Configure Fatigue Analysis Using the SN Method

Use the SN tool to calculate fatigue based on stress life.

1. Click the SN tool.
   The SN tool should be the default fatigue module selected. If it is not, click the arrow next to the fatigue module icon to display a list of available options.

   
   
   Figure 1.

   The SN dialog opens.
2. Select an axial method.
   Solid models are not compatible with multiaxial analysis. Currently, only shell elements are supported.
3. Select the FE model units.
   This unit is necessary because the SN/EN curve might be defined in a different unit, and FEA stress needs to be converted before looking up the fatigue life for a given stress level on the SN curve.
4. Select a stress combination.
   Note: This option is only available for uniaxial analyses.

   Supported options include:

   • abs. max. principal
   • signed vonmises
   • signed max shear
   • max. principal
   • critical plane
5. Based on the scatter of the SN curve, enter a real value between 0 and 1 for the certainty of survival.
6. Define the tension and shear damage models.
   Note: This option is only available for multiaxial analyses.

   You can choose to evaluate one or both of the damage models. Whichever damage from the two models is worse will be displayed after evaluation.

7. Choose a layer selection.
   Note:

   • If the layer is a shell, fatigue calculation will be done both on top and bottom.
   • If the shell layer has just one layer example – i.e. mid or membrane, etc. – then the reported fatigue results will be the same for any of the layer selection options ( Worst, Top, or Bottom).
   • If the model has only solid elements, you can only perform uniaxial fatigue, and Top or Worst should be the layer selection.
8. Select a loading type.
   • Time Series
   • Transient Response
   • Modal Superposition
   • Random (PSD Stresses) - uniaxial with a von Mises or Abs Max Principal stress combination only
   • Random (Input PSD with FRF) - uniaxial with a von Mises or Abs Max Principal stress combination only
   • Sine Sweep - uniaxial with a von Mises stress combination only

   For Random loading, do the following:

   1. Select a damage model from the drop-down menu.
   2. Define the upper limit of the stress range.
      Stress Range Upper Limit (Calculated): Calculates the upper limit of the stress range. This is calculated as 2*RMS Stress*factor (Default factor = 8). The RMS stress is output from the random response subcase. The stress ranges of interest are limited by the above calculated stress. Any stresses beyond the calculated value are not considered in random fatigue damage calculations. Upper stress range can also be input directly via the User Input option.
   3. Define the width of the stress range.
      Stress Range Width (Calculated): Calculates the width of the stress range for which the probability is calculated. The default is 100 and the first bin starts from 0.0 to the calculated width. The width of the stress range is calculated as the upper limit of the stress range / Stress Range Width (Calculated). The stress range width can also be input directly via the User Input option.
   For Sine Sweep loading, choose the number of frequencies to be considered from the frequency response subcase.
   • User Input - Input a value for the number of frequencies to be considered between the first and last frequency.
   • All Frequencies - Consider all the frequencies from the frequency response subcase.
   • Frequency Increment - Input a value to select frequencies based on an increment.