Consider a load history made up of these two load cycles:
- Load Cycle 1: 500 repetitions of Sx = +100.1 to +100.0 ksi
- Load Cycle 2: 1000 repetitions of Sx = +50.1 to +50.0 ksi
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 separat e event would yield
an over-conservative count of 1000 repetitions of about 25 ksi range.
14.3.4. Activate the Fatigue Calculations
Now that you have locations, stresses, events, and material parameters all specified, you can execute
the fatigue calculations at a specified location. The location is specified by either the location number
or the node itself.
Command(s):FTCALC
GUI: Main Menu> General Postproc> Fatigue> Calculate Fatig
14.3.5. Review the Results
Fatigue calculation results are printed in the output. If you have routed your output [/OUTPUT] to a
file (for example Jobname.OUT), then you can review the results by listing that file.
Command(s):*LIST
GUI: Utility Menu> List> Files> Other> Jobname.OUT
If you have input an S-N curve, output is in the form of a table of alternating stress intensities (listed
in decreasing order) with corresponding pairs of event/loadings, as well as cycles used, cycles allowed,
temperature, and partial usage factor. Following that, the cumulative usage factor is shown for that
particular location. This information is repeated for all locations.
As has been just mentioned,FTCALC output shows the contributing pairs of events and loadings for
any given alternating stress-intensity range. This information can help you isolate the transients
(events/loadings) causing the most fatigue damage.
A convenient way to modify your events would be to write all stored fatigue data on Jobname.FATG.
( This option could be executed either before or after FTCALC.) Data are written to Jobname.FATG in
terms of equivalent fatigue module commands. You can modify your events by editing Jobname.FATG;
then use the /INPUT command to reread the modified fatigue commands.
Command(s):FTWRITE
GUI: Main Menu> General Postproc> Fatigue> Write Fatig Data
14.3.6. Other Approaches to Range Counting
Earlier, we discussed the "rain flow" range-counting method. This technique is useful whenever the exact
time-history of various loadings is not known. However, if in your fatigue analysis the time-history is
known, you can avoid the undue conservatism of this procedure simply by running a separate fatigue
analysis [FTCALC] for each sequential event and then adding the usage factors manually.
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