Triaxial Compression Test in DEM Engine

[Shahriar Karim Shafin]

Can I do a triaxial compression test in Chrono DEM Engine??  I can easily do uniaxial compression but I cant put any confined pressure in y and z directions when trying triaxial tests in chrono DEM Engine. How can I keep the pressure constant in a direction  at the time of compression??

[Ruochun Zhang]

The short answer is yes, but it's not a one-liner. If you already have your setup to compress the material, then rather than directly setting the wall pressure, think of creating a servo-controlled boundary problem. You can use the tracker class to track the walls that you use for compression, then you can get the normal force using that tracker, and divide that by the area of the wall, you get the pressure. Adjust the wall velocity (or position) based on the difference between the current pressure and your target pressure. This creates a feedback loop that drives the system toward the desired pressure.

Let me know if there are further questions.

Ruochun

[Shahriar Karim Shafin]

Thanks a lot, but the problem is how can I keep that pressure constant after getting my desired pressure (If I put an axial strain rate in the z-direction after that, the lateral pressure will increase again according to you)? I wanted to apply an isotropic confining pressure of some value to the specimen in all directions to consolidate it to equilibrium and then an axial strain rate on the top wall (z-axis) while maintaining constant lateral pressure (x and y axis) which is also called Standard Triaxial Test.

[Ruochun Zhang]

Then again, I still think it's a servo-controlled boundary problem, or say you need a control policy implemented (i.e. you keep the lateral pressure constant by keeping the lateral walls under servo control during the axial loading phase). During shear you prescribe the top wall’s z-velocity (strain rate control) while continuously (like updating it every N steps or so) adjusting the x/y wall positions/velocities based on the tracker-measured lateral pressure values​. This should be exactly like what's happening under the hood for a physical test, and this control policy shouldn't be too hard to implement with C++ or Python. It's not already in DEME, which is a shame... But if you can contribute such a demo to the project, that'd be great. I hope I understand your question correctly.