This document discusses fatigue in steel structures. It defines fatigue as weakening caused by repeated loads and describes the typical stages of fatigue failure. S-N curves are introduced as a way to present fatigue data by plotting stress versus number of cycles to failure. Factors that influence fatigue properties include surface finish, temperature, residual stresses, and stress concentrators. The AISC steel manual specifications for designing structures to resist fatigue are summarized.

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Fatigue effects on steel
ARCH 6343
REPORT 3
ABHISHEK KOPPOLU
OUTLINE
 Fatigue Definition
 S-N Curves
 Factors effecting Fatigue properties
 Fatigue failure
 Design for fatigue
 AISC Specifications
 Examples
 Conclusions
 References

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Fatigue definition
 Fatigue is the weakening of a material caused by repeatedly
applied loads. It is the progressive and localized structural damage
that occurs when a material is subjected to cyclic loading.
 A component or structure, which is designed to carry a single
monotonically increasing application of static load, may fracture
and fail if the same load or even smaller load is, applied cyclically a
large number of times.
 For example a thin rod bent back and forth beyond yielding fails
after a few cycles of such repeated bending. This is termed as the
‘fatigue failure’
Fatigue
 Examples of structures, prone to fatigue failure, are bridges, cranes,
offshore structures and slender towers, etc., which are subjected to cyclic
loading.
 The fatigue failure is due to progressive propagation of flaws in steel under
cyclic loading.
 The fatigue failure occurs after four different stages, namely:
1. Crack initiation at points of stress concentration
2. Crack growth
3. Crack propagation
4. Final rupture

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Fatigue
Crack width and fatigue failure under cyclic load
S-N curves
 The common form of presentation of fatigue data is by using the S-N curve, where
the total cyclic stress (S) is plotted against the number of cycles to failure (N) in
logarithmic scale.

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S-N curves
 An important characteristic to this plot as seen is the ‘fatigue limit’.
 The point at which the S-N curve flattens off is called the ‘endurance
limit’ or ‘Fatigue limit’
 The significance of the fatigue limit is that if the material is loaded
below this stress, then it will not fail, regardless of the number of times it
is loaded.
 Materials such as Aluminium, copper and magnesium do not show a
fatigue limit; therefore they will fail at any stress and number of cycles.
Other important terms are fatigue strength and fatigue life.
 The fatigue strength can be defined as the stress that produces failure
in a given number of cycles usually 107.
 The fatigue life can be defined as the number of cycles required for a
material to fail at a certain stress.
Factors effecting Fatigue properties
 Surface finish:
 Scratches dents identification marks can act as stress raisers and so
reduce the fatigue properties.
 Electro-plating produces tensile residual stresses and have a significant
effect on the fatigue properties.
 Temperature:
 As a consequence of oxidation or corrosion of the metal surface
increasing, increase in temperature can lead to a reduction in fatigue
properties.

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Factors effecting Fatigue properties
 Residual stresses:
 Residual stresses are produced by fabrication and finishing processes.
 Residual stresses on the surface of the material will improve the fatigue
properties.
 Heat treatment:
 Hardening and heat treatments reduce the surface compressive stresses. as
a result the fatigue properties of the materials are getting affected.
 Stress concentrations:
 These are caused by sudden changes in cross section holes or sharp
corners can more easily lead to fatigue failure. Even a small hole lowers
fatigue-limit by 30%.
Fatigue failure
 Fatigue fracture results from the presence of fatigue cracks, usually initiated by
cyclic stresses, at surface imperfections such as machine marking and slip
steps.
 The initial stress concentration associated with these cracks are too low to
cause brittle fracture they may be sufficient to cause slow growth of the cracks
into the interior.
 Eventually the cracks may become sufficiently deep so that the stress
concentration exceeds the fracture strength and sudden failure occurs.
 The extent of the crack propagation process depends upon the brittleness of
the material under test.

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Fatigue Failure
 In brittle materials the crack grows to a critical size from which it propagates
right through the structures in a fast manner, whereas with ductile materials
the crack keeps growing until the remaining area cannot support the load
and an almost ductile fracture suddenly occurs.
 Failure can be recognized by the appearance of fracture.
 For a typical fracture ,Two distinct zones can be distinguished – a smooth zone
near the fatigue crack itself which, has been smoothened by the continual
rubbing together of the cracked surfaces, and a rough crystalline-looking
zone which is the final fracture.
 Occasionally fatigue cracks show rough concentric rings which correspond to
successive positions of the crack.
AISC Specifications
 Appendix 3 of AISC steel manual discusses about Design for fatigue.
 No evaluation of Fatigue resistance is required if the live load stress range is less
than the threshold stress range, FTH or if the number of cycles of application of
live load is less than 20,000.
 The cyclic load resistance determined by the provisions of this Appendix is
applicable only to structures subject to temperatures not exceeding 3000F
 Table A 3-1 gives the threshold stress ranges.
 Table A3-1.

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Direct stress range FSR

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Design for fatigue
To secure satisfactory fatigue life
 Modification of the design to avoid stress concentration eliminating
sharp recesses and severe stress raisers.
 Precise control of the surface finish by avoiding damage to surface by
rough machining, punching, stamping, shearing etc.
 Control of corrosion and erosion or chemical attack in service and to
prevent of surface decarburization during processing of heat
treatment.
 Surface treatment of the metal.
Conclusions
 Fatigue stresses are material and time dependent stresses and are need
to be considered in order to safe guard the structure.
 The shape of the structure will significantly affect the fatigue life.
 The greater the applied stress range, the shorter the life.
 Design to keep stress below threshold of fatigue limit.

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References
 AISC Steel manual
 http://www.asminternational.org/documents/10192/1849770/05224
G_Chapter14.pdf
 http://nptel.ac.in/courses/105106112/1_introduction/7_Fatigue.pdf
 https://www.engineersedge.com/material_science/fatique_failure.
htm