The FATIGUE module of POST1 combines the effects of stress cycling over many cycles involving all stress components at a point in the structure.
The module 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.
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 summations by Miner's rule.
Fatigue commands referenced in this section are described in Part II: Archived Commands.
The following steps are performed for the fatigue calculations (initiated by the FTCALC command).
Each loading is compared to each other loading to compute a maximum alternating shear stress:
A. First, a vector of stress differences is computed:
(5–1)
where:
{σ}i = stress vector for loading 
i
{σ}j = stress vector for loading 
j
B. Second, a stress intensity (σI (i,j)) is computed based on {σ}i,j, using
C. Then, the interim maximum alternating shear stress is:
(5–2)
D. The maximum alternating shear stress is calculated as:
(5–3)
where Ke is determined by:
Analysis Type Range Ke ELASTIC (based on peak stresses) All 1.0 SIMPLIFIED ELASTIC PLASTIC (based on linearized stress components) σn < 3 Sm 1.0 3 Sm < σn < 3 m Sm 
3 m Sm < σn 
where:
σn = a stress intensity equivalent of 2 
except that it is based on linearized stresses (based
on the output of the FSSECT command), not actual stresses. Sm = design stress-intensity obtained from the Sm versus temperature table. (The table is input using the FP commands inputting Sm1 to Sm10 and T1 to T10). m = first elastic-plastic material parameter (input as M on the FP command) (m >1.0) n = second elastic-plastic material parameter (input as N on the FP command) (0.0 < n < 1.0) There are a total of (L/2) (L-1) loading case combinations, where L is the number of loadings. These loadings are then sorted (the rain flow method), with the highest value of
first.Designate the highest value of
as occurring with loading 
i, event ki together with loading
j, event kj. Let MT be the minimum
number of times that either event ki or event
kj is expected to occur. Compute a usage factor
following Miner's rule as:
(5–4)
where:
fu = usage factor (output as PARTIAL USAGE) MA = number of allowable cycles at this stress amplitude level. Obtained by entering the allowable alternating stress amplitude (Sa) versus cycles (N) table from the Sa axis and reading the allowable number of cycles MA corresponding to 
. (The table is input using the FP commands inputting
S1 to S20 for Sa and N1 to N20 for N). Next, cumulatively add fu to
where 
= output as CUMULATIVE FATIGUE USAGE. Then decrease the number
of possible occurrences of both event ki and event
kj by MT (so that one of them
becomes zero).Repeat step 3, using the next highest value of
until all of the 
values have been exhausted. It may be seen that the number of
times this cycle is performed is equal to the number of events (or less).