UANISOHYPER_INV and SIGINI working together

Greetings,

I am a new user and I would firstly like to say hello, and thanks in advance, to everyone here! I am working in research in the biomechanics field and I am using a few subroutines in a FEA simulation with ABAQUS. However, I have experienced some problems, which are described below.

I am trying to implement a transverse isotropic hyperelastic material with an UANISOHYPER_INV and I also need to declare initial stresses in my simulation (I do it with a SIGINI subroutine). Although both subroutines work alone, when coupled together, they seem not to work. I experience no stress at all, even during the very first moment of the simulation. Does anyone else experience this issue? May it be a bug?

I created another simulation using a simple hyperelastic law (Neo-Hookean) with a UHYPER, and it works perfectly with the SIGINI.

I am currently writing a UMAT. I have finished the Neo-Hookean version but it is quite unclear to me if it is possible to implement initial stresses in the UMAT, without using the SIGINI. I have tried using UMAT and SIGINI, and again although both work alone, they do not when used together. However, I get the correct stresses in the first moment of the simulation, but then they disappear during the first step although the movement is completely restricted (I think it is because of the stress definition in the UMAT). Does anyone have some information on this?

Thanks for everything,

Levrero.

PS: Happy new year!

I havent used SIGINI much but according to the manual stresses from SIGINI should be available in the UMAT in the stress array at the beginning of the analysis. If you dont propagate this stress yourself in the UMAT it will naturally disappear. Also the manual says it is not possible to define initial stresses for fabric materials, which might explain SIGINI not working with UANISOHYPER_INV.