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What is Fatigue Testing?

1. From Latin "Fatigare" meaning "to tire."

2. Engineering terminology: - damage and failure of materials under cyclic loads.

3. 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.


Nomenclature to describe the test parameters involved in cyclic loading
Fatigue is the progressive, localized, permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fractures after a sufficient number of fluctuations. The cyclic stresses are normally well below the yield strength of the material.

 

Typical Fatigue Test Waveform Graph


The process of fatigue consists of three key stages:


1. Initial fatigue damage leading to crack nucleation and crack initiation,

2. Progressive cyclic growth of a crack (crack propagation) until the remaining un-cracked cross section of a part becomes too weak to withstand the loads applied,

3. Final, sudden fracture of the remaining cross section.

 


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
environment.

5. Fretting Fatigue - Cyclic loads combined with frictional sliding.
Fatigue Endurance Limit and Fatigue Strength

Fatigue Endurance Limit Graph

 

 

Cyclic loading generally produces failure however low the stress may be. However, with some materials the S-N curve levels off, suggesting that for these materials a limit of stress (load) can be specified - known as the fatigue limit - below which infinite life can be expected.


The fatigue life is thought to be associated with the phenomenon of strain ageing.
Most non ferrous alloys do not show a fatigue limit. Instead their S-N curves continue to drop at a slow rate (dotted line).


For these types of materials, the fatigue strength is quoted. This is the value of stress to which the material can be subjected to for a given number of cycles (10,000,000 cycles is the value often used).
Strain-Life Approach

Low cycle, high stress fatigue with appreciable plastic deformation. Uses the cyclic strain range versus number of
cycles to failure.

Total life = crack initiation + crack propagation (90% of
life can be crack initiation).

Failure = Typically a crack of predefined size.


The strain-life relationship is as follows:

Fatigue Strain life formula
 

 

 


Where the constants in the equation are:


sf’ is the fatigue strength coefficient


E is the elastic modulus (Young's Modulus)


b is the fatigue strength exponent (Basquin’s exponent)


ef’ the fatigue ductility coefficient and c is the fatigue ductility exponent (the Coffin-Manson exponent)

 


Stress-Life Approach


High cycle, low stress fatigue.
Material deforms elastically.
Cyclic stress range vs number of cycles to failure (S-N Curve)

Total life = crack initiation + crack propagation

Failure = Total separation of specimen

First fatigue design method. Large amount of data available.
Damage-Tolerant Approach
Assumes defects present in material.

Linear Elastic Fracture Mechanics approach ( da/dN vs delta K)
Resistance to Fatigue Crack Growth.

Useful Fatigue life = number of cycles to propagate a crack from an initial size to some critical dimension.

Failure = critical crack size based on fracture toughness of material, limit load for particular structural part, allowable strain, change in compliance of a component etc.

 

 

 

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