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# Fatigue Doubt

## Fatigue Doubt

(OP)
I plan to run analyze a locomotive body structure for + (or) - 20 % vertical loading and use eurocode for aluminium structures for damage calculation .

I plan to run the model for +1.2g and 0.8 g in abaqus.

To calculate the stress range
I plan consider the max principal stress for 1.2g and max. principal stress for 0.8 g.

Stress range = Max Stress at 1.2g - Max Stress at 0.8g
Is my approach of calculating stress range fine ?

In the same element ID where is checked the max.principal, the compressive stresses (min.principal) is zero or negligible?
Hence i ignored the compressive stresses ? is it fine ?

However if i check min. principal in the first place, the maximum compressive stress is on a different location ? So, should i consider the compressive stresses fro stress range calculation ? will it contribute to the fatigue failure ?

### RE: Fatigue Doubt

You're running linear FEM, yes?

so you don't need to rerun cases for factored loads. You can run a 1g loadase, then factor by 1.2 and 0.8 (post-process).

the max principal stress for 1.2g will be 1.2* the max principal stress from 1g run
the max principal stress for 0.8g will be 0.8* the max principal stress from 1g run … and will be the same element
so the stress range will be 1.2g - 0.8g = 0.4g* the max principal stress from 1g run.
and the stress ratio, R = min/max, will be 0.8/1.2 = 0.6667

I think neglecting the compression component is reasonable, but you do need to check/verify that compressions are low
max principal or von mises are both good stresses for fatigue.

### RE: Fatigue Doubt

(OP)

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