The assessment of the fatigue behaviour of welded structures according to DVS 1608 is based on stress values. Therefore, WeldFEM expects its calculation input to be stress values not stress range values. By setting up multiple load cases in Femap that represent specific load scenarios, WeldFEM is able to calculate the stress ratio afterwards. The stress ratio for the stresses transformed into the weld coordinate system is calculated for every element of concern regarding all load cases chosen by the user. Thus, the influence of the stress ratio is taken into account for every element of concern separately. Since the time path of the loads is not known, WeldFEM calculates the stress ratio for an element with the overall maximum and minimum stress acting in it.


In the following, the calculation of the stress ratio is carried out exemplary for both sides of the CQUAD4-element. The FE-model used for this example has been loaded with service loads according to EN 12663.


Step 1) Gathering All Transformed Stress Components

The transformed stress components of both sides of the CQUAD4-element of concern are summarized in the following table:


Load case

TOP side

BOT side

σ⊥ [MPa]

σ|| [MPa]

τ [MPa]

σ⊥ [MPa]

σ|| [MPa]

τ [MPa]

1

60,07

34,32

8,83

-68,45

-32,90

-9,50

2

-60,91

-33,83

-8,57

71,10

34,90

9,45

3

-3,05

1,04

4,24

-0,56

1,76

5,41

4

2,21

-0,55

-3,98

3,21

0,24

-5,46

5

-1,26

0,74

0,39

3,98

3,00

-0,07

6

0,42

-0,25

-0,13

-1,33

-1,00

0,02


Step 2) Extracting The Maximum And Minimum Stress Values

The maximum and minimum stress values of the element of concern are summarized in the following table:


Extreme values

TOP side

BOT side

σ⊥

σ||

τ

σ⊥

σ||

τ

min

-60,91

-33,83

-8,57

-68,45

-32,90

-9,50

max

60,07

34,32

8,83

71,10

34,90

9,45


Step 3) Calculating The Stress Ratio

The maximum and minimum stress values of the element of concern are summarized in the following table:


Stress Ratio

TOP side

BOT side

σ⊥

σ||

τ

σ⊥

σ||

τ

R

-1,014

-0,986

-0,971

-0,963

-0,943

-1,005