Fatigue: Strain-Life

1
Fatigue Strain-Life

• Workshop A12-2

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

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

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.

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

6
Review Fatigue Material Properties

• Select Solid under the Geometry branchIn 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.
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.
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 sf/(ef 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

9
Specify Fatigue Options

• Select the Solution branch and, from the
  Context toolbar, add Tools gt 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

10
Request Fatigue Contour Results

• From the Context Toolbar, add the following
  fatigue results from Contour Results
• Life
• Damage
• Safety Factor
• Biaxiality Indication

8.
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.
12
Request Fatigue Graph Results

• From the Context Toolbar, add the following
  fatigue results from Graph Results
• Fatigue Sensitivity
• Hysteresis
• Hysteresis
• Hysteresis
• Request Hysteresis three times. In the Object
  Tree, there should be Hysteresis, Hysteresis
  2, and Hysteresis 3

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

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

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.

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
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.
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
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.

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.