Materials fatigue is the process by which
materials fail or
fracture when subjected to
repetitive stress /
strain. Fatigue
failures are normally studied in the context of
modern or
manufactured materials, however the concepts apply to
natural materials and
fibers
as well.
History
The mechanism of fatigue was first studied when early railroad car
or engine axles failed unexpectedly. Engineers of the time had a
good understanding of simple stress failures however the resulting
designs did not take into account the then unknown mechanisms of
fatigue failure.
Mechanisms
When metals are stressed the lattice structure deforms along slippage
planes. Within the elastic limit, most of slippage is fully recovered
on unloading. However under cyclic deformations some of the slippages
are permanent and these begin to form microscopic cracks.
These small cracks can act as both stress concentrations and to
effect stress relief, depending on the details of design and
loading. At the micro-level stress cracks develop low pH at the
crack-tip which acts to
accelerate crack growth and is a factor in accelerating fatigue rate
in aqueous, saline or corrosive environments.
Design implications
Many structural metals (iron, steel, titanium exhibit a
distinct 'endurance limit' which is a level of cyclic stress
which can continue for an infinite number of cycles without causing
failure. Some materials (e.g. aluminum, magnesium) do not have
a fixed endurance limit and will eventually fail at any level of
repeated stress, no matter how low. Generally where there is a known
endurance limit, if a structure survives one million cycles, it will
last indefinitely.
The study of fatigue is a substantial branch of materials science.
Because most reciprocating or rotating machinery sees well over
a million cycles in many modes the economics of understanding
fatigue are compelling.
Historically, when fatigue was not well understood, many catastrophic
failures resulted, including for example early iron and steel ships
sometimes broke in half due to repetitive stresses due to wave and
storm motions.
Today many large structures such as bridges are inspected for fatigue
cracks. In situations where it is not practical to completely prevent
fatigue failure, the designer must accommodate the anticipate crack
growth rate, crack-size and ensure that the inspection plan
will find fatigue cracks at or above the critical size for the
design.
High-stress, low weight components, such as engine connecting rods
must be produced and maintained without transverse scratches, which
will induce early failure by creating a stress concentration where
a fatigue crack will be initiated at a lower level than in a smooth
surface.
The fatigue endurance of critical components may also be increased by
creating residual stress in the surface of the part. Usually this
is accomplished by bead blasting the surface, which leaves the
surface in a state of residual compressive stress. By pre-stressing
in compression, the effective tension stress in the surface is
reduced.