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Local axes assigned on surfaces
The model is created related to a global axis system XYZ that is unique for the entire problem. The main property of this local axes axis system is that the local Z’ axe must have the same direction than the normal of the element. 
The ways for defining local axes systems are:
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- If not, X’ axe will be in the direction of the cross product of V and N (VxN).
- Y’ axe will be the cross product of Z’ axe and X’ axe.
Intuitively, this local axes axis system is calculated so as if element is approximately contained in the plane XY, local X’ axe will point towards global X axe. If not, this X’ axe is obtained as orthogonal to global Z axe and local Z’ axe.
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