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The development of the theory of endurance is mainly through the
accumulation and systematization of experimental data. Today we
can count on endurance rather small list of parts, however, those that
are often found (shafts, gears, thread connection, etc.). For the
calculation of new devices and systems we have to perform full-
scale fatigue test.

6.2 Cycles strains and their characteristics

Variable voltage may have established and unstable conditions.
At steady state stress variability over time is periodic. After a
certain period (cycle) time is an exact repetition of the values of
stresses.
The set of all values of stresses during one period is called a
cycle.
Effect of waveform changes in stress and frequency on the
fatigue strength of parts is negligible. The main role is played by
the maximum and minimum values of the stress cycle and their
relation.
Cycle variable stress (fig. 6.2, a) is characterized by:
1) maximum stress  ,
max
2) minimum stress  ,
min

Figure 6.2

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