The document discusses fatigue testing and the phenomenon of material fatigue, which causes failure due to repeated cyclic stresses below a material's ultimate strength. It outlines types of fatigue failure, stages leading to crack propagation, and the importance of conducting fatigue tests to determine the fatigue limit. The fatigue limit, or endurance limit, is defined as the stress level that allows infinite loading cycles without failure.

1. Fatigue Testing (DT)
By -PAWAN KUMAR
PGLF2150056

2. Fatigue
• Fatigue is the condition whereby a material cracks or
fails because of repeated (cyclic) stresses applied
below the ultimate strength of the material. Fatigue
failure often occurs quite suddenly with catastrophic
result.
• Material fatigue is a phenomenon where structures
fail when subjected to a cyclic load.
• Examples of where Fatigue may occur are: springs,
turbine blades, airplane wings, bridges and bones.

3. • Failure of material under fluctuating stresses at
a stress magnitude which is lower than the
ultimate tensile strength of the material called
fatigue failure
• .Fatigue result is brittle appearing fracture,
without any deformation at the fracture.
• It particularly occur without any warning
• It is generally occurs in turbines, aircraft etc.
• A fatigue failure always starts as a small crack
and grows in size under repeated loading of the
stress

4. Types of fatigue failure
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.

5. Stages of fatigue failure
• 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.

7. • Stage of failure

8. Determination of fatigue limit
During fatigue the stress may alternate about
zero, may vary from zero to a maximum or may
vary about some value above - or below - zero.
To quantify the effect of these varying stresses
fatigue testing is carried out by applying a
particular stress range and this is continued until
the test piece fails. The number of cycles to
failure is recorded and the test then repeated at
a variety of different stress ranges.
This enables an S/N curve. This graph shows the
results of testing a plain specimen or a welded
component. The endurance limit of the plain
specimen is shown as the horizontal line - if the
stress is below this line the test piece will last for
an infinite number of cycles. The curve for the
welded sample, however, continues to trend
down to a point where the stress range is
insufficient to cause a crack to propagate from
the intrusion.

9. Determination of fatigue limit
• The fatigue limit, also
known as the endurance
limit or fatigue strength,
is the stress level below
which an infinite number
of loading cycles can be
applied to a material
without causing fatigue
failure

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