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x 
 X  N   P  0  M i x  M  M x   0P  
i
i x xi 
 
 Y  Q   P  0 ;
y 
i y yi   M i y  M  M y   0P   (1.1)
i

 Z  Q   P  0 
i z zi 
 M i z  M  M z    
z 
0
P

i
If the object of study is the core of x axis, the force N is called
x
longitudinal and moments M , M and M - twisting and
z y x
bending, respectively.

1.6 The concept of stress

In order to establish the law of distribution of the internal forces
of elasticity in the body section, the concept of degree of intensity
– stress is introduced.
In the neighborhood of an arbitrary point K of the body section
let choose an elementary area A (fig.1.10, a). The ratio of the
resultant internal forces of elasticity R , resulting in a unit
platform, to the area of this platform  is called the average
A
stress in the neighborhood of K. In levying the platform A we get
the true stress S at the point K (fig.1.10, b)
 R
S  lim .
 A 0  A
Stress - the value of a vector that is characterized by quantity
(module), direction and point of application. Unit of measure of
stress is  S  Pa .

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