# Stress vs Strain

When I assign a velocity to a wall of cube I can see the necking and failure.
But when I use fix deform command even though if I set final strain to 5…!!! there is no necking.
Can I conclude that fix deform is for tiny strain values and estimates that there is no necking in tiny strain values.
Please take a look at the way I calculate stress vs strain.

fix 2 all deform 1 z vel .32 x volume y volume remap x
variable sigmavolume equal c_fz[3]vol10^-5
variable Lzero equal 32.4
variable strain equal (lz-v_Lzero)/v_Lzero
compute perfz all stress/atom pair
compute fz all reduce sum &
c_perfz[1] c_perfz[2] c_perfz[3] &
c_perfz[4] c_perfz[5] c_perfz[6]

thermo 50
thermo_style custom step lz vol v_strain v_sigmavolume c_fz[3]

Thanks.

The strain rate is an input to fix deform, so that's up
to you. Fix deform applies the strain homogenously
to the entire simulation box. Just applying a velocity
to atoms at the ends of a sample is different and typically
a more harsh change to the sample.

Steve

2011/4/15 Reza <[email protected]...>:

Reza, there have been many simulation studies of this sort of problem - that have looked into the details of what happens when you apply different deformation protocols. I’d suggest a Web of Science (or pick your favorite online paper-database) search, with various keywords, to look and see what others have done. That’ll give you a lot of guidance. It can be time consuming, but people do this sort of thing all the time, and probably more useful than asking the question on this list.

Best,
Rob