Help: Indentation Force Starts High from First Step in LAMMPS

LAMMPS LAMMPS General Discussion
1

Hi everyone,
I’m performing a nanoindentation simulation in LAMMPS using a virtual spherical indenter on a metal surface. The problem is — from the very first timestep, the indentation force (fy) value starts very high, instead of gradually increasing after the indenter contacts the surface.
I already tried:

  • Increasing the simulation box height (e.g., ly + 20)
  • Adjusting indenter position above 20 Å of the surface
  • Checking boundary and group definitions

I’m using fix indent command.
Still, the initial force is large.
Can anyone suggest what might cause this issue or how to ensure the indenter begins above the surface with zero initial force?

Thanks in advance!

The script is here:
alloyin.lmp (3.2 KB)

2

The only way this can happen is when the indenter already overlaps with atoms from the first step.

3

I know that. For this reason I placed the indenter some distance of the substrate. I also provide my code. How can i fix that? I don’t want overlap substrate atom at the very first during indentation process.

4

I just opened your script, and it seems to me that you are going too fast trying to perform indentation, which is a relatively difficult simulation, without understanding the basics of MD. For instance, I saw that:

fix             relax all langevin 10 10 0.1 904297
fix             nve_relax all nve
fix		        4 all temp/rescale 100 10 10 1 1

and a bit later :

fix             npt all npt temp 10 10 0.1 iso 0.0 0.0 1.0
fix		        8 all temp/rescale 100 10 10 1 1

while also using all along :

fix		2 lower setforce 0.0 0.0 0.0

This is a mess, and this forum is not the right place to help you, unfortunately. You need a proper tutor.

Simon

5

I’m at my learning platform now. If you don’t mind please give me some outline and guidance that I can be able to solve my mistakes. I will be very grateful to you.
Thank you.

6

I would start by following at the very least the first four tutorials from lammpstutorials.github.io, and then by reading the book of Daan Frenkel (Understanding Molecular Simulation) to understand better what is behind the most important LAMMPS commands.

7

Thank you for your github tutorial. I saw your tutorial. Can you tell me for build a smooth nanoindentation script where from should I start? I saw it full. Give me further information. Thanks in advance.

8

It looks to me, that you are not getting the point that @simongravelle is making here. So let me repeat: your posted input shows a massive lack of understanding for fundamental MD simulation knowledge (and a dangerous ignorance towards warnings printed by the software, too) and thus you should first teach yourself the basics and learn them properly. Just looking at a tutorial is not sufficient, you have to work through all the steps and read up on the parts and commands that you don’t understand until you do. Once you have accumulated a sufficient amount of fundamental knowledge, you don’t need a “template”, but have the knowledge and confidence to write a suitable input from scratch with additional information found in the documentation and examples input decks for similar setups. In the end, you should put every command in your input for a reason because only then you can be certain that the simulation is doing what it should be doing. …and if it does not produce the expected output, you can easily step back and check each step and find out where things go wrong.

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