Workshop 16

Thermal-Stress Analysis of Intersecting PipesIntroductionThis workshop involves the thermal-stress analysis of a cylindrical pipe intersection. Thepipes are surrounded by a fluid and are filled with another fluid. The interaction betweenthe pipes and the surrounding fluids are modeled using surface convection. The internalpressure due to the inner fluid is modeled with a pressure load. Both the thermal andmechanical responses are sought. The mechanical behavior of the part is expected toinfluence the thermal response weakly. Thus a sequential thermal stress analysis isperformed. The thermal analysis model will be developed first and will form the basis ofthe structural analysis model. The structural analysis consists of two steps and willillustrate the use of the restart capability in ABAQUS. The thermal-stress analysis willalso be performed with ABAQUS/Explicit. Only a quarter of the intersection will bemodeled because of symmetry.

Importing and defeaturing the part1. Start a new session of ABAQUS/CAE from the workshops/heatTransfer

directory.2. From the main menu bar, select FileImportPart.3. In the Import Part dialog box, select the file named pipe_int.sat and click

Continue.4. In the Create Part from ACIS File dialog box, review the basic information

about the geometry and click OK to proceed.5. The geometry of the model is shown in Figure W16–1.

Figure W16–1. Initial part geometry

6. A quarter-symmetry model will be used in the analysis. From the main menu bar,select ShapeCut Extrude to create two extruded cuts. For the first cut,select the face highlighted in Figure W16–2 as the plane for the extruded cut.Select the vertical edge highlighted in the figure as the edge that will appearvertical and to the right of the sketch.

Figure W16–2. First extruded cut7. Sketch the section for the extruded cut as shown in Figure W16–3. In the

viewport, click mouse button 2 to complete the operation and in the prompt areaclick Done to proceed. (Mouse button 2 is the middle mouse button on a 3-buttonmouse; on a 2-button mouse, press both mouse buttons simultaneously.) In theEdit Cut Extrusion dialog box, select the direction of cut as shown in FigureW16–2 and select the end condition Through All.

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Select this face

Direction of cut

Edge that will appear vertical and to the right of the sketch

Figure W16–3. Shape of the first cut8. Repeat steps 6 and 7 for the second cut, as shown in Figures W16–4 and W16–5.

Figure W16–4. Second extruded cut

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Select this face

Direction of cut

Edge that will appear vertical and to the right of the sketch

Figure W16–5. Shape of the second extruded cut

9. From the main menu bar, select ToolsRepair. In the Geometry RepairTools dialog box, select the Remove faces tool to remove the fillets shown inFigure W16–6 (you may use the selection filters to facilitate the face selection).Select the faces highlighted in the figure. Remember to toggle on Local stitch inthe prompt area before completing the operation.

Figure W16–6. Fillets to be removed10. The fillets will be removed and the adjacent faces are automatically extended by

ABAQUS/CAE to fill the gaps as shown in Figure W16–7. Rename the modelthermal, and save the ABAQUS/CAE model database as pipe-intersection.

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Remove these faces

Figure W16–7. Final part geometry

Properties and model assemblyThe units used in this model are SI (kg, m, s, N, °C). The pipes are made of a typicalcommercial purity aluminum alloy. The material is assumed to harden isotropically. Thedependence of the flow stress on the temperature is included.

1. Switch to the Property module. 11. From the main menu bar, select MaterialCreate. Name the material

aluminum, and specify the following thermal properties: · Temperature dependent conductivity:

204 W/m°C at 0°C225 W/m°C at 300°C

· Specific heat = 880 J/kg°C· Inelastic heat fraction = 0.0 (for the ABAQUS/Explicit analysis)· Density = 2700 kg/m3

12. Add the following mechanical properties. (These properties will be used in thesubsequent stress analysis.)· Modulus of elasticity = 6.9E10 Pa· Poisson's ratio = 0.33 · Coefficient of thermal expansion = 8.42E-5· Temperature-dependent plasticity:

Read the data from an ASCII text file. Toggle on Use temperature-dependent data, as shown in Figure W16–8, and right-click in the data fieldindicated in the figure. From the list of available options, select Read fromFile. Read the data from the file plasticProps.inp

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Figure W16–8. Reading plastic material properties from a file13. Create a homogeneous solid section named aluminumSection, and assign it

to the part.14. Switch to the Assembly module, and create an instance of the part pipe-int.

Analysis procedure and outputTo simulate the thermal response of the part, a single heat transfer step will be used.

1. Switch to the Step module. 15. From the main menu bar, select StepCreate. In the Create Step dialog box,

select Heat transfer as the general procedure type and create a transient heattransfer analysis step using the following parameters:· Description: Thermal analysis· Total time period = 200· Maximum number of increments allowed = 100 · Initial increment size = 1 · End the step when the temperature change rate is less than 0.5· Maximum allowable temperature change per increment = 10

16. Accept all default ODB output requests. Specify a restart frequency of 5.

Surface film conditionThe conditions to model the surface convection will now be applied.

1. Switch to the Interaction module. 17. From the main menu bar, select InteractionCreate. In the Create

Interaction dialog box, select Film condition as the interaction type and clickContinue. Specify a film condition for the outer surface of the pipe shown inFigure W16–10. Use a film coefficient of 50 W/m2·s°C and sink temperature of20°C.

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Toggle this on

Right click here

Figure W16–10. Surface for outer film condition18. The fluid temperature on the inner surface is time-dependent. Thus, an amplitude

curve is required to prescribe the temperature history. From the main menu bar,select ToolsAmplitudeCreate. Accept the default Tabular type, and clickContinue. Enter the film sink temperature amplitude data points (0, 20),(10, 400), and (200, 400) in the table. Click OK.

19. Create a film condition for the interior surface of the pipe shown in Figure W16–11. Specify a film coefficient of 1200 W/m2·s·°C. Enter a value of 1°C for the sink temperature, and use amplitude curve Amp-1 created earlier forthe sink amplitude. The magnitude of the sink temperature will be the product ofthe specified value and the amplitude.

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Figure W16–11. Surface for inner film condition

Initial conditionsThe pipe is initially at room temperature (20°C).

1. Switch to the Load module. 20. The pipes are initially at a temperature of 20°C. From the main menu bar, select

FieldCreate. 21. In the Create Field dialog box, set the step to Initial, the category to Other, the

type to Temperature, and click Continue. 22. Select the complete model by dragging the mouse across the viewport with the left

mouse button held down. Click Done. 23. In the Edit Field dialog box, enter a value of 20°C for the initial temperature

Magnitude.

Partitioning and meshing the partYou will now generate the finite element mesh. Rather than creating more complicatedpartitions for the sake of generating an all-hex mesh, create a series of simple partitions tosubdivide the part instance into hex- and tet-meshable regions. ABAQUS/CAE willautomatically impose the necessary tie constraints between regions of the mesh that areincompatible.

1. Switch to the Mesh module. 24. From the main menu bar, select ToolsPartition. In the Create Partition

dialog box, select Cell as the partition type and Extend face as the method.Partition the end region shown in Figure W16–12. Use the face highlighted in thefigure as the face to be extended.

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Figure W16–12. First partition25. Similarly create a partition for the other end, as shown in Figure W16–13.

Figure W16–13. Second partition26. Create another partition near the pipe junction using the Define Cutting plane

technique and the Point & Normal method for specifying the partitioning plane.Select the point highlighted in Figure W16–14 as the point through which theplane will pass and the highlighted edge as the normal direction.

Extend this face

Extend this face

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Figure W16–14. Third partition27. Similarly create another partition using the same technique as shown in

Figure W16–15.

Figure W16–15. Fourth (and final) partition

28. From the main menu bar, select MeshControls and assign the Tet elementshape to the region highlighted in Figure W16–16.

Define cutting plane through this point

Edge normal to the cutting plane

Define cutting plane through this point

Edge normal to the cutting plane

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Figure W16–16. Region assigned tet elements

29. From the main menu bar, select MeshElement Type and then select thewhole model as the region to be assigned an element type. In the Element Typedialog box, choose Standard as the element library and Heat Transfer as theelement family. Accept the default element type (DC3D8 elements for hex-meshable regions and DC3D4 for tet-meshable regions).

30. From the main menu bar, select SeedInstance and assign a global seed sizeof 0.05 to the part instance.

31. From the main menu bar, select MeshInstance to generate the part instancemesh. The message shown in Figure W16–17 appears to indicate that tieconstraints will be automatically generated at the interface between the tet and hexelement regions. Click Continue.

Figure W16–17. Warning message regarding tie constraints

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The meshed part is shown in Figure W16–18.

Figure W16–18. Finite element mesh

Thermal analysis1. Switch to the Job module. 32. From the main menu bar, select JobCreate, create a job named pipe-

thermal, and click Continue. Accept the default job parameters, and click OK.33. The nodal temperatures must be written to the results file for them to be read by

the subsequent stress analysis. Currently there is no direct way of requesting thisoutput using the ABAQUS/CAE menus. The output must be requested using theKeywords Editor. From the main menu bar, select ModelEditKeywordsthermal to open the Keywords Editor. Select the last text blockavailable (before the *End step option), and click Add After. Enter thefollowing two lines in the new text block:

*node filent,

34. Click OK to close the Keywords Editor.35. Save the model database.

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36. Open the Job Manager, and submit the job for analysis.

Postprocessing1. Once the analysis completes, click Results in the Job Manager. 37. Plot the contours of nodal temperature by selecting the variable NT11 from the

Field Output dialog box. The contour plot is shown in Figure W16–19.

Figure W16–19. Temperature distribution in the pipes

Stress analysisThe stress analysis consists of two steps. In the first step, a pressure load is applied. In thesecond step, the thermal load is applied. The complete structural analysis will beperformed using two jobs to illustrate the use of the restart analysis capability.The thermal analysis model and properties will form the basis of the stress analysismodel. From the main menu bar, select ModelCopy Model and copy the modelnamed thermal to a new model named stress. From the Model pull down list,select stress. Make the following changes to this model.

1. Enter the Step module. Delete the Heat Transfer step, and create a Static,General step with a time period of 10 and an initial time increment size of 1.

38. Create a set named n-hist consisting of the two vertices on the outer and innersurface of the pipe as shown in Figure W16–20. Request displacement historyoutput for this set.

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Figure W16–20. Vertices belonging to set n-hist39. Enter the Load module. Apply a pressure with magnitude 3.50E6 Pa to the

internal surfaces of the pipe. 40. In the Initial step, define symmetry boundary conditions to each symmetry plane

and a pinned condition to the top face as shown in Figure W16–21, Figure W16–22, and Figure W16–23.Tip: Set the selection filter type to Face to facilitate the selections.

Figure W16–21. XSYMM faces

Select these two vertices

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Select these faces

Figure W16–22. ZSYMM faces

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Select these faces

Boundary condition on this face represents attachment to a larger structure.

Figure W16–23. PINNED face41. Enter the Mesh module. Change the element type assigned to the part instance

regions to C3D4 and C3D8I by choosing the 3D Stress element family.42. From the main menu bar, select ModelEdit Keywordsstress and click

Discard All Edits to delete the two keyword lines added for the temperature fileoutput in the thermal analysis.

43. Enter the Job module. Create a job named pipe-stress, and run the analysis job. 44. Save the model database.45. Once the job completes, enter the Visualization module and plot the contours of

stress and displacement. The displacement magnitude contour plot is shown inFigure W16–24. In this figure the displacement magnification has been set to1.0.

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Select this face

Figure W16–24. Contour of displacement magnitude

Restart analysisThe remainder of the stress analysis will now be performed using the restart analysiscapability. Copy the model stress to stress-restart. For the stress-restart, do thefollowing:

1. Enter the Step module. Create an additional static step for the restart analysis. Setthe step time period to 200 and the initial time increment to 0.2.

46. Modify the history output request for the node set n-hist. In the second analysisstep, add an output request for nodal temperatures.

47. Enter the Load module. Open the Field Manager. For the second analysis step,edit Field-1 (the initial temperature) so that its status is set to Reset to initial.This effectively deactivates the field in this step.

48. Create a new field to apply the temperatures obtained in the thermal analysis. Inthe Edit Field dialog box, specify the values shown in Figure W16–25.

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Figure W16–25. Reading temperatures from the .fil file49. From the main menu bar, select ModelEdit Attributesstress-restart to

edit the model attributes for the restart analysis model. Use the parameters shownin Figure W16–26. Please note that text input to ABAQUS/CAE is case sensitive.

Figure W16–26. Restart analysis model attributes50. Enter the Job module. Create a job named pipe-stress-restart. Set the

job type to Restart in the Edit Job dialog box.51. Save the model database, and submit the analysis job.

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52. When the job completes, plot the Mises stress contours. The plot is shown inFigure W16–27.

Figure W16–27. Mises stress distribution at the end of the analysis

Coupled thermal-stress analysis with ABAQUS/ExplicitYou will now perform the full thermal stress analysis using the explicit dynamics solver.Even though a fully coupled procedure is used, the thermal response has been uncoupledfrom the mechanical response since the inelastic heat fraction has been set to zero. Thus,in effect a sequential analysis is performed. The steps required to complete this analysisare described next.

1. Copy the model named stress to a new model named stress-explicit.53. Enter the Step module. Delete the Static, General step, and create two

Dynamic, Temp-disp, Explicit steps. For the first step use a time period of10 seconds, while for the second step use a time period of 200 seconds.

54. For each step, apply Semi-automatic mass scaling using a scale factor of1.0e8.

55. Enter the Interaction module. Create the following surface film conditions inStep-2: · For the outer surfaces of the pipes, use a film coefficient of

50 W/m2·s°C and sink temperature of 20°C.· For the inner surfaces of the pipes, use a film coefficient of 1200

W/m2·s·°C. Enter a value of 1 °C for the sink temperature and useamplitude curve Amp-1 for the sink amplitude.

56. Enter the Load module. Recall that in dynamic analysis procedures, loads areapplied instantaneously. However, in this problem, a quasi-static response is

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sought. In order to promote a quasi-static response, loads must be appliedgradually. For this purpose create a smooth-step amplitude curve. Name the curveAmp-2; use the points (0,0), (10,1) to define the curve.

57. Apply a pressure load of 3.50E6 Pa in Step-1.Use the amplitude Amp-2 forthe load application.

58. Enter the Mesh module. Change the element library to Explicit, the elementfamily to Thermally Coupled, and the element type to C3D8RT and C3D4T.

59. Enter the Job module. Create a job named pipe-stress-explicit and submit itfor analysis.

Postprocessing (continued)1. Plot the contours for Mises stress and PEEQ on the deformed shape for both the

implicit and explicit analyses. The PEEQ contours for both analyses are shown inFigure W16–28. The results predicted by ABAQUS/Standard andABAQUS/Explicit are nearly identical.

Figure W16–28. PEEQ contours at the end of the analysis

(explicit, left; implicit, right)

60. From the main menu bar, select ToolsPathCreate. Choose the Node listtype, and Continue. Click Select, and select the nodes along the edge shown inFigure W16–29.

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Figure W16–29. Node path

61. From the main menu bar, select ToolXY DataCreate. Select Path from theCreate XY Data dialog box, and click Continue. Examine the various optionsin the XY Data from Path dialog box. Click Plot to display the variation ofPEEQ along the path as shown in Figure W16–30.

Figure W16–30. Path plot of PEEQ (explicit analysis)

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The path starts here

Select the nodes on this edge to define the path