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[Chareyre <bruno.chareyre@xxxxxxxxxxx>]

You are right on the "geotechnical" sign convention. But don't worry, it 
is consistent with elasticity (you just replace outward normals by 
inward normals everywhere in the theory). I prefer to keep the most 
common convention though, with positive tensile stress/strain. I just 
didn't realize this sign convention could be applied to forces.
It is still really non-intuitive for me, but I'm just one.
At least Janek or Olivier got the same intuition than me apparently...

-- 
NormalShearInteraction::{normalForce,shearForce} has undefined orientation
https://bugs.launchpad.net/bugs/493102
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Status in Yet Another Dynamic Engine: Confirmed

Bug description:
NormalShearInteraction::{normalForce,shearForce} should contain forces on the interaction, which can be used for various purposes by other functions. However, different functors use them with different sign

ef2_Spheres_Elastic_ElasticLaw (a.k.a. ElasticContactLaw):

   applyForceAtContactPoint(-currentContactPhysics->normalForce-shearForce, ... )

Law2_Dem3DofGeom_CpmPhys_Cpm:

   applyForceAtContactPoint(BC->normalForce+BC->shearForce, ...)

This is a convention issue, but must be fixed. Should normalForce and shearForce be oriented as it applies on id1 (first case) or id2 (second case)? 2nd case is nicer WRT applyForceAtContactPoint syntax, but that one is arbitrary as well.