How to perform vibration fatigue analysis in Inventor Nastran

Autodesk Support

Oct 8, 2023

Products and versions covered

Issue:

Is there a tutorial for performing a vibration fatigue analysis?

Solution:

The attached document "In-CAD Dynamics Training.zip" goes through an example of performing a modal analysis, frequency response, and vibration fatigue. The file contains the following:

  • "Autodesk Nastran In-CAD Dynamics Training.pdf" - general steps to perform the analyses and view the results
  • "Muffler&Brackets.iam", "Muffler Mount.ipt", "Muffler.ipt" - Inventor assembly and part files used in the example
Note: "In-CAD" is the former name of Inventor Nastran.

Vibration Fatigue is a combination of a random response analysis and a fatigue analysis. (The random response analysis is a combination of a modal analysis and a frequency response analysis. Thus, a vibration fatigue analysis performs four analyses in one.)

The setup of the Vibration Fatigue includes all input to define those four types of analyses. The basic steps are as follows:

  1. Edit the Analysis and set the type to Vibration Fatigue (from the ribbon or the model tree).
  2. When entering the material properties, click "Fatigue" and enter the S-N data.
  3. Enter any desired Modal Setup input (from the model tree) for the random response portion of the analysis.
  4. Enter the Fatigue Setup (from the model tree):
    1. Set the "Approach" to "Stress-Life".
    2. Enter the "Event Duration" as the total time that the model experiences the random vibration. For example, if the model is attached to a shaker table for 20 hours, the Event Duration would be (20 hours) *(60 minutes/hour) *(60 seconds/minutes) =72000 seconds.
    3. The other input is set as desired.
  5. Enter the Damping input (from the model tree) for the random response portion of the analysis.
  6. Apply a load (from the ribbon or the model tree) for the random response portion of the analysis. (The load is usually an "Enforced Motion > Acceleration" applied to the model through a rigid body connector. The magnitude of the acceleration is a unit gravity, such as 386.4 inch/second^2.)
  7. Apply the constraints (from the ribbon or the model tree).
  8. Enter the Dynamics Setup (from the model tree).
    1. Enter the PSD table to define the random vibration load.
    2. Enter the frequencies to vibrate the model for the frequency response portion of the analysis. (The goal is to produce a smooth "sine sweep" result without missing or clipping any peaks or valleys in the graph.)
  9. Run the analysis.
  10. View the stress, displacement, and other results for the desired subcases. The frequency response analysis results are in the "STEP=n" subcases. The random response analysis results are in the PDS=n, RMS OUTPUT, and NPX OUTPUT subcases.
  11. View the "Fatigue > Damage" or "Fatigue > Life" result for either the RMS OUTPUT or NPX OUTPUT subcase. The fatigue results are the same for both subcases, so only one subcase needs to be viewed. (The fatigue results are not shown for the other subcases.)

Products:

Inventor Nastran;

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