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[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

> Bruno, I still have some doubts ;( I will try to explain myself. What 
> I see regarding the shear force is simply the need of a tangent unit 
> vector exactly for the same reason we define a normal one. The 
> direction of the normal force is not dependent upon which id is which 
> because we define the normal vector, but in the shear direction we are 
> not defining any vector of reference... I can see what you mean in the 
> normal direction but not in the shear one. Can you see my point? Am I 
> completely wrong?
We have 3D space, subdivided into a normal direction (1D), and a shear 
plane (2D).
Why do we split like this? Because we have different kn and ks, and more 
generally different behaviours in normal and shear.
We don't need a unit tangent as long as we have isotropic laws for shear 
(same ks in all directions). We can always write fs=ks*us whatever the 
coordinate system, so why not using the global system?
Perhaps try to exhibit a precise mathematical problem, for now I don't 
see any.

Bruno