A Comprehensive Study of Fatigue Failure in Materials: ME Report

This report provides a comprehensive overview of material fatigue, a critical factor in engineering failures. It begins by defining fatigue as the weakening of materials under cyclic stress and explains how it differs from static failure. The report delves into the causes of fatigue, including microscopic flaws, stress concentration, and surface defects, and how these factors contribute to crack initiation and propagation. It outlines the four stages of fatigue failure: crack initiation, crack growth (in two phases), and catastrophic failure. The report also discusses factors influencing fatigue failure, such as tensile stress, stress fluctuation range, and the number of cycles, as well as the impact of geometrical dimensions, microstructural perspectives, and corrosive environments. It explains the S-N curve and endurance limit, which are crucial for designing components with seemingly infinite life. Furthermore, the report covers fatigue testing methods (bending, torsion, and internal pressurization) and the use of software-based fatigue analysis in complex engineering designs, including the use of ANSYS for simulating loading cycles and estimating fatigue life. The report concludes with a list of relevant references.

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What is Fatigue?
Fatigue is the feeling of tiredness and reduce energy levels
while working. Fatigue is caused due to prolonged activity of
the same nature, basically by doing the same thing, in a cycle
or continuously for longer period of time. Fatigue reduces
performance of an individual, making one unable to do the
work which that person can do normally without any problem.
But does Materials also experience Fatigue?
• Yes, they do. It is seen that when materials are stressed
by fluctuating loads(cyclic stress) for long period of time
they tend to fail at a much lower value of its Ultimate
strength [1].
• It is statistically seen that fatigue accounts for nearly 90%
of all engineering failures.

What causes fatigue in Materials?
• Fatigue is observed in ductile materials with clear crack
propagation. Since brittle materials will not give much
deformation and will fail catastrophically after the crack
initiation has taken place.
• In any materials there will be microscopic flaws (E.g.
Voids, slip planes, surface defects, material inclusions
etc.).
• These flaws become region of stress concentration
when the it is subjected to some external forces.
• At this point the stress will become so high that it will
start to form microscopic voids in the material.
• Fatigue typically starts from the surface of any material,
from surface flaws, sharp corners, notches, cracks etc.

How fatigue leads to failure?
There are 4 steps of fatigue failure:
1. Crack initiation – The crack starts from zones of
stress concentrations, from surface or
interfaces.
2. Crack Growth(I) – the crack grows along planes
of high shear stress. This can be viewed as
actually extension of the slip process
3. Crack Growth(II) – the crack gradually
propagates following the direction of maximum
tensile stress
4. Catastrophic failure – finally the effective
stress bearing area reduces and the material
gives into ultimate failure.[2]

Factors influencing Fatigue failure
• 3 factors take a vital role in this:
(i) Tensile Stress,
(ii) Fluctuation range of stress values,
(iii) number of working cycles.
• Geometrical dimensions and
microstructural perspectives influence
fatigue failure. Stress concentration from
both these sources have a detrimental
impact. Residual stresses can influence as
well.
• A Corrosive domain can have a pernicious
effect on fatigue failure as well.
Fatigu
e
Failure
Magnitude of
tensile force
Variation range of
Fluctuating forces
Number of
Cycles

Fatigue life and S-N Curve
• When Fatigue stress value is
plotted against the number of
working cycle for any material we
get the S-N curve
• Endurance limit is that stress
value which lies about 50% of the
yield strength of the material
• It is important that a machine
component is manufactured with
the design stress(factoring the
FOS also) lower than the
endurance limit of the material
for seemingly infinite life.

Fatigue testing (experimental)
Fatigue life of a material is often tested in bending or torsion mode (rather than
tension/compression mode). Flexural tests are convenient. In pipes fatigue tests may
be done by internal pressurization with a fluid (varying the pressure inside between
minimum to maximum in cycles).
In torsional mode the specimen is fixed in
the chuck and either end is twisted in
opposite rotations to create moment.
This is done till the sample fails.
In flexure mode the sample is bent by
the loading point and then returned to
normal. This is repeated till failure. [3]

Fatigue Analysis (software based)
• In real engineering problems when a
component is made it is often complex in
geometry and shapes.
• Any sharp corners (often should be
filleted) becomes a zone of stress
concentration which can reduce the
fatigue life of the component.
• Such parts cannot be tested in
conventional laboratory fixtures. And
thus, engineering simulation packages are
used to simulate loading cycle as and how
applied to the part.
• Such software (Eg: ANSYS, Abaqus, LS-
Dyna etc) can find out the estimated
fatigue life of the component.
Here we see a fatigue life analysis of a pressure
vessel done in ANSYS

References
[1]. Jenkins, C., & Khanna, S. (2005). Mechanics of materials: a modern
integration of mechanics and materials in structural design.
Academic Press.
[2]. Lampman, S. R. (1996). ASM handbook: Volume 19, fatigue and
fracture. ASM International.
[3]. Manujesh, B. J., Rao, R. V., Umashankar, K. S., & Harish, S. R. (2010).
Flexural Fatigue Criterion and Failure Mechanism of Sandwich
Composites in Structural Applications. Advances n Mechanical
Engineering, 411, 340.

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