yade-dev team mailing list archive

[Chiara Modenese <c.modenese@xxxxxxxxx>]

Hi, thanks for reply.

2011/4/4 Jan Stránský <_honzik@xxxxxxxxxx>

>
> Hi,
>
> in PeriIsoCompessor, the Vector3r stiff contains normal ("pseudo")
> stiffnesses for each axis (stiff[i] = stress[i] / strain[i], where stress[i]
> and strain[i] are normal components, too, as Bruno answered in previous
> email). It is not "real" stiffness due to the absence of Poisson's ratio,
> but it is completely sufficient for purpose of PeriTriaxController (you only
> need to predict stress/strain for next step from known value of the dual
> quantity using this normal "pseudo" stiffness).
>
I do not know about you but, actually, I have been using PeriTriaxController
for a while and to be honest I could only use the method which uses the mass
parameter introduced by Bruno (sometimes it was painful as it needs some
calibration, but it works). When I was trying the method which uses the
stiffness, it did not work.

Chiara


>
> Very similar approach is used in Peri3dController (with addition of shear
> components as well), evaluating something like Vector6r stiff instead of
> full 6x6 stiffness matrix.
>
> From my point of view, Vector3r stiff is ok, maybe it could be renamed to
> normalStiff to prevent confusion, but it is only a detail..
>
> regards
> Jan
>
> ______________________________________________________________
> > Od: "Bruno Chareyre" <bruno.chareyre@xxxxxxxxxxx>
> > Komu: yade-dev@xxxxxxxxxxxxxxxxxxx
> > Datum: 04.04.2011 18:25
> > Předmět: Re: [Yade-dev] Stiffness tensor
> >
> >This is more a question for Vaclav or Jan, but here are my two cents:
> >There is no simple definition of stiffness in the general case. The
> >definition you find in periEngines is based on the implicit assumptions
> >that there is no strong heterogeneity of the displacement field, and
> >that rotations of particles are negligible, which can happen if you have
> >more than ~10 contacts per particle (it was the case in Vaclav's PhD).
> >For e.g. Cundall law or HM, it doesn't apply.
> >
> >I don't know precise references but I think the main points of the
> >derivation are, with the assumptions mentioned above:
> >1- relative displacement at contact is du=epsilon*dl, with dl the
> >distance between centers
> >2- forces increments are df = dun*kn+dus*ks
> >3- average df's somehow to get a dsigma (they could be averaged using
> >Love Weber stress, but I think there is a different averaging currently)
> >4- derive dsigma/depsilon = stiffness
> >
> >Bruno
> >
> >
> >On 04/04/11 16:54, Chiara Modenese wrote:
> >> Hi there,
> >>
> >> Do you have any reference which shows the formula for the stiffness
> >> tensor? I see that in the PeriIsoCompressor.*pp there is a vector
> >> called stiff, is this correct?
> >>
> >> Cheers.
> >> Chiara
> >>
> >>
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> >
> >
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>
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