Mechanical APDL Structural Analysis Guide

Chapter 14: Fatigue

Fatigue is the phenomenon in which a repetitively loaded structure fractures at a load level less than
its ultimate static strength. For instance, a steel bar might successfully resist a single static application
of a 300 kN tensile load, but might fail after 1,000,000 repetitions of a 200 kN load.

The primary factors that contribute to fatigue failures include:

• Number of load cycles experienced


• Range of stress experienced in each load cycle


• Mean stress experienced in each load cycle


• Presence of local stress concentrations

A formal fatigue evaluation accounts for each of these factors as it calculates how "used up" a certain
component will become during its anticipated life cycle.

For beam joint fatigue, also refer to the FATJACK solver.

The following fatigue-related topics are available:

14.1. How Fatigue Is Calculated

14.2. Fatigue Terminology

14.3. Evaluating Fatigue

14.1. How Fatigue Is Calculated

The fatigue calculations rely on the ASME Boiler and Pressure Vessel Code, Section III (and Section VIII,
Division 2) for guidelines on range counting, simplified elastic-plastic adaptations, and cumulative fatigue
summation by Miner's rule.

For fatigue evaluations based on criteria other than those of the ASME Code, you can either write your
own macro, or else interface your results with an appropriate third-party program (see the ANSYS
Parametric Design Language Guide for more information on these two features).

The program features the following fatigue-calculation capabilities:

• You can postprocess existing stress results to determine the fatigue usage factors for any solid-element
  or shell-element model. (You can also manually input stresses for fatigue evaluation of line-element
  models.)


• You can store stresses at a preselected number of locations for a preselected number of events and
  loadings within the event.


• You can define stress concentration factors for each location and scale factors for each event.

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