# fix box/relax : circumferential strain vs. axial strain

Dear All,

I am trying to calculate circumferential strain and axial strain of CNT under hydrostatic pressure (using fix box/relax then minimization).

My question is how to print/calculate circumferential (radial) strain of the tube due to hydrostatic pressure?

Many thanks and looking forward your help.

Rajib

Hi,

If the CNT (cylinder) is inside the computational box, and your
applying hidrostatic pressure, then you can meassure the change in
length along the "x" and "y" directions. Then calculate the radius
using the classical Pythagorean theorem (x^2 + y^2)^(1/2) . The
circumferential strain (or radial strain) would be the difference
between the radius before and after the deformation divided by the
initial radius i.e (Rfinal - Rinitial )/Rinitial. I belive you can
calculate this quantity "x" , "y" by using the "fix thermo" , and
using those values then you can define an evaluation function inside
the lammps script .

There may also exist a Tangential component (Shear), but since its
hidrostatic pressure my guess is that only a "radial" and "axial"
deformation must occour

ps: My solution are base on pure intuition

Oscar G.

Hi Oscar,

I was doing similar thing, however, changing simulation box dimensions changes the final strains. Any idea how to solve this?

Rajib

Hi,

Not sure what you mean by "final strains", maybe someone else will be