Evaluation of Alternating Stresses From Constant Amplitude Fatigue
Events
One Event Defined by a Single Fatigue Load
User-defined
load ratio: Assuming that the user-defined load ratio is R, the
program gets one of the peaks from the reference study (considering specified
scale factor) and calculates the other peak by multiplying the first peak
by R. The program then calculates the stress quantity selected in the
Fatigue
properties dialog box and calculates the alternating stress as |S*(1 -
R)|/2 where S is the extreme value of the stress component in the reference
static study.
Single Event Defined by Multiple Fatigue
Loads
The program calculates the alternating stresses for each node by considering
the combination of peaks from different fatigue loads. The fatigue loads
can refer to one or more static studies.
Suppose that you specified fatigue loads
using studies A, B, and C with scale factors FA, FB,
and FC
to define an event. In order to evaluate the alternating stress and the
associated stress ratio at a node, the program proceeds as follows:
It evaluates SA*FA, SB*FB, and SC*FC,
where SA,
SB, and
SC refer to the stress component values
at the node for studies A, B, and C, respectively. The program calculates
the associated values for the SX, SY, SZ, TXY, TXZ, and TYZ components.
Evaluates the differences in the stress quantity
specified for calculating the alternating stresses for all possible combinations
of stress peaks and determines the combination that produces the largest
stress fluctuation.
Evaluates the alternating stress associated with
the event by dividing the stress fluctuation range by 2.
Calculates the stress ratio based on the calculated
stress extremes (Smin
and Smax).
If multiple S-N curves with different stress ratios
are defined, the program uses linear interpolation to extract the proper
number of cycles for the calculated alternating stress and stress ratio.
If a single S-N curve with zero mean is defined
and a mean stress correction method is selected, the program uses the
corrected alternating stress against the S-N curve.
Multiple Events
Evaluating alternating stresses for multiple events depends on whether
the events occur randomly or independently. This option is available in
the Fatigue
dialog box.
No Interaction between Events
The program evaluates the alternating stress separately as explained
above for the different loading types.
Random Interaction between Events
The program evaluates the maximum alternating
stress by considering all possible combinations of fatigue loads at
each node. The program then uses the Miner’s rule and the ASME Boiler
and Pressure Vessel Code to define a set of modified fatigue events.
This approach predicts a higher damage factor than applying the defined
events sequentially unless the stress peaks from different events are
marginally different. In such cases, it is recommended to run two different
studies; one study with no
interaction between events and the otherstudy with random
interaction. The results can reveal that one option gives higher
damage at some locations while the other option gives higher damage at
other locations.
Example 1
User-defined events
Event 1: 8000
cycles stresses alternating between 900
psi and -400 psi
Event 2: 2000
cycles alternating between 700
psi and -700 psi
Modified events
Event 1: 2000
cycles with alternating stress (900 +
700)/2 = 800 psi
Event 2: 6000
cycles with alternating stress (900 +
400)/2 = 650 psi
Example 2
User-defined events
Event 1: 8000
cycles stresses alternating between 900
psi and -400 psi
Event 2: 2000
cycles with alternating between 700
psi -402 psi
Modified events
Event 1: 2000
cycles 900 psi with alternating stress (900
+ 402)/2 = 651 psi
Event 2: 6000
cycles 900 psi with alternating stress (900
+ 400)/2 = 650 psi
The independent event approach is likely to
be more conservative in this case.
Related Topic
Fatigue
Analysis
Fatigue
Plots