Drucker Stability Check using Abaqus

[cthen]

Dear Dr. Bergstrom & all,

I am working with abaqus using the ogden material model and am determining material constants using an own routine. I would like to implement the Drucker stability check into this routine but the output I get for the stability limits does not correspond to what CAE puts out. For uniaxial loading e.g., I tried to check the stiffnes matrix given in the theory manual Abaq. Version 6.5., Sec. 4.6.2, Ogden form, for positive definitness by varrying lamda_1 in steps of delta_lamda1=0.01 in certain limits and checking if the equations employing D11, D22, D12 and D21 are satisfied. For varification purposes, I did assume an imaginary parameter set (mu_1, alpha_1) and assuming N=1. When checking this parameter set in CAE for Drucker stability, I get different stability ranges for nominal strain then given in my output. Can you tell me what I am doing wrong? Do you possibly have any examplarily routine (c, c++, or any other language) that does the stability check which I possibly could use for orientation? Do you have any suggestions on how to approach the stability check?
Thank you so very much for your help in advance!!! I would be very thankful for any advice regarding my problem!

Kind Regards,
Christophe Then

[Jorgen]

Hello Christophe,

Your approach sounds fine to me. How different is your predicted stability range compared to what CAE gives?

I don't have any code that I can give away :(
but I use a similar technique where I check for the response in a few different loading modes over wide range of applied strains. I am not sure it is necessary to match what Abaqus specifies as long as you are satisfied in your own evaluation.

-Jorgen

[cthen]

Dear Jorgen,

thank you so much for your reply! Although it's just a few lines of code, I am pretty sure that the mistake is somewhere on my side and I will try and spend some more time on finding where the deviation comes from. I would go ahead and post it in case I ever find out.
A great forum by the way with lots of helpful hints and thoughts!

Thank you again and all the best!
Kind regards from Germany,
Christophe