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Control the normal stress fluctuation in the direct shear box modeling

Asked by Weimin Song

Dear all,

I'm modeling the direct shear box right now.

I made vertical compression of the assembly with a moving wall until the pre-defined compressing force is reached. And then I made the shear. However, during the shear, the normal force deviated from the pre-defined value.

After I checked the references, I know during the shear process, the desired compressing normal force can be regulated to stay the same by moving the upper plate up or down, depending on the actual compressing force value. However, how to adjust? Anyone can give me some hints?

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Bruno Chareyre (bruno-chareyre) said : 		#1

One possible strategy is to simply impose a force on the top plate and let it equilibrate (it needs to have a mass).
Is is simple but it does not offer the best control usually, due to the inertia of the plate.

Another strategy is to impose de velocity (or displacement): v = f(sm-st), with "sm" the measured force and "st" the target force.
The remaining question is how to define f. One way is to calculate the cumulated stiffness K of all plate-particle interactions, and to use it in:

v=(sm-st)/(K*O.dt)

which will produce a nearly constant force.

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Jérôme Duriez (jduriez) said : 		#2

As a side note, KinemCNLEngine [1] is a dedicated engine for such task for simple shear (as opposed to direct shear) boxes. The corresponding code source [2] may also give some hints (corresponding to Bruno's suggestions)

[1] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.KinemCNLEngine
[2] https://github.com/yade/trunk/blob/e4e757f2e98a620e3177b7a36a1d10f69f6a6a28/pkg/dem/KinemC__Engine.cpp

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