1. Workshop A12-2
Fatigue: Strain-Life

2. Goals
Goal:
In this workshop, a fatigue analysis will be performed using the strain-life approach.
A solid bracket, shown on the left, is constrained on one end and loaded on the other end.
A load of 1000 N is applied on one end
For fatigue calculations, 3000 N will be assumed
Fatigue calculations using the strain-life approach is performed on the part.
A design life of 1e5 cycles is considered
August 26, 2005
Inventory #002266
WSA12.2-2

3. Start Page From the launcher, start Simulation
Change “Open” to “Workbench Projects,” and click on [Browse]
Select the Workbench database “strain-based.wbdb” and click on [Open]
August 26, 2005
Inventory #002266
WSA12.2-3

4. … Start Page
The Workbench Project page will appear. Double-click on the Simulation icon (highlighted) to open the existing Simulation database
A stress analysis has already been set-up and solved for the bracket. Only fatigue-specific steps will be covered in this workshop.
August 26, 2005
Inventory #002266
WSA12.2-4

5. Review Static Analysis Results
Review the mesh by selecting the “Mesh” branch
Note that a fine mesh is specified at four corners of the bracket in anticipation of areas of high stress concentration
Inspect the loads and supports by selecting the “Environment” branch
One end is constrained while a force of 1000 N is applied on the other end
View the static analysis results.
For example, select the “Equivalent Stress” branch to view von Mises stress results 1. 2. 3.
August 26, 2005
Inventory #002266
WSA12.2-5

6. Review Fatigue Material Properties
Select “Solid” under the “Geometry” branch In the Details view, click on the tab next to “Material: Structural Steel” and select “Edit Structural Steel…”
The “Engineering Data” module will appear (next) 4.
August 26, 2005
Inventory #002266
WSA12.2-6

7. … Review Fatigue Material Properties
Select the curve icon next to “Strain Life Parameters”
The Strain-Life data will appear, as shown on the right, with the following data:
“Strength Coefficient” is 920 MPa
“Strength Exponent” is –0.106
“Ductility Coefficient” is 0.213
“Ductility Exponent” is –0.47 5.
August 26, 2005
Inventory #002266
WSA12.2-7

8. … Review Fatigue Material Properties
Under the “Display Curve Type” pull-down menu, “Cyclic Stress Strain” can be selected to plot the stress-strain curve, which uses the following data:
“Cyclic Strength Coefficient” is 1000 MPa
“Cyclic Strain Hardening Exponent” is 0.2
Note: In reality, although there are six parameters, only four are independent: n’ = b/c and H’ = s’f/(e’f b/c) However, it is common practice to derive all six constants from test data and only satisfy this constraint approximately
-0.106/-0.47 = ≈ 0.2
920/(0.213^-0.106/-0.47) = 1303 ≈ 1000
August 26, 2005
Inventory #002266
WSA12.2-8

9. Specify Fatigue Options
Select the “Solution” branch and, from the Context toolbar, add “Tools > Fatigue Tool”
In the newly-added “Fatigue Tool” Details View, make the following changes:
Change “Type” to “Zero-Based”
Change “Scale Factor” to “3”
This multiplies all static analysis results by a specified factor. While the initial linear static analysis was carried out with a load of 1000 N, the fatigue calculations will be based on an applied load of 3000 N.
This feature allows users to scale loads without having to re-run the static analysis, which may be more computationally intensive than the fatigue calculations.
Change “Analysis Type” to “Strain Life”
Leave “Mean Stress Theory” to “None”
For the first run, no modification of strain-life based on mean stress will be accounted for.
Change “Stress Component” to “Signed von Mises”
Leave “Infinite Life” to “1e9” 7.
August 26, 2005
Inventory #002266
WSA12.2-9

10. Request Fatigue Contour Results
From the Context Toolbar, add the following fatigue results from “Contour Results”:
Life
Damage
Safety Factor
Biaxiality Indication 8.
August 26, 2005
Inventory #002266
WSA

11. … Request Fatigue Contour Results
Select the “Damage” object and, in the Details view, change “Design Life” to “1e5”
Select the “Safety Factor” object and, in the Details view, change “Design Life” to “1e5”
9. and 10.
August 26, 2005
Inventory #002266
WSA

12. Request Fatigue Graph Results
From the Context Toolbar, add the following fatigue results from “Graph Results”:
Fatigue Sensitivity
Hysteresis
Request Hysteresis three times. In the Object Tree, there should be “Hysteresis,” “Hysteresis 2,” and “Hysteresis 3”
11.
August 26, 2005
Inventory #002266
WSA

13. … Request Fatigue Graph Results
Select “Hysteresis 2” and, in the Details view, change “Geometry” to the fillet shown on the bottom
Also change “Points per Segment” to “100”
12.
August 26, 2005
Inventory #002266
WSA

14. … Request Fatigue Graph Results
Similarly, select “Hysteresis 3” and, in the Details view, change “Geometry” to the fillet shown on the bottom
Also change “Points per Segment” to “100”
13.
August 26, 2005
Inventory #002266
WSA

15. Perform Fatigue Calculations
Click on the “Solve” icon to initiate the fatigue analysis
Since the linear static analysis has already been completed, only the fatigue calculations need to be run
Review fatigue results.
Plots of “Damage” using isolines is shown on the bottom.
Note that the amount of damage present on the top and bottom fillets are close. This is because although the load is Zero-Based, there is no correction made for tensile vs. compressive stresses
Both “Damage” and “Safety Factor” show that the current design life of 1e5 cycles will not be met.
August 26, 2005
Inventory #002266
WSA

16. Review Fatigue Results
“Hysteresis 2” and “Hysteresis 3” show the cyclic stress-strain behavior at the top and bottom fillets, respectively. As is apparent from the curves, the top fillet is in compression while the bottom is in tension. If “Signed Von Mises” were not used, both results would be the same since “Equivalent (von Mises)” is always positive.
Top Fillet
Bottom Fillet
August 26, 2005
Inventory #002266
WSA

17. Rerun Fatigue Calculations
Select the “Fatigue Tool” and change “Mean Stress Theory” to “SWT”
Mean stress correction will be accounted for both tensile and compressive mean stresses
Rerun the fatigue calculations by clicking on the “Solve” icon
16.
August 26, 2005
Inventory #002266
WSA

18. Review New Fatigue Results
Review “Damage”
This example shows the difference of using no mean stress correction with using SWT.
Note that unlike the case with no stress correction, the top and bottom fillets report different amounts of damage. This is because the top is in compression and the bottom is in tension. With the SWT mean stress correction, compressive mean stresses increase life while tensile mean stresses decrease it.
Top Fillet
Bottom Fillet
August 26, 2005
Inventory #002266
WSA

19. … Review New Fatigue Results
Select “Biaxiality Indication”
Select the “Legend” icon on the Context toolbar. The Legend dialog box will appear, as shown on the right.
Change “Max” value to “1” and “Min” value to “-1”
Change the number of middle colors (+/-) to “3”
Click on “OK”. The contour plot will be modified as shown on the next slide.
19.
August 26, 2005
Inventory #002266
WSA

20. … Review New Fatigue Results
Values of “0” correspond to uniaxial stress, “1” indicates biaxial state of stress, and “-1” relates to pure shear state. This helps users to determine what the stress state is in different regions since the fatigue tests are done assuming a particular state of stress. For this example, the critical fillet regions report values near zero (green), so the fatigue assumptions may be valid if the fatigue testing was done on uniaxial specimens.
August 26, 2005
Inventory #002266
WSA