Hi,
I’m trying to deform a CNT sample using “fix 2 all deform 1 x erate 0.1 remap x units box” where 0.1 is erate value = R
R = engineering strain rate (1/time units)
But I cannot figure out is this strain rate equivalent to a strain rate like 0.1 mm/sec or something like this? I’m using “real” units for the simulation.
What’s the actual meaning of this R? What should be the value of R if I want to 20% extension only?
Let me give you some general advice first. Whenever somebody writes a statement like “I do not know how to do this” or “I don’t understand this” or “I cannot figure this out”, then this is usually an indication that you are either not understanding your research properly or that you are not using the right approach to learn from the available documentation. In both cases, your chances to get the help you are looking for in this kind of forum are slim, since they are not problems with LAMMPS but problems on your side where you need more tutoring or training or need to invest more effort in thinking and looking for solutions.
What you should be doing, in case you have trouble to understand something from the documentation, is to set up some (very!) simple test case, make a hypothesis and see if this is confirmed. If not look at the outcome, compare to the documentation again, and see if you can explain that outcome from the documentation. If that is not consistent, you can ask the kind of question that will make it much more likely to get a meaningful response: “I tried to do X by using the command Y with settings Z (and best provide a very minimal and complete input deck that demonstrates this). Based on the documentation (and best you quote the specific text), I expected to get result A, but instead it get B”. That kind of question shows that you did your ‘homework’ and followed the documentation and you either misread something that is easy to correct or there is indeed an inconsistency in the documentation or a bug in the software and those need to be corrected.
As for fix deform and how to use it. If you had looked at the documentation more carefully, you would have seen and understood that there is only one way how it does the deformation, but multiple ways to provide that information. That means that you can specify the final dimensions after the next “run” command, or the ratio by which those should be changed after the “run” command or a rate by which the dimension is changed in every step. All of this settings will be translated internally to the same setting and hence you cannot specify multiple of those.
Thus if you want a deformation by 20% you can easily specify that and then the rate is determined by the number of time steps and the duration of a timestep. If you also want to apply a specific rate, this is also easily possible, as you have two variables: the length of a timestep and the number of timesteps and you can adjust one or both of them. This is all simple and straightforward and only requires a little bit of thinking and some elementary calculus. Once you have that figured out, it will be equally straightforward to determine which value for erate produces the same deformation.
…and voila, you have found the answer yourself and without getting a lecture about how to do research.