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# Fatigue approach for an aluminium structure

## Fatigue approach for an aluminium structure

(OP)
I need to evaluate a structure for fatigue. The cyclic load give is +/- 20% of vertical acceleration load.

My Approach :

I am planning to use equivalent stress range approach as per euro code 9 guideline for aluminium structure.
(Following euro code guideline for fatigue assessment is an requirement)

1. planning to analysis the model in opti struct - Step 1 for +1.2 g and step 2 for +0.8g .
is my approach of applying the loads in two steps right ?

2. I will determine the max.principal and min principal stresses for +1.2 g

3. I will determine the max.principal and min principal stresses for +0.8 g

4. Among the stress values for +1.2g and +0.8g, Consider the Peak max.principal value and peak min. principal stress to calculate the stress range.

5. The calculated stress range range value is applied in the formula.(See attached image)
(The denominator indicating the reference fatigue strength at 2*10^6 cycles for the considered material is available 71 MPa).

6. The calculated stress range is divided by the reference fatigue strengtha nd checked if its < 1. If the value is less than 1th e design is considered safe.

### RE: Fatigue approach for an aluminium structure

I'd say that's a conservative way to calculate stress range … and if it works why not ?

Less conservative would be to calculate stress range on an element by element basis, and determine the maximum stress range.

A simple approach, since I assume you're using linear FEM, is to use the stress from 1g … what's maximum and minimum ?
then stress range is 1.2*max-0.8*min.

### RE: Fatigue approach for an aluminium structure

(OP)
rb1957 Thank you for your inputs.

### RE: Fatigue approach for an aluminium structure

thank you for the LPS

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