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[Bruno Chareyre <question654408@xxxxxxxxxxxxxxxxxxxxx>]

Question #654408 on Yade changed:
https://answers.launchpad.net/yade/+question/654408

Bruno Chareyre posted a new comment:
>maxFs will be negative when |Fn*phys->tanFrictionAngle| >
|phys->FsMax|. If maxFs is negative, then it will cause a shear failure,
as in line 147[3] maxFs is compared against a vector norm
(scalarShearForce), that is always positive. Should this be allowed to
occur, I mean, a negative maxFs?

If I understand correctly it needs a fix, and the fix is simply to take
maxFs=max(0,maxFs).

>1 - is it the expected behaviour that negative (tensile) normal forces
decrease the shear resistance of the interaction?

Expected by the author of these lines, yes, probably. As Robert suggests it is a conventional Mohr-Coulomb condition. 
More generally, it is just a constitutive assumption, and as such it is not right nor wrong in a general sense.
In PFC the assumption is(*) more to take friction OR adhesion, without summing them at any point. Just another assumption. 

Bruno

(*) or "was", at least, years ago.

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