DESIGNING AGAINST FATIGUE
Fatigue failure account for about 80 % of part failure in
engineering
Occurs subjected to fluctuating loads
Generally, fatigue fractures occurs as a result of crack
which usually start at some discontinuity in the material,
or at other stress concentration location, and then
gradually grow under repeated application of load.
As the crack grows, the stress on the load-bearing crosssection increase until it reaches a high enough level to
cause catastrophic fracture of the part.
DESIGNING AGAINST FATIGUE
Fracture surface which usually exhibits smooth areas
which correspond to the gradual crack growth stage, and
rough areas, which correspond to the catastrophic fracture
stage.
The smooth parts of the fracture surface usually exhibit
beach marks which occurs as a result of changes in the
magnitude of the fluctuating fatigue load.
Fatigue behavior of materials is usually described by
means of the S-N diagram which gives the number of
cycles to failure, N as a function of the max applied
alternating stress, Sa.
DESIGNING AGAINST FATIGUE
DESIGNING AGAINST FATIGUE
Types of fatigue loading
1. Alternating stress
Alternating tension – compression
Stress ratio, R = min / max = -1
2. Fluctuating stress
Positive R value
Greater tensile stress than compressive stress
max = m + a
max = m - a
DESIGNING AGAINST FATIGUE
Many types of test are used to determine the fatigue life of
material
Small scale fatigue test – rotating beam test
Which a specimen subjected to alternating compression and
tension stresses of equal magnitude while being rotate
Data from this result are plotted in the form of S-N
curves
Which the stress S to cause failure is plotted against number
of cycles N
Figure (a) – S-N curves for carbon steel
(b) - S-N curves aluminum alloy
DESIGNING AGAINST FATIGUE
In the majority cases, the reported fatigue strength or endurance
limits of the materials are based on the test of carefully prepared
small samples under laboratory condition.
Such values cannot be directly used for design purposes because the
behavior of a component or structure under fatigue loading does
depend not only on the fatigue or endurance limit of the material
used in making it, but also an several other factors including :
Size and shape of the component or structure
Type of loading and state of stress
Stress concentration
Surface finish
Operating temperature
Service environment
Method of fabrication
DESIGNING AGAINST FATIGUE
Endurance-limit modifying factors
Se = kakbkckdkekfkgkhSe’
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 = operating T factor (accounts for diff. in working T & room T)
ke = loading factor (differences in loading types)
kf = stress concentration factor
kg = service environment factor (action of hostile environment)
kh = manufacturing processes factor (influence of fabrication
parameters)
DESIGNING AGAINST FATIGUE
DESIGNING AGAINST FATIGUE
ka = Surface finish factor
The surface finish factor, ka, is introduced to account for
the fact that most machine elements and structures are not
manufactured with the same high-quality finish that is
normally given to laboratory fatigue test specimens.
DESIGNING AGAINST FATIGUE
kb = Size factor
Large engineering parts have lower fatigue strength than
smaller test specimen
Greater is the probability of finding metallurgical
flaws that can cause crack initiation
Following values can be taken as rough guidelines :
kb = 1.0 for component diameters less than 10 mm
kb = 0.9 for diameters in the range 10 to 50 mm
kb = 1 – [( D – 0.03)/15], where D is diameter expressed
in inches, for sizes 50 to 225 mm.
DESIGNING AGAINST FATIGUE
kc = Reliability factor
Accounts for random variation in fatigue strength.
Published data on endurance limit, represent 50 % survival
fatigue test.
Since most design require higher reliability, the published
data must be reduced by the factor of kc
The following value can be taken as guidelines
kc = 0.900 for 90% reliability
kc = 0.814 for 99 % reliability
kc = 0.752 for 99.9 % reliability
DESIGNING AGAINST FATIGUE
kd = Operating 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 4500C
kd = 1
At higher operating temperature
kd = 1 – 5800( T – 450 ) for T between 450 and 550oC, or
kd = 1 – 3200( T – 840 ) for T between 840 and 1020oF
DESIGNING AGAINST FATIGUE
ke = Loading factor
Accounts for the difference in loading between lab. test and
service.
During service – vibration, transient overload, shock
From experience show that repeated overstressing can
reduce the fatigue life
Different type of loading, give different stress distribution
ke = 1 for application involving bending
ke = 0.9 for axial loading
ke = 0.58 for torsional loading
DESIGNING AGAINST FATIGUE
kf = Stress concentration factor
Accounts for the stress concentration which may arise when
change in cross-section
kf = endurance limit of notch-free part
endurance limit of notched part
Low strength, ductile steels are less sensitive to notched
than high-strength steels
DESIGNING AGAINST FATIGUE
kg = Service environment factor
Accounts for the reduced fatigue strength due to the action
of a hostile environment.
DESIGNING AGAINST FATIGUE
kh = Manufacturing process factor
Accounts for the influence of fabrication parameter
Heat treatment, cold working, residual stresses and
protective coating on the fatigue material.
kh difficult to quantify, but important to included.
DESIGNING AGAINST FATIGUE
Endurance limit/Fatigue strength
The endurance limit, or fatigue strength, of a given material
can usually be related to its tensile strength, as shown in
table 2.2.
The endurance ratio, defined as (endurance limit/ tensile
strength), can be used to predict fatigue behavior in the
absence of endurance limits results.
From the table shows, endurance ratio of most ferrous
alloys varies between 0.4 and 0.6
DESIGNING AGAINST FATIGUE
Other fatigue-design criteria
Safe-life or finite-life
Design is based on the assumption that the component is
free from flaws, but stress level in certain areas is higher
than the endurance limit of the material
Means that fatigue-crack initiation is inevitable and the
life of the component is estimated on the number of
stress cycles which are necessary to initiate crack
DESIGNING AGAINST FATIGUE
Fail-safe design
Crack that form in service will be detected and repaired before
they can lead to failure.
Employed material adapted with high fracture toughness, crack
stopping features and reliable NDT program to detect crack.
Damage-tolerant design
Is an extension of fail-safe criteria and assume that flaws exist in
the component before they put in service.
Fracture mechanics techniques are used to determine whether such
crack will grow large enough to cause failure before they are
detected during periodic inspection.
DESIGNING AGAINST FATIGUE
Selection of materials for fatigue resistance
In many application, the behavior of a component in
service is influence by several other factor besides the
properties of the material used in its manufacture.
This is particularly true for the cases where the component
or structure is subjected to fatigue loading.
The fatigue resistance can be greatly influenced by the
service environment, surface condition of the part, method
of fabrication and design details.
In some cases, the role of the material in achieving
satisfactory fatigue life is secondary to the above
parameters, as long as the material is free from major
flaws
DESIGNING AGAINST FATIGUE
Steel and cast iron
Steel are widely used as structural materials for fatigue
application as they offer high fatigue strength and good
processability at relatively low cost.
The optimum steel structure for fatigue is tempered
martensite, since it provide max homogeneity
Steel with high hardenability give high strength with
relatively mild quenching and hence, low residual stresses,
which is desire in fatigue applications.
Normalized structure, with their finer structure give better
fatigue resistance than coarse pearlite structure obtained by
annealing.
DESIGNING AGAINST FATIGUE
Nonferrous alloys
Unlike ferrous alloy, the nonferrous alloys, with the
exception of titanium, do not normally have endurance
limit.
Aluminum alloys usually combine corrosion resistance,
light weight, and reasonable fatigue resistance
Fine grained inclusion-free alloys are most suited for
fatigue applications.
DESIGNING AGAINST FATIGUE
Plastics
The viscoelasticity of plastics makes their fatigue behavior
more complex than that of metals.
Fatigue behavior of plastics is affected by the type of
loading, small changes in temperature and environment
and method of fabrication
Because of their low thermal conductivity, hysteretic
heating can build up in plastics causing them to fail in
thermal fatigue or to function at reduces stiffness level.
The amount of heat generated increases with increasing
stress and test frequency.
This means that failure of plastics in fatigue may not necessarily
mean fracture
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Composite materials
The failure modes of reinforced materials in fatigue are
complex and can be affected by the fabrication process
when difference in shrinkage between fibers and matrix
induce internal stresses.
However from practical experiences, some fiber reinforced
plastics are known to perform better in fatigue than some
metal, refer table 2.2.
The advantage of fiber-reinforced plastics is even more
apparent when compared on a per weight basics.
As with static strength, fiber orientation affects the fatigue
strength of fiber reinforced composite
DESIGNING AGAINST FATIGUE
In unidirectional composites, the fatigue strength is
significantly lower in directions other than the fiber
orientation.
Reinforcing with continuous unidirectional fibers is more
effective than reinforcing with short random fibers.