1 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 2

16.0 Release

The Fatigue Module of ANSYS Mechanical

Workshop 2 Fatigue : Strain Life

2 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 2

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

Goals

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1. Launch the Workbench project file: a. From toolbar, click on [Import…]

b. Browse to the workshop_input_files folder in the training material, and select the Workbench database “strain-based.machdat” and click on [Open]

Start Page [1]

a.

b.

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2. Open the Simulation file: • The Workbench Project page will appear. Double-click on the Setup

(highlighted) to open Mechanical • Note : A stress analysis has already been set-up and solved for the bracket. Only fatigue-specific steps

will be covered in this workshop.

Start Page [2]

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Review [1]

3. Review the mesh: • Select the “Mesh” branch • Note : that a fine mesh is specified at four corners of the bracket

in anticipation of areas of high stress concentration

4. Inspect the loads and supports: • Select the “Static Structural” branch • Note : One end is constrained while a force of 1000 N is applied

on the other end

5. View the static analysis results: • Select the “Equivalent Stress” branch to view von

Mises stress results. View the other results as well.

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6. Review the fatigue properties of the material: a. Select “Solid” under the “Geometry” branch

b. In the Details view, click on the tab next to “Material: Structural Steel” and select “Edit Structural Steel…”.The “Engineering Data” module will appear

Review [2]

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c. 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

Review [3]

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Review [4]

d. In Engineering Data, Plot the stress-strain curve, which uses the following data by selecting the “Cyclic Stress Strain” curve:

– “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 = 0.2255 ≈ 0.2

• 920/(0.213^-0.106/-0.47) = 1303 ≈ 1000

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Fatigue Tool

7. Insert Fatigue Tool:

1. Highlight the “Solution” branch, RMB > Insert > Fatigue > Fatigue Tool

8. Specify “Fatigue Tool” Details:

a. Change “Type” to “Zero-Based”

b. 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.

c. Change “Analysis Type” to “Strain Life”

d. Leave “Mean Stress Theory” to “None”

– For the first run, no modification of strain-life based on mean stress will be accounted for.

e. Change “Stress Component” to “Signed von Mises”

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9. Request Fatigue Contour Results: a. Life

b. Damage

– in the Details view, change “Design Life” to “1e5”

c. Safety Factor

– in the Details view, change “Design Life” to “1e5”

d. Biaxiality Indication

Fatigue Results [1]

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Fatigue Results [2]

e. Fatigue Sensitivity

f. Hysteresis (3 times) • Note : Request Hysteresis three times. In the Object Tree, there

should be “Hysteresis,” “Hysteresis 2,” and “Hysteresis 3”

g. Select “Hysteresis 2” and, in the Details view, change “Geometry” to the fillet shown on the bottom. Also change “Points per Segment” to “100”

f.

g.

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h. 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”

Fatigue Results [3]

h.

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10. Click on the “Solve” icon to initiate the fatigue analysis: • Note : Since the linear static analysis has already been completed, only the fatigue calculations need to

be run

11. Review fatigue results: a. Plots of “Damage” using isolines is shown on the bottom.

– Note : 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.

Solve

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b. “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.

Review Fatigue Results

Top Fillet Bottom Fillet

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Rerun Fatigue Calculations

12. Select the “Fatigue Tool” and change “Mean Stress Theory” to “SWT”. • Note : Mean stress correction will be accounted for

both tensile and compressive mean stresses

13. Rerun the fatigue calculations by clicking on the “Solve” icon.

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14. 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.

Review New Fatigue Results [1]

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15. Select “Biaxiality Indication”: a. Click on the Max Value on the contour legend and change it to 1

b. Click on the Min Value on the contour legend and change it to -1

c. Click on one of the legend color bars and use the “+”“-” signs to decrease the number of contour bands to 5

d. The contour band should now resemble the one shown below.

Review New Fatigue Results [2]

d. c. a.

b.

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• 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.

Review New Fatigue Results [3]