Yade

Maximum strain rate in a triaxial test

Asked by Alice Ezzeddine

I am trying to simulate a monotonic periodic triaxial test using Yade, and i am wondering about the choice of the maximum strain rate that i should use. In the tutorial [1], it says that we should choose a higher maximum strain rate on the radial direction than the axial to allow faster deformation along x,y and better maintain stresses. It seems logical to do so, but I am wondering :
How much higher should it be? Should the ratio between the lateral and axial maximum strain rate follow a certain criteria to better maintain stresses throughout the tests?

[1] : https://gitlab.com/yade-dev/trunk/blob/master/doc/sphinx/tutorial/06-periodic-triaxial-test.py

Thank you in advance!

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Best Fabio (fabiogabri) said : 		#1

Hi Alice,
In the example case the test is triaxial with constant confinement stress. So the strainRate you apply vertically has a final strain goal while the strain rate in the x and y direction only has the function of keeping the stress constant. Under "normal" conditions, a granular material has a ratio between the axial and transverse strain that is not constant with the deformation (it depends on the stress ratio, the degree of densification, the angle of friction, the packing ...). You can find some information here for regular packings [1]. Moreover, it can instantly have very different values, especially if few particles are used in the sample.
A ratio of 10 between z-strainrate and strain rate x and y I guess is simply a precautionary value to ensure that the sample remains stable under these conditions. You can also track the unbalanced forces for this purpose.
About the strainrate value you can check in the source code here [2]. Yade controls it in two ways: with a method based on the stiffness of the sample [3] and one based “inertial walls" having a certain mass (and therefore inertia) during their movement [4]. Depending on the dyncell true or false option, you can use the [2] or [3] method. Also pay attention to the value of mass as written here [5].

Cheers
Fabio

[1] Rowe PW. The stress–dilatancy relation for static equilibrium of an assembly of particles in contact. Proceedings of Royal Society of London A 1962; 269:500–527
[2] https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/PeriIsoCompressor.cpp#L174
[3] https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/PeriIsoCompressor.cpp#L214
[4] https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/PeriIsoCompressor.cpp#L219
[5] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.PeriTriaxController.mass

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Alice Ezzeddine (aliceezzeddine) said : 		#2

Thanks Fabio, that solved my question.