1. Workshop A12
Fatigue

2. Workshop A12 – Goals Goal:
In this workshop our goal is to perform a Fatigue analysis of the connecting rod model (ConRod.x_t) shown here. Specifically, we will analyze two load environments: 1) Constant Amplitude Load of 4500 N, Fully Reversed and 2) Random Load of 4500N.
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3. . . . Workshop A12 - Start Page From the launcher start Simulation.
Choose “Geometry > From File “ and browse to the file “ConRod.x_t”.
When DS starts, close the Template menu by clicking the ‘X’ in the corner of the window.
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4. . . . Workshop A12 – Preprocessing
Change the working unit system to metric (m, kg, Pa …).
“Units > Metric (m, kg, Pa, C, s)”
Verify the material is set to “Structural Steel”.
Highlight the “Part 1” in the geometry branch.
If not, click in the “Material” field and “browse”. 1. 2. 3.
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5. . . . Workshop A12 – Preprocessing
Select the “Structural_Steel” material and then click [Open]. 4.
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6. . . . Workshop A12 – Preprocessing
Apply the following boundary conditions (see next page):
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7. . . . Workshop A12 – Preprocessing
Highlight the Environment branch.
Highlight the connecting rod surface shown…
Insert a force load.
“RMB > Insert > Force”
From the detail window change to “Components” and “Z = N”. 5. 6. 7. 8.
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8. . . . Workshop A12 – Preprocessing
Highlight the Environment branch.
Highlight the connecting rod surfaces shown…
Insert a cylindrical support.
“RMB > Insert > Cylindrical Support”
From the Details of “Cylindrical Support” window:
Set Radial=“Fixed”, Axial=“Free”, Tangential=“Free” 9.
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9. . . . Workshop A12 – Preprocessing
Highlight the Environment branch.
Highlight the connecting rod surface shown…
Insert a fixed support.
“RMB > Insert > Fixed Support”
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10. . . . Workshop A12 – Solution Setup
Add results to Solution:
Highlight the solution branch.
RMB > Insert > Stress > Equivalent (von Mises).
RMB > Insert > Deformation > Total.
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11. . . . Workshop A12 – Solution Setup
Insert fatigue tool:
Highlight the solution branch.
RMB > Insert > Fatigue > Fatigue Tool.
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12. . . . Workshop A12 – Solution Setup
From the Details of “Fatigue Tool” window:
Specify a Fatigue Strength Factor (Kf) of .8 (material data represents a polished specimen and the in-service component is cast).
Specify fully reversed loading to create alternating stress cycles.
Specify a stress-life fatigue analysis (No mean stress theory needs to be specified since no mean stress will exist – fully reversed loading).
Specify that Von Mises stress will be used to compare against fatigue material data.
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13. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool:
Insert “Safety Factor”:
RMB > Insert > Fatigue > Safety Factor.
From the Details of “Safety Factor” window:
Set the Design Life to 1e6 cycles.
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14. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool (cont.):
Insert “Fatigue Sensitivity”:
RMB > Insert > Fatigue > Fatigue Sensitivity
From the Details of “Fatigue Sensitivity” window:
Specify a minimum base load variation of 50% (an alternating stress of 2250N) and a maximum base load variation of 200% (an alternating stress of 9000N).
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15. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool (cont.):
Insert “Biaxiality Indication”:
RMB > Insert > Fatigue > Biaxiality Indication
Solve
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16. . . . Workshop A12 – Results View Results
Highlight and plot the “Total Deformation” result.
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17. . . . Workshop A12 – Results
Highlight and plot the “Equivalent Stress” result.
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18. . . . Workshop A12 – Results
Highlight and plot the “Safety Factor” result for a design life of 1e6 cycles.
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19. . . . Workshop A12 – Results
Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%.
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20. . . . Workshop A12 – Results
Find the sensitivity of available life with respect to loading for a maximum base load variation of 400%. Note, must resolve to obtain the new Fatigue Sensitivity results.
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21. . . . Workshop A12 – Results
Highlight and plot the “Biaxiality Indication” result. Note, the stress state near the critical location is not far from uniaxial (.1~.2), which gives an added measure of confidence since the material properties are uniaxial. Recall, a biaxiality of zero corresponds to uniaxial stress, a value of –1 corresponds to pure shear, and a value of 1 corresponds to a pure biaxial state.
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22. . . . Workshop A12 – Solution Setup
Insert a second fatigue tool to analyze a random load of 4500N. Assume that we have strain gauge results that were collected experimentally from the component and that we know that a strain gauge reading of 200 corresponds to an applied load of 4500N:
Highlight the solution branch.
RMB > Insert > Fatigue > Fatigue Tool.
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23. . . . Workshop A12 – Solution Setup
From the Details of “Fatigue Tool 2” window:
Specify a Fatigue Strength Factor (Kf) of .8 (material data represents a polished specimen and the in-service component is cast).
Specify fatigue loading as coming from a scale history and select scale history file containing strain gauge results over time (browse and open the “SAEBracketHistory.dat” file).
Define the scale factor to be .005 (we must normalize the load history so that the FEM load matches the scale factors in the load history file):
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24. . . . Workshop A12 – Solution Setup
From the Details of “Fatigue Tool” window (cont.):
Specify Goodman theory to account for mean-stress effects.
Specify that a signed Von Mises stress will be used to compare against fatigue material data (use signed since Goodman theory treats negative and positive mean stresses differently).
Specify a bin size of 32 (Rainflow and Damage matrices will be of dimension 32x32).
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25. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool 2:
Insert “Life”:
RMB > Insert > Fatigue > Life
Insert “Safety Factor”:
RMB > Insert > Fatigue > Safety Factor
From the Details of “Safety Factor” window:
Set the Design Life to 1000 cycles.
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26. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool (cont.):
Insert “Fatigue Sensitivity”:
RMB > Insert > Fatigue > Fatigue Sensitivity
From the Details of “Fatigue Sensitivity” window:
Specify a minimum base load variation of 50% (an alternating stress of 2250N) and a maximum base load variation of 200% (an alternating stress of 9000N).
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27. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool (cont.):
Insert “Biaxiality Indication”:
RMB > Insert > Fatigue > Biaxiality Indication
Insert “Rainflow Matrix”:
RMB > Insert > Fatigue > Rainflow Matrix
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28. . . . Workshop A12 – Solution Setup
Add results to the Fatigue Tool (cont.):
Insert “Damage Matrix”:
RMB > Insert > Fatigue > Damage Matrix
From the Details of “Damage Matrix” window:
Set the Design Life to 1000 cycles.
Solve
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29. . . . Workshop A12 – Results View Results
Highlight and plot the “Life” result.
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30. . . . Workshop A12 – Results
Highlight and plot the “Safety Factor” result for a design life of cycles.
If the loading history corresponded to the loading experienced by the part over a months time, the damage and FS will be at a design life of 1000 months. Note that although a life of only 112 loading blocks is calculated, the needed scale factor (since 1000=.64) is only .64 to reach a life of 1000 blocks.
Note, the “scale factor” is the scale factor for the loading to make it meet the life of 1000 months.
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31. . . . Workshop A12 – Results
Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%.
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32. . . . Workshop A12 – Results
Highlight and plot the “Biaxiality Indication” result.
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33. . . . Workshop A12 – Results
Highlight and plot the “Rainflow Matrix” result.
Here, one can see from the rainflow matrix that the majority of the cycle counts are for low mean stress and low stress amplitude (range).
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34. . . . Workshop A12 – Results
Highlight and plot the “Damage Matrix” result.
Although, from the previous slide, one saw that most of the counts were for the low mean and range bins, these do not cause the most damage at the critical location, as shown in this damage matrix.  Instead, the 'medium' stress amplitude cycles cause the most damage at the critical location.
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