Workshop 1
FEA Fatigue of a Notched Steel Tube in Bending
Introduction
In this workshop, you will configure a simple fatigue analysis using the default loading
and an example material.
Goals
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Review the FEA model and results
Start the fe-safe Graphical User Interface (GUI)
o Select a working directory (project directory)
o Reset the default settings
Open the FEA solution in fe-safe
o Remove all groups from the list to analyze
Confirm the default loading
Assign the material
Request output
Run fe-safe
Review results
o Plot the signed von Mises stress history
o View the Log(Life) contour
Required Files
This workshop is based on the Abaqus ODB format; additional FEA formats for this
workshop are not available at this time.
Find the required files in the .\TUBE-BENDING\Abaqus directory of the provided
workshop files.
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Review the FEA Model and Results (optional)
The model for this example is a tube with a hole, as shown below.
Use the FEA pre/postprocessor to review the input file and FEA solution, for instance:
tube-BENDING-*.inp and tube-BENDING-*.odb.
1. Inspect the model and results:
a. What is the Young’s modus of the material STEEL in the input file?
_____________________________________
b. What is the thickness of the undeformed tube? _________________________
c. What is the order of magnitude of the stress solution? ___________________
2. Review the field output. At what position in each element is the S variable written?
_____________________________________________________________________
3. What other field output was requested? ____________________________________
4. For which step/increment were the field output variables written?
____________________________________________________________
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Start the fe-safe GUI
1. Start fe-safe using the start menu or desktop shortcut:
2. When prompted, select an appropriate project directory such as
./TUBE-BENDING/Abaqus.
3. Click Create.
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4. From the menu bar, select Tools  Clear Data and Settings.
5. Toggle on Select all and click OK.
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Open the sample FE model
1. From the menu bar, select File  FEA Solutions  Open Finite Element
Model…
a. This will display the file selection dialog box.
b. Select tube-BENDING.odb and click Open.
2. In the Pre-Scan File dialog box, click Yes.
3. In the Quick Select area of the dialog box, select Stresses and Last increment
only.
4. Click Apply to Dataset List to apply the selection.
5. Click OK to load the selected stress solution.
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As fe-safe loads the model, information about the file and the data it contains is appended
to the file ./model/reader.log in the project directory. This information is also
displayed in the Message Log window:
Note: if an element is reported as not supported, this is related to the mesh and surface
detection functions of the FEA solution import, not the actual FEA results. In the case of
DCOUP3D, the element is not a structural element and solutions for that element will not
be analyzed for fatigue.
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When the model has finished loading, the Loaded FEA Models Properties dialog box
appears as shown below.
6. Set the Stress Units to kPa as shown above and then click OK.
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A dialog box will appear prompting: Do you want to edit the group list?:
7. Click Yes.
8. Click All in the Manage Groups dialog box to move all of the groups out of the
list of Analysis Groups as shown above.
9. Click OK.
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A summary of the loaded model appears in the Current FE Models window, showing the
loaded datasets and element group information:
Note: If the window does not appear as shown above, then expand the tree view to show
more details.
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Confirm the Default Loading
1. Select the Loading Settings tab from the Fatigue from FEA window to access
the fatigue loading.
2. Confirm the default loading appears as shown below:
Note: If the fatigue loading is not displayed as above, right-click in Loading Settings
and select Expand All:
If the loading is still not shown, select File  Loading  Open FEA Loading File and
browse to the loading definition provided: Default-Loading.ldf. Click Open.
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Assign the Material
Group Parameters will be used to complete the following step. Note the column headers
(e.g. Subgroup, Surface Finish, Material, etc.). The material will be applied to the
default group; in this case, i.e., the entire FEA mesh.
Defining the material
1. Use the Analysis Settings tab to close Loading Settings.
2. In the Material Databases window, expand the local database.
3. From the local database, select the material SAE_950C-Manten.
4. In the Group Parameters area of Analysis Settings, double-click the Material
column header.
The Change material dialog box will appear:
5. Click Yes.
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Request Output
1. In the Fatigue from FEA window, click Exports….
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2. The Exports and Outputs dialog will be displayed:
Note: The Contours tab has several variable types selected by default. Only one of these
applies to the default algorithm, so only finite life in the format of LOG10(Life) will be
exported.
3. Click the Worst-Item Histories tab:
4. Select Von Mises Stress.
5. Click OK.
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Running the analysis
Once the Fatigue from FEA window has been configured as shown, the FEA Fatigue
analysis is ready to run.
1. Click Analyse.
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2. A summary of analysis parameters is displayed:
3. Check that the analysis is configured as shown above, and then click Continue.
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As the FEA Fatigue Analysis is run, information about the job and the results are written
to the file ./jobs/Job_01/fe-results/tube*Results.log in the project
directory. This information is also displayed in the Message Log window:
When complete, the Analysis completed dialog box will appear:
Note: Results from other FEA solvers may vary slightly due to mesh and element
formulations.
4. Make a note of the worst life element.node combination listed for later use:
________________________________________________________
5. In the Analysis completed dialog box (above), click Open results folder.
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The output file and histories file will be shown; for example:
Review the results
Review the requested history outputs in fe-safe.
1. Use the open results folder to drag-and-drop the generated history
*histories.txt in the Loaded Data Files window:
2. Right-click in the Loaded Data Files window to select Expand All.
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3. Right-click on the signal von Mises stress for Element and select Plot:
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4. In the resulting plot, click the icon to Show min/max values:
The max stress is 326.5 MPa and the min stress is 326.5. This is only possible because
of the signed von Mises stress: in this case the sign comes from hydrostatic stress. A
single solution of 326.5 MPa in tension on one side of the symmetric tube was used to
create the fatigue loading. The default fatigue loading is a fully reversed loading, using a
user defined signal of 1 and then −1 to reverse the solution. This is only possible because
the solution is linear and elastic. Loading will be discussed more later in this course.
Note: the order of Min/Max output to the signal may differ in some cases, but that will
not change the fatigue life calculation.
5. Close the plot window.
6. From the main menu bar, select File Exit to exit fe-safe.
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View the LOGLife contour:
Use the appropriate FEA postprocessor to view the output file. Plot the contour of
LOGLife in reversed rainbow and turn off any averaging of the results. Query the
element of lowest life to determine the node on the element with the lowest LOGLifeRepeats exported. For instance, these steps can be accomplished using Abaqus/Viewer as
discussed in the next section.
Postprocessing in Abaqus
The following are some recommended practices for postprocessing in Abaqus/Viewer.
1. Enter the fe-results directory for this project:
./TUBE-BENDING/Abaqus/jobs/job_01/fe-results
2. Start a new session of Abaqus/Viewer using the following command:
abaqus viewer
where abaqus is the command used to run Abaqus.
3. In the Start Session dialog box, click Open Database.
4. In the Open Database dialog box, browse to the fe-safe results file, for instance:
tube*Results.odb and click OK.
5. You may be prompted to upgrade the ODB file:
If this is the case, click Yes to convert the database to the format required by the
Abaqus/Viewer release being used.
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6. In the toolbox, click
(or select Plot→Contours→On Deformed Shape from
the main menu bar) to view a contour plot of the LOGLife-Repeats, as shown
below.
Note: There is no displacement variable written to the fe-safe results frame so the model
appears undeformed. This is expected in fatigue results, which are by definition not due
to any one frame in a solution but instead from a history of cycles including at least two
samples in a time history.
7. In the toolbox, click
(or select Options→Contour from the main menu bar)
to change the Contour Plot Options.
8. In the Contour Plot Options dialog box, click on the Color & Style tab.
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9. Set the Spectrum to Reversed rainbow.
10. Click Apply and the contour plot will update.
11. In the Contour Plot Options dialog box, click on the Limits tab.
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12. Ensure that Auto-compute is selected for both the max and min limits.
13. If necessary, toggle off Show location for the max limit.
14. Toggle on Show location for the min limit:
15. Click OK to close the dialog box and update the contour plot.
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The Min life is shown in the contour, but the value is still being averaged which is not
suggested for fatigue results.
16. In the toolbox, click
(or select Result → Options from the main menu bar)
to change the Result Options.
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17. In the Result Options dialog box, click on the Computation tab.
18. Toggle off Average element output at nodes.
19. Click OK to close the dialog box and update the contour plot.
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20. Rotate the part so that you can see the Minimum contour location.
Note: For your convenience a script is provided to automate these postprocessing steps.
Find the file in this workshop’s files: ./TUBE-BENDING/ABAQUS.
To apply the postprocessing steps discussed above using a script, select File → Run
Script and browse to the provided script: ./TUBE-BENDING/ABAQUS/postprocessing_LOGLife.py and click OK.
Select the appropriate viewport when prompted. Click OK.
21. Exit Abaqus/Viewer by selecting File→Exit from the main menu bar.
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