Material orientation in VUMAT

[MarcoSteva]

Hi everybody,

I’m facing a problem in the implementation of a VUMAT for an hyperelastic anisotropic material, similar to the one implemented as *ANISOTROPIC HYPERELASTIC, HOLZAPFEL. In particular, I’ve noticed that varying the local material orientations using the *ORIENTATION keyword I’m obtaining incoherent results, and sometimes the error "Bad Material definition in element number 1: zero or negative initial dilatational modulus caused by bad material data".
It looks like that when ABAQUS is running the initial data check phase imposing a set of fictitious strains, these strains are applied in the local coordinate system, while the stretchNew variables passed into the subroutine are in the global coordinate system. So, it could happen to obtain a dilatation of the material (J=det(F)>1) even if the strains are compressive.

I’m working on two input files:
- the first one has a rectangular orientation in which the local x,y,z axes have the same direction of the global ones; this one is working fine.
- the second one has a rectangular orientation in which the local the y-axis and the z-axis have the opposite direction of the global ones; this one is giving the Bad Material definition error.

Is this an issue any of you has ever faced?

Thanks for your help,

Marco

[oati]

Hi Marco,

Did you finally manage to find a solution to your "bad material definition" problem?

I am currently facing the same problem with a crystal plasticity model implemented as VUMAT.
When the material is oriented along the global coordinate system the calculation works fine.
On the other hand, when different material orientations are used for individual elements, Abaqus/Explicit gives the Error: "Bad material definition in element number ...." for some elements.

Thus, I would be very grateful if you could tell me how you solved the problem?

Thank you in advance.

Markus