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[chiara modenese <c.modenese@xxxxxxxxx>]

Hi Sergei,

I think that the global damping (the one at the contact level) as it is now
implemented in Yade (class ViscoelastiPM) is wrong in the shear direction.

At the moment we do the following (I only refer to the shear direction):

First we rotate Fs_tot(old);
Then:
deltaFelastic=ks*deltaUs;
Fvisc=cs*deltaVrel_n;
Fs_tot(new)=deltaFelastic+Fvisc+Fs_tot(old);

Then we check Mohr-Coulomb on Fs_tot(new);

The wrong thing (I suppose) is that we store Fs_tot including the viscous
component and then we go for the next step. Instead we should only store the
elastic part and then add the viscous part if we pass the Mohr-Coulomb
criterion (Bruno was right in pointing this out). Otherwise the final effect
is that we are not dissipating energy but only changing the amplitude and
the frequency of the oscillation. I did a comparison between the analytical
solution, Yade code and what I coded for the shear direction (I took a
simple example to do that). I attach the comparison.
If you think in the normal direction we do exactly the same. We work out the
normal elastic force as:

Fn_tot_elastic=kn*Un_tot;
Fvisc=cn*deltaVrel_n;
Fn_tot=Fn_tot_elastic-Fvisc; (minus or plus depending on how we work out the
relative velocity)

Next step we get a new Fn_tot_elastic that does not include the hystory of
the viscous force, and then we simply add the incremental current viscous
force.

This is a total formulation but we could use the incremental one also for
the normal part (as in Bruno's notes). So you see that in the normal
direction there is no history of the viscous force. And this is correct, in
fact Un_tot (as well as Us_tot) includes the damping effect since it is a
result of the motion.

I wrote a new class that adjusts the implementation of the damping in the
shear direction as explained above. Should I commit it? Or would you prefer
to modify your existing one (ViscoelasticPM)? If you agree with me, of
course.
Any comments would be appreciated.

Cheers, Chiara

Attachment: Damping.pdf
Description: Adobe PDF document