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  
3) average stress   max min , (6.1)
m
2
  
4) amplitude cycle   max min , (6.2)
a
2

5) cycle asymmetry coefficient r  min . (6.3)

max
From formulas (6.1) and (6.2) we see that
     , (6.4)
max m a
     . (6.5)
min m a
In case, if    and     we have symmetrical stress
max min
cycle (fig. 6.2, b). Here with   ,      , r   .
0
1
m a max
Cycle stress, as shown in fig. 6.2 c, called the zero (pulsating). In
this case,
 
0
   ,   0 ,     max  , r  .
max min m a
2 2
Constant static stress (Fig. 6.2, d) can be formally considered as a
particular case of an alternating series of characteristics
0
       ,   , r  1.
max min m a
Designations  are usually used when talking about the normal
stresses under cyclic stretching-compression or bending. For
example under cyclic torsional letter  should be replaced 
while maintaining the underlying indices.
The boundaries of endurance  or  , indicate where the index
r r
r corresponds to the ratio of asymmetry cycle (e.g., symmetric
cycle  ,  ; with the zero – ,  ).
 1  1 0 0

6.3 Methods for determining the boundary of
endurance. Curve fatigue

Value border endurance depends on the type of deformity,
physical and mechanical properties of the material cycle
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