elastic-perfectly plastic material

Asked by azim

Dear all,
i am using [1] material and as you know it is for elastic materials.
i want to use an elastic materials that yields in a perfect plastic after reaching to a certain effective stress {elastic-Perfectly plastic material}.
has anyone worked with such a material? (if yes) what is the name of the material in Yade?

[1]O.materials.append(FrictMat(young=young,poisson=0.3,frictionAngle=radians(compFricDegree),density=2600,label='spheres'))

thanks in advance
Azim

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Luc Scholtès
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Jérôme Duriez (jduriez) said :
#1

Hi,

There may be some confusion here.

1. FrictMat's doc [*] says "Elastic material with contact friction", and I think "with contact friction" is as much important as "Elastic material" for a proper understanding of this material.

The contact description usually obtained with FrictMat, through e.g. Ip2_FrictMat_FrictMat_FrictPhys and Law2_ScGeom_FrictPhys_CundallStrack, actually *is* elastic perfectly plastic in the tangential direction.

2. Generally speaking, I think the discussion would be simpler if you disregard for now constitutive relations for (volume) materials, and stick to contact laws expressing contact forces in terms of relative displacements between contacting particles.

In this framework, which contact behavior would you like to describe ?

Jerome

[*] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.FrictMat

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Bruno Chareyre (bruno-chareyre) said :
#2

Hi,
[1] is exactly elastic-perfectly plastic.
Maybe your question is about an elastic-perfectly plastic _bulk behavior_, but bulk behavior is not an input parameter of DEM (hence answer #1).
As far as I know it is very difficult to obtain such a bulk behavior with DEM.
Regards
Bruno

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azim (mirzavand) said :
#3

thank you both bruno and jerome,
you are right.i checked it. it's elastic-perfectly plastic.
i have another question if you please help me:
Q) How do i know the point that my material yields in plastic domain? (friction angle or stress pattern)

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Bruno Chareyre (bruno-chareyre) said :
#4

>Q) How do i know the point that my material yields in plastic domain?

What does that mean?

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azim (mirzavand) said :
#5

for some stresses domains contacts are elastic. when we do load/unload the behavior is linear and returnable. but after a specific stress when we do unloading some strains remain as residual strain, (plastic strain) and the path of unloading differ from the elastic one.
i want to know the relation between friction angle and stresses with plastic domain???

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Bruno Chareyre (bruno-chareyre) said :
#6

Still unclear for me I'm sorry.
Would you give a mathematical expression of the quantity you are after?

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Jan Stránský (honzik) said :
#7

Hi Azim,
do you mean
Fs,max = Fn * tan(phi) # [1]
?

cheers
Jan

[1] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.Law2_ScGeom_FrictPhys_CundallStrack

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Best Luc Scholtès (luc) said :
#8

Hi,

My guess:

You want to know how to relate the yielding stress of your material (e.g. deviation from linearity of the stress strain curve for a uniaxial compression test) with the interparticle properties of your contact model (e.g. cohesion and friction angle).

If that's what you are after, you need to perform simulations on a particle packing (e.g. a triaxial compression) to see how the interparticle parameters (e.g. friction angle) affect the stress strain response of your material and thus its macromechanical properties (e.g. yielding stress). The relation between interparticle parameters and macromechanical properties is not direct in DEM (unless you use regular packings of particles). That's usually how we calibrate DEM model: trial and error to find the combination of interparticle parameters which gives the desired macromechanical properties. Of course, this micro-macro relationship is also a function of the packing structure (porosity, particle size distribution, homogeneity, etc...).

Have a look at this article to get some ideas about the calibration procedure:

https://link.springer.com/article/10.1007%2Fs10035-016-0687-0

I might be off topic though since your question is not very clear...

Luc

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azim (mirzavand) said :
#9

Dear all, Dear Jérôme, Bruno, Jan, Luc,

Thank a lot for your kind responses and your valuable time.
I have been busy working on my code which made me delayed to come back here.

Luc, thanks for your notes on calibration; I guess that is exactly what I need.

Let me try what you proposed. I will ask any question may arise during my calibration in a new thread.

Thanks Azim.

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azim (mirzavand) said :
#10

Thanks Luc Scholtès, that solved my question.