comparing 2d and 3d structural analysis workshop 4.2

Comparing 2D and 3D Structural Analysis

Workshop 4.2

March 26, 2005Inventory

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Workshop 4.2 - Goals

• Workshop 4.2 consists of a 2 part assembly representing a pressure cap and retaining flange (full model shown below).

• We will solve the model in 2 ways, as a 90 degree symmetry sector and as a 2D axisymmetric model (shown on next page).

• Our goal is to compare the 2 methods both for consistency and for economy.

Pressure Cap

Retaining Ring

Full Model


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Workshop 4.2 - Geometry

• Shown here are the 3D sector model and the 2D axisymmetry model.

Pressure Cap

Retaining Ring


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Workshop 4.2 - Assumptions

• Assumptions:– The retaining ring is fixed at its mounting holes.

– The contact region between the parts is frictionless.

– The base of the pressure cap is constrained using a compression only support.

– Note: due to the presence of the bolt holes the structure is not truly axisymmetric. Part of our goal is to determine the validity of the axisymmetric assumption in this case.


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Workshop 4.2 - Start Page

• From the launcher start Simulation.

• When DS starts, close the Template menu by clicking the ‘X’ in the corner of the window.


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Workshop 4.2 – Geometry Setup

• Before importing the geometry highlight the “Geometry” branch and change the “Analysis Type” preference to “2D” in the details.

• Choose “Geometry > From File . . . “ and browse to the file “Axisym_pressure_2D.x_t”.


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Workshop 4.2 – Preprocessing

1. Set the working unit system to the metric mm system.

– “Units > Metric (mm, Kg, N, C, s)”.

2. Highlight Parts 1 and 2 in the tree and rename “Retaining Ring” and “Pressure Cap”.

3. In the details for each part, change their “Behavior” to “Axisymmetric”.

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…Workshop 4.2 – Preprocessing

4. From details for the “Pressure Cap” and import the material “Stainless Steel”.

4


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Workshop 4.2 – Contact

• Highlight the “Contact Region” and notice the target contains a single edge. We will add a second edge to insure all possible contact is detected.

Additional target edge to be added (shown dashed)


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…Workshop 4.2 – Contact

5. Click in the “Target” field then select the 2 edges of the pressure cap shown here.

6. “Apply” the new selection.

Select Edges

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Note: if you have difficulty selecting the edges of the Pressure Cap, use the “hide” feature to hide the retaining ring during selection.


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…Workshop 4.2 – Contact

7. In the Contact Region detail change the “Type” to “Frictionless”.

8. Highlight the “Mesh” branch, RMB and “Preview Mesh” (note the speed with which the 2D mesh is generated as well as the density).

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Workshop 4.2 – Environment

9. Highlight the Environment branch.

10. Select the 3 inside edges of the Pressure Cap.

11. “RMB > Insert > Structural > Pressure”.

12. Set the pressure magnitude = 0.1 MPa.

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…Workshop 4.2 – Environment

13. Highlight the bottom edge of the pressure cap.

14. “RMB > Insert > Compression Only Support”.

1314


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15. Select the middle line on the top of the retaining ring.

16. “RMB > Insert > Fixed Support”.

Remember, the axisymmetric assumption here is that the retaining ring is a continuous solid. Actually there are bolt holes around its circumference. For this reason, when the model was created in DesignModeler this separate line was intentionally created to provide a location to add our support.

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Workshop 4.2 – Solution

• Highlight the Solution branch, RMB and insert:

17. Stress > Equivalent (von-Mises)

18. Deformation > Total

19. Switch to body select mode, select the pressure cap and repeat steps 16 and 17.

– Solve

17 18

Note, the last two results are now scoped to the pressure cap. This will allow us to isolate its response.

19


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…Workshop 4.2 – Solution

20. “OK” the weak spring message.– Note: due to the fact that the pressure cap is constrained using

frictionless contact and a compression only support, weak springs are added to prevent rigid body motion.

20

Notes on axisymmetry:

1. Notice that the model lies completely in +X space with the Y axis as the axis of revolution. This is required for axisymmetry.

2. Axisymmetry assumes that the model is a complete 360 degree model. For this reason no constraints in the X direction are required. The portion of the pressure load acting in the +X direction is assumed to be offset by an equal portion in the –X direction.


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Workshop 4.2 – Postprocessing

• Highlight each of the result objects to inspect the response.– Note: due to meshing and machine variations, results may not match

exactly those shown here.

• For future reference, highlight the “Equivalent Stress 2” result (scoped) and note the maximum value here:________________


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…Workshop 4.2 – Postprocessing

• Highlight the Solution branch, “RMB > Insert > Solution Information > Solution Information”.

The graphics window will change to the Worksheet view. Scroll to the bottom of the solution information and note the Elapsed Time (this will vary by machine).

Elapsed Time = ___________________________

Note, CP time represents the sum for all processors used. In multiprocessor machines it will generally exceed elapsed time.


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Workshop 4.2 – 3D Symmetry Model

• Close the current project (you may save the current 2D Simulation if desired).

• We’ll now set up and solve the 3D symmetry model using the same boundary conditions.


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Workshop 4.2 - Start Page

• From the launcher start Simulation.

• When DS starts, close the Template menu by clicking the ‘X’ in the corner of the window.


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Workshop 4.2 – Geometry Setup

• Choose “Geometry > From File . . . “ and browse to the file “Axisym_pressure_3D.x_t”.


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Workshop 4.2 – Preprocessing

1. The working unit system should still be set to the metric mm system.

– “Units > Metric (mm, Kg, MPa, C, s)”. 1

• Note, once again rename the 2 parts in the model “Retaining Ring” and “Pressure Cap”


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…Workshop 4.2 – Preprocessing

2. From details for the “Pressure Cap” and import the material “Stainless Steel”.

2


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Workshop 4.2 – Contact

3. Highlight the “Contact Region” branch and change the “Type” to “Frictionless”.

4. Highlight the “Mesh” branch, RMB and “Preview Mesh”.• Refer to p. 4.1-11 to compare the 2D mesh.

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Workshop 4.2 – Environment

5. From the Environment branch highlight the 6 faces representing the planes of symmetry (cut planes).

6. RMB > Insert > Frictionless Support.

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Note, frictionless supports provide constraints in the normal direction. This is used to model the symmetry condition.

Note: there are six (6) faces to select.


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7. Highlight the bottom face of the pressure cap, “RMB > Insert > Compression Only Support”.

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8. Highlight the 3 inside faces on the pressure cap, “RMB > Insert > Pressure”.

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9. Change the Magnitude to “0.1” in the detail window. 9


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10. Highlight the 3 cylindrical faces of the bolt holes, “RMB > Insert > Fixed Support”.

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Workshop 4.2 – Solution

• Highlight the Solution branch, RMB and insert:

11. Stress > Equivalent (von-Mises)

12. Deformation > Total

13. Switch to body select mode, select the pressure cap and repeat steps 16 and 17.

– Solve

11 12

As before, the last two results are scoped to the pressure cap.

13


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Workshop 4.2 – Postprocessing

• As before highlight each of the result objects and inspect the response.

• For reference, highlight the “Equivalent Stress 2” result (scoped) and note the maximum value here:________________


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…Workshop 4.2 – Postprocessing

• Highlight the Solution branch, “RMB > Insert > Solution Information > Solution Information”.

The graphics window will change to the Worksheet view. Scroll to the bottom of the solution information and note the Elapsed Time (this will vary by machine).

Elapsed Time = ___________________________


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Workshop 4.2 – Comparison

• Using the example shown in the exercise we now compare analyses (note, your actual results may vary from those shown here. Also, your solution times will almost certainly differ from those shown here.

• Maximum von-Mises Stress Results:– Axisymmetric = 0.829 MPa

– 3D Symmetry = 0.749 MPa

Note, meshing differences account for the results difference (see next page). Recall that the 2D model resulted in a more refined mesh than the 3D. The next page shows the results from a more refined 3D model.

• Elapsed Time:– Axisymmetric = 8.0 seconds

– 3D Symmetry = 40.0 seconds


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…Workshop 4.2 – Comparison

• Maximum von-Mises Stress Results:– 3D Symmetry (refined) = 0.852 MPa

• Elapsed Time:– 3D Symmetry (refined) = 578.0 seconds

Results using a more refined mesh with the 3D symmetry model

comparing 2d and 3d structural analysis workshop 4.2

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