Complementary Shear stress

When a body is subjected to shear stress then to keep body in equilibrium a another resisting shear stress is developed on anther surface of body. This stress in form of couple. Both couple forms opposite direction to each other to kept body in equilibrium. This resisting shear stress is called as "Complementary Shear stress".

In above fig.
q = applied shear stress.
q' = developed complementry shear stress.