how to apply loading with constant stress ratio

Hi All,

I’d like to deform a orthogonal simulation box in a way that the stress ratios between the three principle directions remain constant during the whole loading process. That is if the stress is Px, Py, Pz in x,y,z direction, I want to keep Py/Px and Pz/Px constant. It seems that “fix deform” command will not do the trick since it could only control the strain rate rather than stress increment. I’m wondering if there is such command in lammps for this kind of loading condition? Any suggestions will be greatly appreciated. Thank you.

Best regards,

Zhi

Hi All,

I'd like to deform a orthogonal simulation box in a way that the
stress ratios between the three principle directions remain constant
during the whole loading process. That is if the stress is Px, Py, Pz
in x,y,z direction, I want to keep Py/Px and Pz/Px constant. It seems
that "fix deform" command will not do the trick since it could only
control the strain rate rather than stress increment. I'm wondering if
there is such command in lammps for this kind of loading condition?
Any suggestions will be greatly appreciated. Thank you.

In addition to the fix deform, what about using a barostat (npt,
press...) with thermo-variables as target pressure ?
(never did it, just a suggestion)

Julien.

Not clear on what you are asking.

Fix deform changes the box size/shape in a prescribed way

over time. If you use its variable option you have a lot

of flexibility in what the x,y,z dimesions (e.g. of an otrhogonal

box) would be over time.

But if you are asking to adjust the box size based on the

measured stress components Pxx, Pyy, Pzz, then that

is barostatting, not fix deform, So you need to look

at fix npt and its options. I don’t think it can do what

you are asking however.

We could possibly add a variable option to fix npt

to allow the target pressure (e.g. in x,y,z) to

be computed on the fly (e.g. as a function of other stress

components). E.g. we’ve done this for the thermostat

target temp in fix langevin. It would be trickier for

pressures b/c they fluctuate so much.

Steve