Presentation: Advanced Modeling of UHMWPE

[Jorgen]

http://polymerfem.com/polymer_files/topics/TNmodel.png
I recently gave a presentation titled: "An Advanced Thermomechanical Constitutive Model for UHMWPE" at the Society of Engineering Science (SES) annual technical meeting. In this presentation I discussed a new material model framework for thermoplastic materials that aims to be both accurate and numerically efficient. The presentation can be downloaded from the attached link.

In the presentation I compared the predictions from the Three Network Model to the predictions from the Hybrid Model. Both of these models are commercially available (http://usermaterial.polymerfem.com/) from Veryst Engineering.

- Jorgen

[Ajaj]

Hello Pfem people, I'm very interested in trying this TNM model but i'm struggling to use it, i think my algorithm to compute it is wrong.
So i used this presentation and Dr Jorgen paper from open journal there: http://journals.tdl.org/ijscs/index.php/ijscs/article/view/2350/2027.

-The aim is to go VUMAT so in my framework F the transformation, B and C are the same symetrical tensors (no rotations).
-I'm trying to make it work step by step. So i have a working neo hookean spring (from vumat_nh). And i want to add effects one by one once they're working.
So i'm trying to make work a spring/dashpot branch. The problem i'm having is i'm expecting an evolution of behaviour with strain rate but i'm having a very "on/off behaviour". At all slow speed the material is pure elastoplastic, once i reach tau/S >1 gammapoint is big and the stress doesn't go above S. At one intermediate speed i have a smooth "thermoplastic" look (for example from 1 to .1 s-1) and above i'm purely hyperelastic. I understand it is pretty OK for a real spring dashpot system btit really has no chance to match my thermoplastic data even with a 2 or 3 network s must have something wrong. Here is my prototype code in MATLAB, easier to read than fortran ;). It is conceptual i removed some trick to manage zers and such things for reading purpose.

F=B=C= transformation/cauchy tensors
S Stress in branch.
tauf elastic limit. m plastic power coef.

%Sigma norm (frobenius norm of Stress deviator)
Sd = dev(S);
tau = normfro(Sd);
% GammapointA/ta evaluation no temp or pressure correction
Gpst = (tau^m)/(tauf^m);
%Viscoplastic transformation rate (symlose)
Fvp = Gpst * inv33(Fe) * Sd * F;
%Viscoplastic transformation direct euler integration
FvN = Fv + Fvp*dt;
%Elastic transformation evaluation
end
FeN = F * inv33(FvN) + K*(J-1)*eye(3);

%New stress evaluation
Je = det(FeN);
SN = mu * Je^(-5/3) * dev(FeN*FeN'); % + K*(Je-1)*eye(3);

return
end

The only relaton i add to the paper (so i'm supposing the mistake is here) is that viscoplastic transformation is the integral of viscoplastic speed:
FvN = Fv + Fvp*dt;
What am i doing wrong ?

There is a conceptual thing i'm missing too, why each TNM branch as it's own bulk stress management ? Why it does not take plastic transformation in it too ?