What is fatigue and fatigue strength .

1. Fatigue
Fatigue is the weakening of a material caused by repeatedly applied loads. It is the
progressive and localised structural damage that occurs when a material is
subjected to cyclic loading.
Characteristics
.Fatigue failures occur when metal is subjected to a repetitive or fluctuating
stressand will fail at a stress much lower than its tensile strength.
•Fatigue failures occurwithout any plastic deformation(no warning).
•Fatigue surface appears as a smooth region, showing beach markor origin of
fatigue crack.
( Fig.) Fatigue in crankshaft

2. Factors causing fatigue failure
Basic factors
1) A maximum tensile stress of sufficiently high value.
2) A large amount of variation or fluctuation in the applied stress.
3) A sufficiently large number of cycles of the applied stress
Additional factors
• Stress concentration • Corrosion
• Temperature • Overload
• Metallurgical structure • Residual stress
• Combined stress
The S-N curve
Engineering fatigue datais normally represented by means of S-N curve, a plot of
stressS against the number of cycle, N.
•Stress can be→ σ a, σ max, σ min
.σ m, R or A should be mentioned.
•S-N curveis concerned chiefly with fatigue failure at high numbers of cycles
(N > 105cycles) →high cycle fatigue (HCF).
•N< 104 or 105 cycles →low cycle fatigue (LCF).
•N increases with decreasing stress level.
Fatigue of materials

3. In materials science, fatigue is the progressive and localised structural damage
that occurs when a material is subjected to cyclic loading. The maximum
stress values are less than the ultimate tensile stress limit, and may be below
the yield stress limit of the material.
Characteristics of fatigue
●The process starts with dislocationmovements, eventually forming persistent slip
bands that nucleate short cracks.
●The greater the applied stress, the shorter the life.
●Damage is cumulative. Materials do not recover when rested.
●Fatigue is a stochasticprocess, often showing considerable scatter. Fatigue life
scatter tends to increase for longer fatigue lives.
●Fatigue life is influenced by a variety of factors, such as temperature, surface
finish, presence of oxidizingor inertchemicals, residual stresses, contact (fretting),
etc.
Fatigue strength
Number of cycles of stress of a specific character that a specimen of material can
withstand before failure of a specific nature occurs. Number of
cycles required (usually 10 million) to cause a failure decreases as the level of
stress increases. Also called fatigue limit, it is affected by the environmental
factors such as corrosion.