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Fatigue effects on steelARCH 6343REPORT 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