Learning Forum

[aqibdrmc]

Hi everyone,nI am trying to use bilinear isotropic hardening for PLA in ansys for a model. I want to know the tangent modulus value for PLA that can be used in Ansys. Is there anyone who can help me in this regard? As the stress-strain curve of PLA is downward in plastic region, even if I take a slope value from any point in plastic region, its negative and I can not use negative values for tangent modulus in Ansys. I am really stuck with this problem and can not proceed further. Any kind of advice is welcome. I really need it. I dont know how to get the tangent modulus value for PLA.nThank younKind RegardsnAqibn

[peteroznewman]

Please review this discussion.nPlease review this discussion.nIt includes a video which shows that the negative slope in the test data is due to necking. The Engineering stress-strain curve, which uses the initial cross-sectional area, went negative because the instantaneous cross-sectional area was reduced in the necking region. The True stress-True strain curve might always have remained positive.nThere are material models that can accept a negative slope, but you may not need them. You can simulate necking using bilinear plasticity with a zero tangent modulus and plot force-displacement data that will produce a negative slope even though the stress-strain curve is flat.n

[aqibdrmc]

Hi Peter,nThank you for your reply. I am trying to use the tangent value of PLA for stent simulation. I have tried to use zero tangent modulus but it when the balloon is removed, the stent shows bent deformation only on one side. n

[peteroznewman]

Please show what you mean by bent deformation only on one side.n

[aqibdrmc]

nHi peter,nnthis is the model. I have used friction less connection between balloon's top surface and stent's bottom surface. The stent has friction less support on highlighted 12 faces. I have used displacement (X=1.5,Y=0,Z=0) on the bottom surface of the balloon with cylindrical co-ordinate system. The radius of the stent is 1.5mm and I am trying to expand it by increasing the balloon radius. The model works fine with steel and Magnesium with bilinear isotropic hardening. But it does not work for PLA with tangent modulus of zero. If i expand the radius by 0.5mm, the simulation works for PLA and I get deformation on one side as the picture.nMoreover, if I increase the displacement of the balloon to 1.5, the simulation fails only for PLA. Steel and magnesium does not show this type of deformation.n