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Imaginary element will be cut out with the size dl and dl (fig.
m t
9.1), by using two meridional and two normal adjacent sections.
On the faces of the element normal stresses will operate:  -
m
longitudinal or meridional stress  - cross or circular tension.
t
Denote  the radius of curvature of the arc of the meridian, and
m
after  - the radius of curvature of the normal section.
t
Construct the equilibrium equation of the selected element shell.
To do this, all the forces acting on it, will design the direction
normal n n to the surface element
d d
pdl  dl  2  dl sin m  2  dl sin t  0 . (9.1)
m t m m t t
2 2
Consider the smallness of the size of the item, we take:
d d d d
sin m  m ; sin t  t ; (9.2)
2 2 2 2

dl dl
d  t , d  m . (9.3)
m t
 
m t
Substituting the representation (9.2) and (9.3) in the equilibrium
equation (9.1), we obtain
dl dl
pdl dl    dl t    dl m  0. (9.4)
m t m m t t
 
m t

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