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Why the VVF of GTN model within Abaqus keeps 0

I calculated an example of unaxial tension using GTN model offered by Abaqus. I followed all the instructions found in CAE User's and Analysis User's manual in Abaqus documentation. No error or warning was issued but the result seemed ridiculous, since VVF, VVFG and VVFN keeped 0 throughout the process while equivalent plastic strain keeped increasing(which showed the material had come into plastic deformation). I don't know what went wrong, looking forward to you reply.

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Emilio Martínez Pañeda's picture

Dear thu_ckx (Why should I use my full name as my user name? - http://imechanica.org/node/460)

Which values have you assigned to the GTN model parameters? Did you use a top-down or local approach to obtain them or just used some random numbers?

Have you tried to reproduce the benchmark problem of the documentation?

Regards,

Emilio Martínez Pañeda 

Thanks for your reminder. I've been working on another tough problem recently which I will post up later. I'm sorry I failed to reply immediatly. The material property was from literature. For elastic property I used isotropic elasticity with E=2.1E11Pa and v=0.29; For plastic I used Johnson-Cook plasticity with A=2.1E9Pa, B=3.78E9Pa, n=0.25, m=1 and melting temp=200000(my model didn't involve thermal effect so for this part I used random data); q1=1.5, q2=1 and q3=2.25; I activated the void nucleation suboption with mean effective plastic strain=0.3, standard deviation=0.1 and volumn fraction=0.04. My model had only one element with element type C3D8. The load was uniform pressure with user amplitude, whose initial value=-4.5E9. And user amplitude was amp=t**(1/6)*sin(2*pi*t). I looked up the example in the Benchmarks. The parameters it uses are nondimensionalized, which makes no difference in my opinion. And I did have run my model with dimensionless parameters just like the Benchmarks example, the void volumn fraction still kept 0. Where is the problem?

   I've solved the problem. It seems the plastic property of the material must be isotopic, you can't use Johnson-Cook plastic instead, though it's isotropic also.

   to "For plastic I used Johnson-Cook plasticity with A=2.1E9Pa, B=3.78E9Pa,
n=0.25, m=1 and melting temp=200000(my model didn't involve thermal
effect so for this part I used random data)"

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