Error when calculating a hyperelastic model

[MechanicPW]

I still get an error that the system matrix has negative eigenvalues. It doesn't calculate even a single increment.

I've read thet this error message usually means that I should improve the mesh. But I quite don't know what would need an inmprovement in my case.

I use Hex, structured mesh, linear elements. I've read in the documentation that hybrid elements are recommended for hyperelasticity, so I use hybrid elements with reduced integration.

In the step module I've turned the nonlinear geometry on. The minimum increment size is 0.001 (I tried smaller).

Thanks in advance for any help.

 

[vumat721]

First make sure your material model is okay. For a hyperelastic model, you can use the "Evaluate" function in CAE. If that runs, then your material model is probably okay and you can move on to further debugging.

 

[MechanicPW]

Thanks for reply. The problem is solved.

This is my first time with hyperelasticity and a just enetered wron costant values (in the table the unit wasn't mentioned). I figured that the unit is MPa and now it completes the analysis.