Workshop 13

Nonlinear Static Analysis of a Pump AssemblyIntroductionIn this workshop you will use ABAQUS/CAE to complete the pump assembly model andto submit it for analysis. You will begin by defining the analysis steps and the outputrequests associated with these steps. Next, you will define the interactions between thedifferent components of the model. You will also apply bolt and pressure loads andboundary conditions to the model. Finally, you will create and submit a job for analysisand evaluate the analysis results.

Defining the analysis stepsThe analysis history of the pump assembly consists of two steps: a step that simulates thepre-tensioning in the bolts, followed by a step that simulates the pressurization of thebolted assembly.

1. Open the model database file PumpAssy.cae. Switch to the Step module, andin the context bar select pump_ribs from the list of available models.

2. Create a general static step named PreloadBolts. Activate geometricnonlinearity (toggle on Nlgeom), and specify an initial time increment of 0.05and a total time period of 1.0.

3. Create a second general static step named Pressure. Insert this step after thestep named PreloadBolts. Specify an initial time increment of 0.1 and atotal time period of 1.0.

4. Activate the contact diagnostic printout for both the steps. From the main menubar, select OutputDiagnostic Print. In the Diagnostic Print dialog box,click in the blank area under the Contact column for both steps so that tickmarks appear, thus activating contact diagnostic output for these steps.

5. Click OK to close the dialog box.

Defining contact and constraintsThe mere proximity of the model components does not indicate that the part instanceswill interact during the analysis. Thus, unless explicitly specified, the individualcomponents of an assembly will not interact with one another. For any loads to beproperly transmitted between the components, you must define interactions between thecomponents. In structural analysis problems the most common method of transferringloads between unconnected regions of a model assembly is through contact interactions.To define a contact interaction between any two bodies, however, you must first identifythe regions of each body that will be involved in contact (e.g., define surfaces). Your nexttask in this workshop, therefore, will be to define surfaces on each component that will beinvolved in contact.

Defining the surfacesIn ABAQUS/CAE a surface can be created either on a part that has underlying geometry(such a surface is known as a geometry-based surface) or on a part that does not haveunderlying geometry (e.g., an orphan mesh; such a surface is known as a mesh-basedsurface). Since this assembly consists of both imported geometry and an orphan mesh,you will create both types of surfaces.

1. Switch to the Interaction module.6. Make only the pump housing visible using the Assembly Display Options

dialog box (ViewAssembly Display Options).7. Create a surface named PumpBot by following the steps given below:

A. From the main menu bar, select ToolsSurfaceManager.B. In the Surface Manager, click Create.C. The Create Surface dialog box appears. Select Mesh as the surface type.

Name the surface PumpBot, and click Continue.D. You will be prompted for the regions to define the surface. For a mesh-based

surface, you can either select the elements individually or select a group ofelements by specifying the maximum deviation in the face angle betweenadjacent elements. The face angle method is in general a more efficient way ofchoosing elements to define a surface. Hence, in the prompt area select byangle as the selection method and enter a face angle of 5 degrees.

E. Click any element face on the bottom of the pump. All the element faces onthe bottom of the pump will then be highlighted in red, as shown in Figure W13–1. Click Done in the prompt area when you are finished.

Figure W13–1. Surface PumpBot8. Similarly, create a surface named PumpBolts that defines a surface in the region

of the pump that will come into contact with the heads of the bolts. Select thesurface using a face angle of 5 degrees, as shown in Figure W13–2.

9. Next, define a surface in the region where the internal pressure will be applied tothe pump. Name the surface PumpInside. Using the face angle method with amaximum deviation of 24.1 degrees, select the element faces shown in Figure W13–3.

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Tip: Use [Shift]+Click to select more than one item at a time. Select as many regions aspossible using the face angle technique; then select any remaining regions individually.Zoom in as necessary to facilitate your selections. To deselect any unintentionallyselected regions, use [Ctrl]+Click.

Figure W13–2. Surface PumpBolts

Figure W13–3. Surface PumpInside

10. Use the Assembly Display Options dialog box to restore the visibility of thecover and to suppress the visibility of the pump housing.

11. Define a geometry-based surface named CoverTop on the region of the coverwhere it contacts the gasket.

12. Define a surface named CoverInside that defines the region where thepressure load will be applied as shown in Figure W13–4.

13. Create a surface for each of the four holes in the cover as shown in Figure W13–5. Name these surfaces BoltHole-1 through BoltHole-4.

Note: Keep track of the order in which you define the bolt hole surfaces since later youwill have to create corresponding surfaces on the bolt shanks. You should save your

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current view (ViewSaveUser 1) to make it easier when defining the bolts surfaceslater.

14. Restore the visibility of the gasket and suppress the visibility of the cover usingthe Assembly Display Options dialog box. Define surfaces on the top andbottom regions of the gasket. Name these surfaces GasketTop andGasketBot, respectively.

15. Suppress the visibility of the gasket, and restore the visibility of the bolts. Set theview to the user-defined view (ViewViews Toolbox and click 1 in theViews dialog box).

16. Create a surface for each bolt thread corresponding to each bolt hole surface in thecover, as shown in Figure W13–5. Name the surfaces BoltThread-1 throughBoltThread-4.

17. Create a single surface that includes the regions directly under the heads of all thebolts as shown in Figure W13–5. Name this surface BoltHeads.

18. Save your model database as PumpAssy.cae.

Figure W13–4. Surface CoverInside

Figure W13–5 Bolt-related surfaces

Defining the contact interactionNow that you have defined the surfaces that will be involved in contact, you can definethe contact interactions between the different components. Defining contact interactionsin ABAQUS/CAE involves choosing the surfaces involved contact and defining contactproperties (friction, etc.) for each interaction. Follow the steps given below to define thecontact interactions for this model.

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surface BoltThread

surface BoltHeads

surface BoltHole

surface CoverInside

1. From the main menu bar, select InteractionPropertyCreate to create acontact property named Friction.

19. In the Edit Contact Property dialog box, select MechanicalTangentialBehavior. Choose the Penalty friction formulation, and define a coefficient offriction of 0.2. Click OK to close the dialog box.

20. From the main menu bar, select InteractionCreate to create anABAQUS/Standard surface-to-surface contact interaction in the Initial step.Name the interaction Pump-Bolts.

21. Click Surfaces in the prompt area to select the regions involved in contact usingthe surfaces defined earlier. In the Region Selection dialog box, choose thesurface PumpBolts as the master surface (since it is continuous) and the surfaceBoltHeads as the slave surface (since it has a relatively finer mesh and isdiscontinuous). Use the Small sliding formulation, and accept all other defaultinteraction settings.

22. Create another ABAQUS/Standard surface-to-surface contact interaction in theInitial step between the bottom of the gasket and the top of the cover. Name theinteraction named Cover-Gasket. Choose the surface CoverTop as themaster surface (since the underlying elements are much stiffer) and GasketBotas the slave surface. Use the Small sliding formulation, and accept all otherdefault settings.

Defining tie constraintsTie constraints will be used to tie the gasket to the pump. You will also define tieconstraints to simulate the effect of the bolt threads when fastened to the bolt holes.

1. From the main menu bar, select ConstraintCreate. Name the constraintPumpGasket. Select Tie as the constraint type and click Continue.

23. The list of previously defined surfaces appears in the Region Selection dialogbox; select the surface PumpBot as the master surface and the surfaceGasketTop as the slave surface. Accept all the default settings in the EditConstraint dialog box, and click OK to close the dialog box.

24. In a similar fashion, define tie constraints between each bolt thread and itscorresponding bolt hole. In each case, select the bolt hole to be the master surfaceand the bolt thread to be the slave surface. Name the constraints Tie-1 throughTie-4. In the Edit Constraint dialog box, specify a distance of 0.07 as theposition tolerance and toggle off Adjust slave node initial position for eachconstraint.

Tip: After creating the first tie constraint between the bolt and the bolt holes, copy theconstraint and edit the region selections.

25. Save your model database as PumpAssy.cae.

Defining loads and boundary conditionsYour next task will be to define the loads and boundary conditions that will act on thestructure.

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Applying bolt loadsIn ABAQUS/CAE assembly or bolt loads are applied across user-defined pre-tensionsections in the bolt. When modeling the bolt with solid elements, the pre-tension sectionis defined as a surface in the bolt shank that effectively partitions the bolt into tworegions. Follow the steps given below to create the pre-tension sections in the bolts.

1. Switch to the Load module.26. Using the Assembly Display Options dialog box, suppress the visibility of all

part instances except for the bolts.Define a datum plane for the purpose of partitioning the bolts at the location where thepre-tension sections will be defined.

27. From the main menu bar, select ToolsDatum. In the Create Datum dialogbox, select Plane as the datum type.

28. Select Offset from plane as the method, and click OK. Offset the datum planefrom the bottom of the bolts (select the bottom face of any bolt). Click EnterValue in the prompt area. Flip the arrow indicating the offset direction ifnecessary so that it points from the bottom of the bolt toward the bolt head. Enteran offset distance of 0.5.

29. Partition the bolts using this datum plane. From the main menu bar, selectToolsPartition. Create a Cell partition using the Use datum plane method.Select all the bolts as the cells to be partitioned.

Note: The bolt meshes will have to be regenerated as a result of this partition. Switch tothe mesh module after creating the partition. Respecify the edge seeds for the bolts with 8elements along the edges and recreate the bolt meshes.

30. To view the partitions, switch the rendering style to wireframe .31. To define the direction along which the bolt load will be applied, create a datum

axis along the principal Z-axis. From the main menu bar, select ToolsDatum.Select Axis as the type and Principal axis as the method. Click Z-axis in theprompt area.

32. From the main menu bar, select LoadCreate to define the bolt load. TheCreate Load dialog box appears. Select the step named PreloadBolts as thestep in which the load will be applied. Select Bolt load as the load type as shownin Figure W13–6. Click Continue.

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Figure W13–6. Create Load dialog box

33. In the prompt area, click Geometry as the region type.34. Select the internal surface of any bolt (e.g., as shown in Figure W13–7). This

surface defines the pre-tension section.

Figure W13–7. Bolt internal surface

35. Select the datum axis defined earlier when prompted for a datum axis parallel tothe bolt centerline.

36. Enter a bolt load of 500 lb in the Edit Load dialog box. Accept all other defaultsettings in the dialog box.

37. Repeat steps 8 through 12 for the other bolts.Tip: After creating the first bolt load, copy the load and edit the region selections.

38. Typically, you want to tighten the bolts to a predefined load level (pre-tensioning)and then “freeze” the deformation in the subsequent load steps (pressure loading,heating up, etc). To do this, proceed as follows:A. From the main menu, select LoadManager. The load manager appears in

the viewport.B. Select any bolt load in the Pressure step by clicking Propagated and click

Edit in the dialog box.C. In the Edit Bolt Load dialog box, select Fix at current length as the

method and click OK in the dialog box.D. Repeat steps B and C for the other bolt loads.

Applying the pressure load and boundary conditions1. Display the pump housing using the assembly display options. Switch the render

style to shaded .39. From the main menu bar, select LoadCreate to define a pressure load named

PumpLoad in the step named Pressure. Apply the load to surfacePumpInside. Specify a load magnitude of 1000 psi.

40. Similarly, apply a pressure of 1000 psi to the surface CoverInside. Name theload CoverLoad.

41. Assume that the bottom of the cover is welded against a rigid plate. From themain menu bar, select BCCreate.

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42. In the Create Boundary Condition dialog box, select Symmetry/Antisymmetry/Encastre as the boundary condition type and the Initial step asthe step in which to apply the boundary condition and click Continue.

43. In the prompt area, click Select in Viewport to permit direct selection of theaffected region in the viewport. Select the bottom region of the cover and clickDone in the prompt area. In the Edit Boundary Condition dialog box, selectEncastre as the boundary condition type and click OK to close the dialog box.

44. Apply symmetry boundary conditions to the cover and gasket as follows:A. From the main menu bar, select BCCreate.B. In the Create Boundary Condition dialog box, accept Symmetry/

Antisymmetry/Encastre as the boundary condition type and the Initialstep as the step in which to apply the boundary condition. Click Continue.

C. In the prompt area, select Geometry as the region type. Select the faces onthe symmetry planes of the cover and gasket using [Shift]+Click and clickDone in the prompt area. In the Edit Boundary Condition dialog box,select YSYMM as the boundary condition type and click OK to close thedialog box.

45. Similarly, apply symmetry boundary conditions to the nodes on the symmetryplane of the pump housing. For this part the region type is Mesh.

46. Save your model database.

Creating and submitting a job for analysisNow you are ready to create and submit the model for analysis. You will use the Jobmodule to create an analysis job and submit it for analysis.

1. Switch to the Job module.47. Create a job named PumpAnalysis. Enter any suitable job description. Accept

all other default job settings.48. Submit the job for analysis.

Note: This analysis job takes about 30 minutes to complete on a Windows 2000 machinewith a 1700 MHz processor. Monitor the job for several minutes to ensure it is runningproperly. Then proceed with the next lecture before continuing with this workshop.

Visualizing the analysis results

After the analysis is complete, you will review the results in the Visualization module.For the kind of analysis you just performed, some of the interesting results would be thedistribution of the sealing pressure in the gasket at different stages of operation, thehistory of the bolt force in the bolts, the deformed shape and stresses in the bolts, etc.

1. Switch to the Visualization module, and open the PumpAnalysis outputdatabase file.

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49. Look at the undeformed and deformed plot of the model. You will notice that thepump housing does not undergo large deformations because of its stiffness. Mostof the deformation takes place in the gaskets and the bolts. Use the CreateDisplay Group dialog box to look at the deformed shape in the bolts and gasketsonly.

50. Plot the Mises stresses in the model. Use the Create Display Group dialogbox to plot the Mises stresses in the bolts only.

51. Plot a contour of the sealing pressure in the gasket. First use the Create DisplayGroup dialog box to view the gasket only. The sealing pressure in the gasket isthe S11 component of the stress tensor. From the main menu bar, selectResultField Output. In the Field Output dialog box select the S11component and click OK.

52. Plot an animation of the sealing pressure in the gasket. Specify a minimum valueof 0.0 and a maximum value of 325 in the Limits tabbed page of the ContourOptions dialog box. From the main menu bar, select AnimateTime Historyto animate the results.

53. Create a path plot of the sealing pressure along a critical location in the gasket.Follow the steps given below:A. From the main menu bar, select ToolsPathCreate. The Create Path

dialog box appears in the viewport.F. Click Continue in the dialog box.G. From the pull down menu of the part instances in the Edit Node List Path

dialog box, select GASKET-1. Click Select in the dialog box.H. You will be prompted to select the nodes from the viewport to be inserted into

the path. Pick the nodes as shown in Figure W13–8. Click Done in theprompt area when the selection is complete. Click OK in the Edit Node ListPath dialog box.

Figure W13–8. Node path

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beginning of path

end of path

I. From the main menu bar, select ToolsXY DataCreate. Select Path inthe Create XY data dialog box and click Continue.

J. Browse the settings in the XY Data from Path dialog box. In the Y-valuesframe, click Step/Frame.

K. In the Step/frame dialog box, select the last frame of the step namedPressure. Click OK.

L. Make sure that Field output variable is set to S11 in the XY Data fromPath dialog box. Click Plot in the dialog box to view the path plot.

M. Save the plot as Step-2.N. Similarly, create a path plot of the sealing pressure for the same set of nodes

for the PreloadBolts step. Save the plot as Step-1.O. View the saved plots. From the main menu bar, select ToolsXY

DataManager. In the XY Data Manager dialog box, select both thesaved plots using [Shift] +Click and click Plot in the dialog box. The plotshould look similar to the one shown in Figure W13–9.

Figure W13–9. Sealing pressure along the path

From this figure it is clear the gasket unloads during the pressurization step. In thisexercise, however, nominal bolt and pressure loads were applied. At these load levels, themaximum pressure in the gasket after the bolt load step is approximately 330 psi, whichis less than the largest pressure specified in the loading data (6835.4 psi). Thus, during thepressurization step, the gasket unloads elastically; i.e., along the loading curve.

Optional analysisTo observe the distinct loading and unloading behaviors of the gasket, make thefollowing modifications to the model:

1. Apply a load of 12000 lb to each bolt.

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54. Apply a pressure load of 10000 psi.Rerun the analysis and postprocess the results. You will notice that in this case the peakpressure in the gasket after the bolt load step is approximately 8000 psi, which is greaterthan the largest pressure value specified in the loading data. In the pressurization step,then, the gasket unloads along the unloading curve. To see this, create a pressure vs.closure curve as described below:

1. Using field data, create and save X–Y curves of S11 (i.e., pressure) and E11 (i.e.,closure) versus time at the integration points of the element indicated in Figure W13–10.

Since the element has four integration points, eight curves are created.

Figure W13–10. Element used for pressure-closure curve

55. Create a new curve by combining the pressure and closure curves of a givenintegration point (say integration point 1) into a single curve. The resulting curveis shown in Figure W13–11 and clearly illustrates the distinct loading andunloading behaviors of the gasket. The response predicted by ABAQUS followsthe material data very closely.

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Choose this element

Figure W13–11. Pressure-closure predicted by ABAQUS at higher load levels

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largest pressure specifiedin data