Mechanical APDL Structural Analysis Guide

14.2. Fatigue Terminology

A location is a node in your model for which fatigue stresses are to be stored. You would typically
choose locations that represent points on the structure that would be susceptible to fatigue damage.

An event is a set of stress conditions that occur at different times during a unique stress cycle. See
Guidelines for Obtaining Accurate Usage Factors (p. 430) later in this chapter for more information.

A loading is one of the stress conditions that is part of an event.

The alternating stress intensity is a measure of the difference in stress state between any two loadings.
The program does not adjust the alternating stress intensity for mean-stress effects.

14.3. Evaluating Fatigue

You perform a fatigue evaluation in POST1, the general postprocessor, after you have completed a
stress solution.The procedure normally consists of five general steps:

1. Enter POST1 (/POST1) and resume your database.


2. Establish the size (the number of locations, events, and loadings), define the fatigue material prop-
   erties, identify stress locations, and define stress concentration factors.


3. Store stresses at locations of interest for various events and loadings; assign event repetitions and
   scale factors.


4. Activate the fatigue calculations.


5. Review the results.

The following fatigue-evaluation topics describe each step in detail and provide additional information:

14.3.1. Enter POST1 and Resume Your Database

14.3.2. Establish the Size, Fatigue Material Properties, and Locations

14.3.3. Store Stresses and Assign Event Repetitions and Scale Factors

14.3.4. Activate the Fatigue Calculations

14.3.5. Review the Results

14.3.6. Other Approaches to Range Counting

14.3.7. Sample Input

14.3.1. Enter POST1 and Resume Your Database

In order to perform a fatigue evaluation, you need to follow these steps:

1. Enter POST1.
   Command(s):/POST1
   GUI: Main Menu> General Postproc


2. Read your database file (Jobname.DB) into active memory. (If your intended fatigue evaluation is
   a continuation of an ongoing session, then Jobname.DB will already be in memory.) A results file
   (Jobname.RST) with nodal stress results should also be available; you will read results data into
   memory later.
   Command(s):RESUME
   GUI: Utility Menu> File> Resume from

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Fatigue