# Question on indentation forces

Hi I have tried to compare two indentation cases in (i) by using a rigid diamond indenter and by using a realistic potential function of Carbon and in (ii) by using an imaginary indenter using fix indent command with a function F® = K(r-R)2 where K is the force constant (1 KeV/Å3).

I obtained the indentation forces on the indenter in both cases. However, while comparing them, I noticed the indentation forces in the former case to be double of the later in each case. I couldn’t think of an explanation for this though? Any suggestions will be appreciated!!

SG.

Hi I have tried to compare two indentation cases in (i) by using a rigid
diamond indenter and by using a realistic potential function of Carbon and
in (ii) by using an imaginary indenter using fix indent command with a
function F(r) = K(r-R)2 where K is the force constant (1 KeV/Å3).

I obtained the indentation forces on the indenter in both cases. However,
while comparing them, I noticed the indentation forces in the former case to
be double of the later in each case. I couldn't think of an explanation for
this though? Any suggestions will be appreciated!!

Typo doesn’t seems to be the case, only the ensemble was different, and the methodology was simply as follows:

Case 1 in NVE ensemble

region tool sphere 100 185 100 30 units box
fix forces tool setforce NULL NULL NULL
variable wfy equal f_forces[2]
variable force equal (v_wfy)*1.602 #in nN

Case 2 in NVT ensemble
fix 4 moving indent 1000.0 sphere 100 185 100 30 units box

variable force equal f_4[2]*1.602 #in nN

Perhaps you should demonstrate to this list first why do you expect both methods to render the same results. And I don’t mean by just saying “I think that should be the case” but by providing convincing arguments that show people here it is worth investing some time thinking about your issues because you have all other potential problems under control in your input and setups.

Carlos

Carlos, I don’t really the notion with what you want me to reply for. I already wrote, "I couldn’t think of an explanation for this though? " I didn’t said whether they should be same or different. Its my query and in my understanding, fix indent is a purely repulsive potential whereas in case of a rigid diamond indenter, I am not sure whether repulsion and attraction are added together or not? But in both cases, it makes sense that when a material is indented under the same conditions, it should ideally indicate the same indentation forces.

Carlos, I don't really the notion with what you want me to reply for. I
already wrote, "I couldn't think of an explanation for this though? " I
didn't said whether they should be same or different. Its my query and in
my understanding, fix indent is a purely repulsive potential whereas in
case of a rigid diamond indenter, I am not sure whether repulsion and
attraction are added together or not?

Ok so if you are not sure why not starting with more simple cases that you
may be able to troubleshoot better? Avoid the Hertzian spherical tip and
start with a simple plane indenter for example. The plane geometry can be
very easy to compute analytically. This exercise will answer your second Q
above.

But in both cases, it makes sense that when a material is indented under
the same conditions, it should ideally indicate the same indentation
forces.

Ahhh, the MACRO-intuition argument... Well, you are working at the
nanoscale where many other effects could play roles that will manifest
differently even for very similar initial conditions and systems. But then
first ask (and prove) yourself if the repulsive continuum indenter maps the
diamond one in the MACRO limit.
Carlos

Carlos, I don’t really the notion with what you want me to reply for. I already wrote, "I couldn’t think of an explanation for this though? " I didn’t said whether they should be same or different. Its my query and in my understanding, fix indent is a purely repulsive potential whereas in case of a rigid diamond indenter, I am not sure whether repulsion and attraction are added together or not? But in both cases, it makes sense that when a material is indented under the same conditions, it should ideally indicate the same indentation forces.

But there is no proof that you do have the same conditions except your say-so. So the possibility of a typo or a badly designed input remains.

And that is exactly what you asked for: possible explanations.

Axel.