Introduction to fatigue

1. FATIGUE OF METALS
BY
HARISH M R

2. CONTENTS
 Introduction
 Factors causing fatigue failure
 Stress Cycles
 The S-N Curve
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3. INTRODUCTION
Fatigue failure in a bolt
Fatigue failure occurs at the outer rim
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4. Failure of crankshaft journal
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5. INTRODUCTION
 Fatigue failures are widely studies because it
accounts for 90% of all service failures due to
mechanical causes.
 Fatigue failures occur when metal is subjected to a
repetitive or fluctuating stress and will fail at a
stress much lower than its tensile strength.
 Fatigue failures occur without any plastic
deformation (no warning).
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6. FACTORS CAUSING FATIGUE FAILURE
 A maximum tensile stress of sufficiently high value.
 A large amount of variation or fluctuation in the applied
stress.
 A sufficiently large number of cycles of the applied
stress.
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7. Tensile stress +
Compressive stress -
a) Completely reversed cycle of stress
(sinusoidal)
(b) Repeated stress cycle
(c ) Irregular or random stress cycle
DIFFERENT TYPES OF FLUCTUATING STRESS
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8. STRESS CYCLES
max
min
m


a
+
_ cycles
Fatigue stress cycle
Nomenclature of stress
parameter in fatigue loading
  max  min
 or 
r
Maximum stress, max
Minimum stress, min
Stress range
Alternating stress
 
 
 max   min
2 2
a
Mean stress
 
 max  min
2
m
Stress ratio
max
R 
 min
Amplitude ratio
 1 R
 m 1 R
A 
 a
Eq.1
Eq.2
Eq.3
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9. THE S-N CURVE
 Engineering fatigue data is
normally represented by
means of S-N curve, a plot
of stress S against the
number of cycle, N.
 Stress can be a, max, min
 m , R or A should be
mentioned.
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Fatigue limit or endurance limit is normally
defined at 107 or 108 cycles. Below this limit,
the material presumably can endure an
infinite number of cycle before failure.

10. REFERENCE
 “Fundamentals of Metal Fatigue Analysis” by
Julie A Bannantine
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11. THANK YOU FOR YOUR TIME
ANY QUESTIONS?
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