Mechanical APDL Structural Analysis Guide

14.3.3.4. Guidelines for Obtaining Accurate Usage Factors

Structures are usually subjected to a variety of maximum and minimum stresses, which occur in unknown
(or even random) order. Therefore, you must take care to achieve an accurat e count of the number of
repetitions of all possible stress ranges, in order to obtain a valid fatigue usage factor.

The program automatically calculates all possible stress ranges and keeps track of their number of oc-
currences, using a technique commonly known as the "rain flow" range-counting method. At a selected
nodal location, a search is made throughout all of the events for the pair of loadings (stress vectors)
that produces the most severe stress-intensity range. The number of repetitions possible for this range
is recorded, and the remaining number of repetitions for the events containing these loadings is de-
creased accordingly. At least one of the source events will be "used up" at this point; remaining occur-
rences of stress conditions belonging to that event will subsequently be ignored. This process continues
until all ranges and numbers of occurrences have been considered.

Caution

It can be surprisingly easy to misuse the range-counting feature of the fatigue module. You must

take pains to assemble events carefully if you want your fatigue evaluation to yield accurate usage

factors.

Consider the following guidelines when assembling events:

• Understand the internal logic of the range-counting algorithm. See POST1 - Fatigue Module in the
  Mechanical APDL Theory Reference for more details on how the program performs range counting.


• Because it can be difficult to predict the exact load step at which a maximum (or minimum) 3-D stress
  state occurs, good practice often requires that you include several clustered loadings in each event,
  in order to successfully capture the extreme stress state. (See Figure 14.2:Three Loadings in One
  Event (p. 428).)


• You will obtain consistently conservative results if you include only one extreme stress condition
  (either a local maximum or a local minimum) in any given event. If you group more than one extreme
  condition in a single event, you will sometimes generat e unconservative results, as illustrat ed by the
  following example:

Consider a load history made up of two slightly different cycles:

Load Cycle 1: 500 repetitions of Sx = +50.0 to -50.1 ksi

Load Cycle 2: 1000 repetitions of Sx = +50.1 to -50.0 ksi

These load cycles will obviously sum to 1500 repetitions having an alternating stress intensity of about
50 ksi. However, carelessly grouping these loadings into only two events will result in an inaccurate
range count. Let's see how this would happen:

Event 1, loading 1: Sx = 50.0 500 repetitions

loading 2: Sx = -50.1

Event 2, loading 1: Sx = 50.1 1000 repetitions

loading 2: Sx = -50.0

The possible alternating stress intensities are:

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Fatigue