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One more question about orientation in a VUMAT

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I am working on a non-linear transversely isotropic compressible hyperelastic model. The aim is to simulate, with C3D8 elements, the behaviour of a wet (i.e. with no matrix) fibre bundle, inside a fabric composite reinforcement. I use the *ORIENTATION keyword to define a coordinate system whose first axis is aligned with the prefered direction in the undeformed configuration.

I have been working on it for a while but I am still not familiar with orientations, as I have heard many opposite ideas about them. So here are the three main questions I would like to answer : when *ORIENTATION is used, and when the material undergoes finite transfomation,

(a) in which coordinate system are the defGradNew and stretchNew tensors coordinates expressed? abq doc says : [I]The constitutive model is defined in a corotational coordinate system in which the basis system rotates with the material[/I], but there exist different ways to rotate with the material when finite deformation appear (green-nagdy, jaumann).

(b) in which coordinate system do we have to define the stressNew components?

(c) If the prefered direction coincides with the [1 0 0] vector in the undeformed configuration, what will be its coordinates in the current configuration ?

Thanks by advance!


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1) defGradNew is standard deformation gradient, stretchNew is the left stretch tensor. They are defined in the standard way.

2) stressNew has to be in a corotational reference frame.

3) If a=[1 0 0] , then it becomes: F a


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