New question #689676 on Yade:
https://answers.launchpad.net/yade/+question/689676

Hi All,

I want to get the macroscale stress of a local area in a sample. 
Here are my thoughts.
How to relate the macroscale stress with the microscale stress? 
I'm thinking to use the idea of the representative volume element, something 
like the measurement sphere in PFC.
sigma(macro) = 1/V*sigma(per-particle) 
because in YADE, we can get the per-particle stress for each particle[1-2]. so 
it's very easy to get the macroscale stress for a local area. (#### although we 
can use the equation: sigma(macro)= 1/V*f*l,where f is the contact force, l is 
the branch vector.#####)

In[1], it mentioned body stress has three principal stress and the associated 
principal directions.
In order to use the equation above, I need to add the stress in each direction. 
I don't know whether the principal directions were in the local coordinate 
system or the global coordinate system. If they are in the global coordinate 
system the equation would be easy to calculate. If they are in the local 
coordinate system, it will be very complex. (convert all the coordinate into 
global system and then calculate the macroscale stress)

my question is: how can I know the principal directions is in the global or 
local system?

thanks,
Yong


References:
[1]https://yade-dem.org/doc/yade.wrapper.html?highlight=recorder#yade.wrapper.VTKRecorder
[2]https://yade-dem.org/doc/yade.utils.html?highlight=bodystress#yade._utils.bodyStressTensors

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