Hello Everyone,

I am trying to implement Holzapfel-Gasser-Ogden (HGO) model using a UMAT in abaqus. I am able to specify the fiber directions using direction cosines in a global coordinate system. I am using these values in calculating stress in the term diff(W,I4)*FN(tensor product)FN, W is the stored energy function . But I could not understand, how to specify fiber directions in a local coordinate system. This is required if I want to assign fiber orientation in the circumferential and axial directions in an artery.

Abaqus manual says that a local coordinate system is rotated by average rigid body motion( R in F=VR). First I am trying out simple shear in XY plane of a cube. I have done this in the global coordinate system, Next, I specified a local coordinate system using orientation option and gave the direction cosines for reference fiber direction N=(0.707,0.707,0) along the diagonal. Now as the cube is sheared local coordinate system rotates and I am calculating the reference fiber direction in the local coordinate system through N = RN. But when I looked at the stress values in global and local coordinate system they donot seem to be related through stress= R*stress*R.

Have I missed something in the abaqus manual. Is there a way to access fiber directions in UMAT by specifying them in the input file. More generally, how to specify local fiber direction in UMAT and how to rotate them into the local coordinate system.

I appreciate any suggestions