Associating S-N Curves to a Material

To solve a fatigue study, you need to define S-N curves for each material applied to your model. You can enter alternating stress - cycle data to define a new S-N curve, or you can use an existing S-N curve from the material database. S-N curves are only used for fatigue studies.

Some materials in the material library have S-N curves already assigned. These material are identified by an (SN) suffix at the end of their names in the list box. The reference of the SN curves is the Atlas of Fatigue Curves, ASM International.
Materials with stress-strain curves are identified by (SS), and materials with both SN curves and stress-strain curves are identified by (SS,SN).

To assign an S-N curve to a material:

  1. In a Fatigue study, add a fatigue event.
  2. In the Material dialog, on the Fatigue SN Curves tab, in the Source box do the following:
    1. For Interpolate, set the scheme for interpolating alternating stresses against the number of cycles of the S-N curve.
    2. Select Define and select a curve from the list. Curves that are already defined have the stress ratio value attached to their names (for example R = -1). Curves that are not defined are tagged as (Not Defined). Select an undefined curve to populate with data.
      You can define up to 10 S-N curves for different values of stress ratios R to a specific material.
  3. In the Table data box, do the following:
    1. In the Stress ratio (R) box, enter the stress ratio associated with the curve.

      Stress Ratio

      R = Smin / Smax, where Smin is the minimum stress in the stress cycle and Smax is the maximum stress.

      Mean Stress

      Smean= (Smax + Smin) / 2
    2. Set the unit of stress.
    3. Populate the curve data by entering Alternating Stress values versus number of cycles to failure due to fatigue, or click File to import data from the Simulation S-N Curve library.
      To import curve data points stored in a (*.dat) file, click File in the Function Curves dialog.The format of the curve data points File (*.dat) should have two columns: Number of Cycles (first column) and Alternating Stress (second column).

      To add a new row, double-click the last cell under the Points column.

      To delete a row, highlight it and click the Delete key or right-click it and select Delete.

    4. Click View to graph the data or Save to save the curve to a data file (*.dat).
      Preview box shows the curve as it is defined.
    5. Type a source of reference for the SN curve data in Source.
  4. Click Apply.