The document outlines the history and mechanisms of fatigue testing as a failure mechanism since the nineteenth century, particularly noting its significance in rail travel and rotating components. It discusses how fatigue failure occurs under cyclic stress, the factors that influence fatigue life, and the necessity for fatigue testing in design to predict service life accurately. Additionally, it highlights the relationship between stress cycles and material performance, emphasizing that higher strength materials do not necessarily improve fatigue life.

1. Fatigue Testing

2. History Regarding Fatigue
• Fatigue as a specific failure mechanism
recognized since the early part of the nineteenth
century.
• The development of rail travel that resulted in a
major increase of interest in this type of fracture.
• The premature failure of wagon axles led to
investigating fatigue failure under rotating
loading.
• This led to the design of the first standardized
rotating fatigue test.

3. Wohler rotating fatigue test

5. Fatigue Testing
• Failure under cyclic or repeated stress.
• The value of stress is lower than the static
stress required to cause fracture.
• It occurs in metals and non metals a like.
• Generally characterized by local crack
propagation.
• The component often showing no sign of
failure before the final fracture.

6. • Most fatigue failure occurs in relatively
common components such as gear teeth,
crankshaft, axel and so on.
• Catastrophic failures causing extensive
lives.
• In practice ninety percent of all services
failures are due to fatigue.
• Most of failures are due to poor design.

8. • Data obtained under constant load are
normally plotted on S-N curve.
• S is the amplitude of alternating stress.
• N is the No. of cycles to failure.
• Two types of S-N curve
• Fatigue limit and endurance limit.
• Better to draw S-N-P graph.

12. Variables affecting fatigue life
• Amplitude of the stress cycle:
• Surface condition:
• Effect of T: High fatigue strength at low T
• Frequency of stress cycle: little effect on
fatigue life. lowering frequency reduced
fatigue life.
• Environment:

13. • Failure can occur at a fluctuating load well
below the yield point of the metal and
below the allowable static design stress.
The number of cycles at which failure
occurs may vary from a couple of
hundreds to millions. There will be little or
no deformation at failure and the fracture
has a characteristic surface, as shown in
Fig.2.

14. • Why Do a Fatigue Test?
• In many applications, materials are subjected to vibrating
or oscillating forces.
• The behavior of materials under such load conditions
differs from the behavior under a static load.
• Because the material is subjected to repeated load
cycles (fatigue)
• in actual use, designers are faced with predicting fatigue
life, which is defined as the total number of cycles to
failure under specified loading conditions.
• Fatigue testing gives much better data to predict the in-
service life of materials.

15. • It should be mentioned that, in service, few structures
experience purely static loads and that most will be
subjected to some fluctuations in applied stresses
• Therefore be regarded as being fatigue loaded.
Motorway gantries, for example, are buffeted by the
slipstream from large lorries and offshore oilrigs by wave
action.
• Process pressure vessels will experience pressure
fluctuations and may also be thermally cycled.
• If these loads are not accounted for in the design, fatigue
failure may occur in as few as a couple of tens of cycles
or several million and the result may be catastrophic
when it does.

17. • It may be thought that the use of a higher strength
material will be of benefit in increasing fatigue life.
• The rate of crack propagation, however, is determined
by Young's Modulus - a measure of the elastic behaviour
of the metal - and not simply by tensile strength.
• Alloying or heat treatment to increase the strength of a
metal has very little effect on Young's Modulus and
therefore very little effect on crack propagation rates.
• Since the bulk of a welded component's life is spent in
propagating a crack, strength has little or no influence on
the fatigue life of a welded item.
• There is thus no benefit to be gained by using high
strength alloys if the design is fatigue limited.

18. • The figure shows several types of loading that could initiate a
fatigue crack.
• The upper left figure shows sinusoidal loading going from a tensile
stress to a compressive stress.
• For this type of stress cycle the maximum and minimum stresses
are equal.
• Tensile stress is considered positive, and compressive stress is
negative.
• The figure in the upper right shows sinusoidal loading with the
minimum and maximum stresses both in the tensile realm.
• Cyclic compression loading can also cause fatigue. The lower figure
shows variable-amplitude loading, which might be experienced by a
bridge or airplane wing or any other component that experiences
changing loading patterns.
• In variable-amplitude loading, only those cycles exceeding some
peak threshold will contribute to fatigue cracking.

19. • There are two general types of fatigue
tests conducted. One test focuses on the
nominal stress required to cause a fatigue
failure in some number of cycles. This test
results in data presented as a plot of
stress (S) against the number of cycles to
failure (N), which is known as an S-N
curve. A log scale is almost always used
for N.

20. • The data is obtained by cycling smooth or notched specimens until
failure.
• The usual procedure is to test the first specimen at a high peak
stress where failure is expected in a fairly short number of cycles.
• The test stress is decreased for each succeeding specimen until
one or two specimens do not fail in the specified numbers of cycles,
• which is usually at least 107
cycles. The highest stress at which a
runout (non-failure) occurs is taken as the fatigue threshold.
• Not all materials have a fatigue threshold (most nonferrous metallic
alloys do not) and for these materials the test is usually terminated
after about 108 or 5x108 cycles.

21. • Since the amplitude of the cyclic loading has a
major effect on the fatigue performance, the S-N
relationship is determined for one specific
loading amplitude. The amplitude is express as
the R ratio value, which is the minimum peak
stress divided by the maximum peak stress.
(R=σmin/σmax). It is most common to test at an
R ratio of 0.1 but families of curves, with each
curve at a different R ratio, are often developed.