Need help with Principal Stresses Field Output equation

[jczarne]

I want to plot the Gerber Interaction Formula for fatigue life requirements in Abaqus.

If I go to field outputs and create my own field output is there anyway to make an equation using the principle stresses.
The Gerber equation is

alternat stress/ + (mean stress/yield stress)^2 = 1
endurance limit


.5(max prin - min prin) + (0.5(max prin + min prin)^2 = 1
----------------------- -------------------------
endurance stress (yield stress)


what would I use as the values in Abaqus for the max and min principle stress???? How would it be entered into a formula...meaning

the way the stress components are entered.

example
"s1f6_S" for a basic stress component


Thanks much

 

[gurmeet2003]

jczarne,

There is one point that is not clear in your mail. In general stresses are 3D. How do you propose to apply the above forumlas to 3D.

However the terms for the max and min principal are obtained as follows:

s1f6_S.getScalarField(invariant=MAX_PRINCIPAL)
S1f6_s.getScalarField(invariant=MIN_PRINCIPAL).

Gurmeet