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FATIGUE MECHANICAL LIFE
Fatigue is due to cyclic loading and
unloading of one kind or the other.
It is due to the presence of discontinuities
in the material. Mostly fatigue failure is
progressive and plastic in nature. It is due
to the nucleation, growth and propagation
of a micro crack at the point of a
discontinuity. Plain low carbon steels have
unlimited fatigue life. Nonferrous &
ferrous materials have limited fatigue life.
Fatigue is mostly due to tensile stresses
and is random as well as sudden without
any warning. 90 % of the service failures
are due to fatigue. There is lot of
information on fatigue failures. It is being
continued because of complex nature of
fatigue failures. This results in loss of life
and property. Avoid fatigue failures by a
proper selection of material & surface
finish. Stress raisers, residual stresses,
reliability, surrounding environment and
temperature affects fatigue. Fatigue is due
to cyclic loading and unloading. However,
the fatigue reduces by proper selection of
fatigue resistant material like composites.
Further, fatigue also decreases by drilling a hole at
the point of a probable crack. Use of laser
peeing and high frequency mechanical
impact (HFMI) treatment of welds reduce
fatigue. Use stress strain fatigue life
approaches for plastic and elastic
deformations respectively. This short
review paper cannot treat the vast subject
thoroughly. The reader may consult
consult more references for additional
Under cyclic loading and unloading, failure is due to fatigue. Fatigue/endurance limit (σe) represents a stress level. Below which the material does not fail even after infinite number of cycles. Fatigue is reduction in strength due to a progressive and localized structural damage. Fatigue takes place in a moving component only. For example, in automobiles, ships , aircraft wings and nuclear reactors, jet engines, and turbines.
First time, fatigue became known as early 1800 in Europe. It was observed that bridge and railroad components were cracking subjected to repeated loading[1-10].
Three basic factors to cause fatigue are
(i) a sufficiently high tensile stress
(ii) a large variation in the applied stress
(iii) a sufficiently large number of repetitions in loading and unloading.
The nominal maximum stress which causes fatigue is much less than the ultimate tensile strength of a brittle material. It is less than the yield stress of a ductile material. If the stress present is above a certain threshold value, microscopic cracks will start at the points of stress concentrations. For example, like a scratch, key way, square holes or sharp corners. The crack then travels along weaker points. Ultimately it results in a fracture. Fatigue is thus a progressive plastic failure. This phenomenon occurs in three phases namely
(i) crack initiation
(ii) crack propagation
(iii) catastrophic overload failure
There are two types of materials experiencing fatigue. One type which has a fixed endurance limit as plain low carbon steels. These steels do not undergo fatigue even for infinite life. It happens when the actual stress present in the component is slightly less than the fatigue limit. There are also brittle or ductile materials which do not have a fixed fatigue limit. For example, Cast iron, Copper, Aluminum and their alloys. The design for such materials for a fixed number of cycles 5 x 108(500 million cycles). If the component has 750 RPM with one reversal per cycle, it will have a life of about four years. If the RPM increases, life will reduce [1-16].
Thus importance of fatigue is that it directly governs the useful life of a component under cyclic loading. Continuous research is there on fatigue. It is because number of well-known catastrophic fatigue failures which took place all over the world. Avoid fatigue failures by a proper selection of material. Surface finish, stress raisers, residual stresses, reliability, surrounding environment and temperature help in the selection of material.
Salient features of fatigue include randomness and sudden failure without any warning. It is mostly due to tensile stress and the presence of a stress raiser. There is a strong affect of the surrounding environment & temperature. Surface finish and residual stresses also affect it. Fatigue reduces by proper selection of fatigue resistant material like
(ii) drilling a hole at the point of a probable crack
(iii) use of laser peeing and use of high frequency mechanical impact (HFMI) treatment of welds.
Out of different fatigue design approaches, stress life and strain life has been used for plastic and elastic deformations respectively [17-24].