Learning Forum

[andrewtck96]

I have been trying to simulate a compression test for large strain of a Polylactic Acid specimen using LS-DYNA. I defined the material as hyperelastic, using the curve fitting for the Mooney-Rivlin 5 parameter model, and including isotropic elasticity data to calculate the young's modulus.

In the set up in Ansys Mechanical, the top and bottom platens are set as structural steel, while the specimen is set as PLA.

However, the simulation does not run. All the elements and nodes are deleted at time = 0s because of negative volume. Based on what I have read in the forum, this is due to the meshing of the model, but I have not been able to tweak the mesh settings such that the simulation can run.

I've included screenshots of the material properties, the mesh and analysis settings, as well as the output of negative volumes that is shown in the solver window.

Can anyone point me in the right direction to set up the simulation in Ansys Mechanical? I would be really grateful.

[Ashish Khemka]

Hi Array,nnNegative Volume error message appears if materials undergo extremely large deformations such as soft foams an element may become so distorted that the volume of the element is calculated as negative. You can stop this error by reducing the time step size by changing the Time Step Safety Factor from default value (0.9) to lesser values (0.5 or 0.1).nnRegards,nAshish Khemkan

[andrewtck96]

Thanks for the advice, Ashish!nI have already tried changing the hourglass control to 4 or 5, based on the information from this LS-DYNA page on negative volumes. The simulation was able to run, although the results were not very accurate.nI will try reducing the time step safety factor and post an update here once I have done so.Regards,nAndrewn

[andrewtck96]

I have succeeded in running the simulation by reducing the time step safety factor to 0.5. The result was also much more realistic that the simulations with Hourglass controls 4 and 5.nnThank you so much !n