Crystal orientation issue

[majidmech]

Hi

I am trying to use Neper for microstructure generation and then use it with Abaqus UMAT crystal plasticity. The problem that I have at the moment is that I don't understand the cell orientations in the .tess file for the Rodrigues case.

To be clear, for the UMAT, I need two vectors in the crystal coordinate system and their equivalent direction vectors in the global coordinate system. How can I use the *ori values in the tess file for the case of Rodrigues to calculate those vectors?

[mkasemer]

@majidmech

The orientations are by default printed as Rodrigues vectors. The orientation output can be printed using various parameterizations, including various flavors of Euler angles and angle-axis parameterizations.

To get what you need, I suggest converting any of the parameterizations printed from Neper to a rotation matrix, and using said matrix to transform two vectors in the sample frame to the crystal frame using standard vector transformation techniques, facilitated through the calculation of the rotation matrix.

[mkasemer]

The above suggestion is, of course, much more direct than what I described.

[majidmech]

Thanks All. Your suggestion helped me a lot. So I can readily use these rotmat components for my crystal coordinates.

[majidmech]

Hi Romain,
I've been working today on this crystal orientation topic and I've tested multiple cases in Neper with only one crystal, to better relate the crystal and sample coordinate systems. Through my tests, I think that maybe what you have told needs to be rephrased as below, correct me if I am wrong:

• The vector of components 11 12 13 corresponds to the sample direction parallel to the crystal X direction.
• The vector of components 21 22 23 corresponds to the sample direction parallel to the crystal Y direction.
• The vector of components 31 32 33 corresponds to the sample direction parallel to the crystal X direction.

Here is the orientations as rotation matrix
*ori
rotmat:active
0.835261851153 0.508799068169 0.208473375371 -0.545422340404 0.718640865283 0.431358061631 0.069657092940 -0.474002969374 0.877763678006

I've drawn vectors (11 12 13), (21 22 23) and (31 32 33) against sample coordinates to better visualize the issue:

[rquey]

Yes, I think you are correct, I phrased it wrong. Anyway, here is what I usually write:

By construction, its rows contain the coordinates of the crystal coordinate system vectors in the reference coordinate system, and similarly, its columns contain the coordinates of the reference coordinate system vectors in the crystal coordinate system.

[majidmech]

Thanks for your kind replies. I can go on then.