  It was experimentally confirmed that if steel cyclic stress reduces to less
than tensile yield stress values, i.e. in case of high-cycle fatigue, the
mechanism of fracture changes from dislocation to vacancy one. This conclusion
was based on the fact that steel density determined by the method of liquid
displacement is less than that of steel in both initial condition and after
fracture under the cyclic loads exceeding tensile yield stress values. In the
latter case steel hardness increases, whereas the steels fractured under the
cyclic stresses less than their tensile yield stress values show no change in
hardness. It means that in such a case metal fractures without strain
hardening, i.e. undergoes brittle fracturing developing by vacancy mechanism
rather than by dislocation one. As a result such steel obtains the structure
and properties similar to those appearing after its exposure to radiation, i.e.
friability and brittleness. The results obtained allow us to explain the
presence of a horizontal section in the W\"ohler fatigue failure diagram
