Law of the shearing stresses couple
Let us determine normal and shearing stresses at two mutually perpendicular planes. For the inclined under
For the mutually perpendicular plane under To study the material mechanical properties irrespective of the specimen dimensions a diagram in the coordinates “stress - deformation” Applying the formulae (2.3) and (2.4), we get
Analyzing the obtained results we can see that, firstly,
i.e. the sum of the normal stresses in two mutually perpendicular areas is constant and equal to the principal stress, secondly,
So, at two mutually perpendicular areas shearing stresses (law of the shearing stresses couple) equal by value and reverse at their signs act. The law of shearing stresses couple (reciprocation) is valid not only for the monoaxial, but for any other tensed condition: biaxial and volumetric.
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angle plane by the formulae (2.3) and (2.4) we have:
angle value, we can determine the normal and shearing stresses either from the equilibrium condition of the upper or lower bar parts (Fig.2.6 c) or by the formulae (2.3) and (2.4) replacing 
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is applied. The diagrams in question differ only by scales.
