In order to perform a fatigue evaluation, the program must know the stresses at different events and loadings for each location, as well as the number of repetitions of each event. You can store stresses for each combination of location, event, and loading, using the following options:

The following command sequences schematically illustrate how to store stresses. In some situations, you might prefer to use LCASE instead of SET.

You can use more than one method of storing stresses in an event. Each of these methods is explained in detail below.

FS,201,1,2,1,-2.0,21.6,15.2,4.5,0.0,0.0
FS,201,1,2,7,450.3

SET,1               ! Define data set for load step 1
FSNODE,123,1,1      ! Stress vector at node 123 assigned to event 1,
                    !  loading 1.
SET,2               ! Define data set for load step 2
FSNODE,123,1,2      ! ...event 1, loading 2
SET,3               ! ...load step 3
FSNODE,123,1,3      ! ...event 1, loading 3

      PATH,Name,2        ! Define the path using the two surface nodes
      PPATH,1,391
      PPATH,2,395
      SET,1
      FSSECT,,1,1        ! Store stresses at nodes 391 and 395

Use the following options to list, plot, or delete stored stresses.

This option assigns the number of occurrences to the event numbers (for all loadings at all locations of the event). It can also be used to apply scale factors to all of the stresses that make up its loadings.

An example of this command usage is given below:

FE,1,-1         ! Erase all parameters and fatigue stresses formerly          
                          !  used for event 1.
FE,2,100,1.2    ! Assign 100 occurrences to events 2, 3 and 4,
*REPEAT,3,1     !  and scale by 1.2.
FE,5,500        ! Assign 500 occurrences to event 5.

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 accurate count of the number of repetitions of all possible stress ranges, in order to obtain a valid fatigue usage factor.

The ANSYS program automatically calculates all possible stress ranges and keeps track of their number of occurrences, 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 decreased accordingly. At least one of the source events will be "used up" at this point; remaining occurrences of stress conditions belonging to that event will subsequently be ignored. This process continues until all ranges and numbers of occurrences have been considered.

Consider the following guidelines when assembling events:

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:

The possible alternating stress intensities are:

Sorting these combinations by decreasing alternating stress intensity gives:

The range counting then proceeds as follows:

Thus, only 1000 repetitions of about 50 ksi range would be counted, instead of the known 1500 cycles. This error results solely from improper assembly of events.

If the loadings had each been described as separate events (such that E1,L1 E1; E1,L2 E2; E2,L1 E3; and E2,L2 E4), then the following range counts would be obtained:

Cumulative fatigue damage in this case would properly be calculated for 1500 repetitions of about 50 ksi range.

Consider a load history made up of these two load cycles:

It is readily apparent that the worst possible combination of these cycles would yield 500 repetitions having alternating stress intensity of about 25 ksi range.

Proceeding as in the above example, grouping these loadings into two events would produce an accurate count of 500 repetitions of about 25 ksi range. Treating each loading as a separate event would yield an over-conservative count of 1000 repetitions of about 25 ksi range.