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