How to control the pressure on servo wall during the confining process in triaxial compression test

Submitted by deepakpawar.2310 on Wed, 09/11/2019 - 14:18

Hello everyone

I got stuck with working of servo wall during the confining process in my triaxial simulations.
I do want to maintain the 1 MPa stress on the each of the servo wall( i have six faces) before going for the shearing process

I am using following commands for the servo wall
variable pressure equal 64 # 60 N /64 mm^2 = 1MPa
variable servo_vel equal 0.0000001
variable servo_kp equal 10
variable servo_ki equal 0
variable servo_kd equal 0
fix xplane1servo all mesh/surface/stress/servo file mesh005/x-.stl type 1 scale 1 move 0.001 0. 0. stress on com -0.004 0. 0. ctrlPV force axis 1. 0. 0.
target_val ${pressure} vel_max ${servo_vel} kp ${servo_kp} ki ${servo_ki} kd ${servo_kd}

In my case the timestep is 1e-7 and particle size is 1.5 mm

Not able to get that prescribed value of confining stress on the wall, what would be the reason for the same.
Although I tried for varying the values of Kp and target velocity as well

and also it possible to use servo wall as to velocity controlled confine the granular assembly.

Thank you

alberthappy | Mon, 02/24/2020 - 11:22

Dear Deepak,

did you solve the problem? I tried to do the same thing as you were doing. I calculated the target force according to the prescribed pressure. When I run the simulation and measured the pressure, I observed a quite significant derivation of the pressure value. Did you figure out why?

Fenglei Qi

tjleps | Tue, 03/03/2020 - 17:16

Have you tried much larger target velocities? At this velocity it will take 1e9 time steps before the wall has moved 1% of a particle diameter, a more appropriate value would probably be on the order of 1-10 m/s.