Fatigue Module Updates

For users of the Fatigue Module, COMSOL Multiphysics® version 6.0 brings a new feature to assess fatigue under random vibrations and additional S-N curve types. Read more about these additions below.

Fatigue Under Random Vibrations

In random loading, loads are defined using a statistical representation in the form of a power spectral density (PSD). This is a useful modeling approach in situations where the loads acting on a structure can no longer be expressed deterministically. The computed stress response PSD makes it possible to extract meaningful quantities for fatigue evaluation. The new Random Vibration feature in the Fatigue interface computes a fatigue usage factor during a specified time period and computes the total fatigue life of a structure when subjected to PSD loading. These quantities are computed using the Palmgren–Miner damage summation rule, and you can use different cycle counting models and different fatigue evaluation criteria. Depending on the characteristics of the underlying PSD loading, you can choose from two cycle counting models: the Dirlik model, for broad band loading, or the Bendat model, for narrow band loading.

To compute fatigue damage, the cycle counting is complemented with a model for fatigue evaluation:

  • Basquin's model
  • An approximate (parameterized) S-N curve
  • An S-N curve, specified as an interpolation function, for example

In the Random Vibration feature, you can define any linear stress measure on which to base the fatigue computations, or you can select a built-in multiaxial Equivalent von Mises Stress (EVMS) measure. The Random Vibration feature provides several different results that emanate from the PSD loading. These help you evaluate the structure with regard to the risk for fatigue failure. In the figure below, the number of peaks per unit time and the RMS of stress are shown.

The geometry of a bracket model showing fatigue in the Prism color table. The fatigue life of a bracket geometry.

Additional S-N Curve Types in Cumulative Damage

The Cumulative Damage feature has been extended with additional S-N curve types in this software version. In addition to using an S-N curve for amplitude (alternating) loading (with an optional, analytical mean stress correction), you now have the option of providing the load ratio dependence or mean stress dependence directly. This is useful in situations where you have access to experimental data that encompasses mean stress effects. The available S-N curve types are:

  • S-N curve with R-value dependence
  • S-N curve with mean stress dependence
  • S-N curve for amplitude stress

New Tutorial Model

COMSOL Multiphysics® version 6.0 brings one new tutorial model to the Fatigue Module.