Questions about fix mesh/surface/stress/servo

Submitted by southeast1012 on Sun, 11/03/2019 - 14:55

Hello everyone
I am trying to conduct triaxial test using LIGGGHTS. My boundary is a cube made up of six STL files ,the size of one STL file is 0.2 meter. In the begining, they are crossed with each other, and particles are inserted in the center region, a cube with side length of 0.1 m.
I used servo command to accomplish isotropic consolidation.After that, I compute wall's position, unfix servo wall and fix new wall on the top and bottom.Using move/mesh command to move the top and bottom walls while lateral walls remain servo walls.
My question is that, When I use :
fix servo all mesh/surface/stress/servo file plate.stl type 1 com 0. 0. 0. ctrlPV force axis 0. 0. 1. target_val ** vel_max **
the mesh in one direction is a square with a side length of 0.2m,but my sample is much smaller than it. When I choose target_value of force , the area used in force=sigma*area is the area of the mesh or the area of sample?
Briefly, if I want my sample subjected to confining pressure of 200Kpa, the target_value is 8000N or 2000N ?
Sorry for my poor english,any suggestions are welcome.
Best regards

Rachel | Mon, 11/11/2019 - 03:51


The target force should be your sigma*(area of sample on one face). You need to update the area during the shearing process, which mean the target force needs to be updated during your simulation.

southeast1012 | Thu, 11/14/2019 - 12:47

Thanks for your reply, the target value was updated every N steps successfully with run_every style and fix_modify command. I think I managed it :-)

mkr2020 | Thu, 06/18/2020 - 21:47

Hi Southeast,

We are trying to begin our first triaxial simulation. We used servo walls similar to what you had described. We try to reproduce results in a paper produced using PFC#D. However, the stresses we get are smaller by a factor of two the three. The simulation runs without problems.

Would really appreciate if you could share your experience. Would it be possible to share an input script?