95 - Understanding Displacement Constraints.pdf

7/27/2019 95 - Understanding Displacement Constraints.pdf

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StructuralConstraintsUnderstandingDisplacementConstraintsLecture

UnderstandingDisplacementConstraints.mp3

Understanding Displacement Constraints

Constraints are applied to restrict the degrees of freedom of

selected entities.

Limit degrees of freedom

Prevent rigid body motion

Sufficient/Proper Constraints required to solve analyses

Settings:

Free Fixed Prescribed

Before Displacement Constraints


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Six degrees of freedom:

Translational (3)1. forward/backward2. left/right3. up/down

Rotational (3)1. Yaw2. Pitch3. Roll

Translation only for Solid Elements

After Displacement ConstraintsLectureNotes

Displacement Constraints

Displacement Constraints (sometimes referred to simply as constraints) are used tolimit the degrees of freedom of an analysis model and to prevent rigid body motion (inmost cases). Constraints can be applied to surfaces, edges, and points. Analyses inMechanica solve specific equations, and these equations rely on certain assumptions.For a static problem, it is assumed that the model is in equilibrium, so the force isproportional to the displacement. In a dynamic or vibration problem, the sum of the


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inertial, damping, elastic, and external forces are also in balance. As such, an

insufficiently constrained model can make the analysis impossible to solve.

From a practical perspective, constraints can also enable an engineer to simplify amodel. Rarely is an engineer able to analyze a single complex system such as anengine or super-structure in one analysis. For example, consider a book. The book issitting on a desk, and the desk sits on the floor. The floor is held up by a building andthe building is anchored into the ground, which belongs to a planet floating in space.Clearly, if someone wanted to model how the book deformed under its own weightwhile sitting on the desk, the model would include only the handbook with constraintspreventing it from moving downwards. Similarly, if you wanted to model a driveshaft,you can remove the bearings and assign constraints in their place and perform ananalysis on the simplified model. If necessary, an analysis on the bearings could be

done separately.

When a constraint is defined on an entity, each degree of freedom can be assigned

one of three values:

Free: This setting designates that the selected entity is free in this degree offreedom.

Fixed: This setting designates that the selected entity cannot move in thisdegree of freedom.

Prescribed: The user specifies a discrete amount of deformation that the entitywill move in. This is used in situations where the user does not know themagnitude of the external load, but does know the desired deformation.

Any model in space has six degrees of freedom:

Translational degrees of freedom (3)1. forward/backward2. left/right3. up/down

Rotational degrees of freedom (3)1. Yaw2. Pitch3. Roll

Because of the way solid elements are created in Mechanica, they only have threeunique degrees of freedom (three translation) because any face on a tetrahedralcannot rotate without translation in one of the three directions. Because of this,Mechanica will ignore rotational constraints on solids. However, rotational constraints

can still be used on shell and beam idealizations.


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Best Practices

When a model is insufficiently constrained for an analysis, it can manifest itself inseveral ways. Most often, the Mechanica solver engine will report that the model isinsufficiently constrained for the analysis. If this occurs, the best way to tackle theproblem is to run a constrained modal analysis with rigid mode search. An animation

of the rigid body motion should expose which component or degree of freedom iscausing the problem.

Constraints in a model will generate a reaction force if there is an attempt to deform

the model in the direction being constrained. For example, a constraint that limits

motion in the X and Y directions will create reaction forces with X and Y components,

but no Z component. In other words, reactions can only be evaluated where a degree

of freedom is being constrained. In a prescribed displacement constraint, the reaction

force reported is the force required to produce that displacement.

UnderstandingDisplacementConstraintsDemonstrationUnderstandingDisplacementConstraints_demo.mp4UnderstandingDisplacementConstraintsProcedure

Procedure: Understanding Displacement Constraints

Scenario

Create a displacement constraint.

CreateDispCons hb_support.prt

Task 1. Open the Mechanica application and create two displacement constraints.

1. Click Applications > Mechanica.

2. Click Displacement Constraint from the Mechanica toolbar.

3. Verify that the World radio button and WCS is selected in the Coordinate System

area of the dialog box.4. Press CTRL and select the surface regions around the holes as shown.

5. Select Free Translation for the X and Z translational degrees of freedom.

6. Leave the Y translational degree of freedom set to Fixed Translation .

7. Select Free Rotation for the X, Y, and Z rotational degrees of freedom.

8. The dialog box should appear as shown. Click OK to create the Constraint and close

the dialog box.


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10. Verify that the World radio button and WCS are selected in the CoordinateSystem area of the dialog box.

11. Press CTRL and select the inner surfaces of both holes as references as shown.


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12. Select Free Translation for the Y translational degree of freedom.

13. Leave the X and Z translational degrees of freedom set to Fixed Translation .


15. The dialog box should appear as shown. Click OK to create the Constraint and

close the dialog box.


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16. The model now has icons present for both constraints on each of the holes as

shown.

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

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

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

3. From the main menu, click File > Close Window.

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

This completes the procedure.

UnderstandingDisplacementConstraintsExerciseExercise: Using Prescribed Displacement Constraints

Objectives

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

Identify when using a prescribed displacement can be useful. Determine the force required to produce a given prescribed displacement.

ScenarioIn most situations, a Mechanica model will have external loads applied to it, and the

results reported show what happens to the model under those external loads. There aresituations however when the user knows how much deflection the model is enduring, butnot the magnitude of the force causing it. In this kind of situation the user can prescribe a

displacement using the displacement constraints functionality.

In this exercise, a PC graphics card is meant to be assembled into a motherboard, while

having the port aligned with one of the external slots in the PCs casing. As the engineerin this situation, you want to know what will happen if the slot in the PC case is not in line

with the slot on the motherboard. The assumption is that the end of the board (where thesocket is) will have to deflect by 1 mm in order to fit into position. You would also like to

know what force is required to create such a deflection.

PrescribedDisp pc_card.asm


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Task 1. Select a simplified representation of the model and start the Mechanicaapplication.

1. Click View Manager from the main toolbar.

2. Right-click Mech_Model in the list of Simp Rep Names and select Set Active.

3. Click Close from the View Manager window.

4. Click Applications > Mechanica .

Note the constraints that already exist along the bottom edge of the model.

These are the constraints used to fix the PC_CARD.ASM in the motherboard.

Task 2. Create a displacement constraint that prescribes a -1 mm deflection of theboard in the Y direction.


2. Type Misaligned_Card in the Name field.

3. In the Member of Set area of the dialog box, click New.

4. Type FORCED_DISPLACEMENT in the Name field of the Constraint Set Definition

dialog box as shown in the figure.

5. Click OK to complete the Constraint Set Definition and close the dialog box.


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6. Verify that the World radio button and WCS are selected in the Coordinate System

area of the dialog box.

7. Select Edges/Curves from the References drop-down menu.

8. Select the bottom edge of the MOUNT.PRT as shown in the figure below.

9. Select Free Translation for the X and Z translational degrees of freedom.

10. Select Prescribed Translation for the Y translational degree of freedom, type -1 for its value, and verify that the units field is set to mm.


12. The dialog box should appear as shown in the figure. Click OK to create theConstraint and close the dialog box.


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Task 3. Create a Measure to measure the reaction force.

1. Click Simulation Measure from the Mechanica toolbar.

2. Click New in the Measures dialog box.3. Type Force_Y in the Name field.

4. From the Quantity drop-down menu, select Force.

5. Verify that the second drop-down menu is set to Reaction At Constraint.

6. From the Component drop-down menu, select Y.

7. Verify that the Coordinate System is set to WCS.


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8. In the Spatial Evaluation area of the dialog box, click Select Reference , select

the constraint you created in the previous task as shown in the figure, and click OK.

9. The dialog box should appear as shown in the figure. Click OK to create the

Measure and close the dialog box.

10. Verify that Force_Y exists as a measure in the Measures dialog box and clickClose.

Task 4. Create and run a static analysis for the model.


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1. From the main toolbar, click Mechanica Analyses/Studies .

2. Click File > New Static....

3. In the Name field, type prescribed_disp_static.

4. Select the Combine Constraint Sets check box.

5. Press CTRL and select the ConstraintSet1 and FORCE_DISPLACEMENT

constraints.

6. Note that there are no Loads to select.

7. Accept all of the other settings in the dialog box by clicking OK to complete thestatic analysis definition and close the dialog box.

8. Verify that prescribed_disp_static is selected in the Analyses and Design Studies

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

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

Note that even in the absence of a Load Set, Mechanica did not issue a

warning. This is because one of the Constraint Sets (specifically the

FORCE_DISPLACEMENT Constraint Set) has a prescribed displacement in it.

The analysis should complete in a few minutes.

Task 5. Examine the prescribed_disp_static analysis results, save the model and

erase it from memory.

1. Examine the results in the Run Status dialog box. In particular, make note of theForce_Y measure results.

Note that the value reported for the measure Force_Y is -.955 N.

2. Verify that prescribed_disp_static is still selected in the Analyses and Design

Studies dialog box and click Results in the Analyses and Design Studies dialogbox to start Results mode.


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3. In the Display Type area of the dialog box, select Graph from the drop-down menu.

4. On the Quantity tab, select Displacement from the first drop-down menu, Y fromthe second drop-down menu, and mm from the units drop-down menu if necessary.

5. In the Graph Location area of the dialog box, click Select Reference and selectthe same curve you selected for the Misaligned_Card constraint as shown in the

figure. Click OK to complete the selection and OK to accept the start point referred

to by the Information window.

6. The dialog box should now appear as shown in the figure. Click OK and Show to

show the resulting graph.


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7. Review the resulting graph.

Examine the resulting graph closely you should find the curve on the graph

as a straight line at -1 on the Y axis. This confirms that the prescribed

displacement constraint is behaving as expected.


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8. Click Copy from the main toolbar in the Results window.

9. In the Display Type area of the dialog box, select Fringe from the drop-downmenu.


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10. On the Quantity tab, verify that the first drop-down menu is set to Displacement,

select Magnitude from the second drop-down menu, and verify that the unit drop-down menu is set to mm.

11. Select the Display Options tab.

12. Select the Deformed check box, type 1 in the Scaling field and clear the % check

box.

13. The dialog box should now appear as shown in the figure. Click OK and Show to

show the resulting fringe plot.

14. Review the resulting fringe plot.

Close examination of the fringe plot should show physically, as well as

through the fringe plot legend and colors, that the edge has indeed deflected

1 mm in the Y direction.


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15. When you are through reviewing the results, click File > Exit Results > No to

exit the Result window without saving any results.

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

17. From the main menu, click File > Close Window.18. Click File > Erase > Not Displayed > OK to erase the model from memory.

19. If necessary, click Close to close the Run Status dialog box.

This completes the exercise.

Documents

95 - Understanding Displacement Constraints.pdf