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.