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Re: Granular ratchetting explained

 

Actually, I cannot give an answer too, ATM. In fact, what is usually called
and implemented as Hertz-Mindlin formulation is not really what happens in
the real case of two elastic bodies in mutual contact. It is a _simplified_
solution, computationally convenient, to avoid the complete one proposed by
Mindlin which instead would adopt an hysteretic behavior in the shear
direction.
Given the simplified formulation, what I see is that some energy is created
performing that simple cycle loop, having a residual shear force at the end
of it.

Now:

1) I am not sure that the way the granular ratcheting is avoided in the
linear elastic case suits also the non linear elastic one as and if it makes
sense in my case to set the concerned bool as True. I will see (thanks for
the link about hypoelastic models).

2) The bool granular ratcheting is almost part of each contact law which
derives from ScGeom (IICR NormalInelasticityLaw, ViscoleasticPM... ) but
does it really make sense for all of them?

Bruno, one last question on the topic. In your comment you make reference to
the "good" ratcheting which is described in the literature (though not yet
well understood) and which relates to the sliding of the contacts. The good
ratcheting involves contact sliding, the bad ratcheting is not concerned
with the sliding of the contacts. What is the link between the twos? Could
you elaborate a little bit more or give me references about "bad"
ratcheting? Cheers.

Chiara





> I would have a question about the ratcheting. I have run the same test also
>> for HM and of course I do not get the same good result as in the case of the
>> linear formulation. The reason is that in HM the tangential stiffness
>> depends on the normal force (or say on the overlapping alike), and this is
>> different from the initial one once I come back with the rotation (say at
>> the last step). Hence shear force is not equal to zero although granular
>> ratcheting is avoided. Do you think this is expected? I have not really
>> investigated too much the granular ratcheting in the literature. Is this
>> phenomenon perhaps addressing only the linear elastic case?
>>
> I don't know your model enough to give a precise answer. Based on what you
> write, it seems you could proove irreversible behaviour in cycles even the
> analytical way?
> Variable stiffness makes me wonder if it is not something linked to the
> well known issue in hypoelastic models, see e.g.
> http://linkinghub.elsevier.com/retrieve/pii/S0749641903001347.
>
> Quoting :
> "it is demonstrated that all hypothetical materials with non-potential
> finite elastic or hypo-elastic constitutive relations can create an energy
> from nothing, i.e. work as perpetual motion machines. This gives a
> ‘physical’ proof of necessity of potential conditions in general finite
> elasticity and hypo-elasticity and their extensions to finite
> viscoelasticity."
>
> Cheers.
>
> Bruno
>
>
>
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