# Simple question: calculating kn

I have read in many journal papers using YADE [1] that kn = E*(Ra*Rb)/(Ra+Rb). However, when I use YADE and calculate kn, I get kn = 2*E*(Ra*Rb)/(Ra+Rb). I also read [2] and I did not see where the 2 is coming from. Can you help me identify it?

Thanks!

[1] A DEM model for soft and hard rocks: Role of grain interlocking on strength

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2020-03-05
 Marcus Moravia (mgmoravia) said on 2020-02-27: #1

Hi,
the normal stiffness in the contact is calculated by the harmonic mean of the individual rigidities of the respective bodies. As the calculation is for two bodies in contact, behold, the number 2 appears in the numerator! I hope this helps,

Marcus.

 Jérôme Duriez (jduriez) said on 2020-03-02: #2

Since FrictMat.young has no physical meaning when using Ip2_FrictMat_FrictMat_FrictPhys (which is understated here), another reason could be "why not 2, or even 10" ;-)

This would just modify the input value for this stress-like stifness parameter. Where any input value, for any formulation with 2 or 10 or.. , would be equally as correct as long as they give satisfactory results.

PS : Would you feel more confortable reading [1] ? (which does reflect the source code at [2], and your observations)

 Chareyre (bruno-chareyre-9) said on 2020-03-02: #3

> FrictMat.young has no physical meaning

This is defying my understanding of what can be called a physical meaning.
:)
papers, anyway.
Bruno

Le lun. 2 mars. 2020 16:33, Jérôme Duriez <
<email address hidden>> a écrit :

> Question #689015 on Yade changed:
>
> Jérôme Duriez proposed the following answer:
> Since FrictMat.young has no physical meaning when using
> Ip2_FrictMat_FrictMat_FrictPhys (which is understated here), another
> reason could be "why not 2, or even 10" ;-)
>
> This would just modify the input value for this stress-like stifness
> parameter. Where any input value, for any formulation with 2 or 10 or..
> , would be equally as correct as long as they give satisfactory results.
>
> PS : Would you feel more confortable reading [1] ? (which does reflect
> the source code at [2], and your observations)
>
> [1]
> [2]
>
> --
>
> _______________________________________________
> Post to : <email address hidden>
>

 Jérôme Duriez (jduriez) said on 2020-03-03: #4

Indeed, it may be better to state that FrictMat.young in this case does not have the same physical meaning, or physical interpretation, than Hooke's law Young modulus.

 Chareyre (bruno-chareyre-9) said on 2020-03-05: #5

Young in FrictMat does have the meaning of Young modulus in Hooke's law for
that material. «material» refering to granular material, not solid
constituent.
Bruno

Le mar. 3 mars. 2020 09:27, Jérôme Duriez <
<email address hidden>> a écrit :

> Question #689015 on Yade changed:
>
> Jérôme Duriez posted a new comment:
> Indeed, it may be better to state that FrictMat.young in this case does
> not have the same physical meaning, or physical interpretation, than
> Hooke's law Young modulus.
>
> --
>
> _______________________________________________