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Re: [Question #688979]: period triaxial test under cyclic shear load

 

Question #688979 on Yade changed:
https://answers.launchpad.net/yade/+question/688979

    Status: Open => Answered

Jan Stránský proposed the following answer:
Hello,

> How to apply the shear stress

as you proposed, change in shear strain **should** result in a change of
shear stress

> Is it possible to use O.cell.velGrad=Matrix3(0,0,-0.01,0,0,0,0,0,0) to
control the shear strain rate and periodically change the direction to
produce cyclic shear stress?

yes (see above)

> This process does not change the engine's goal and stressmask, is it
for this reason that the effective stress keeps unchanged at 100kpa
during the shear process?

yes

> What is the solution?

this is the intended behavior, so please specify what you want to solve

> In materials we learn that pure shear volume does not change.

yes, only normal components of deformation has influence on volume

> How to control the volume of the sample to remain unchanged in the periodic boundary? I used the peritriaxcontroller engine to simulate the undrained triaxial test.
> If the shear stress is applied to the zx surface, how can the volume be controlled unchanged?
> Is there any need for control here?

you have 6 independent components of strains / stresses. For each, either strain or stress is prescribed. Only normal components has influence on volume. To control volume, you can
- prescribe all 3 normal strains (stressMask=0) to fully control strain(rate), than it is easy to prescribe it to preserve volume (sum of strain increments has to be zero)
- prescribe all 3 normal stresses (stressMask=7) -> than you have no control over volume, volume is just a result
- prescribe some stresses and at least one strain - then according to actual volume you can control the prescribed strain(s) to preserve volume (or to minimize the difference between actual and desired volume)

> By the way ,Sand is not elastic material,

well, it depends pretty much on definition, model, conditions, load, purpose of modelling..........
I would not be that decisive in general.

> granular material has shear volume deformation, if the volume remains
constant, so theoretically the effective stress should change.

reality is the worst enemy of theory :-) illustration:
> Under the action of cyclic shear load, the effective stress gradually decreases as the number of cycles increases.
>  the effective stress should change. -> indeed, it should increase, "In general, the denser the soil, the greater the amount of volume expansion under shear." [2]

on one hand you expect decrease of stress, on the other hand granular
matter could expand (increasing normal stress at constant volume), so
stress can do (both theoretically and in reality) anything :-) depending
on many factors..

cheers
Jan

[4] https://en.wikipedia.org/wiki/Dilatancy_(granular_material)

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