# yade-users team mailing list archive

## Re: the relationship of poisson ratio and tangential stiffness

```Hello Vaclav,

Thank you for your comments. I can understand it. You know, from the mechanical concept, the shear modulus (G) can be represented by E/2*(1+v) for homogeneous isotropic materials, here E is young modulus, v is poisson ratio. So, can we decide G_over_E is equal to 1/2*(1+v) directly? Can we use 1/2*(1+v) to calculate kt/kn?
Thank you so much.

Best
Liqing

At 2010-09-20 17:21:39，"Václav Šmilauer" <eu@xxxxxxxx> wrote:

>> I have checked the relationship of the physical parameters and the
>> required parameters in Yade. I am confused how to get the shear
>> stiffness (tangential stiffness) from the poisson ratio and normal
>> stiffness correctly. You know, according to the description in
>> Vaclav's thesis page 52. I get the poisson ratio is not equal to the
>> Kt/Kn (shear stiffness/normal stiffness) directly. However, in
>> Ip2_2xCohFrictMat_CohFrictPhys, the shear stiffness is calculated just
>> from Kn*poisson ratio. You know, in PFC theory, the users always need
>> to give the shear stiffness directly.  I know the poisson ratio (v)
>> depends on the dimensionless ratio Kn/Kt. But I don't know how to get
>> this formula (v=Kt/Kn). How to decide the coefficient?
>
>Hi,
>
>the way to compute Kn/Kt depends on the Ip2 functor in use.
>
>In the case of the Cpm model which you refer to (page 52), I was
>directly specifying the Kn/Kt ratio in the CpmMat class (I call it
>G_over_E, but G/Kt=E/Kn), the ElastMat::poisson is not used at all.
>
>Admittedly, the ElastMat::poisson is quite confusing, maybe replacing it
>by the kt/kn ratio (ktDivKn) would be better. I think it is there for
>historical reasons, though other people might know better.
>
>Cheers, v
>
>
>
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