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 from Veryst Engineering.
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....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 ?
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. It is conceptual i removed some trick to manage zers and such things for reading purpose.
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