DESIGN AGAINST FLUCTUATING LOAD

DESIGN OF MACHINE ELEMENTS
DESIGN AGAINST FLUCTUATING LOADSDESIGN AGAINST FLUCTUATING LOADS
Prepared by:
PAVAN GANDHI (150053119005)
PRIYANK GANDHI (150053119006)
HARSH DHARAIYA (150053119007)
DHARAMJEET JADEJA (150053119008)

STRESS CONCENTRATION
 Whenever a machine component changes the shape of its cross-section, the
simple stress distribution no longer holds good. This irregularity in the stress
distribution caused by abrupt changes of form is called stress concentration.
 A stress concentration (stress raisers or stress risers) is a location in an object
where stress is concentrated. An object is strongest when force is evenly
distributed over its area, so a reduction in area, e.g., caused by a crack, results
in a localized increase in stress.
 A material can fail, via a propagating crack, when a concentrated stress
exceeds the material's theoretical cohesive strength. The real fracture strength
of a material is always lower than the theoretical value because most materials
contain small cracks or contaminants that concentrate stress.
 It occurs for all kinds of stresses in the presence of fillets, notches, holes,
keyways, splines, surface roughness or scratches etc.

THEORETICAL OR FORM STRESS
CONCENTRATION FACTOR
 The theoretical or form stress concentration factor is defined as the
ratio of the maximum stress in a member (at a notch or a fillet) to the
nominal stress at the same section based upon net area.
 Mathematically, theoretical or form stress concentration factor,
 The value of Kt depends upon the material and geometry of the part.

FATIGUE STRESS CONCENTRATION
FACTOR
• When a machine member is subjected to cyclic or fatigue loading, the
value of fatigue stress concentration factor shall be applied instead of
theoretical stress concentration factor.
• Mathematically, fatigue stress concentration factor,

NOTCH SENSITIVITY
 Notch Sensitivity: It may be defined as the degree to which the
theoretical effect of stress concentration is actually reached.
 Notch Sensitivity Factor “q”: Notch sensitivity factor is defined as the
ratio of increase in the actual stress to the increase in the nominal stress
near the discontinuity in the specimen.
Where, Kf and Kt are the fatigue stress concentration factor and
theoretical stress concentration factor.
 The stress gradient depends mainly on the radius of the notch, hole or
fillet and on the grain size of the material.

METHODS TO REDUCE STRESS
CONCENTRATION
• The presence of stress concentration can not be totally eliminated but it
may be reduced to some extent.
• A device or concept that is useful in assisting a design engineer to visualize
the presence of stress concentration and how it may be mitigated is that of
stress flow lines.
• The mitigation of stress concentration means that the stress flow lines shall
maintain their spacing as far as possible.
• Some of the changes adopted in the design in order to reduce the stress
concentration are as follows:
• Avoid abrupt changes in cross section
• Place additional smaller discontinuities adjacent to discontinuity
• Improve surface finish

 In Fig. (a), we see that stress lines tend to bunch up and cut very close to
the sharp re-entrant corner. In order to improve the situation, fillets may
be provided, as shown in Fig. (b) and (c) to give more equally spaced flow
lines.
 It may be noted that it is not practicable to use large radius fillets as in
case of ball and roller bearing mountings. In such cases, notches may be
cut as shown in Fig. (d).

• Following figures show the several ways of reducing the stress concentration
in shafts and other cylindrical members with shoulders, holes and threads :
• The stress concentration effects of a press fit may be reduced by making more
gradual transition from the rigid to the more flexible shaft.

FACTORS TO BE CONSIDERED WHILE
DESIGNING MACHINE PARTS TO AVOID
FATIGUE FAILURE
• The following factors should be considered while designing machine parts
to avoid fatigue failure:
• The variation in the size of the component should be as gradual as
possible.
• The holes, notches and other stress raisers should be avoided.
• The proper stress de-concentrators such as fillets and notches should be
provided wherever necessary.
• The parts should be protected from corrosive atmosphere.
• A smooth finish of outer surface of the component increases the fatigue
life.
• The material with high fatigue strength should be selected.
• The residual compressive stresses over the parts surface increases its
fatigue strength.

ENDURANCE LIMIT AND FATIGUE FAILURE
 It has been found experimentally that when a material is subjected to
repeated stresses, it fails at stresses below the yield point stresses. Such
type of failure of a material is known as fatigue.
 The failure is caused by means of a progressive crack formation which are
usually fine and of microscopic size. The failure may occur even without any
prior indication.
 The fatigue of material is effected by the size of the component, relative
magnitude of static and fluctuating loads and the number of load reversals.

 A standard mirror polished specimen, as shown in figure is rotated in a fatigue
testing machine while the specimen is loaded in bending.
 As the specimen rotates, the bending stress at the upper fibers varies from
maximum compressive to maximum tensile while the bending stress at the
lower fibers varies from maximum tensile to maximum compressive.
 In other words, the specimen is subjected to a completely reversed stress
cycle. This is represented by a time-stress diagram as shown in Fig. (a).

 Endurance or Fatigue limit (σe) is defined as maximum value of the
completely reversed bending stress which a polished standard specimen can
withstand without failure, for infinite number of cycles.
 It may be noted that the term endurance limit is used for reversed bending
only while for other types of loading, the term endurance strength may be
used when referring the fatigue strength of the material.
 It may be defined as the safe maximum stress which can be applied to the
machine part working under actual conditions.
 We have seen that when a machine member is subjected to a completely
reversed stress, the maximum stress in tension is equal to the maximum
stress in compression as shown in Fig.(a). In actual practice, many machine
members undergo different range of stress than the completely reversed
stress.
 The stress verses time diagram for fluctuating stress having values σmin and
σmax is shown in Fig. (c). The variable stress, in general, may be considered
as a combination of steady (or mean or average) stress and a completely
reversed stress component σv.

 The following relations are derived from Fig. (c):
σa =
σmax σmin
2
Alternating stress
Mean stress
σm =
σmax σmin
2
+

FACTORS AFFECTING ENDURANCE LIMIT
1) SIZE EFFECT:
•The strength of large members is lower than that of small specimens.
•This may be due to two reasons.
•The larger member will have a larger distribution of weak points than the
smaller one and on an average, fails at a lower stress.
•Larger members have larger surface Ares. This is important because the
imperfections that cause fatigue failure are usually at the surface.
Effect of size:
•Increasing the size (especially section thickness) results in larger surface
area and creation of stresses.
•This factor leads to increase in the probability of crack initiation.
•This factor must be kept in mind while designing large sized components.

 2) SURFACE ROUGHNESS:
• Almost all fatigue cracks nucleate at the surface of the members.
• The conditions of the surface roughness and surface oxidation or corrosion
are very important.
• Experiments have shown that different surface finishes of the same
material will show different fatigue strength.
• Methods which Improve the surface finish and those which introduce
compressive stresses on the surface will improve the fatigue strength.
• Smoothly polished specimens have higher fatigue strength.
• Surface treatments. Fatigue cracks initiate at free surface, treatments can
be significant
• Plating, thermal or mechanical means to induce residual stress.
 3) EFFECT OF TEMPERATURE:
• When the mechanical component operates above the room temperature,
its ultimate tensile strength, and hence endurance limit decrease with
increase in temperature.

 4) Effect of metallurgical variables;
• Fatigue strength generally increases with increase in UTS
• Fatigue strength of quenched & tempered steels (tempered martensitic
structure) have better fatigue strength
• Finer grain size show better fatigue strength than coarser grain size.
• Non-metallic inclusions either at surface or sub-surface reduces' the
fatigue strength.

S-N DIAGRAM
 Fatigue strength of material is determined by R.R. Moore rotating beam
machine. The surface is polished in the axial direction. A constant bending
load is applied.

 A record is kept of the number of cycles required to produce failure at a given
stress, and the results are plotted in stress-cycle curve as shown in figure.
 A little consideration will show that if the stress is kept below a certain value the
material will not fail whatever may be the number of cycles.
 This stress, as represented by dotted line, is known as endurance or fatigue limit
(σe).
 It is defined as maximum value of the completely reversed bending stress which a
polished standard specimen can withstand without failure, for infinite number of
cycles (usually 107 cycles).

RELATIONSHIP BETWEEN ENDURANCE
LIMIT AND ULTIMATE STRENGTH
Se =′
0.5Sut
100 ksi
700 MPa
Sut ≤ 200 ksi (1400 MPa)
Sut > 200 ksi
Sut > 1400 MPa
Steel
0.4Sut
Se =′
Sut < 60 ksi (400 MPa)
Sut ≥ 60 ksi24 ksi
160 MPa Sut < 400 MPa
Cast iron Cast iron

RELATIONSHIP BETWEEN ENDURANCE
LIMIT AND ULTIMATE STRENGTH
Se =′
0.4Sut
19 ksi
130 MPa
Sut ≥ 48 ksi
Sut ≥ 330 MPa
Aluminum
For N = 5x108
cycle
Copper alloys
Se =′
0.4Sut
14 ksi
100 MPa
Sut ≥ 40 ksi
Sut ≥ 280 MPa
Copper alloys
For N = 5x108
cycle

CORRECTION FACTORS FOR SPECIMEN’S
ENDURANCE LIMIT
Se = kakbkckdkekfSe’
Where,
•Se = endurance limit of component
•Se’ = endurance limit experimental
•ka = surface finish factor (machined parts have different finish)
•kb = size factor (larger parts greater probability of finding defects)
•kc = reliability / statistical scatter factor (accounts for random variation)
•kd = loading factor (differences in loading types)
•ke = operating T factor (accounts for diff. in working T & room T)
•kf = stress concentration factor

CORRECTION FACTORS FOR SPECIMEN’S
ENDURANCE LIMIT
 Surface Factor, Ka
The rotating beam test specimen has a polished surface. Most components
do not have a polished surface. Scratches and imperfections on the surface
act like a stress raisers and reduce the fatigue life of a part. Use either the
graph or the equation with the table shown below.
ka= A (Sut)b

 Size factor, kb
• Larger parts fail at lower stresses than smaller parts. This is mainly due to
the higher probability of flaws being present in larger components.
• The diameter of the rotating beam specimen is 7.62 mm. if the diameter
or size of the component is more, the surface area is more, resulting in
greater number of surface defect. Hence the endurance limit of
component reduce with the increase in size of component.
• For solid round cross section
d ≤ 0.3 in. (8 mm) kb = 1
0.3 in. < d ≤ 10 in. kb = .869(d)-0.097
8 mm < d ≤ 250 mm kb = 1.189(d)-0.097
If the component is larger than 10 in., use kb = .6

 Reliability factor, kg
• The reliability factor is depends upon the reliability requirement of the
mechanical component.
• The reliability correction factor accounts for the scatter and
uncertainty of material properties (endurance limit).

• Load factor, kc
Pure bending kc = 1
Pure axial kc = 0.7
Combined loading kc = 1
Pure torsion kc = 1 if von Mises stress is used, use 0.577 if
von Mises stress is NOT used.
 Temperature factor
• Accounts for the difference between the test temperature and operating
temperature of the component
• For carbon and alloy steels, fatigue strength not affected by operating
temperature – 45 to 4500
C ke = 1
• At higher operating temperature
• ke = 1 – 5800( T – 450 ) for T between 450 and 550o
C, or
• ke = 1 – 3200( T – 840 ) for T between 840 and 1020o
F

DESIGN PROCESS – FULLY REVERSED
LOADING FOR INFINITE LIFE
• Determine the maximum alternating applied stress, σa, in terms of
the size and cross sectional profile
• Select material → Sy, Sut
• Use the design equation to calculate the size
• Se
• Kf σa =• n
• Choose a safety factor → n
• Determine all modifying factors and calculate the
endurance limit of the component → Se
• Determine the fatigue stress concentration factor, Kf
• Investigate different cross sections (profiles), optimize for size or
weight
• You may also assume a profile and size, calculate the alternating stress
and determine the safety factor. Iterate until you obtain the desired
safety factor

DESIGN FOR FINITE LIFE
Sn = a (N)b
equation of the fatigue line
N
S
Se
106
103
A
B
N
S
Sf
5x108
103
A
B
Point A
Sn = .9Sut
N = 103
Point A
Sn = .9Sut
N = 103
Point B
Sn = Sf
N = 5x108
Point B
Sn = Se
N = 106

DESIGN FOR FINITE LIFE
Sn = a (N)b
log Sn = log a + b log N
Apply conditions for point A and B to find the two
constants “a” and “b”
log .9Sut = log a + b log 103
log Se = log a + b log 106
a =
(.9Sut)
2
Se
b =
.9Sut
Se
1
3
log
Sn
Kf σa =
n
Design equation
Calculate Sn and replace Se in the design equation
Sn = Se (
N
106
)
⅓ (
Se
.9Sut
)log

FLUCTUATING STRESSES
 The failure points from fatigue tests made with different steels and
combinations of mean and variable stresses are plotted in figure as functions
of stress amplitude(σa) and mean stress (σm).
 The most significant observation is that, in general, the failure point is little
related to the mean stress when it is compressive but is very much a function
of the mean stress when it is tensile.
 In practice, this means that fatigue failures are rare when the mean stress is
compressive (or negative). Therefore, the greater emphasis must be given to
the combination of a variable stress and a steady (or mean) tensile stress.
Mean stress
Alternating
stress
σm
σa
Se
SySoderberg line
Sut
Goodman line
Gerber curve
Sy
Yield line

GOODMAN METHOD FOR COMBINATION
OF STRESSES:
• A straight line connecting the endurance limit (σe) and the ultimate strength
(σu), as shown by line AB in figure given below follows the suggestion of
Goodman.
• A Goodman line is used when the design is based on ultimate strength and
may be used for ductile or brittle materials.

Now from similar triangles COD and PQD,

SODERBERG METHOD FOR COMBINATION OF
STRESSES
 A straight line connecting the endurance limit (σe) and the yield strength
(σy), as shown by the line AB in following figure, follows the suggestion of
Soderberg line.
 This line is used when the design is based on yield strength. the line AB
connecting σe and σy, as shown in following figure, is called Soderberg's
failure stress line.
 If a suitable factor of safety (F.S.) is applied to the endurance limit and yield
strength, a safe stress line CD may be drawn parallel to the line AB.

MODIFIED GOODMAN DIAGRAM:
 In the design of components subjected to fluctuating stresses, the
Goodman diagram is slightly modified to account for the yielding failure
of the components, especially, at higher values of the mean stresses.
 The diagram known as modified Goodman diagram and is most widely
used in the design of the components subjected to fluctuating stresses.

MODIFIED GOODMAN DIAGRAM FOR
FLUCTUATING AXIAL AND BENDING
STRESSES
+σm
σa
Sut
Safe zone
- σm
C
Sy
Safe zone
Se
- Syc
Finite life
Sn
1=
Sut
σa σm
+
Fatigue, σm > 0Fatigue, σm ≤ 0
σa =
Se
nf
σa + σm =
Sy
ny
Yield
σa + σm =
Sy
ny
Yield
nfSe
1
=
Sut
σa σm
+ Infinite life

DESIGN EXAMPLE
R1 R2
10,000 lb.
6˝6˝12˝
D = 1.5dd
r (fillet radius) = .1d
A rotating shaft is carrying 10,000 lb force as
shown. The shaft is made of steel with Sut =
120 ksi and Sy = 90 ksi. The shaft is
rotating at 1150 rpm and has a machine
finish surface. Determine the diameter,
d, for 75 minutes life. Use safety factor
of 1.6 and 50% reliability.
Calculate the support forces, R1 = 2500, R2 = 7500 lb.
A
The critical location is at the fillet, MA = 2500 x 12 = 30,000 lb-in
σa =Calculate the alternating stress, Mc
I
=
32M
πd 3
=
305577
d 3
σm = 0
Determine the stress concentration factor
r
d
= .1
D
d
= 1.5
Kt = 1.7

DESIGN EXAMPLE
Assume d = 1.0 in
Using r = .1 and Sut = 120 ksi,
q (notch sensitivity) = .85
Kf = 1 + (Kt – 1)q = 1 + .85(1.7 – 1) = 1.6
Calculate the endurance limit
Cload = 1 (pure bending)
Crel = 1 (50% rel.)
Ctemp= 1 (room temp)
Csurf = A (Sut)b
= 2.7(120)
-.265
= .759
0.3 in. < d ≤ 10 in. Csize = .869(d)-0.097
= .869(1)
-0.097
= .869
Se = Cload Csize Csurf Ctemp Crel (Se) = (.759)(.869)(.5x120) = 39.57 ksi′

DESIGN EXAMPLE
Design life, N = 1150 x 75 = 86250 cycles
Sn = Se (
N
106
)
⅓ (
Se
.9Sut
)log
Sn = 39.57 (
86250
106
)
⅓ (
39.57
.9x120
)log
= 56.5 ksi
σa =
305577
d 3
= 305.577 ksi n=
Sn
Kfσa
=
56.5
1.6x305.577
= .116 < 1.6
So d = 1.0 in. is too small
Assume d = 2.5 in
All factors remain the same except the size factor and notch sensitivity.
Using r = .25 and Sut = 120 ksi,
q (notch sensitivity) = .9
Kf = 1 + (Kt – 1)q = 1 + .9(1.7 – 1) = 1.63
Csize = .869(d)-0.097
= .869(2.5)
-0.097
= .795 Se = 36.2 ksi→

DESIGN EXAMPLE
σa =
305577
(2.5)3
= 19.55 ksi
n =
Sn
Kfσa
=
53.35
1.63x19.55
= 1.67 ≈ 1.6
d = 2.5 in.
Check yielding
n =
Sy
Kfσmax
=
90
1.63x19.55
= 2.8 > 1.6 okay
Se = 36.2 ksi Sn = 53.35 ksi→

DESIGN AGAINST FLUCTUATING LOAD

DESIGN AGAINST FLUCTUATING LOAD

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