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Paul Joseph Wischt

Stress Strain Curve Question


Great tool! I have a few technical questions.

1. What are the units of the output Pressures? (Press, Pxx. Pyy, ...., Pyz)

2. I would like to establish a constant stretch rate to the polymer. I successfully did that by applying a strain rate. However, the resulting Pressure value fluctuate between positive and negative values. Is there a way to apply an increasing positive Force (or Pressure) in one direction?

Essentially I want a positive increasing Pressure and the resulting increasing box area so I can form a stress strain curve.




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    Alejandro Strachan


    Thanks for the interest in the tool. Ben Haley can answer the unit question better, it depends on how we setup the run and I do not remember from the top of my head. You can check this yourself, at the top of the LAMMPS output there is a "units" command that determined the overall units of the calculation and you can check in the LAMMPS website the specifics of each unit type.

    Regarding the creation of the stress-strain curve. While it is possible to use a barostat and increase pressure gradually, you will still have fluctuations and my experience is that it is best to control strain (box length) in the direction of the load. The stress will fluctuate but you should see a net increase or decrease depending on deformation and you can use these curves to extract information like stiffness or yield/flow stresses. You can perform a running average of the stress to get a smoother curve, but stress fluctuations are physical and they decrease with the square root of the number of atoms in the system, so larger systems will show smaller fluctuations.

    Hope this helps,



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    Benjamin P Haley

    Hi Paul,

       The tool uses real units in LAMMPS, so the pressure unit is atmospheres.


    Ben Haley

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