Hello everyone,
I have built a single layer graphene sheet in Materials studio and converted in LAMMPS file. My goal is to test uniaxial tensile strength of graphene. At first I used fix deform command to stretch the simulation box in x-direction. But the graphene continued to stretch untill when the box length is equal to the twice the initial box length and the strength is around 200 GPa. I stopped the simulation after that and the sheet still did not fracture. But from the literature we know that fracture stress of graphene is around 120 GPa and strain is 0.18-0.27. So, my simulation setup is clearly wrong.
So, now I am trying to set one layer of atoms in the graphene sheet fixed. I used region command in LAMMPS to fix the atoms but LAMMPS showed me an error saying I have to use create_box command first. But I am reading the data file from read_data command. I need an advice about how to fix a portion of atoms when I am creating the box from read_data command. I am using CVFF potential, not tersoff or AIREBO potential.
I can send the LAMMPS input file if required.
EDIT:
Actually, I could break the graphene sheet by changing the bond stretch potential from harmonic to morse. But still, the stress is around 25 GPa, which is far lower than published values. However, the failure strain is around 0.27, which seems accurate. What can cause this low value of stress?
Your question is quite long. Let me give you some hints and if you want please feel free to send me the script through a private message.
After your read_data, you need to define a lattice, it can be as simple as the typical
lattice sc 5.0 orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
Then you need to define a region
region sheet block 0.0 100.0 0.0 100.0 -5.0 5.0
Then on that region, which should include all your graphene sheet, you should create two subregions as an example
region R2region cylinder z 1000 1000 740 -50 50 units box
region R3region cylinder z 1000 1000 750 -50 50 units box
you need to create groups
group R2 region R2region
group R3 region R3region
then you will fix just the slice between those two subregions by defining a new group
group fixedboundaries subtract R3 R2
and finally over that region you will impose zero force, zero speed
fix 4 fixedboundaries setforce 0.0 0.0 0.0
velocity fixedboundaries set 0.0 0.0 0.0
If you want, send me the script through a private message.
Hello Carlos,
Thank you so much for replying. I have included the code that you sent me in my code. But still the stress is very low. I have sent you the code in personal message.
I could solve my issue with stress-strain calculation with graphene. Thank you Carlos for helping me.
Hello Doan,
Sorry for the late answer. I used 3.35 angstrom thickness for graphene simulation. That is well tested from experiment and also from simulation.
region fixleft block INF INF INF INF ${lim1} ${lim2} units box
group fixleft region fixleft
velocity fixleft set 0 0 0
fix fixleft fixleft setforce 0 0 0
Hello abdullah can you mail me your code for generating a graphene structure
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