Page 66 - 4560

static load. Consumer example of this load can be repeated bending
wire segment when trying to break it. Obviously, the wire fiber in
turn finds itself in the stretched and the compressed zones (fig. 6.1).
After a certain amount of bending wire failure occurs, although the
same static steps of the load do not lead to fracture.

Figure 6.1
The phenomenon of weakening or destruction of the material
under prolonged exposure to stress that cyclically changes over
time is called fatigue.
The ability of materials and structures to resist the actions of
repeated (cyclic) loads is called endurance.
Fatigue is the result of landslides and nucleation of microcracks
in adverse oriented structural components of the material (crystal
grains in metals, fibers and matrix composites, polymer
molecular chains, etc.). Increasing, cracks merge into one trunk
crack (macrocrack). When the size of the crack reaches a critical
backbone, it begins to grow spontaneously, leading to the
destruction of parts or design.
There are two types of fatigue:
 Much cyclic, characterized by damage and destruction of
the material due to the large number of loading cycles (more
than 105) at stresses lower liquid limit;
 little cyclic is observed at relatively small number of cycles
3
5
(10 – 10 ), when the load causing plastic deformation.
For the destruction of fatigue stress is enough variable. It is
also necessary to stress had a certain value. The greatest stress that
the material can withstand without destroying almost infinite
number of stress cycles is called the limit of endurance.

66