Workshop 7

Step Definition and Loads: Pipe Creep Model

Defining steps and specifying output requests You will now define the analysis steps. For this simulation you will define two static,general steps. In the first step a pressure load is applied; in the second step a transientanalysis is carried out to determine the creep of the pressurized vessel. In addition, you will specify output requests for your analysis. Moreover, sinceinteractions, loads, and boundary conditions can be step dependent, analysis steps must bedefined before these can be specified.To begin this workshop, start a new session of ABAQUS/CAE from theworkshops/pipeCreep directory. Open the database containing the pipe creepmodel.

To define a step:1. From the Module list located under the toolbar, select Step to enter the Step

module. 2. From the main menu bar, select StepCreate to create an analysis step. In the

Create Step dialog box that appears, name the step Pressure and accept theGeneral procedure type. From the list of available procedure options, acceptStatic, General. Click Continue.

3. In the Edit Step dialog box that appears, enter the following step description:Apply internal pressure. Accept the default settings, and click OK.

4. From the main menu bar, select StepCreate to create another analysis step.Insert the new step after the one created earlier. In the Create Step dialog boxthat appears, name the step Creep and accept the General procedure type. Fromthe list of available procedure options, select Visco. Click Continue.

5. In the Edit Step dialog box that appears, enter the following step description:Transient creep. Set the time period for the step to 4.38E5 hours(approximately 50 years). Use initial and minimum time increments of 1.0 hourand a maximum time increment of 4.38E5 hours. Set the tolerance for themaximum difference in the creep strain increment (CETOL) to 1.0E5 and themaximum number of increments to 1000.

Since you will use ABAQUS/Viewer to postprocess the results, you must specify theoutput data you wish to have written to the output database (.odb) file. Defaulthistory and field output requests are selected automatically by ABAQUS/CAE foreach procedure type. This output is sufficient for the first step (Pressure). For thesecond step (Creep), however, we require only the following output:

· The displacements, stresses, and creep strains (written as field data to the outputdatabase file every 2 increments).

· The displacements for the point shown in Figure W7–1 (written as history data everytwo increments).

Figure W7–1. Region for restricted outputThe history output request requires a set to be defined. Follow the steps outlinedbelow to define a set and request output.

To define a set:1. From the main menu bar, select ToolsSetCreate. In the Create Set dialog

box, name the set Out and click Continue. 6. Select the point indicated in Figure W7–1.7. Click Done in the prompt area when the appropriate region is highlighted in the

viewport.

To specify output requests to the output database file:1. From the main menu bar, select OutputField Output Requests

Manager. In the Field Output Requests Manager, select the cell labeledPropagated in the column labeled Creep. The information at the bottom of thedialog box indicates that preselected default field output requests have been madefor this step.

W7.2

set Out

8. On the right side of the dialog box, click Edit to change the field output requests.In the Edit Field Output Request dialog box that appears: A. Click the arrow next to Stresses to show the list of available stress output.

Accept the default selection of stress components and invariants. B. Click the arrow next to Strains to show the list of available strain output.

Toggle off PE, PEEQ, and PEMAG.C. Toggle off Forces/Reactions and Contact. D. Accept the default Displacement/Velocity/Acceleration output. E. Save the output every 2 increments.F. Click OK.G. Click Dismiss to close the Field Output Requests Manager.

9. Modify the history output by selecting OutputHistory OutputRequestsManager. In the History Output Requests Manager, selectthe cell labeled Created in the column labeled Pressure if it is not alreadyselected. On the right side of the dialog box, click Edit. A. Toggle on Set name as the domain and, from the list of available sets,

choose Out.H. Toggle off Energy in the Output Variables region.I. Select the displacement components (U under Displacement/Velocity/

Acceleration).J. Save the output every 2 increments.K. Click OK.L. Click Dismiss to close the History Output Requests Manager.

Prescribing boundary conditions and applied loadsSymmetry conditions must be applied to the two symmetry planes in the model. Inaddition, a single point must be restrained in the vertical direction to prevent rigid bodymotion.Both the pipe and the pressure vessel are assumed to be operating under an internalpressure of 1.4E7 Pa. In addition, the pipe and pressure vessel are subject to end cap loadconditions. This implies that for any cut through the model, the equivalent load due to thepressure on the cap can be applied as traction loads on the cut section. Hand calculationsprovide that the equivalent traction loads are: 8.281E6 Pa for the pressure vessel and7.682E6 Pa for the pipe. Furthermore, depending on the proximity of the cuts to thecritical stress region, the boundary conditions could include multipoint constraints thatwould require that plane cut sections remain plane. For this exercise we will assume thatthe cuts are made a sufficient distance from the area of interest, and this last requirementwill be ignored.The pipe is at a uniform initial temperature of 540ºC.

To prescribe boundary conditions:1. From the Module list located under the toolbar, select Load to enter the Load

module.

W7.3

10. From the main menu bar, select BCCreate to prescribe boundary conditionson the model. In the Create Boundary Condition dialog box that appears,name the boundary condition X-SYMM and select Initial as the step in which itwill be applied. Accept Mechanical as the category and Symmetry/Antisymmetry/Encastre as the type. Click Continue. You may need to rotate the view to facilitate your selection in the following steps.

11. Select ViewRotate from the main menu bar (or use the tool from thetoolbar), and drag the cursor over the virtual trackball in the viewport. The viewrotates interactively; try dragging the cursor inside and outside the virtualtrackball to see the difference in behavior.

12. Select the regions of the model indicated in Figure W7–2 using [Shift]+Click.Click Done in the prompt area when the appropriate regions are highlighted inthe viewport, and toggle on XSYMM in the Edit Boundary Condition dialogbox that appears. Click OK to apply the boundary condition.

Figure W7–2. XSYMM boundary condition region

Arrows appear on the face indicating the constrained degrees of freedom. TheXSYMM boundary condition constrains the degrees of freedom necessary toimpose symmetry about a plane X = constant; after the part is meshed and the jobis created, this constraint will be applied to all the nodes that occupy the region.

W7.4

XSYMM regions

13. Repeat steps 2 through 4 to apply a ZSYMM boundary condition to the regionshown in Figure W7–3. Name the boundary condition Z-SYMM.

Figure W7–3. ZSYMM boundary condition region

To satisfy the end cap condition on the intersecting pipe, apply a displacement constraintnormal to the entire face of the free end of the pipe. This action will constrain the modelagainst rigid body motion, and the equivalent traction loads will be generated as reactionforces.

14. From the main menu bar, select BCCreate. In the Create BoundaryCondition dialog box that appears, name the boundary condition EndCap, andselect Initial as the step in which it will be applied. Accept Mechanical as thecategory and select Displacement/Rotation as the type. Click Continue.

15. Select the region of the model indicated in Figure W7–4 using the cursor. ClickDone in the prompt area when the appropriate region is highlighted in theviewport, and toggle on U2 in the Edit Boundary Condition dialog box thatappears. Click OK to apply the boundary condition.

W7.5

ZSYMM region

Figure W7–4. U2 boundary condition region

To apply a pressure load: 1. From the main menu bar, select LoadCreate to prescribe the internal pressure

load. In the Create Load dialog box that appears, name the load InternalPressure and select Pressure as the step in which it will be applied. AcceptMechanical as the category, and select Pressure as the type. Click Continue.

16. Select the surfaces associated with the interior of the pipe and pressure vesselusing the cursor; the region is highlighted in Figure W7–5. When the appropriatesurfaces are selected, click Done in the prompt area.

W7.6

Fix U2 at top of pipe

Figure W7–5. Surface to which internal pressure will be applied

17. Specify a uniform pressure of 1.4E7 in the Edit Load dialog box, and click OKto apply the load. Arrows appear on the model faces indicating the applied load.Next, apply a pressure load to impose the end cap condition on the pressurevessel.

18. Repeat steps 1 through 3 above to apply a pressure of 8.281E6 Pa to the regionhighlighted in Figure W7–6. Name the load Vessel End Cap.

W7.7

inner surface

Figure W7–6. Surface to which end cap pressure will be applied

To apply an initial temperature:1. From the main menu bar, select FieldCreate to prescribe the initial

temperature. In the Create Field dialog box that appears, name the fieldInitialTemp and select Initial as the step in which it will be applied. SelectOther as the category and Temperature as the type. Click Continue.

19. Select the entire model as the region to which the field will be applied.20. Click Done in the prompt area when the appropriate region is highlighted in the

viewport.21. Specify a uniform temperature of 540ºC in the Edit Field dialog box, and click

OK to apply the field.22. Save your model database, and exit your ABAQUS/CAE session.

W7.8

Apply end cap pressure to this surface