I am running some simple tensile tests on an elongated (200x10x5) specimen with fix deform in the x. I was testing to see if there were any stress concentrations at the root or tip of the elongated sample. Given that the boundary conditions are periodic I would not expect concentrations. To check for concentrations I create three regions (root, middle, tip) and get the px term in each region, divided by the volume, to get the stress in that region. I think get the slope of stress, strain to get a modulus in each region:
This averages to a modulus of 141 (based on the end regions being 10% of the volume each), which is close to the total modulus using pxx. But why am I see concentrations at the root and tip? Is this boundary effects? I think not as it is periodic, but still confused.
Hi Liam, I’m not familiar with the EAM pair style’s implementation. But my guess is that because EAM is a many-body potential, the virial is computed not from the individual forces, but using the total force on each atom, \vec{F} \otimes \vec{r}. This would end up favoring the contribution of the atoms near the boundaries of the unit cell, where |\vec{r}| is largest. The total virial stress of the system would be correct, but this is one of the cases where computing a local virial contribution from a region may not be so useful.
The PBC or absolute atom positions should not have any effect on per-atom stress. The reason is that EAM potential is a function only of relative positions. We explain this further in the associated paper (see compute stress/atom doc page).
The small effect you are seeing may be statistical fluctuations or it may be an artifact of how you are driving the system.
I suggest doing a test of a fully equilibrated case, no fix deform, remove all unnecessary stuff. Use ave/chunk to generate 1D stress profiles.
This makes sense Dr. Thompson. I did a test without the deform and fixed my region command and it cleared up the findings. The results are now as expected.