISH command) d) Activate results from the initial analysis. (Remember, we need the displacements from the initial analysis.) Change the jobname back to the initial one Resume the initial database (RESUME command or Toolbar > RESUM_DB) Enter POST1 and read in the desired set of results (SET command or General Postproc > -Read Results-) October 30, 2001 Inventory #001571 6-10

Submodeling ...General Procedure e) Initiate cut-boundary interpolation: General Postproc > Submodeling > Interpolate DOF Or use the CBDOF command This maps displacements from the initial model to the cut-boundary nodes (on the .node file) and writes out D commands to jobname.cbdo. Interpolate October 30, 2001 Inventory #001571 6-11

Submodeling ...General Procedure f) Exit POST1, change to the submodel jobname, and resume the submodel database. Main Menu > Finish (or FINISH command) Utility Menu > File > Change Jobname (or /FILNAM command) Toolbar > RESUM_DB (or RESUME command) g) Apply cut-boundary displacements to the submodel: Enter PREP7 and read input from the .cbdo file written in step (e) above. Utility Menu > File > Read Input from Or /INPUT command October 30, 2001 Inventory #001571 6-12

Submodeling ...General Procedure 4. Analyze the submodel Ensure that all loading conditions have been duplicated on the submodel. Then save the database and solve the analysis. Toolbar > SAVE_DB (or SAVE command) Solution > -Solve- Current LS (or SOLVE command) Review the submodel results using standard postprocessing procedures. SX stresses Stress deviation SDSG October 30, 2001 Inventory #001571 6-13

Submodeling ...General Procedure 5. Verify the cut-boundary distance This is an important step since submodeling theory is based on the assumption that the cut boundary is far away from the stress concentration region. You can do this by comparing stress or strain results at the cut boundary: Compare path plots at that location between the initial model and submodel. Use the query option. List the results. Etc. October 30, 2001 Inventory #001571 6-14

Submodeling ...General Procedure SX queries for the plate-with-a-hole model are shown below. Near the submodel boundary, the SX values show good agreement, indicating that the boundary is far enough away from the stress concentration. SX = 63 to 113 psi for the initial model SX = 70 to 109 psi for the submodel October 30, 2001 Inventory #001571 6-15

Submodeling B. Workshop This workshop consists of the following problem: W5. Crankshaft Please refer to your Workshop Supplement for instructions. October 30, 2001 Inventory #001571 6-16