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How do I set the stiffness of FrictMat?

Asked by 孙灿

I need a material that has friction, but is non-sticky, and can set the stiffness. FrictMat most likely met my request. In [1], kn can set the stiffness, but in [2] it means that the stiffness cannot be set in FrictMat, why? If FrictMat does not meet my requirements, what materials can meet my requirements?

[1]https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.Ip2_FrictMat_FrictMat_FrictPhys.kn
[2]https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.FrictMat

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Jan Stránský
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Jan Stránský (honzik) said : 		#1

Hello,

> FrictMat most likely met my request.

yes

> In [1], kn can set the stiffness, but in [2] it means that the stiffness cannot be set in FrictMat, why?

You can "set stiffness" in both cited places.

In FrictMat, you use young parameter [3].
"Actual stiffness" is FrictPhys.kn, which belongs to specific interaction.
Value of FrictPhys.kn is determined from materials of interacting particles, i.e. from two (possibly equal) values of FrictMat.young.
How the two values of FrictMat.young are combined into one FrictPhys.kn is determined by Ip2_FrictMat_FrictMat_FrictPhys.kn [1].

Cheers
Jan

[3] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.FrictMat.young
[4] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.FrictPhys.kn

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孙灿 (suncan) said : 		#2

Do you mean to set the stiffness indirectly by setting the Young's modulus? Is it only necessary to set the Young's modulus to complete the stiffness setting?

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Jan Stránský (honzik) said : 		#3

> Do you mean to set the stiffness indirectly by setting the Young's modulus?

No.
I mean to set the stiffness directly by setting "young" parameter of FrictMat.
It is, as documentation say [3], not necessarily the Young's modulus..

> Is it only necessary to set the Young's modulus to complete the stiffness setting?

"young" is always set (its default value is 1e9, [3]) so "the stiffness setting" is "completed" essentially.

Cheers
Jan

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孙灿 (suncan) said : 		#4

So as long as the Young's modulus is set, the stiffness is actually set. So how to find the Young's modulus according to the required stiffness, can I use the formula to calculate?

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Best Jan Stránský (honzik) said : 		#5

> So as long as the Young's modulus is set, the stiffness is actually set.

Yes and no.
So as long as the "young" parameter (not Young's modulus) is set, the stiffness is actually set.
"young" parameter is set always.

As mentioned in previous answers, "young" is a normal modulus, not necessarily Young's modulus.

> So how to find the Young's modulus according to the required stiffness, can I use the formula to calculate?

"calibration", "model fitting" and similar are your keywords.
"young" and "macroscopic stiffness" are linearly related, but the specific values is influenced by factors like actual packing, "poisson" parameter of FrictMat etc.

A good start is to run the simulation with a few different parameters and investigate its influence.

Cheers
Jan

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孙灿 (suncan) said : 		#6

Thanks Jan Stránský, that solved my question.