Chapter 6 fatigue failure loading

83
CHAPTER
6
Fatigue Failure Resulting from Variable Loading:
6–1 Fatigue in Metals 6–8 Stress Concentration and Notch Sensitivity
6–2 Fatigue-Life Method 6–9 Fluctuating Stresses
6–3 The Stress-Life Method 6–10 Failure Criteria for Fluctuating Stress
6–4 Fracture Mechanics Method 6–11 Torsional Fatigue Strength
6–5 The Endurance Limit 6-12 Combinations of Loading Modes
6–6 Fatigue Strength 6-13 Cumulative Fatigue Damage
6–7 Endurance Limit Modifying Factors
6–1 Introduction to Fatigue in Metals :
 When machine parts are subjected to time varying loading, their behavior is entirely
different from what they could behave when they are subject to static loading. These and
other kinds of loading occurring in machine members produce stresses that are called
variable, repeated, alternating, or fluctuating stresses.
 Machine members are found to have failed under the action of repeated or fluctuating
stresses.
 The actual maximum stresses were well below the ultimate strength of the material, and
quite frequently even below the yield strength. The most distinguishing characteristic of
these failures is that the stresses have been repeated a very large number of times. Hence
the failure is called a fatigue failure.
6–2 Fatigue-Life Methods:
 The three major fatigue life methods used in design and analysis are the:
1. Stress-Life Method
2. Strain-Life Method
3. Linear-Elastic Fracture Mechanics Method
 These methods attempt to predict the life in number of cycles to failure, N, for a speciﬁc level of
loading. Life of 𝟏 ≤ 𝑵 ≤ 𝟏𝟎 𝟑
cycles is generally classiﬁed as low-cycle fatigue, whereas
high-cycle fatigue is considered to be 𝑵 > 𝟏𝟎 𝟑
cycles.
 The stress-life method, based on stress levels only, is the least accurate approach, especially for
low-cycle applications. However, it is the most traditional method, since it is the easiest to
implement for a wide range of design applications, has ample supporting data, and represents
high-cycle applications adequately. It can be summarized as:
 High Cycle Fatigue ( N > 1000)
 It is based on stress levels.
 Predictions of life are based upon nominal stresses in a component
 Use empirical correction factor (surface finish, groove, …)
 The least accurate approach, but most used method, since it is the

84
easiest to implement for a wide range of design applications.
 The strain-life method involves more detailed analysis of the plastic deformation at localized
regions where the stresses and strains are considered for life estimates. This method is especially
good for low-cycle fatigue applications.
 Low Cycle Fatigue (N < 1000)
 Involves more detailed analysis of the plastic deformation at localized
regions where the stresses and strains are considered for life estimates.
 The Linear-Elastic Fracture Mechanics Method:
 It assumes a crack is already present and detected.
 It is then employed to predict crack growth with respect to stress
intensity.
 It is most practical when applied to large structures in conjunction
with computer codes and a periodic inspection program.
6–3 Stress-Life Method:
 To determine the strength of materials under the action of fatigue loads, four types of tests are
performed: tension, torsion, bending, and combinations of these.
 In each test, specimens
are subjected to repeated
forces at specified
magnitudes while the
cycles or stress reversals
to rupture are counted.
 For the rotating-beam
test, a constant bending
load is applied, and the
number of revolutions
(stress reversals) of the
beam required for failure
is recorded. The first test
is made at a stress that is somewhat under the ultimate strength of the material. The second test
is made at a stress that is less than that used in the first. This process is continued, and the
results are plotted as an S-N diagram.
 The figure blow is the S-N diagram for typical UNS G41300 steel.

85
 It should be noted that from the N-S diagram for the case of steel:
 A knee occurs around 𝑵 = 𝟏𝟎 𝟔
and beyond this knee failure will not occur, no matter
how great the number of cycles.
 The strength corresponding to the knee is called the endurance limit Se , or the fatigue
limit.
 For nonferrous metals and alloys, the graph of S-N diagram will never be horizontal.
 Meaning of N:
 A stress cycle (𝑵 = 𝟏) constitutes a single application and removal of a load and then
another application and removal of the load in the opposite direction.
 Thus, 𝑵 = 𝟎. 𝟓, means the load is applied once and then removed, which is the case with
the simple tension test.
 A body of knowledge available on fatigue failure from 𝑵 = 𝟏 to 𝑵 = 𝟏𝟎 𝟑
cycles is
generally classified as low-cycle fatigue.
 A stress cycles greater than = 𝟏𝟎 𝟑
, is classified as high-cycle fatigue.
 The boundary between the finite-life region and the infinite-life region can be defined
only for a specific material such as steel as shown above in which it lies somewhere
between 𝑵 = 𝟏𝟎 𝟔
𝐚𝐧𝐝 𝑵 = 𝟏𝟎 𝟕
 S-N method does not work well in low-cycle application, where the applied strains have
a significant plastic component.
 Mischke has analyzed a great deal of actual teat data from several sources and concluded that
endurance limit can be related to tensile strength.
 For Steel:
𝑺 𝒆
,
= {
𝟎. 𝟓𝑺 𝒖𝒕 𝑺 𝒖𝒕 ≤ 𝟏𝟒𝟔𝟎 𝑴𝑷𝒂
𝟕𝟎𝟎 𝑴𝑷𝒂 𝑺 𝒖𝒕 > 𝟏𝟒𝟔𝟎 𝑴𝑷𝒂
, where: 𝑺 𝒖𝒕 : : minimum tensile strength, and 𝑺 𝒆
,
: : rotating-beam specimen itself.
 Aluminum and Magnesium alloys do not have an endurance limit, and the fatigue strength is
usually based on 5(108
) cycles of stress reversal and it is given in table A-26 and A-27.
 Endurance limits for various classes of cast irons, polished or machined, are given in table A-24
 We could also use for Cast Iron and Cast Steel:
𝑺 𝒆
,
= {
𝟎. 𝟒𝟓𝑺 𝒖𝒕 𝑺 𝒖 ≤ 𝟔𝟎𝟎 𝑴𝑷𝒂
𝟐𝟕𝟓 𝑴𝑷𝒂 𝑺 𝒖𝒕 > 𝟔𝟎𝟎 𝑴𝑷𝒂
 The most widely used fatigue‐testing device is the R. R. Moore high–speed rotating-beam
machine. This machine subjects the specimen to pure bending (no transverse shear) by means of

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weights.
 The specimen is very carefully machined and polished, with a final polishing in an
axial direction to avoid circumferential scratches.
 Other fatigue‐testing machines are available for applying fluctuating or reversed axial stresses,
torsional stresses, or combined stresses to the test reversed axial stresses, torsional
stresses, or combined stresses.
6–5 The Strain-Life Method:
 The best approach yet advanced to
explain the nature of fatigue failure is
called by some the strain-life method.
 A fatigue failure almost always
begins at a local discontinuity such as
a notch, crack, or other area of stress
concentration.
 When the stress at the discontinuity
exceeds the elastic limit, plastic strain
occurs.
 If a fatigue fracture is to occur, there must exist cyclic plastic strains.
 Thus we shall need to investigate the behavior of
materials subject to cyclic deformation.
 Figure 6–2 has been constructed to show the
general appearance of these plots for the first few
cycles of controlled cyclic strain.
 In this case the strength decreases with stress
repetitions, as evidenced by the fact that the
reversals occur at ever-smaller stress levels. As
previously noted, other materials may be
strengthened, instead, by cyclic stress reversals.
 The graph has been reproduced as Fig. 6–2, to
explain the graph, we first define the following
terms:
 Fatigue ductility coefficient 𝜺 𝑭
′
is the true strain corresponding to fracture in one reversal
(point A in Fig. 6–3). The plastic-strain line begins at this point in Fig. 6–2.

87
 Fatigue strength coefﬁcient 𝝈 𝑭
′
is the true stress corresponding to fracture in one
reversal (point A in Fig. 6–3). Note in Fig. 6–2 that the elastic-strain line begins at
𝝈 𝑭
,
𝑬
.
 Fatigue ductility exponent c is the slope of the plastic-strain line in Fig. 6–2 and is the
power to which the life 2N must be raised to be proportional to the true plastic-strain
amplitude. If the number of stress reversals is 2N, then N is the number of cycles.
 Fatigue strength exponent b is the slope of the elastic-strain line, and is the power to
which the life 2N must be raised to be proportional to the true-stress amplitude.
 Now, from Fig. 6–3, we see that the total strain is the sum of the elastic and plastic
components. Therefore the total strain amplitude is half the total strain range:
∆𝜺
𝟐
=
∆𝜺 𝒆
𝟐
+
∆𝜺 𝒑
𝟐
(𝒂)
The equation of the plastic-strain line in Fig. 6–2 is:
∆𝜺 𝒑
𝟐
= (𝜺 𝑭
,
) (𝟐𝑵) 𝒄 (𝟔 − 𝟏)
The equation of the elastic strain line is:
∆𝜺 𝒆
𝟐
=
𝝈 𝑭
,
𝑬
(𝟐𝑵) 𝒃 (𝟔 − 𝟐)
Therefore, from Eq. (a), we have for the total-strain amplitude:
∆𝜺
𝟐
= (𝜺 𝑭
,
) (𝟐𝑵) 𝒄
+
𝝈 𝑭
,
𝑬
(𝟐𝑵) 𝒃 (𝟔 − 𝟑)
, which is the Manson-Cofﬁn relationship between fatigue life and total strain.
6–6 The Linear-Elastic Fracture Mechanics Method: See Textbook Section 6-6 (p270-274).
 A fatigue failure has an appearance similar to a brittle fracture, as the fracture surfaces are flat
and perpendicular to the stress axis with the absence of necking.
 The fracture features of a fatigue failure, however, are quite different from a static brittle
fracture arising from three stages of development:
 Stage I: Is the initiation of one or more micro-cracks due to cyclic plastic deformation followed
by crystallographic propagation extending from two to five grains about the origin.
 Stage II: Progresses from micro-cracks to macro-cracks forming parallel-like fracture surfaces
separated by longitudinal ridges. The plateaus are generally smooth and normal to the direction
of maximum tensile stress. These surfaces can be wavy dark and light bands referred to as beach
marks or clamshell marks.
 Stage III: Occurs during the final stress cycle when the remaining material cannot support the
loads, resulting in a sudden, fast fracture cannot support the loads, resulting in a sudden, fast

88
fracture.
6–7 The Endurance Limit:
For steels, simplifying our observation of Fig. 6–4, we will estimate the endurance limit as:
𝑆 𝑒
,
= {
0.5𝑆 𝑢𝑡 𝑆 𝑢𝑡 ≤ 100 𝑘𝑝𝑠𝑖 (1460 𝑀𝑃𝑎)
100 𝑘𝑝𝑠𝑖 𝑆 𝑢𝑡 > 100 𝑘𝑝𝑠𝑖
700 𝑀𝑃𝑎 𝑆 𝑢𝑡 > 1460 𝑀𝑃𝑎
(6 - 8)
, where Sut is the minimum tensile
strength. The prime mark on 𝑆 𝑒
,
in this
equation refers to the rotating-beam
specimen itself. We wish to reserve the
unprimed symbol Se for the endurance
limit of any particular machine element
subjected to any kind of loading. Soon
we shall learn that the two strengths may
be quite different. Soon we shall learn
that the two strengths may be quite
different.
6–8 Fatigue Strength:
 The purpose of this section is to develop methods
of approximating the S-N diagram for steels when the
results of the simple tension test are known, in the high
cycle region.
 High-Cycle fatigue extends from 𝑵 = 𝟏𝟎 𝟑
to
𝑵 = 𝟏𝟎 𝟔
(the endurance limit life 𝑵 𝐞 )
 Let the equation for the S-N line be written as:
(𝑺 𝒇
,
)
𝑵
= 𝝈 𝑭
,
(𝟐𝑵) 𝒃 (𝟔 − 𝟗)
 At 103
cycles:
(𝑺 𝒇
,
)
𝟏𝟎 𝟑
= 𝝈 𝑭
,
(𝟐. 𝟏𝟎 𝟑) 𝒃
= 𝒇𝑺 𝒖𝒕
, where f is the fraction of Sut represented by (𝑺 𝒇
,
)
𝟏𝟎 𝟑
cycles . Solving for f gives:
𝒇 =
𝝈 𝑭
,
𝑺 𝒖𝒕
(𝟐. 𝟏𝟎 𝟑) 𝒃
= 𝒇𝑺 𝒖𝒕 (𝟔 − 𝟏𝟎)
 Now, from Eq., 𝝈 𝑭
′
= 𝝈 𝟎 𝜺 𝒎
, with 𝜺 = 𝜺 𝑭
′
. If this true-stress–true-strain equation is not known,
the SAE approximation for steels with 𝑯 𝑩 ≤ 𝟓𝟎𝟎 may be used:

89
𝝈 𝑭
′
= 𝑺 𝒖𝒕 + 𝟓𝟎 𝒌𝒑𝒔𝒊 𝐨𝐫 𝝈 𝑭
′
= 𝑺 𝒖𝒕 + 𝟑𝟒𝟓 𝑴𝑷𝒂 (𝟔 − 𝟏𝟏)
 To find b, substitute the endurance strength and corresponding cycles, 𝑺 𝒆
′
, 𝐚𝐧𝐝 𝑵 𝒆, respectively
into Eq. (6–9) and solving for b:
𝒃 = −
𝐥𝐨𝐠(𝝈 𝑭
′
𝑺 𝒆
′⁄ )
𝐥𝐨𝐠(𝟐𝑵 𝒆)
(𝟔 − 𝟏𝟐)
 Thus, the equation 𝑺 𝑭
′
= 𝝈 𝑭
′
( 𝟐𝑵 ) 𝒃
is known. For example, if𝑺 𝒖𝒕 = 𝟏𝟎𝟓 𝒌𝒑𝒔𝒊 and 𝑺 𝒆
′
=
𝟓𝟐. 𝟓 𝒌𝒑𝒔𝒊 at failure:
𝐄𝐪. (𝟔 − 𝟏𝟏) ⇒ 𝝈 𝑭
′
= 𝟏𝟎𝟓 + 𝟓𝟎 = 𝟏𝟓𝟓 𝒌𝒑𝒔𝒊
𝐄𝐪. (𝟔 − 𝟏𝟐) ⇒ 𝒃 = −
𝐥𝐨𝐠(𝟏𝟓𝟓 𝟓𝟐. 𝟓⁄ )
𝐥𝐨𝐠(𝟐 . 𝟏𝟎 𝟔)
= −𝟎. 𝟎𝟕𝟒𝟔
𝐄𝐪. (𝟔– 𝟏𝟎) ⇒ 𝒇 =
𝟏𝟓𝟓
𝟏𝟎𝟓
(𝟐. 𝟏𝟎 𝟑)−𝟎.𝟎𝟕𝟒𝟔
= 𝟎. 𝟖𝟑𝟕
and for Eq. (6–9), with 𝑺 𝒇
′
= (𝑺 𝒇
′
)
𝑵
𝑺 𝒇
′
= 𝟏𝟓𝟓(𝟐𝑵)−𝟎.𝟎𝟕𝟒𝟔
= 𝟏𝟒𝟕𝑵−𝟎.𝟎𝟕𝟒𝟔 (𝐚)
 The process given for finding f can be repeated for various ultimate strengths. Fig. 6–5 is a plot of
f for 𝟕𝟎 ≤ 𝑺 𝒖𝒕 ≤ 𝟐𝟎𝟎 𝒌𝒑𝒔𝒊. To be conservative, for 𝑺 𝒖𝒕 < 𝟕𝟎 𝒌𝒑𝒔𝒊 , let f 0.9.
 For an actual mechanical component, 𝑺 𝒆
′
is reduced to 𝑺 𝒆 (see Sec. 6–9) which is less
than 𝟎. 𝟓 𝑺 𝒖𝒕. However, unless actual data is available, we recommend using the value of f found
from Fig. 6–5. Equation (a), for the actual mechanical component, can be written in the form:
𝑺 𝒇 = 𝒂𝑵 𝒃 (𝟔 − 𝟏𝟑)
, where N is cycles to failure and the constants a and b are defined by the points 𝟏𝟎 𝟑
, (𝑺 𝒇)
𝟏𝟎 𝟑
and 𝟏𝟎 𝟔
, 𝑺 𝒆 with (𝑺 𝒇)
𝟏𝟎 𝟑 = 𝒇𝑺 𝒖𝒕 . Substituting these two points in Eq. (6–13) gives:
𝒂 =
(𝒇𝑺 𝒖𝒕) 𝟐
𝑺 𝒆
(𝟔 − 𝟏𝟒)
𝒃 = −
𝟏
𝟑
𝐥𝐨𝐠 (
𝒇𝑺 𝒖𝒕
𝑺 𝒆
) (𝟔 − 𝟏𝟓)
 If a completely reversed stress 𝝈 𝒂 is given, setting 𝑺 𝒇 = 𝝈 𝒂 in Eq. (6–13), the number of cycles-
to-failure can be expressed as:
𝑵 = (
𝝈 𝒂
𝒂
)
𝟏 𝒃⁄
(𝟔 − 𝟏𝟔)
 Low-cycle fatigue is often defined (see Fig. 6–1) as failure that occurs in a range of 𝟏 ≤ 𝑵 ≤
𝟏𝟎 𝟑
cycles. On a log-log plot such as Fig. 6–1 the failure locus in this range is nearly linear
below 𝟏𝟎 𝟑
cycles. A straight line between 𝟏𝟎 𝟑
, 𝒇 𝑺 𝒖𝐭 and 𝟏, 𝑺 𝒖𝒕 (transformed) is conservative,

90
and it is given by:
𝑺 𝒇 ≥ 𝑺 𝒖𝒕 𝑵(𝐥𝐨𝐠 𝒇) 𝟑⁄
𝟏 ≤ 𝑵 ≤ 𝟏𝟎 𝟑 (𝟔 − 𝟏𝟕)
EXAMPLE 6–1:
Given a 1050 HR steel, estimate:
1. The rotating-beam endurance limit at 106
cycles.
2. The endurance strength of a polished rotating-beam specimen corresponding to 104
cycles to
failure
3. The expected life of a polished rotating-beam specimen under a completely reversed stress of 55
kpsi.
EXAMPLE 6-2:

91
6–9 Endurance Limit Modifying Factors:
 It is unrealistic to expect the endurance limit of a mechanical or structural member to match the
values obtained in the laboratory. Some differences include:
1. Material: composition, basis of failure, variability.
2. Manufacturing: method, heat treatment, fretting corrosion, surface condition, stress
concentration.
3. Environment: corrosion, temperature, stress state, relaxation times.
4. Design: size, shape, life, stress state, stress concentration, speed, fretting, galling.
 Marin identified factors that quantified the effects of surface condition, size, loading,
temperature, and miscellaneous items. The Marin equation is therefore written as:
𝑺 𝒆 = 𝒌 𝒂 𝒌 𝒃 𝒌 𝒄 𝒌 𝒅 𝒌 𝒆 𝒌 𝒇 𝑺 𝒆
′ (𝟔 − 𝟏𝟖)
, where:
𝒌 𝒂 = surface condition modification factor
𝒌 𝒃 = size modification factor
𝒌 𝒄 =load modification factor
𝒌 𝒅 = temperature modification factor
𝒌 𝒆 =reliability factor
𝒌 𝒇 =miscellaneous-effects modification factor
𝑺 𝒆
′
=rotary-beam test specimen endurance limit
𝑺 𝒆 = endurance limit at the critical location of a machine part in the geometry and condition of use
 To account for the most important of these conditions, we employ a variety of modifying
factors, each of which is intended to account for a single effect. Or when endurance tests of parts
are not available, estimations are made by applying Marin factors to the endurance limit.

92
1. Surface Factor
𝑘 𝑎
Surface modification factor depends on the quality of the finish of the actual
part surface and on the tensile strength of the part material. Surface factor can
be calculated using the following formula:
𝑘 𝑎 = 𝑎 𝑆 𝑢𝑡
𝑏 (6 − 19)
, where 𝑆 𝑢𝑡 is the minimum tensile strength and a and b are to be found in
Table 6–1. Table 6.1
Surface Finish
Factor a
Exponent b
Sut, kpsi Sut, MPa
Ground 1.34 1.58 -0.085
Machined or CD 2.70 4.51 -0.265
HR 14.4 57.7 -0.718
As-forged 39.9 272 -0.995
2. Size Factor
𝑘 𝑏
i.Round bar in bending and in rotating: For bending and torsional loadings
there is a size effect, so:
𝑘 𝑏 = {
0.879𝑑−0.107
0.11 ≤ 𝑑 ≤ 2 𝑖𝑛
0.91𝑑−0.157
2 < 𝑑 ≤ 10 𝑖𝑛
1.24𝑑−0.107
2.79 ≤ 𝑑 ≤ 51 𝑚𝑚
1.51𝑑−0.157
51 < 𝑑 ≤ 254 𝑚𝑚
(6-20)
For axial loading there is no size effect, so:
𝑘 𝑏 = 1 (6 − 21)
ii. Round bar in bending is not rotating : The approach to be used here
employs an effective dimension 𝑑 𝑒, obtained by equating the volume of
material stressed at and above 95% of the maximum stress to the same
volume in the rotating-beam specimen.
𝐴0.95𝜎 =
𝜋
4
[𝑑2
− 0.95𝑑2] = 0.0766𝑑2 (6 − 22)
For non-rotating solid or hollow rounds, the 95 percent stress area is twice
the area outside of two parallel chords having a spacing of 0.95d, where d is
the diameter. Using an exact computation, this is
𝐴0.95𝜎 = 0.01046𝑑2 (6 − 23)
, with 𝑑 𝑒 in Eq. (6–22), setting Eqs. (6–22) and (6–23) equal to each other
enables us to solve for the effective diameter. This gives:
𝑑 𝑒 = 0.370𝑑 (6 − 24)

93
iii. Noncircular cross section is used: A rectangular section of dimensions
ℎ × 𝑏 has 𝐴0.95𝜎 = 0.05ℎ𝑏 . Using the same approach as before:
𝑑 𝑒 = 0.808(ℎ𝑏)
1
2⁄ (6 − 25)
Table 6–2 provides 𝐴0.95𝜎 areas of common structural shapes undergoing
non-rotating bending.
3. Loading Factor,
kc
𝑘 𝑐 = {
1 𝑏𝑒𝑛𝑑𝑖𝑛𝑔
0.89 𝑎𝑥𝑖𝑎𝑙
0.59 𝑡𝑜𝑟𝑠𝑖𝑜𝑛
(6-26)
For axial load we could also use the following formula:
𝑘 𝑐 = {
1 𝑆 𝑢𝑡 > 1520 𝑀𝑃𝑎
0.923 𝑆 𝑢𝑡 > 1520 𝑀𝑃𝑎
4. Temperature
Factor kd
𝑘 𝑑 = 0.975 + 0.432(10−3)𝑇𝐹 − 0.115(10−5)𝑇𝐹
2
+ 0.104(10−8)𝑇𝐹
3
− 0.595(10−12)𝑇𝐹
4(6 − 27)
Two types of problems arise when temperature is a consideration:
i. If the rotating- beam endurance limit is known at room temperature,
then use:
𝑘 𝑑 =
𝑆 𝑇
𝑆 𝑅𝑇
(6 − 28)

94
from Table 6–3 or Eq. (6–27) and proceed as usual.
ii. If the rotating-beam endurance limit is not given, then compute it using
Eq. (6–8) and the temperature-corrected tensile strength obtained by
using the factor from Table 6–4. Then use 𝑘 𝑑 = 1.
5. Reliability
Factor ke
ke = 1 − 0.08za (6 − 29)
, where za is deﬁned by Eq. (20–16) and values for any desired reliability can be
determined from Table A–10. Table 6–4 gives reliability factors for some
standard specified reliability.

95
6. Miscellaneous-
Effects Factor kf
Read your textbook p.288
7. Stress
Concentration and
Notch Sensitivity
The factor Kf is commonly called a fatigue stress-concentration factor, and
hence the subscript f. So it is convenient to think of Kf as a stress-concentration
factor reduced from Kt because of lessened sensitivity to notches. The resulting
factor is defined by the equation:
𝐾𝑓 =
𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑠𝑡𝑟𝑒𝑠𝑠 𝑖𝑛 𝑛𝑜𝑡𝑐ℎ𝑒𝑑 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
𝑠𝑡𝑟𝑒𝑠𝑠 𝑖𝑛 𝑛𝑜𝑡𝑐ℎ − 𝑓𝑟𝑒𝑒 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
(𝑎)
Notch sensitivity q is defined by the equation:
𝑞 =
𝐾𝑓 − 1
𝐾𝑡 − 1
𝑜𝑟 𝑞 𝑠ℎ𝑒𝑎𝑟 =
𝐾𝑓 − 1
𝐾𝑡 − 1
(6 − 30)
The fatigue stress-concentration factor, Kf then:
𝐾𝑓 = 1 + 𝑞(𝐾𝑡 − 1) 𝑜𝑟 𝐾𝑓𝑠 = 1 + 𝑞 𝑠ℎ𝑒𝑎𝑟(𝐾𝑡𝑠 − 1) (6 − 31)
For steels
and 2024
aluminum
alloys, use
Fig. 6–20 to
find q for
bending and
axial loading.
For shear
loading, use
Fig. 6–21.
Figure 6–20
has as its
basis the
Neuber
equation,
which is
given by:

96
𝐾𝑓 = 1 +
𝐾𝑡 − 1
1 + √𝑎 𝑟⁄
(6 − 32)
, where √ 𝑎, is deﬁned as the Neuber constant and is a material constant.
Equating Eqs. (6–30) and (6–32) yields the notch sensitivity equation:
𝑞 =
1
1 +
√ 𝑎
√ 𝑟
(6 − 33)
For steel, with Sut in kpsi, the Neuber constant can be approximated by a
third-order polynomial ﬁt of data as:
√ 𝑎 = 0.245 799 − 0.307 794((10−2)𝑆 𝑢𝑡 + 0.150 874(10−4)𝑆 𝑢𝑡
2
− 0.266 978(10−7)𝑆 𝑢𝑡
3 (6 − 34)
To use Eq. (6–32) or (6–33) for torsion for low-alloy steels, increase the
ultimate strength by 20 𝑘𝑝𝑠𝑖 in Eq. (6–34) and apply this value of √ 𝑎.
EXAMPLE 6–3:
A 1015 hot-rolled steel bar has been machined to a diameter of 1 in. It is to be placed in reversed axial
loading for 70 000 cycles to failure in an operating environment of 550°F. Using ASTM minimum
properties, and a reliability of 99 percent, estimate the endurance limit and fatigue strength at 70 000
cycles.

97
EXAMPLE 6-4:
A rotating shaft supported in a ball bearings at
A and D and loaded by the non-rotating force F
as shown in figure below. If all fillets are 3-mm
radius, the shaft rotates, the load is stationary
and the material is machined from AISI 1050
cold-drawn steel, estimate the life of the part.
Known that, the bending moment for the shaft
is shown in figure b.

98
6–11 Characterizing Fluctuating Stresses:

99
Fluctuating stresses in machinery often
take the form of a sinusoidal pattern
because of the nature of some rotating
machinery.
It has been found that in periodic patterns
exhibiting a single maximum and a single
minimum of force, the shape of the wave
is not important, but the peaks on both the
high side (max.) and low side (min.) are
important.
In characterizing the force pattern. If the
largest force is 𝑭 𝒎𝒂𝒙 and the smallest
force is 𝑭 𝒎𝒊𝒏 , then a steady component
and an alternating component can be
constructed as follows:
𝑭 𝒎 =
𝑭 𝒎𝒂𝒙 + 𝑭 𝒎𝒊𝒏
𝟐
𝑭 𝒂 =
𝑭 𝒎𝒂𝒙 − 𝑭 𝒎𝒊𝒏
𝟐
, where Fm is the midrange steady component of force, and Fa is the amplitude of the alternating
component of force. Figure 6–7 illustrates some of the various stress-time traces that occur. The
following relationships and definitions are used when discussing mean and alternating stresses:, some of
which are shown in Fig. 6–7d, are:
𝝈 𝒎𝒊𝒏 = 𝐦𝐢𝐧𝐢𝐦𝐮𝐦 𝐬𝐭𝐫𝐞𝐬𝐬 𝝈 𝒎 = 𝒎𝒊𝒅𝒓𝒂𝒏𝒈𝒆 𝒄𝒐𝒎𝒑𝒐𝒏𝒆𝒏𝒕
𝝈 𝒎𝒂𝒙 = 𝐦𝐚𝐱𝐢𝐦𝐮𝐦 𝐬𝐭𝐫𝐞𝐬𝐬 𝝈 𝒂 = 𝒂𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆 𝒄𝒐𝒎𝒑𝒐𝒏𝒆𝒏𝒕
𝝈 𝒓 = 𝐫𝐚𝐧𝐠𝐞 𝐨𝐟 𝐬𝐭𝐫𝐞𝐬𝐬 𝝈 𝒔 = 𝒔𝒕𝒂𝒕𝒊𝒄 𝒐𝒓 𝒔𝒕𝒆𝒂𝒅𝒚 𝒔𝒕𝒓𝒆𝒔𝒔
The following relations are evident from Fig. 6–7:
𝝈 𝒎 =
𝝈 𝒎𝒂𝒙 + 𝝈 𝒎𝒊𝒏
𝟐
𝝈 𝒂 = |
𝝈 𝒎𝒂𝒙 − 𝝈 𝒎𝒊𝒏
𝟐
| (𝟔 − 𝟑𝟓)
In addition to Eq. (6–35), the stress ratio, R:
𝑹 =
𝝈 𝒎𝒊𝒏
𝝈 𝒎𝒂𝒙
(𝟔 − 𝟑𝟔)
and the amplitude ratio, A:
𝑨 =
𝝈 𝒂
𝝈 𝒎
(𝟔 − 𝟑𝟔)

100
 A steady or static stress is not the same as the mean stress. In fact, it may have any value between
min and max. The steady state exists because of a fixed load or preload applied to the part, and it
is usually independent of the varying portion of the load.
Notes:
 For simple loading, it is acceptable to reduce the endurance limit by either dividing the un-
notched specimen endurance limit by Kf or multiplying the reversing stress by Kf (More safe
because it gives less life cycles).
 For combined loading, which may involve more than one value of fatigue-concentration factor,
the stresses are multiplied by Kf.
 In the case of absence of a notch, 𝝈 𝒂 and 𝝈 𝒎 are equal to the nominal stresses 𝝈 𝒂𝒐 and 𝝈 𝒎𝒐
induced by loads Fa and Fm, respectively.
 In the case of presence of a notch they are 𝑲 𝒇 𝝈 𝒂𝒐 and 𝑲 𝒇 𝝈 𝒎𝒐, respectively, as long as the
material remains without plastic strain. In other words, the fatigue stress concentration factor
𝑲 𝒇 is applied to both components.
 When the steady stress component is high enough to induce localized notch yielding, the designer
has a problem. The material properties ( Sy and Sut) are new and difﬁcult to quantify. The nominal
mean stress method (set 𝝈 𝒂 = 𝑲 𝒇 𝝈 𝒂𝒐 𝒂𝒏𝒅 𝝈 𝒎 = 𝝈 𝒎𝒐 ) gives roughly comparable results to the
residual stress method, but both are approximations. For the purposes of this course, for ductile
materials in fatigue, the steady stress component stress-concentration factor Kfm as:
𝑲 𝒇𝒎 =
{
𝑲 𝒇 𝑲 𝒇|𝝈 𝐦𝐚𝐱,𝒐| < 𝑺 𝒚
𝑺 𝒚 − 𝑲 𝒇 𝝈 𝒂𝒐
|𝝈 𝒎𝒐|
𝑲 𝒇|𝝈 𝐦𝐚𝐱,𝒐| > 𝑺 𝒚
𝟎 𝑲 𝒇|𝝈 𝐦𝐚𝐱,𝒐 − 𝝈 𝒎𝒊𝒏,𝒐| > 𝟐𝑺 𝒚
(𝟔 − 𝟑𝟕)
 To avoid the localized plastic strain at a notch, set 𝝈 𝒂 = 𝑲 𝒇 𝝈 𝒂𝒐, and 𝝈 𝒎 = 𝑲 𝒇 𝝈 𝒎𝒐.
 If the plastic strain at a notch cannot be avoided, then use Eqs. (6–37); or conservatively, set 𝝈 𝒂 =
𝑲 𝒇 𝝈 𝒂𝒐, and use 𝑲 𝒇 𝒎 = 𝟏 , that is, 𝝈 𝒎 = 𝝈 𝒎𝒐.
6–12 Fatigue Failure Criteria for Fluctuating Stress:

101
 Varying both the midrange stress and the
stress amplitude, or alternating component,
will give some information about the fatigue
resistance of parts when subjected to such
situations.
 Three methods of plotting the results of such
tests are in general use and are shown in
figures 6.8, 6.9, and 6.10.
 𝝈 𝒎 Plotted along the x-axis.
 All other components of stress plotted on the
y-axis.
 The modified Goodman diagram (MGD)
consists of the lines constructed to 𝑺 𝒆 (or 𝑺 𝒇)
above or below the origin.
 𝑺 𝒚 is plotted on both axes, because 𝑺 𝒚 would be the criterion of failure if 𝝈 𝒎𝒂𝒙 exceeded 𝑺 𝒚.
 Useful for analysis when all dimension of the part are known and the stress components can be
easily calculated. But it is difficult to use for design when the dimension are unknown.
 The x-axis represents the ratio of the midrange strength 𝑺 𝒎 to the ultimate strength.
 The y-axis represents the ratio of the alternating strength to the endurance limit.
 The line BC represents the
modified Goodman criterion
of failure.
 Existence of midrange stress
in the compressive region has
little effect on the endurance
limit.
 Any Stress state, such as at the
one at A, can be described by
the minimum and maximum
components or by the
midrange and alternating
components.

102
 Safety is indicated whenever the point described by the stress components lies below the
constant-life line.
 When 𝝈 𝒎 is compression,
failure occurs whenever 𝝈 𝒂 =
𝑺 𝒆 or whenever 𝝈 𝒎𝒂𝒙 = 𝑺 𝒚𝒄
 The diagrams are constructed
for analysis and design
purposes.
 They are easy to use and the
results can be scaled off
directly.
 here existed a position which
divided safe from unsafe combinations of 𝝈 𝒂 and 𝝈 𝒎
 In design work for the force amplitude and the mean force can usually be calculated or
determined.
 Considering the modified Goodman line as a criterion, point A represents a limiting point with an
alternating strength Sa and midrange strength Sm. The slope of the load line shown is defined as =
𝑺 𝒂 𝑺 𝒎⁄ . The criterion equations are:
1) Soderberg line:
𝑆 𝑎
𝑆 𝑒
+
𝑆 𝑚
𝑆 𝑦
= 1 (6 − 38)
𝜎 𝑎
𝑆 𝑒
+
𝜎 𝑚
𝑆 𝑦
=
1
𝑛
(6 − 39)
2) The modified Goodman criterion:
𝑆 𝑎
𝑆 𝑒
+
𝑆 𝑚
𝑆 𝑢𝑡
= 1 (6 − 40)
𝜎 𝑎
𝑆 𝑒
+
𝜎 𝑚
𝑆 𝑢𝑡
=
1
𝑛
(6 − 41)
3) The Gerber failure criterion:
𝑆 𝑎
𝑆 𝑒
+ (
𝑆 𝑚
𝑆 𝑢𝑡
)
2
= 1 (6 − 42)
𝑛𝜎 𝑎
𝑆 𝑒
+ (
𝑛𝜎 𝑚
𝑆 𝑢𝑡
)
2
= 1 (6 − 43)
4) The ASME-elliptic:
(
𝑆 𝑎
𝑆 𝑒
)
2
+ (
𝑆 𝑚
𝑆 𝑦
)
2
= 1 (6 − 44)
(
𝑛𝜎 𝑎
𝑆 𝑒
)
2
+ (
𝑛𝜎 𝑚
𝑆 𝑦
)
2
= 1 (6 − 45)
5) The Langer first-cycle-yielding:
𝑆 𝑎 + 𝑆 𝑚 = 𝑆 𝑦 (6 − 46)
𝜎 𝑎 + 𝜎 𝑚 =
𝑆 𝑦
𝑛
(6 − 47)

103
 The failure criteria are used in conjunction with a load line, 𝒓 = 𝑺 𝒂 𝑺 𝒎⁄ = 𝝈 𝒂 𝝈 𝒎⁄ .
 Principal intersections are tabulated in Tables 6-6 to 6-8 (see your textbook).
EXAMPLE 6-5:

104
Torsional Fatigue Strength under Fluctuating Stresses:
The existence of a torsional steady-stress component not more than the torsional yield strength
has no effect on the torsional endurance limit, provide the material is ductile, polished, notch-
free, and cylindrical.
The torsional fatigue limit decreases monotonically with torsional steady stress when the material
has stress concentration, notches or surface imperfections.
In constructing the Goodman diagram:𝑺 𝒔𝒖 = 𝟎. 𝟔𝟕𝑺 𝒖𝒕
Also, from chapter 5, 𝑺 𝒔𝒚 = 𝟎. 𝟓𝟕𝟕𝑺 𝒚𝒕 from distortion-energy theory, and the mean load factor
kc is given by equation 6-26 or 0.577 for torsion.
Combinations of Loading Modes:
 How do we proceed when the loading is a mixture of axial, bending, and torsional loads?
 This type of loading introduces a few complications in that there may now exist combined
normal and shear stresses, each with alternating and midrange values, and several of the
factors used in determining the endurance limit depend on the type of loading. There may
also be multiple stress-concentration factors, one for each mode of loading. The problem of
how to deal with combined stresses was encountered when developing static failure theories.
 The distortion energy failure theory proved to be a satisfactory method of combining the

105
multiple stresses on a stress element into a single equivalent von Mises stress. The same
approach will be used here:
1) The first step is to generate two stress elements—one for the alternating stresses and one for the
midrange stresses.
2) Next, apply the appropriate fatigue stress concentration factors to each of the stresses; i.e., apply
(𝑲 𝒇)
𝒃𝒆𝒏𝒅𝒊𝒏𝒈
for the bending stresses, (𝑲 𝒇𝒔)
𝒕𝒐𝒓𝒔𝒊𝒐𝒏
for the torsional stresses, and (𝑲 𝒇)
𝒂𝒙𝒊𝒂𝒍
for the
axial stresses. Then calculate an equivalent von Mises stress for each of these two stress
elements,𝝈 𝒂
′
, and 𝝈 𝒎
′
. For the endurance limit, 𝑺 𝒆, use the endurance limit
modifiers, 𝒌 𝒂 , 𝒌 𝒃, 𝒂𝒏𝒅 𝒌 𝒄 for bending stress only and do not use 𝒌 𝒄 nor divide by 𝑲 𝒇 or 𝑲 𝒇 𝒔 . If
axial stress is present divide the alternating axial stress only by 𝒌 𝒄 = 𝟎. 𝟖𝟓 . For the special
case of combined bending, torsional shear, and axial stresses:
𝝈 𝒂
′
= {[(𝑲 𝒇)
(𝝈 𝒂) 𝒃𝒆𝒏𝒅𝒊𝒏𝒈 + (𝑲 𝒇)
𝒂𝒙𝒊𝒛𝒍
(𝝈 𝒂) 𝒂𝒙𝒊𝒂𝒍
𝒐. 𝟖𝟓
]
𝟐
+ 𝟑 [(𝑲 𝒇𝒔)
(𝝉 𝒂)𝒕𝒐𝒓𝒔𝒊𝒐𝒏]
𝟐
}
𝟏
𝟐
(𝟔 − 𝟒𝟖)
𝝈 𝒎
′
= {[(𝑲 𝒇)
(𝝈 𝒎) 𝒃𝒆𝒏𝒅𝒊𝒏𝒈 + (𝑲 𝒇)
𝒂𝒙𝒊𝒛𝒍
(𝝈 𝒎) 𝒂𝒙𝒊𝒂𝒍]
𝟐
+ 𝟑 [(𝑲 𝒇𝒔)
(𝝉 𝒎) 𝒕𝒐𝒓𝒔𝒊𝒐𝒏]
𝟐
}
𝟏
𝟐
(𝟔 − 𝟒𝟗)
3) Finally, select a fatigue failure criterion {modified Goodman, Gerber, ASME-elliptic, or
Soderberg [see Eq. (6–38) to (6–47)]} to complete the fatigue analysis.
4) Conservative check for localized yielding using von Mises stresses, as:
𝝈 𝒂
′
+ 𝝈 𝒎
′
=
𝑺 𝒚
𝒏
(𝟔 − 𝟓𝟎)
5) For first-cycle localized yielding, the maximum von Mises stress is calculated. Then substitute
σmax and τmax into the equation for the von Mises stress. A simpler and more conservative method
is to add Eq. (6–48) and Eq. (6–49). That is, 𝝈 𝒎𝒂𝒙
′
= 𝝈 𝒂
′
+ 𝝈 𝒎
′
.
Example 6-6:
A 38 mm diameter bar has been machined from AISI 1050 CD steel. This part is to withstand a
fluctuating tensile load varying from zero to 71.2kN. Because of the ends and the fillet radius, a fatigue
stress-concentration factor 𝑲 𝒇 is 1.85 for 𝟏𝟎 𝟔
or larger life. Find 𝑺 𝒂, 𝑺 𝒎, and the factor of safety
guarding against fatigue (𝒏 𝒇) and first-cycle yielding (𝒏 𝒚) using (a) Gerber method and (b) Goodman

106
method.
6–15 Varying, Fluctuating Stresses; Cumulative Fatigue Damage:
The method used here amounts to a variation of the rain-ﬂow counting technique. The Palmgren-Miner
cycle-ratio summation rule, also called Miner’s rule, is written as:
𝐶 = ∑
𝑛𝑖
𝑁𝑖
(6 − 51)
, where ni is the number of cycles at stress level σi and Ni is the number of cycles to failure at stress
level σi. The parameter C has been determined by experiment; it is usually found in the range 0.7 <
𝑐 < 2.2 with an average value near unity.
Using the deterministic formulation as a linear damage rule we write:
𝐷 = ∑
𝑛𝑖
𝑁𝑖
(6 − 52)
, where D is the accumulated damage. When D = c = 1 , failure ensues. And the total number of cycles
is obtained as:
𝐷 = ∑
∝𝑖
𝑁𝑖
≥
1
𝑁
𝑎𝑛𝑑 ∝𝑖=
𝑛𝑖
𝑁
(6 − 53)

107
Problem
6–30
A machine part will be cycled at ±48 𝑘𝑝𝑠𝑖 for 4(103
) cycles. Then the loading will be changed to
±38 𝑘𝑝𝑠𝑖 for 6(104
) cycles. Finally, the load will be changed to ±32 𝑘𝑝𝑠𝑖. How many cycles of operation
can be expected at this stress level? For the part, 𝑆 𝑢𝑡 = 76 𝑘𝑝𝑠𝑖, 𝑓 = 0.9 , and has a fully corrected
endurance strength of 𝑆 𝑒 = 30 𝑘𝑝𝑠𝑖. (a) Use Miner’s method. (b) Use Manson’s method.

108
Problem
6–31
A rotating-beam specimen with an endurance limit of 50 kpsi and an ultimate strength of 100 kpsi
is cycled 20 percent of the time at 70 kpsi, 50 percent at 55 kpsi, and 30 percent at 40 kpsi. Let
𝑓 = 0.9 and estimate the number of cycles to failure.
Problem
6–8
A solid round bar, 25 mm in diameter, has a groove 2.5-mm deep with a 2.5-mm radius
machined into it. The bar is made of AISI 1018 CD steel and is subjected to a purely
reversing torque of 200 N · m. For the S-N curve of this material, let f = 0.9 . (a) Estimate
the number of cycles to failure. (b) If the bar is also placed in an environment with a
temperature of 450 o
C , estimate the number of cycles to failure.

109
Problem
6–17
The cold-drawn AISI 1018 steel bar shown in the ﬁgure is subjected to an axial load
fluctuating between 800 and 3000 lbf. Estimate the factors of safety ny and nf using (a) a
Gerber fatigue failure criterion as part of the designer’s fatigue diagram, and (b) an ASME-
elliptic fatigue failure criterion as part of the designer’s fatigue diagram.

Chapter 6 fatigue failure loading

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