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Figure 6.4 - Wohler curve

Load these samples gradually reduced. Obviously, each less
loaded sample  , , ,...  will withstand the increasing
II III IV
number of cycles to failure N , N , N ,...  . Processed data
II III IV
obtained building fatigue curve (Wohler curve). Carried to the
fatigue curve (fig. 6.4) horizontal asymptote cuts on the axis 
of the segment, which is the border of endurance  .
 1
Generally, to determine whether  or  certain conduct when
r r
the base test (number of cycles) above which does not lead to
the destruction of the sample. For steel it is 107 cycles, and for
8
non-ferrous metals – 10 .
Based on the large number of tests established close relationship
between the endurance limit in bending and in stretching the
boundaries of endurance – compression  and torsion  :
 1p  1
  0,7 ;   0,58 .
 1p  1  1  1

6.4 The main factors affecting the fatigue strength

From experiments it is known that the fatigue strength
significantly affect stress concentration, size and condition of the
workpiece surface.
Stress concentration - a phenomenon local increase in stress areas
near the abrupt change in the size of parts, holes, recesses,
notches, etc. Apart form the details on the impact fatigue
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