Defining Fatigue Loading

You can define the fatigue loading history of a durability analysis case in terms of one or more events. The following types of fatigue event are available:

  • Superposition, which provides summation of the product of loads and signals.
  • Sequence of frames, which generates a loading history based on frames added to the event in a particular order.

Before you begin:
  • You must have the appropriate role to perform a durability analysis.
See Also
About Fatigue Loading
  1. From the Durability section of the action bar, click Fatigue Loading .
  2. Do either of the following to define an event:
    • Click to add a new Sequence of Frames event.
    • Click to add a new Superposition event.
    The dialog box adds the event to the loading history and includes a set of fatigue event attribute options (fields, repeats, and scale) with default values. You can customize these options later in the process.
  3. Add frames to a sequence of frames event by doing the following:
    1. Right-click the Sequence of events row, and select Add frames.
      The Frames dialog box appears, displaying all the steps solved in each structural analysis case.
    2. For each step or load case, specify the increments to be included:

      • All increments: Include every increment in the step.
      • All except first increment: Exclude the first increment for steps where the base state is not part of the fatigue loading.
      • Last increment: Include only the last increment. You might want to use this option when several steps might be added.

      The frame is added to the event with default settings for scale and offset.
    3. Click OK to close the Frames dialog box.
  4. Add a frame to a superposition event by doing the following:
    1. Right-click the Superposition row, and select Add frame.
      The Frames dialog box appears, displaying all the steps solved in each structural analysis case. The desired stress solution is the end state of a static step or load case, so only the last increment is available.
    2. Specify the step for which you want to include the last increment.
    3. Click OK to close the Frames dialog box.
      A Frames row is added to the event.
    4. Specify signal values for the frame:

      • To specify the signal values directly in the fatigue loading definition, right-click the Frames row and select Add signal values. You can specify the signal values in a comma-delimited list in the Signal row that appears.
      • To specify signal values using a signal file in the database, right-click the Frames row and select Attach signal document. The app prompts you to search for a suitable engineering document. When you select the signal document you want to attach, the app plots its data in the Channel dialog box to enable you to review the signal before you attach it. Click OK to attach the signal document.

    The app adds the frame to the event with default settings for scale and offset.
  5. Click OK.
    You can further refine your fatigue loading history using any of the following steps:
  6. Complete the set of events that comprises your fatigue history. You can do any of the following to create additional events and present them in the right order:
    • Click or to append additional sequence or frames or superposition events.
    • Click an event and click or to move it up or down in the fatigue history.
    • Click an event and click to delete it from the fatigue history.

    Tip: You can also duplicate, reorder, and delete events by right-clicking an event and selecting the appropriate option from the list that appears.

  7. Right-click any of the following attributes of an event, and select Edit to adjust the selected behavior (or double-click the attribute):
    OptionDescription
    Fields Type of data used for the fatigue analysis. Sequence of frames events can use stress data or stress and strain data; superposition events can use stress data only.
    Repeats Number of stress cycles or stress-strain cycles in the selected event.
    Scale Multiplier that increases or decreases the stress or strain in the selected event.
  8. Right-click an event and select Time History or Duration to add a time history property or duration to an event.
  9. Right-click an event and select Temperature to add temperature settings to an event, then double-click the Default temperature field to customize the temperature value for this event.
  10. Right-click a sequence of frames event, and select either of the following options to define the temperature for the event:
    • Select Temperature for matching stress/strain frames to specify that, for each fatigue item (such as nodes and element nodes), the maximum temperature over all these frames be used to evaluate the material properties. This method is generally conservative.
    • Select Select frame for temperatures to define the event temperature using an increment from a step in the preceding structural analysis case. This method is useful when the temperatures over the whole model rise monotonically over a sequence of frames, and therefore using the last frame makes sense.
  11. Overlay an event on top of one of the events in your fatigue history.

    For example, you can superimpose an engine RPM pressure load cycle on top of the startup/shut down and main drive mode cycles.

    1. Right-click the event and select either Overlay superposition or Overlay sequence of frames.
      The overlaid event appears as a child of its parent event in the fatigue history.
    2. Add frames to the overlaid event to complete its definition.
  12. Right-click Inter-event transitions to turn the effects of fatigue cycles that occur between fatigue events on or off.
  13. To include residual stresses in the fatigue loading:
    1. Right-click Fatigue Loading and select Residual stress.
    2. From the Frames panel that appears, select a frame.
    3. Click OK.
  14. Click OK.