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VUMAT error when the element local coordinate system is varied

VUMAT error when the element local coordinate system is varied

VUMAT error when the element local coordinate system is varied

(OP)
Dear Friends,

I have been working with ABAQUS VUMAT for quite some time now. Recently I came across a very strange issue that I am hoping that someone could shed some light on.
I have a very simple isotropic compressible Neo-Hookean Hyperelastic VUMAT (taken from a textbook) which I tested on a single cubic element. This element was simply stretched in one direction.
To better understand about ABAQUS corotational coordinate system and how the stretch and deformation gradient tensors are stored, I varied the orientation of the element's initial local coordinate system and performed identical loading cases.
For local coordinate systems that were identical or slightly varying from the global coordinate system, the jobs could be solved and the predicted stress values were identical to the analytical solution (AWESOME!).
However, I found that for some other initial local coordinate systems, ABAQUS exited with error because of the failed VUMAT data check. In particular, this is the reoccurring error:
***ERROR: Bad Material definition in element number 1 instance CUBE-1: zero or
negative initial dilatational modulus caused by bad material data.
Please check your material input and any initial conditions if
necessary.

I have no idea what is going on. I only found a very few posts in internet forums who had exactly the same problem (see the links below) and they all remain unsolved:
https://abaqus.yahoogroups.narkive.com/poLEOXLU/ma...
https://polymerfem.com/forum/finite-element-modeli...

If anyone ever experienced a similar issue and could give me some advice, it would be really appreciated. Thank you very much in advance.

Regards,

Willsen

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