Trouble with running a uniaxial compression test

[sdantu2]

Hello everyone! I’m an undergraduate researcher exploring uniaxial compression simulations for sugar (currently) and for energetics (eventually) in FEPX. I’ve generated a Neper tessellation and meshed it successfully, but I’m encountering confusion while creating my .cfg file. Because real sugar (sucrose) is monoclinic, and FEPX only supports(from what I have been able to gather from the documentation) fcc, bcc, hcp, and bct, I’m attempting to use the bct crystal type to approximate sugar’s lower symmetry, however I am struggling with how best to assign lattice parameters (a0, ca) and elastic constants (c11, c12, c13, c44, c66) so they reasonably reflect published data on sugar’s monoclinic properties. I’m also unsure about defining the slip systems, slip hardening parameters, and interaction matrices for this approximate bct setup.

On top of that, I’d like to simulate a high‐strain‐rate compression test similar to what you’d see in a Kolsky (Split‐Hopkinson) bar experiment. Since FEPX is quasi‐static, I plan to approximate high‐rate loading by applying a large velocity or strain rate on the top face over a very short total simulation time, while fixing the bottom face. However, I’m not entirely sure how to specify these boundary conditions correctly—how to constrain the bottom vs. the top faces, whether to use a velocity or strain‐rate boundary, and how to choose increments or total time that best emulate a short, rapid event. If anyone has advice on mapping monoclinic sugar to a bct configuration (selecting an appropriate c/a ratio, approximate stiffness constants, and slip parameters), or on setting up Kolsky bar–style boundary conditions in FEPX, I would greatly appreciate your insights. Thank you in advance!

Additionally here is the .cfg file that I have made so far:

### FEPX Configuration File

## Material Parameters

    number_of_phases 1

    phase 1

      crystal_type bct
      c_over_a 0.81

      c11 28.0e3
      c12 12.0e3
      c13 9.0e3
      c44  8.0e3
      c66 8.0e3

      m 0.02d0
      gammadot_0 1d0

      hard_type isotropic
      h_0 100.0
      g_0 10000.0
      g_s 2000.0

## Boundary Conditions

    set_bc strainrate 33 0.001
    set_bc vel x0y0z0 x 0 y 0
    set_bc vel x1y0z0 x 0 y 0

## Steps

    number_of_steps 1
    target_strain33 0.02
    dstrain 0.001

## Printing Results

    print coo
    print ori
    print stress
    print strain
    print forces

[mkasemer]

FEPX does not support monoclinic crystals currently. You can either implement, or you will have to approximate via a different crystal system. You will have to determine the elastic constants on your own, perhaps by holding the bulk modulus similar between the two systems.

Overall, FEPX is in no way set up to handle high rate phenomena. The models implemented in our code base do not consider the deformation physics that appear at high strain rates. Again, this would need further development.