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1
   . (4.4)
2
On the other side, the diagonal elongation is caused by the
action of the principal stresses, we find from the formula (3.25):
1
          ,
1  1 2 3 
E

or taking into account the relation (4.2), we obtain

    1   . (4.5)
1
E
Equating the right parts (4.4) and (4.5), we obtain:
E
   . (4.6)
 2 1  
E
We introduce the notation G  . The value G is called
 2 1  
the shear modulus or modulus of the second kind. Shear modulus
of the material is a physical constant that characterizes its ability to
resist elastic deformation in shear.
The relationship between  and  is called Hooke’s law in
shear
  G . (4.7)
Substituting formulas (4.1) and (4.3) in Hooke's law (4.7), we
obtain
Qh
 S  , (4.8)
GF
that force of shear is directly proportional to absolute shear and the
distance between the planes and shear stiffness is inversely
proportional to the shear GF .

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