i have square rve with 2 reference point for X and Y dir. I applied displacement BC to ref point 1 and 2 to perform shear test.

using XYdata i got reaction force RF0 for reference point2 of Ydir, area=1,so tau= RF0, or 

should i add the RF0 of reference point X also to get tau

and how to get shear strain from the displacement data of reference points