What is Fatigue ?
Fatigue the word is derived From Latin "Fatigare" meaning "to tire."Fatigue is engineering term which is the weakening or damage and failure of a material caused by repeatedly applied loads.The fatigue strength or endurance limit of material is used in the design of parts subjected to repeated alternating stresses over an extended period of time.
The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.. 90 percent of machine components fail due to fatigue.For Example It is difficult to break a wire by stretching but if we apply a cyclic load and bend unbend the wire a number of times it breaks easily.

What is Fatigue Test ?
Fatigue testing is defined as the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations.

Specimens are tested to failure using different loads. The number of cycles is noted for each load. The results of such tests are plotted as graphs of applied stress against the logarithm of the number of cycles of failure. The curve is known as S-N curve. The tests are carried out on special fatigue testing machines.

Types Of Fatigue Failure:-
1. Mechanical Fatigue - fluctuations in externally applied stresses or strains.
2. Creep Fatigue - Cyclic loads at high temperatures.
3. Thermo mechanical Fatigue - fluctuations in temperature as well as
stresses and strains.
4. Corrosion Fatigue - Cyclic loads in a chemically aggressive or embrittling
5. Fretting Fatigue - Cyclic loads combined with frictional sliding.
Fatigue Endurance Limit and Fatigue Strength.

Stages of Fatigue :-
1.Crack initiation:- Micro cracks in the structure of the material are initiated or already existed in the structure due to flaws, impurities or small surface notches grow into macro cracks.
2.Crack propagation:- Macro crack grows steadilyunder repeated load.
3.Unstable crack growth:- Crack grows at a very rapid rate and failure occurs when the net section is too small to resist the repeated fatigue load.

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