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[Question #689676]: coordinate system, local or global


New question #689676 on Yade:

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?



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