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The value of the required safety factor   1,4 3n   depends on
several factors (appointment details, working conditions, the
accuracy of existing loads, etc.) and selected in accordance with
the accepted norms, standards, technical specifications or current
experience in operating details.
Safety factor n is the ratio of the boundary endurance defined for
the details  , to the nominal value of the maximum stress that
rд
occurs in the danger point details. Nominal voltage is the value
that is determined by methods of strength of materials (without
concentration, etc.).
It is very easy to determine n in case of a symmetric cycle, as
endurance border material at these cycles are known:
   
– when bent n   1д   1   ; (6.13)
 k 
a  a
   
– when torsion n   1д   1   . (6.14)
 k 
a  a
In the asymmetric cycle situation is complicated boundary
condition is characterized by two quantities: the average stress
and the corresponding boundary amplitude. To determine the
coefficients of safety we use limit stress diagram (diagram Hay).
It is built in the coordinates  -  (fig. 6.5). Inside the region
a m
bounded by the coordinate axes  ,  and curves of stress,
a m
there is no fracture in an infinite number of loading cycles.
Usually the limit stress diagram schematized. Remove an area
where the maximum stress      exceeds the yield
max m a
stress limit of the material. To do this, through the point D ,

corresponding to the liquid limit, hold a straight angle 45 (fig.
6.5), the equation which
       . (6.15)
max m a т
The initial section of the chart replacing line passing through the
two points corresponding to the symmetric limit cycle
A   0;     1  and limiting the pulsing cycle
m
a
B      0  2 . The equation of this line is of the form
m
a
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