What the objective stress rate used in FEAP?

[kangjingtian]

Dear Professors:
I am trying to understand the code for Ogden material in wder3f.f
After carefully reading the code, I found that the objective stress rate used in FEAP differs from that in ABAQUS.

Let's say: I material description, we derived the 4th-order stiffness tensor C_IJKL.
              and push forward in spatial description by c_ijkl=1/J * F_iI*F_jJ *  F_kK*F_lL * C_IJKL.   ABAQUS uses (jaumann rate)/J.

I want to ask:
1. For Ogden material (wder3f.f), does FEAP use the stiffness in the spatial description as the code shown?
2. If I want to use FEAP to calculate hyperelasticity material using UMAT. Which objective stress should I use to code the stiffness matrix?

Thanks for your time

Jingtian

[Prof. S. Govindjee]

FEAP does not use a hypo-elastic formulation.  FEAP's built-in elements/models are all hyper-elastic based.

For writing your own user hyper-elastic models, you should compute the Cauchy stress and c_ijkl = (1/J) F_iA F_jB F_kC F_lD C_ABCD,
where C_ABCD is 4 d^2W/(dC_AB dC_CD).  FEAP will take care of the rest.  You do not need to think about objective-rates etc. to use FEAP's user material options.

[kangjingtian]

Thank you for your response, Prof. S. Govindjee. Have a nice day!
Jingtian

FEAP does not use a hypo-elastic formulation.  FEAP's built-in elements/models are all hyper-elastic based.

For writing your own user hyper-elastic models, you should compute the Cauchy stress and c_ijkl = (1/J) F_iA F_jB F_kC F_lD C_ABCD,
where C_ABCD is 4 d^2W/(dC_AB dC_CD).  FEAP will take care of the rest.  You do not need to think about objective-rates etc. to use FEAP's user material options.