[lammps-users] Regarding uniaxial tensile test

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1

Hello all,
I am running a tensile simulation on Lammps (15 Apr 2020) and facing some problems with my results. I am deforming on the Z-direction with a strain rate of 10^9/s. The results in the output file are showing length is increasing in all directions. According to the tensile test, the length should be an increase in the deformation direction (z) and decreases in the remaining directions (x and y). And one more thing I am getting negative stress in the z-direction. I am not getting where I have done a mistake. Do you maybe have some suggestions for me to solve that problem?
Here is my Lammps Input File:

##---------------INITIALIZATION-------------------------------
units metal
dimension 3
boundary p p p
atom_style atomic
newton on
##---------------ATOM DEFINITION------------------------------
read_data indu
##---------------FORCE FIELDS---------------------------------
pair_style eam/alloy
pair_coeff * * Zhou_TiTaNbZrMo1.eam.alloy Ti Ta Nb Zr Mo
##---------------SETTINGS-------------------------------------
timestep 0.0005
variable ts equal 0.0005
##---------------COMPUTES-------------------------------------
compute 1 all stress/atom NULL
compute 2 all reduce sum c_1[1] c_1[2] c_1[3]
variable Lx equal lx
variable Ly equal ly
variable Lz equal lz
variable Vol equal vol
fix 1 all npt temp 300 300 0.05 x 0 0 0.5 y 0 0 0.5 z 0 0 0.5
thermo 2000
##---------------RELAXATION--------------------------------------
run 10000
##---------------DEFORMATION--------------------------------------
unfix 1
reset_timestep 0
fix 1 all npt temp 300 300 0.05 x 0 0 0.5 y 0 0 0.5
variable srate equal 1.0e9
variable srate1 equal “v_srate / 1.0e12”
fix 4 all deform 1 z erate ${srate1} units box remap x
run 100
##---------------THERMO-OUTPUTS--------------------------------------
variable ConvoFac equal 1/1.0e4
variable sigmaxx equal c_2[1]*v_ConvoFac/v_Vol
variable sigmayy equal c_2[2]v_ConvoFac/v_Vol
variable sigmazz equal c_2[3]v_ConvoFac/v_Vol
variable StrainPerTs equal v_srate1v_ts
variable strain equal v_StrainPerTsstep
thermo 100
thermo_style custom step temp v_strain v_sigmaxx v_sigmayy v_sigmazz pe ke lx ly lz vol
##---------------DEFORMATION--------------------------------------
dump 1 all atom 2000 tensile_test.lammpstrj
run 100000
output-file
Step Temp v_strain v_sigmaxx v_sigmayy v_sigmazz PotEng KinEng Lx Ly Lz Volume
100 4178.3118 5e-05 2.7346411 2.6703664 0.7596709 -98518.82 7931.2097 53.946879 54.352424 103.42493 303256.74
200 6771.2969 0.0001 -3.1592439 -3.7711233 -4.5920966 -96160.172 12853.176 54.892914 55.282898 103.4301 313873.04
300 5802.3534 0.00015 -11.687118 -10.999384 -13.628464 -94862.892 11013.942 55.559089 56.124735 103.43527 322535.9
400 5863.6657 0.0002 3.2026904 3.4139507 1.088236 -94660.385 11130.324 55.759725 56.208915 103.44044 324202.37
500 4406.4443 0.00025 -1.5293336 0.52705793 -2.5856379 -94605.679 8364.2475 55.680551 56.322608 103.44561 324413.07
600 3742.2075 0.0003 -10.331542 -10.644514 -10.428027 -95201.241 7103.4031 55.692076 55.989866 103.45078 322579.39
700 3395.7427 0.00035 -1.1937191 -0.87210977 -2.1582255 -95166.395 6445.7486 55.802464 55.938773 103.45595 322939.97
800 4270.3592 0.0004 2.935509 2.5867492 1.0649779 -94958.204 8105.9327 56.117894 55.957445 103.46112 324890.07
900 3414.0645 0.00045 3.7990271 3.3667051 2.2185578 -95500.825 6480.5267 55.830082 55.807228 103.4663 322372.23
1000 3194.1234 0.0005 -2.5394564 -2.1994481 -2.2342687 -95715.881 6063.0379 55.579244 55.601952 103.47147 319759.37
1100 3602.8771 0.00055 -1.9174245 -3.8484007 -6.265048 -96338.082 6838.9281 55.481296 55.366446 103.47664 317859.76
1200 2672.2552 0.0006 1.8199292 1.2651353 1.5484497 -96731.98 5072.4353 55.271131 55.339646 103.48181 316518.24
1300 2355.062 0.00065 -3.0421154 -2.6432658 -3.5080611 -96947.84 4470.3439 55.131732 55.228714 103.48698 315102.81
1400 2195.4317 0.0007 2.2860798 2.1743296 2.0103172 -96861.036 4167.3359 55.05989 55.233907 103.49215 314737.52
1500 3004.0847 0.00075 2.989903 3.0858079 2.3067693 -97110.294 5702.3092 55.047074 55.223856 103.49732 314622.72
1600 2222.5342 0.0008 -1.4661569 -2.0178143 -0.91044868 -97237.734 4218.7816 54.910001 54.859951 103.50249 311786.77
1700 1906.6967 0.00085 -0.11058375 -0.11382097 -0.96581517 -96935.135 3619.2635 55.173725 55.141396 103.50767 314907.19
1800 2007.1774 0.0009 2.0670948 1.7765419 1.6734895 -97124.666 3809.9944 54.920302 55.060882 103.51284 313018.71
1900 1815.1031 0.00095 0.95231182 0.86135043 0.87057208 -97554.992 3445.4019 54.78547 54.776943 103.51801 310655.53
2000 1767.9439 0.001 2.0866611 1.6955155 1.8539869 -97862.505 3355.885 54.73772 54.734729 103.52318 310161.06
2100 2132.7162 0.00105 2.0939662 1.8334463 1.6277002 -98025.541 4048.2905 54.739544 54.771047 103.52835 310392.71
2200 1685.3505 0.0011 2.7803127 2.7637338 3.1239593 -97958.886 3199.1074 54.444043 54.645117 103.53352 308022.7
2300 1940.5547 0.00115 -12.144686 -10.298113 -11.77299 -97921.106 3683.5322 54.527642 54.751333 103.53869 309110.74
2400 1866.2655 0.0012 3.963259 3.5122948 3.3254222 -98019.841 3542.5176 54.496098 54.69948 103.54386 308654.76

2

The problem is that you’re using npt in all directions (x, y and z). If you’re deforming in z direction you shouldn’t do npt in z direction.

3

Thank you for your reply. As per your suggestion I modify the NPT condition (fix 1 all npt temp 300 300 0.05 x 0 0 0.5 y 0 0 0.5). But I am getting the same output as I mentioned in the previous email.

4

rather than worrying about the box dimensions, you should be asking why your temperature is so very far away from the desired target temperature.
that means that either there is something wrong with your potential file and settings or that your initial structure was very far away from equilibrium and thus you have not reached equilibrium at all when starting your production simulation. the number of timesteps for your run commands is on the small side in general.

axel.

5

sorry, I don’t have time to tutor you or to debug/review your work. that is something you need to discuss with your adviser/supervisor or (experienced) colleagues.

just some general advice. if you generated a potential from a paper, then you should first test this for each element individually.
that is best done by comparing results with files that have been tested and are documented to work with LAMMPS.
There are verified potentials in the potentials folder of LAMMPS and others are available through databases.
You should get similar energies, relaxed lattice constants, elastic constants etc. with these potential files than with yours.
Once you have verified the potential file, you can look into the geometry in your input, if that still is a problem.

but as I said before, this is something you either already should know how to do or need to ask whoever is advising/tutoring you to be taught.

axel.