How to connect the material parameters with contact stiffness for polyhedra?

Asked by jasperb on 2020-03-11

Hey

i read https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/Polyhedra.cpp#L498 which says below:

#########################################
         Real Kna = mat1->young;
 Real Knb = mat2->young;
 Real Ksa = mat1->young*mat1->poisson;
 Real Ksb = mat2->young*mat2->poisson;
 Real frictionAngle = math::min(mat1->frictionAngle,mat2->frictionAngle);
        contactPhysics->tangensOfFrictionAngle = math::tan(frictionAngle);
 contactPhysics->kn = Kna*Knb/(Kna+Knb);
 contactPhysics->ks = Ksa*Ksb/(Ksa+Ksb);
#########################################

I am wondering why the value of young could be assigned to the Kn directly? I am speculating that the name "young" does not have any specific meaning here,just represents the stiffness. Is that right?
If so, is there any method implemented in Yade actually connects the stiffness with the material parameters(such as E,shear modulus G) for a polyhedron? (distinguished from calculating the stiffness using Young's moduluus, diameter and poison's ratio for sphere contacts)

Thanks

Jasper

Question information

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Last query:
2020-03-11
Last reply:
2020-03-11
Jan Stránský (honzik) said : #1

Hello,

> I am wondering why the value of young could be assigned to the Kn directly?

why not? :-)

> just represents the stiffness.

yes

> the name "young" does not have any specific meaning here ...

The specific meaning is that it represents the normal stiffness :-)

if you mean that "young" should be Young's modulus, than yes, it may not be the case here.
The name is inherited from FrictMat and has the meaning of stiffness in normal direction, not necessarily Young's modulus.

have a look at a similar discussion [1]

> If so, is there any method implemented in Yade actually connects the stiffness with the material parameters(such as E,shear modulus G) for a polyhedron? (distinguished from calculating the stiffness using Young's moduluus, diameter and poison's ratio for sphere contacts)

as usually in DEM, you have to calibrate the parameters to obtain desired response (if I got the paragraph correctly)

cheers
Jan

[1] https://answers.launchpad.net/yade/+question/681383

Jérôme Duriez (jduriez) said : #2

If you read source code, [*] may be relevant as well.

You will see that PolyhedraMat.young and PolyhedraPhys.kn may even have different "units" than usual (and possibly variable ones, depending on the Law2*.volumePower)

[*] https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/Polyhedra.cpp#L559

Chareyre (bruno-chareyre-9) said : #3

 > is there any method implemented in Yade actually connects the stiffness
with the material parameters(such as E,shear modulus G) for a polyhedron?

Hi,
Hertzian theory of contact is the answer to your question in the case of
spheres. And (surprise surprise) it leads to non linear elasticity and
vanishing stiffness in unconfined materials. So even for spheres there is
no answer to your question (no single stiffness value due to non-linearity).
There is no established theory for polyhedral particles AFAIK.
Cheers
Bruno

On Wed, 11 Mar 2020 at 14:09, Jan Stránský <
<email address hidden>> wrote:

> Question #689263 on Yade changed:
> https://answers.launchpad.net/yade/+question/689263
>
> Status: Open => Answered
>
> Jan Stránský proposed the following answer:
> Hello,
>
> > I am wondering why the value of young could be assigned to the Kn
> directly?
>
> why not? :-)
>
> > just represents the stiffness.
>
> yes
>
> > the name "young" does not have any specific meaning here ...
>
> The specific meaning is that it represents the normal stiffness :-)
>
> if you mean that "young" should be Young's modulus, than yes, it may not
> be the case here.
> The name is inherited from FrictMat and has the meaning of stiffness in
> normal direction, not necessarily Young's modulus.
>
> have a look at a similar discussion [1]
>
> > If so, is there any method implemented in Yade actually connects the
> stiffness with the material parameters(such as E,shear modulus G) for a
> polyhedron? (distinguished from calculating the stiffness using Young's
> moduluus, diameter and poison's ratio for sphere contacts)
>
> as usually in DEM, you have to calibrate the parameters to obtain
> desired response (if I got the paragraph correctly)
>
> cheers
> Jan
>
> [1] https://answers.launchpad.net/yade/+question/681383
>
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