Error in necking and fracture behaviour during tension test

LAMMPS LAMMPS Beginners
1

Respected Lammps Users, I am simulating the tensile behaviour of copper cube using the script below but after reading it thoroughly I am failed to get which command actually control necking and fracture behaviour. As the starin rate is applied to the specimen it elongates and thins itself like a wire but there in no necking and stress concentrated zones as it seems just elongating and thinning as if it is compressing from the two dimensions and elongating in the third dimension and even no fracture is shown along with no poisons effect. The script is below. Which command specifically control this issue.
“”“”# Input file for uniaxial compressive loading of single crystal copper.

------------------------ INITIALIZATION ----------------------------

units metal
dimension 3
boundary p p p
atom_style atomic
variable latparam equal 3.61

----------------------- ATOM DEFINITION ----------------------------

lattice bcc ${latparam}
region whole block 0 15 0 15 0 15
create_box 1 whole

region upper block INF INF INF INF INF INF units box
lattice bcc ${latparam} orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
create_atoms 1 region upper

------------------------ FORCE FIELDS ------------------------------

pair_style meam
pair_coeff * * library.meam Cu Cu.meam Cu

------------------------- SETTINGS ---------------------------------

compute csym all centro/atom bcc
compute peratom all pe/atom

######################################

EQUILIBRATION

reset_timestep 0
timestep 0.005
velocity all create 300 12345 mom yes rot no
fix 1 all npt temp 300 300 1 iso 0 0 1 drag 1

Set thermo output

thermo 100
thermo_style custom step lx ly lz press pxx pyy pzz pe temp

Run for at least 10 picosecond (assuming 1 fs timestep)

run 4000
unfix 1

Store final cell length for strain calculations

variable tmp equal “lx”
variable L0 equal {tmp} print "Initial Length, L0: {L0}"

######################################

DEFORMATION

reset_timestep 0

fix 1 all npt temp 300 300 1 y 0 0 1 z 0 0 1 drag 1
variable srate equal 1.0e10
variable srate1 equal “v_srate / 1.0e12”
fix 2 all deform 1 x erate ${srate1} units box remap x

Output strain and stress info to file

for units metal, pressure is in [bars] = 100 [kPa] = 1/10000 [GPa]

p2, p3, p4 are in GPa

variable strain equal “(lx - v_L0)/v_L0”
variable p1 equal “v_strain”
variable p2 equal “pxx/10000”
variable p3 equal “pyy/10000”
variable p4 equal “pzz/10000”
fix def1 all print 100 “{p1} {p2} {p3} {p4}” file Cu_comp_100_comp_100.def1.txt screen no

Use cfg for ovita post processing

dump 1 all custom 250 dump.Cu.comp.* id xs ys zs c_csym c_peratom fx fy fz

Display thermo

thermo 100
thermo_style custom step v_strain temp v_p2 v_p3 v_p4 ke pe press

run 4000

######################################

SIMULATION DONE

print “All done”
“”“”
Also in the Lammps documentation the NPT ensemble is not explained for the “”“” 1 y 0 0 1 z 0 0 1 drag 1"“”" what these referes to ? “”““fix 1 all npt temp 300 300 0.05 x 0 0 0.5 y 0 0 0.5"””" How these parameters work.
In some scripts two dimensions of the geometry is fixed and the third is just stretching to fracture. In some scripts one dimension is streching and the other two moves inward to express necking.
Which command specifically control it?
How to enable ATC to look for stress concentration contour plots?

2

Please carefully study the post: Please Read This First: Guidelines and Suggestions for posting LAMMPS questions
and that includes expanding all the black triangles to read all the details.

Your post is violating several of those, most importantly the process of properly quoting text files (just look at it. does it seem right to you? does it look exactly as the text you copied??).

You don’t seem to grasp how simulations work:

  • you set up your test geometry
  • you select your potential and simulation settings
  • you run the simulation and observe and analyze properties
  • if you don’t get the expected results that either your model, settings, and method are not suitable for the simulation you did or your expectations are not correct.

If there was a “give me the results I want” kind of command, there would be very little meaning in doing simulations in the first place. It is important to understand that the results you get are for the models and methods you are using, not for the material itself.

They are documented. The Nose-Hoover fixes have many options that can be combined in many ways, it requires careful reading and checking to figure out the meaning of each of them. There will be no section where you can find an example explained specifically for just you.

This just confirms that you need to study the documentation specifically for fix npt with far more care and attention to detail.

This is a completely unrelated question and should be posted in the “LAMMPS Installation” category. But - yet again - underlines the need to spend more time and care to read the LAMMPS manual. Enabling optional packages during compilation of LAMMPS is explained there with great detail and thus your question redundant.