The document discusses design against fluctuating loads and fatigue failure. It introduces stress concentration factors and how to reduce stress concentrations through geometric design changes. It describes fluctuating stresses and how materials can fail under cyclic loading even at stresses below the yield stress. Various methods for analyzing fatigue life are presented, including endurance limits, S-N curves, and approaches like the Soderberg, Goodman and Gerber lines for evaluating finite and infinite fatigue life based on fluctuating stresses and mean stresses. Materials examples for components subjected to these conditions are given.

1. DESIGN AGAINST FLUCTUATING LOADS
Presenting by-
Deepak DolakashariaDeepak Dolakasharia (107)(107)
Rijuan HussainRijuan Hussain (134)(134)
Niraj KakatiNiraj Kakati (142)(142)
Rupjyoti BaruahRupjyoti Baruah (263)(263)
Feroz Ahmed MazumderFeroz Ahmed Mazumder
(017)(017)
for the subject-
Don Bosco School of TechnologyDon Bosco School of Technology1.1.

2.  Stress concentration and factor, Notch sensitivity.Stress concentration and factor, Notch sensitivity.
 Reduction of stress concentration & Fluctuating stresses.Reduction of stress concentration & Fluctuating stresses.
 Endurance limit.Endurance limit.
 Soderberg, Goodman lines, Gerber equations & ModifiedSoderberg, Goodman lines, Gerber equations & Modified
Goodman diagrams.Goodman diagrams.
Monday, March 28, 2016
2
 Introduction

3. Monday, March 28, 2016
3
 What is StressWhat is Stress
ConcentrationConcentration ??
 Technique called
Photo-elasticity.
 Localisation of high
stress due to
irregularities and abrupt
change in size.
 Stress concentration
factor is used, Kt or Kc


4. Monday, March 28, 2016
4
 Why StressWhy Stress
concentrationconcentration ??
 Variation in properties
of materials,
 Load applications,
 Discontinuities in the
component,
 Machining scratches,
 Abrupt change in
cross sections,

5. Monday, March 28, 2016
5
 How to reduce stressHow to reduce stress
concentrationconcentration ??
—By providing a specific geometric shape to the components
through understanding the flow analogy.
 Additional notches and
holes in tension members.
 Fillet radius, undercutting
and notch for bending
member.
 Drilling additional holes for
shaft.
 Reduction of stress
concentration in threaded
members.

6. Monday, March 28, 2016
6
 Fluctuating stressFluctuating stress
 Forces subjected are not static but, vary with time called fluctuating stress, which causes
fatigue failure.
 For design analysis stress-time relationship is used, like sine curve.
or

7. Monday, March 28, 2016
7
 Fatigue failureFatigue failure
 Observed that materials fail under
fluctuating stress at stress magnitude,
lower than ultimate tensile strength.
 Even lower than yield stress sometimes.
 Magnitude of stress causing Fatigue
failure decreases as the no. of stress
cycle increase.
 This phenomenon results in fatigue
failure.
 Is defined as time delayed fracture under
cyclic loading.
 It depends upon no. of cycle, mean
stress, stress amplitude, corrosion, creep
Played with a small wire during childhood, but didn't know the reasonPlayed with a small wire during childhood, but didn't know the reason

8. Monday, March 28, 2016
8
 FatigueFatigue
testtest
 Beam of circular c-s subjected to bending moment.
 The amplitude of this cycle is given as-

9. Monday, March 28, 2016
9
 Endurance limitEndurance limit
 Definition.
 10,00,000 cycles is sufficient for
endurance limit.
 Fatigue life.
 Fatigue failure testing done in
Rotating Beam testing machine.
 S-N curve.
 S-N curve for steel.
 Endurance limit not exactly a
property of ultimate tensile
strength.

10. Monday, March 28, 2016
10
 Low & High cycle fatigueLow & High cycle fatigue
 Earlier S-N curve drawn was
for small cycles.
 There are two main regions.
 Fatigue failure at less than
1000 cycles is low cycle
fatigue and
 & at more than 1000 is high
cycle fatigue.
 Failures like setscrews, studs
on truck, etc are e.g. of low
cycle and spring, ball bearings
are high cycle.
 Low cycle involves plastic

11. Monday, March 28, 2016
11
 Notch SensitivityNotch Sensitivity
Notches are stress riser
Elastic stress concentration
factor,
 the susceptibility of a material to succumb to the damaging effects of stress raising notches in
fatigue loading.
when the material has no sensitivity to notches, q=0; Kf =
1 and
oror

12. Monday, March 28, 2016
12
 Endurance limit calculationEndurance limit calculation
SSee = K= Kaa * K* Kbb * K* Kcc * K* Kdd * S* See''
Se' = Endurance Strength of material specimen under laboratory condition
Se = Endurance Strength of material specimen under actual running
condition
Two separate notations are used for endurance limit,
Ka = Surface Correction factor
Kb = Size Correction factor
Kc =Reliability Correction factor
Kd = Temperature Correction
factor
Size factor Ka
Kd is calculated
as
Se’ has different value for
different materials

13. Monday, March 28, 2016
13
 Design for finite & infiniteDesign for finite & infinite
lifelife
 Design for completely
reversed stress are divided
into finite and infinite life
fatigue.
 For infinite life endurance
limit is the criterion.
 Amplitude stress <
endurance limit for longer
no. of cycles.
 For finite life S-N curve is

14. Monday, March 28, 2016
14
 Soderberg, Goodman, GerberSoderberg, Goodman, Gerber
 GERBERLINE- Aparabo lic
curve jo ining Se o n the o rdinate
to Sut o n the abscissa is calle d
G e rbe r line .
 SODERBERG LINE- Astraig ht
line jo ining Se o n the o rdinate
and Syt o n the abscissa is calle d
the So de rbe rg line .
 GOODMAN LINE- Astraig ht line
jo ining Se o n the o rdinate and Sut
o n the abscissa is calle d
 It is seen that mean stress has an effect on fatigue failure when with
stress amplitude.Using straight line equations
Soderberg, Goodman
equations and Gerber will
be-

15. Monday, March 28, 2016
15
 Modified Goodman lineModified Goodman line
Components subjected to fluctuating axial or bending
stresses.
Components subjected to fluctuating torsion shear
stresses.

16. Monday, March 28, 2016
16
 Materials select & anMaterials select & an
ExampleExample
Steel and Cast iron
Aluminum alloys
Plastics

17. T
H
A
N
K
‘U’
Monday, March 28, 2016 EVERY ONE