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# How does a Fatigue analysis software use a FEM model in life estimation calculations?

## How does a Fatigue analysis software use a FEM model in life estimation calculations?

(OP)
Hi All

I just need to know how does a fatigue analysis software use the FEM model we load in it to calculate the life, cause i don't find it on manuals

### RE: How does a Fatigue analysis software use a FEM model in life estimation calculations?

When you perform fatigue analysis, static structural simulation is evaluated first to calculate stresses and strains under single load of given value. Then results obtained from it are combined with material models and load cycle characteristic to calculate fatigue life.

### RE: How does a Fatigue analysis software use a FEM model in life estimation calculations?

you input your fatigue loadcases into the FEM, get your fatigue stresses, and "analyze them" (typically miner's rule, cumulative damage). There are specialised codes (ncode) for doing this directly, or you hand calc it.

### RE: How does a Fatigue analysis software use a FEM model in life estimation calculations?

Assumptions: large structure, elastic analysis

1. Determine the no. of load input points or channels.
2. Collect the load histories for these channels.
3. Perform unit load analysis in FEA for each channel.
4. Software then determines simultaneous max and mins
across all channels. This results in a reduced
set of loads but hopefully will contain all significant
events.
5. Using the simul. max mins and the unit loads applies superposition
to determine the x,y & maybe Z elastic stress histories for
each element.
6. determine the critical stress direction (max principal histograms,
2D or 3D ) for each element. Then calculate the normal stress
history in this critical direction for each element.
7. Rainflow cycle count this normal stress for each element.
8. Applying a Neuber type plasticity correction to determine element's
elastic and plastic closed stress-strain hysteresis loops.
9. Do a Palmgren-Miner damage estimate for each element.
10. Select a user determined number of max damaged elements. Number
depends on computer available.
11. Repeat steps 5 to 10 for all the max damaged elements using the
and superimposed.
12. Take life predictions and adjust by a user determined (from
experience) safety factor.

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