Specifying the Fatigue Algorithm for a Material

You can specify a fatigue algorithm to describe how fatigue progresses through one of the materials in your model.

When you select a fatigue algorithm, the dialog box describes its characteristics in the line under the Algorithm list. This line serves as a quick reminder of the behavior characteristics for that algorithm. For example, when you select the Brown-Miller algorithm, the shorthand in the description reads LCF + HCF, Strain Based, and CP. This information indicates that the algorithm models both low cycle fatigue and high cycle fatigue (LCF+HCF), it uses strain data for its calculations, and it is a critical plane method (CP). If CP is missing, the algorithm does not include a critical plane search.

This task shows you how to:

Before you begin: Open the material in the Material Definition: Simulation Domain dialog box.

Define a Finite Life Fatigue Algorithm

  1. From the material options in the left side of the Material Definition: Simulation Domain dialog box, click Fatigue algorithm.
  2. From the Life Regime options, select Finite Life.
  3. From the Algorithm options, select one of the following.
    OptionDescription
    Brown-Miller A critical plane multi-axial fatigue algorithm, using planes perpendicular to the surface, and at 45° to the surface.
    Normal StrainCalculates fatigue life based on the normal strain.
    Cast IronA fatigue algorithm for analysis of cast iron materials.
    Maximum Shear StrainA critical plane multi-axial fatigue algorithm that uses planes parallel to the surface.
    Normal StressCalculates fatigue life based on the normal stress.
    Von MisesThe von Mises life algorithm uses the von Mises stress as the damage parameter.
    Manson McKnight OctahedralA multiaxial fatigue model that allows for a multiaxial stress state and stress-stress effects to be accounted for.
    Uniaxial Strain LifeAn algorithm for analyzing uniaxial stresses that uses the elastic-plastic strain amplitude to calculate the fatigue life.
    Uniaxial Stress LifeAn algorithm for analyzing uniaxial stresses that uses the stress amplitude to calculate the fatigue life.
  4. Specify the Mean stress correction method, if any. The full set of mean stress correction methods for all fatigue algorithms follows; some of these methods might not be available for your selected fatigue algorithm.
    OptionDescription
    GoodmanThe Goodman relation describes the interaction of mean and alternating stresses on the fatigue life.
    GerberThe Gerber relation describes the interaction of mean and alternating stresses on the fatigue life.
    WalkerThe Walker mean stress correction is widely used in crack growth calculations.
    R-RatioUses multiple S-N curves dependent on stress ratio (or R-ratio)
    Morrow Does this.
    Smith Watson TopperThe Smith-Watson-Topper (SWT) mean stress correction can show the relationship between strain amplitude and mean stress for any specified endurance.
    Maximum Shear StrainProposes that cracks will initiate on planes that experience the maximum shear strain amplitude.
    Smith-Watson-Topper using Downing-Life curve(For cast iron only.) Enables you to select a user-defined correction model.
    User-definedEnables you to select a user-defined correction model.
    User-Defined using Strain-Life curve(For cast iron only.) Enables you to select a user-defined strain-life curve as a mean stress correction model.

    You can also select None to include no mean stress correction in the fatigue algorithm.

  5. If you selected Smith-Watson-Topper using Downing-Life curve, User-defined, or User-Defined using Strain-Life curve, do the following:
    1. Click in the dialog box.
    2. Enter your search criteria (a curve definition or user-defined correction model) in the top bar.
    3. Select your curve or model from the search results and click OK.
  6. Click OK.

Define an Infinite Life Fatigue Algorithm

  1. From the material options in the left side of the Material Definition: Simulation Domain dialog box, click Fatigue algorithm.
  2. From the Life Regime options, select Infinite Life.
  3. From the Algorithm options, select one of the following.
    OptionDescription
    Brown-Miller Critical plane multi-axial fatigue algorithm, using planes perpendicular to the surface and at 45° to the surface.
    Normal StrainAlgorithm that calculates fatigue life based on the normal strain.
    Normal StressAlgorithm that calculates fatigue life based on the normal stress.
    Von MisesAlgorithm that uses the von Mises stress as the damage parameter.
  4. Specify the Fatigue reserve factor envelope, if any. The full set of fatigue reserve factor envelope models for all infinite life fatigue algorithms follows; some of these methods might not be available for your selected fatigue algorithm.
    OptionDescription
    Goodman Describes the interaction of mean and alternating stresses on the fatigue life.
    Gerber Describes the interaction of mean and alternating stresses on the fatigue life.
    R-Ratio Uses multiple S-N curves dependent on stress ratio (or R-ratio)
    User-defined User-defined fatigue reserve factor envelope model.
  5. If you selected User-defined, do the following:
    1. Click in the dialog box.
    2. Enter your search criteria (a user-defined correction model) in the top bar.
    3. Select your model from the search results and click OK.
  6. Click OK.