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Re: twist Moment and Bending Moment in cohesionlessMomentRotation


Hello Bruno,

In my model, When I am using Law2_ScGeom_CohFrictPhys_ElasticPlastic, it always occurs the errors. It cannot run. If I close the moment law, it is ok. If I open it, the error must be shown.
If I use the traditional Triaxial-test and open the moment function, it is normal. If I delete the constrain along one axle and close the moment function, it is normal. If I delete the constrain along one axle and open the moment function, it is abnormal. I am so confused that.
In the other hand, it only contains the elastic moment algorithms in Law2_ScGeom_CohFrictPhys_ElasticPlastic. whether it should be recovered if unloading. If do that, I think there will be some problem. The plastic rotation deformation may not be shown here. Do you agree with me?
Could you possibly give your opinion? Thank you.

I have not tried the Cpm model. Because it may not be considered the cohesion at the re-contact point. It should be fit to the rock type perfectly. But in my model, the cohesion at the re-contact point must be implemented. So if I can solve the moment problem with your help, I think your model should be better.


At 2010-10-21 00:34:12,"Bruno Chareyre" <bruno.chareyre@xxxxxxxxxxx> wrote:

>> I am still investigating the cohesionlessMomentRotation.
>> It is written in the sources files that this code has been "verified 
>> with the paper of Plassiard in GM".
>> However, Plassiard considers in his paper "and in his thesis" only the 
>> rolling part of the relative rotation of particles (and thus only 
>> bending moment), whereas in the code rolling and twist part of the 
>> relative rotation are considered (and thus bending and twist moment).
>There would be no contradiction here, if only it was possible to set 
>twist stiffness = 0 (so the law with bending and twist would "contain" 
>the law with bending alone).
>For now, Ktwist=Kbending=Kr, so it is indeed not possible...
>> My question: is there a particular reason for that? What is the 
>> motivation of the person who wrote this code? I don't say that is bad 
>> or good, but I would like to have an idea about advantages and 
>> disavantages, and physical meaning for considering a twist moment.
>As soon as there is a finite area of contact between the solids (i.e. 
>always), it makes sense to include twist resistance I think. The testing 
>has been done by Boon IIRC.
>By the way, I think rolling resistance, Ktwist!=Kbending, and other 
>features would be ideally implemented in 
>Law2_ScGeom_CohFrictPhys_CohesionMoment, which is now the cleanest, and 
>shortest code for moments.
>Funky factors η, α, β, would be contained in a collection of Ip 
>functors, since they are not needed by the law itself, which just needs 
>4 stiffnesses, and the definition of maximum values for each force/moment.
>Any help on this task of unifying duplicates would be welcome.
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