How to calculate the load (force) values of the simulation box during uniaxial compression test

[niki-MASc]

Dear LAMMPS Team

Given the below link, thanks for the helpful explanations on the forum.

Calculating stress-strain/force-displacement of granular solid

Hi Aksel, My responses below (note that I don’t want to get wrapped up in someone else’s research, so unless you have additional LAMMPS related Q’s, which most of these are not, this will probably be my last answer) From your script, these are related since you used compute reduce sum - and should give the same value if they are normalized correctly. I suggest reading up on the compute stress/atom and pressure documentation - and search past postings if need be. Looking at the dimensions of …

For the compression test (-x) of the cubic simulation box through applying an engineering strain rate, I also want to plot the load-Displacement curve.
As far as I know, (Engineering stress)=(force)/(original cross sectional area), which means we need to consider constant cross sectional area to calculate the engineering stress values during the whole test time. Thus, is it correct to use below formula for MD simulations to calculate the load values for the entire system (simulation box) on every step? (By applying the correct unit conversion)

Fx = (-pxx)(A=Ly.Lz, the cross sectional area on the last step of equilibrium phase)

Best,
Niki

[niki-MASc]

I am also wondering if my below questions are correct or not!

1- May I also use compute reduce sum command to compute fx for the entire simulation box? If yes, are these commands correct?
compute 1 all reduce sum fx
variable Fx equal “c_fx”
thermo_style custom v_Fx

2- If I’m not mistaken, the maximum component of force is on the (x) direction on this simulation. Thus, is it correct to consider fmax outputs through thermo_style equal to Fx values for the simulation box?

[anazarahari]

Hi Niki,
Mark Tschopp has a great tutorial on the uniaxial compression (and other mechanical tests) in LAMMPS. Have you given it a try? You can find the link here. (As far as I remember, he used some unit conversion in his input)

[niki-MASc]

Hi Alireza

Thanks for your kind reply. Yes, I read them before, and those tutorials were truly helpful for my research. His main concentration was on the stress results to plot stress-strain curve, which is quite important in materials science. However, he has not mentioned load calculation method. My supervisor suggested that both stress-strain and load-displacement curves can be useful if we can have them.

Best,
Niki