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between the axis and the field lines (fig. 1.4, b).

Bending moment assume positive if the points of the first
quadrant they cause tensile stress. Particularly in fig. 1.4, a the
bending moments M , M are shown positive.
z y
Dividing the first equation (1.1) for the second, we obtain
M
tg  y . (1.2)
M
z
The normal stress at any point (y, z) cross-section of the
beam is determined by the formula
M y M
      z  z y . (1.3)
M M
y z J J
y z
We get neutral line equation, considering it as a set of points
( y z ) section in which the normal stresses are zero. Putting in
,
0 0
0
(1.3),   we have
M M
y z
z  y  0,
J 0 J 0
y z
from here
M y J
y    z z   kz . (1.4)
0 0 0
M J
z y
This is the equation of the line with angular coefficients
M y J J
k  tg   z  z tg . (1.5)
M J J
z y y
So to build a neutral line, you need to hold it at an angle  to the
axis z (fig. 1.4, b).
From (1.5) we see that in general, the angle  is not equal
to the angle  , i.e. the neutral line is perpendicular to the plane
of action of bending moment. It can be perpendicular to this plane
only J  J (cross-section, with more than two axes of
z y
symmetry - a circle, a ring, any right polygon), so the beam cross
sections with the following concepts oblique bend graded.
Formula (1.3), taking into account relations (1.1) lead to
another species

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