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Structural Constraints Using Symmetry Constraints  – Exercise 

Exercise: Using Symmetry Constraints

Objectives

After successfully completing this exercise, you will be able to:

  Identify how a model should be cut for use with cyclic symmetry.

  Identify when loads on a model need to be modified due to symmetry.

ScenarioIn cyclic symmetry the cut used to slice model has to be precise. The constraint requiresthat both surfaces selected map to each other when rotated about the axis of symmetry.

A model may contain more than one patterned feature, and these circular patterns may

not repeat themselves at the same interval. So the use of an incorrect cut and a correct

cut will be examined.

When symmetry is applied, the model is reduced to a smaller size and some loads thatwere on the model need to be modified. Instances when a load should be modified and

when it should be left a alone will be examined.

SymmExer  hat.prt

Task 1. Enter the Mechanica application and create a Cyclic Symmetry Constraintwith different cut features.

1. From the Main Menu, click Edit > Resume > Resume Last Set.

Note that the CYCLIC_SYM_CUT_1 cut feature has been resumed. It

results in a piece of geometry that is NOT cyclically symmetric. You willattempt to apply a cyclic symmetry constraint to the model and use it in an

analysis.

2. Click Applications > Mechanica .

3. Click Symmetry Constraint from the Mechanica toolbar.

4. Select Cyclic from the Type drop-down menu.5. Query select the hidden surface on the far side of the slice as the first reference as

shown in the figure.

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6. Right-click anywhere in the display area and select Second Side.

7. Select the surface on the near side of the slice as the second reference as shown inthe figure.

8. Click OK to complete the Symmetry Constraint.

9. From the Main toolbar, click Mechanica Analyses/Studies .

10. Verify that cyclic_modal is selected in the Analyses and Design Studies dialog

box and click Start Run > Yes to start the design study.

Note that the analysis fails. A message in the Diagnostics dialog boxindicates that the shape is not appropriate for the cyclic symmetry

constraint.

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11. Click Close to close the Diagnostics dialog box and Close to close the Analyses

and Design Studies dialog box.

12. Select the Constraint from the display area, right-click and select Delete > Yes to

delete the erroneous constraint as shown in the figure.

13. From the Main Menu, click Applications > Standard.

14. Right-click the CYCLIC_SYM_CUT_1 feature in the model tree and select Delete

> OK as shown in the figure to delete the cut.

15. From the Main Menu, click Edit > Resume > Resume Last Set.

Note that the CYCLIC_SYM_CUT_2 cut feature has been resumed. Thistime, the resulting geometry is cyclically symmetrical.

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16. Click Applications > Mechanica.

17. Click Symmetry Constraint from the Mechanica toolbar.

18. Select Cyclic from the Type drop-down menu.

19. Query select the hidden surface on the far side of the slice as the first reference asshown in the figure.

20. Right-click anywhere in the display area and select Second Side.

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21. Select the surface on the near side of the slice as the second reference as shown

in the figure.

22. Click OK to complete the Symmetry Constraint.

23. From the Main toolbar, click Mechanica Analyses/Studies .

24. Verify that cyclic_model is selected in the Analyses and Design Studies dialog

box and click Start Run > Yes > Yes to start the design study.

Note that this time the analysis completes successfully without errors.

Note: The best practices warn about using symmetry with a modal analysis,

in this case it is being used purely as a tool to see if the analysis will run tocompletion with different references selected for cyclic symmetry.

Task 2. Save the model and erase it from memory.

1. Click Close > Close to close all of the open dialog boxes.

2. Return to the Standard Pro/ENGINEER mode by clicking Applications > Standard.

3. Click Save from the main toolbar and click OK to save the model.

4. From the Main Menu, click File > Close Window.

5. Click File > Erase > Not Displayed > OK to erase the model from memory.

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Task 3. Open the FRAME.PRT part model, run the complete model static analysiswith the existing Loads and Constraints and note the results.

1. Click Open from the Main toolbar, select FRAME.PRT and click Open.

2. Click Applications > Mechanica .

Note that the model is simply supported with a load of 200 N acting in thenegative Y direction.

3. From the Main toolbar, click Mechanica Analyses/Studies .

4. Verify that complete_model_static is selected in the Analyses and Design Studies

dialog box and click Start Run > Yes to start the design study.

5. Click Display Study Status once the analysis is started.

6. When the analysis complete, examine the resulting measure values in the RunStatus dialog box.

In particular, note that the analysis reports a maximum Von Mises Stress of 8.92 MPa and a maximum displacement of 3.025 X10-3 mm.

7. Click Close to close the Run Status dialog box.

8. From the Main Menu, click Applications > Standard.

Task 4. Resume a cut feature (and with it a Mirror Symmetry Constraint), run themirror_static analysis on the model and note the erroneous results.

1. From the Main Menu, click Edit > Resume > Resume Last Set.

Note that the MIRROR_SYM_CUT cut feature has been resumed and it cuts

away half of the model.

2. Click Applications > Mechanica .

When the cut was originally created, the model was brought into Mechanica

and a Mirror Symmetry Constraint was associated with it. When the cut wassuppressed, the Mirror Symmetry Constraint was suppressed with it. Whenyou resumed the cut feature with the Resume Last Set functionality, the

Mirror Symmetry Constraint was also resumed with it and appears now in

the Mechanica analysis model.

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3. From the Main toolbar, click Mechanica Analyses/Studies .

4. Select mirror_static from the Analyses and Design Studies dialog box and click

Start Run > Yes to start the design study.

5. Click Display Study Status once the analysis is started.

6. When the analysis complete, examine the resulting measure values in the Run

Status dialog box.

In particular, note that the analysis reports a maximum Von Mises Stress of 16.3 MPa and a maximum displacement of 6.06 X10-3 mm. These values are

almost exactly double the results from the analysis on the whole model. Youwill investigate and correct for this discrepancy in the next few steps.

7. Click Close > Close > Close to close all of the open dialog boxes.

Task 5. Examine and modify the existing load, rerun the mirror_static analysis andnote the results.

1. Click the + next to Loads/Constraints near the bottom of the model tree, thenclick the + next to Load Set LoadSet1.

2. Right-click Load1 in the model tree and select Edit Definition.

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3. Examine the load definition in the Force/Moment Load dialog box. Click Advanced to see more information about how the load is applied.

The -200 N load is applied as a Total Load and is distributed Uniformly

across the surface: the total load of -200 N is applied regardless of the sizeof the surface it is applied to. The value truly apparent to this surface (now

that we've cut half of it away) is actually -100 N.

4. In the Force area of the dialog box, type -100 in the Y field.

5. Click OK to complete editing the Force/Moment Load.

6. From the Main toolbar, click Mechanica Analyses/Studies .

7. Select mirror_static from the Analyses and Design Studies dialog box and click

Start Run > Yes > Yes to start the design study.

8. Click Display Study Status once the analysis is started.

9. When the analysis complete, examine the resulting measure values in the Run

Status dialog box.

In particular, note that the analysis reports a maximum Von Mises Stress of 8.16 MPa and a maximum displacement of 3.03 X10-3 mm. These values are

almost exactly the value that resulted when the -200 N load was applied tothe entire model.

The conclusion here is that loads cut by a plane of symmetry need to be re-examined. If the load is a distributed load (for example, pressure, assignedas a force/unit area) then no modification is needed. If the load is applied as

a total load, then the load magnitude adjusted.

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Task 6. Save the model and erase it from memory.

1. Click Close > Close > Close to close all of the open dialog boxes.

2. Return to the Standard Pro/ENGINEER mode by clicking Applications > Standard.

3. Click Save from the main toolbar and click OK to save the model.

4. From the Main Menu, click File > Close Window.5. Click File > Erase > Not Displayed > OK to erase the model from memory.

This completes the exercise.