Hello everyone. Need some help with a UMAT.

[Rohit32]

Hi,
I have been "googling" such forums and came across this one. Seems like it has helped out quite a few people on the subject. Hope y'all can give me some tips and directions too.
My problem is that of a circular cylinder with pipe pressure inside, finite length, both ends fixed. The material is user defined, based on an orthotropic SEDF. And hence the UMAT. The material being modelled is basically the aortic wall, with the goal of studying aneurysms. The material is incompressible, which has been enforced both in the UMAT and through the use of hybrid 3D solid elements (needs to be solid since cylinder is a thick cylinder).
The strain energy function being proposed is in terms of the components of the right cauchy green strain tensor C.
The UMAT gives me all sorts of errors, ranging from system errors 8,6,11 (11 is incorrect memory referencing). If it runs beyond that, it gives too much distortion at integration points and says convergence not predicted and aborts. Any pointers into trouble shooting?
Regards,
Rohit

[Jorgen]

Hello Rohit,

First, what is "SEDF"?
There can be many different reasons for your problems. One direct comment is that you should not make an incompressible UMAT.You need to allow for some compressibility.

- Jorgen

[Rohit32]

Hi. I was waiting on a automated message on my inbox in case there was a reply. Dint find one and possible deleted it on my spam. Sorry for the late reply again.

Well, by sedf, i meant strain energy density function. Yes, Im still having problems with the umat. Infact the thing does not even run for one increment of the very first step and gives me a "signal 6" abnormal termination error. Been breaking my head trying to figure out why. Any pointers?

[Jorgen]

Unfortunately there is no magic trick for debugging a UMAT :(

I would add a bunch of print statements at various strategic positions inside your code and then run Abaqus to see that is what is printed out. That way you should be able to figure out the problem...

- Jorgen

[Rohit32]

Hi Jorgen, thats what I have been doing for a while. There was an error in taking the inverse of the DFGRD1 at the first increment. Am trying to find out if its with the subroutine for inverse or otherwise. There may be other possible errors too. Will keep you guys informed on how it shapes up.
I have another question; if i write a PYTHON script with some parameters defined to be continous, within certain limits; can i use this in sync with the UMAT. In the sense, the parameters have nothing to do with the geometry of the model, but infact are parameters that go into the UMAT. Can this be done without any sever glitches?
Regards,
Rohit

[Jorgen]

I don't quite understand your question. Are you planning to call python from within your UMAT? If so, that should work just fine.

- Jorgen

[doctorant59]

Hello Dr,

I'am trying to write a UMAT for ABAQUS,
In my UMAT i have to call the result of the previous step to use them in the calculation in the current step.
for exemple
for the first step i will fix lamda for the first step
then i will use lamda to calculate the stress
then i will calculate the reel valu of lamda with the stress
for the second step i need to call the reel valu of lamda calculated in the first step.

[Tompa]

Hi
I might have misunderstood your problem, but can't you just store the values you want to use later as a "solution dependent state variable" STATEV?