[lammps-users] doubt regarding BC'S and loading

Hi to all
I am trying to do on a tensile test. I am having some doubts, as I am giving the ramp velocity, and created fix setforce for a group of atom where I am constraining the atoms. but after complition of the run the box is compressing instead of expanding. and another doubt is even at the constrained region shrinking of the atom is taking place as I had given fixed boundary in Z direction for those atoms. I think I am doing some thing wrong specially in the boundary and setforce command. If someone can tell me where I am going wrong.

thank you and warm regards

Vijay Kumar Sutrakar

Input file’s details are as

dimension 3
units metal
boundary s s fs

atom_style atomic
lattice fcc 3.6150
region box block 0 5.0 0 5.0 0 20.0
create_box 1 box
create_atoms 1 box

pair_style eam
pair_coeff * * ./Cu_u3.eam

group box1 region box

region 1 block 0.0 5.0 0.0 5.0 0.0 1.0
group BCs region 1
region 7 block 0.0 5.0 0.0 5.0 1.0 50.0
group Load region 7
group mobile subtract all BCs

dump 1 all atom 100 ./dump.vel_vz

fix 1 all nvt 10.0 10.0 1
thermo 10
run 100
unfix 1

fix 1 BCs setforce 0.0 0.0 0.0
velocity mobile ramp vz 0.0 10.0 z 2.0 49.0

fix 2 all nve
timestep 0.001
thermo 100
run 5000

See the examples/crack/in.crack script for an example
of tensile pull on a sample.