Failure mechanics: Fatigue Failure

i. Fracture
ii. Fatigue failure
iii. creep
Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com)
1

Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com)
Part 2
2

 Fatigue is a form of failure that occurs in structures subjected to
dynamic & fluctuating stresses (e.g., bridges, aircraft, & machine
components).
 Under these circumstances it is possible for failure to occur at a stress
level considerably lower than the tensile or yield strength for a static
load.
 The term “fatigue” is used because this type of failure normally
occurs after a lengthy period of repeated stress or strain cycling.
 Fatigue is important inasmuch as it is the single largest cause of
failure in metals, estimated to comprise approximately 90% of all
metallic failures; polymers & ceramics (except for glasses) are also
susceptible to this type of failure.
Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com) 3

 Furthermore, fatigue is catastrophic & insidious, occurring very
suddenly & without warning.
 Fatigue failure is brittle like in nature even in normally ductile metals,
in that there is very little, if any, gross plastic deformation associated
with failure.
 The process occurs by the initiation & propagation of cracks, &
ordinarily the fracture surface is perpendicular to the direction of an
applied tensile stress.

• Cyclic stress distribution of forces (a stress)
that changes over time in a repetitive fashion.
• When cyclic stresses are applied to a material,
even though the stresses do not cause plastic
deformation, the material may fail due to
fatigue.
• Fatigue failure is typically modeled by
decomposing cyclic stresses into mean &
alternating components.
• Mean stress is the time average of the principal
stress.
• The definition of alternating stress varies
between different sources.
• It is either defined as the difference between
the minimum & the maximum stress, or the
difference between the mean & maximum
stress.

 It can be developed in four stages.
 Crack nucleation;
 Crack-growth;
 Ultimate failure.
 Crack Nucleation:
 Fatigue failure begins with formation of a small crack, generally at some
point on the external surface

 Crack Growth:
 The crack formed on surface is then develops slowly into the material in a direction
roughly perpendicular to the main tensile axis.
 Ultimately the cross- sectional area of the member will have been so reduced that it
can no longer withstand the applied load & ordinary tensile fracture will result path
which the crack has followed
 Fracture:
 A fatigue crack ‘front’ advances a small amount during each stress cycle & each
increment of advance is shown on the fracture surface as a minute ripple line.
 These ripple lines radiate out from the origin of fracture as a series of
approximately concentric arcs.
 These individual ripples are visible only by very high-powered metallographic
methods. Few ripples much larger than the rest & shows the general

 The figure shows mechanism of fatigue failure in three stages.
 The crack propagates slowly from the source, the fracture surface rub together
due to pulsating nature of the stress & so the surface become burnished.
 Fatigue failure in metals is very easy to identify.
 The fatigue cracks are not result of brittle fracture but of plastic slip.

 A number of theories (or mechanism) proposed for
fatigue failure are as follow
 Orowan’s theory
 Wood’s theory
 Cottrell & hull theory
 Mott theory

 Orowan’s theory
 The metal is considered to contain small, weak regions, which may be areas
of favourable orientation for slip or areas of high stress concentration due to
metallurgical notches such as inclusions.
 If the loading or the stress is such that the total plastic strain in the weak
region exceeds the critical value, a crack is formed.
 Mott theory
 Mott suggested a model involving the cross slip of screw dislocations & as a
result a column of metal is extruded from the surface & a cavity is left behind
in the interior of the crystal which is source of fatigue crack & ultimately the
fatigue fracture occurs

 Wood interprets microscopic observation of slip produced by fatigue as
indicated that slip bands are the result of a systematic build-up of fine slip
movement, corresponding movements of the order of 10^-5 to 10^-4 cm,
which are observed for static slip bands.
 According to wood, the back & fort fine slip movement of fatigue could
build up notches or ridges at the surface as shown in figure.
 These notches act as stress raisers & this way starts a fatigue crack which
ultimately leads to fatigue fracture.

 A model involving the interaction of edge dislocation on slip systems was
suggested by Cottrell & Hull.
 Two different slip systems when work with different directions & planes of
slip produce slip step at surface, thereby forming intrusions & extrusions as
shown in figure
 Fatigue crack starts from intrusions at the surface.

 The method of testing the metal for fatigue was developed by Wohler.
 The figure shows a typical experimental set up for Wohler’s setup.
 The specimen is in the form of a cantilever & loaded at one end
through ball bearing.
tension on bottom
compression on top
counter
motor
flex coupling
specimen
bearing bearing

 It is rotated by means of a high speed motor to which a counter is
attached to count the number of rotations.
 At any instant, the upper surface of the specimen is under tension &
lower surface is under compression, with the neutral axis.
 In one rotation, the specimen undergoes two cyclic fluctuations of
stress.
 The number of cycles to cause failure will vary with the applied stress.
 Higher the stress, lower will be the cycles to cause failure.
 Similarly, if the stress is lowered, more number of cycles required to
cause failure.

 The basic method of presenting engineering fatigue data is by means
of the S-N curve, a plot of stress ‘S’ against the number of cycles to
failure ‘N’.
 The value of stress that is plotted can be alternating stress, maximum
stress, or minimum stress.
 Most determinations of the fatigue properties of materials have been
made in completed reverse bending.

 For a few important engineering materials such as steel & titanium, the S-N
curve becomes horizontal at a certain limiting stress.
 Below this limiting stress, which is called the fatigue limit or endurance
limit, the material can endure an infinite number of cycles without failure.
 Most non ferrous metals, like aluminium, magnesium & copper alloys, have
an S-N curve which slopes gradually downward with increasing number of
cycles.
 These materials do not have a true fatigue limit because the S-N curve
never becomes horizontal.
 In in such cases it is common practice to characterize the fatigue properties
of material by giving the fatigue strength at an arbitrary number of cycles,
e.g. 10^8 cycles.

 Fatigue strength is seriously reduced by following factors:
 Notch effect
 Surface effect
 Corrosion fatigue
 Thermal fatigue
 Pre-stressing

 Fatigue strength of material is reduced by presence of notch in the
material.
 E.g. Machine element it contains fillets, keyways, screw threads and
holes.
 Fatigue cracks in structure parts usually starts at such geometrical
irregularities.
 The effect of fatigue is generally studied by specimens containing a
‘V’ notch or a circular notch.
 The effect of notches on fatigue strength is determined by comparing
the S-N curves of notched and un-notched specimens.

 Fatigue failure can be produced by fluctuating thermal stress under conditions
where no stress are produced by mechanical causes.
 Thermal stress result when the change in dimensions of a member as the result of
a temperature change.
 For the simple case of a bar with fixed end supports, the thermal stress developed
by a temperature change ‘dT’ is
 σ = α*E*dT
 where ,
 α = Linear thermal coefficient of expansion.
 E = Elastic modulus
 If failure occurs by one application of thermal stress, the condition is called
thermal shock.
 However, if failure occurs after repeated application of thermal stress it is called
as Thermal fatigue.

 All fatigue failures start at the surface. The factors which affect on the
surface of a fatigue specimen are roughly divided into 3 categories:
 Surface roughness
 Changes in surface properties
 Surface residual stress
 Surface roughness:
 Smoothly polished specimen in which the fine scratches are oriented parallel
with the direction of the principle tensile stress, give the highest values in
fatigue tests.
 Such polished specimen are usually used in laboratory fatigue tests and are
known as ‘par bars’.

 Change in surface properties:
 Fatigue failure is dependent on the surface conditions.
 Decarburization of the surface of heat-treated steel reduces fatigue
performance.
 Carburizing and nitriding makes steel surface stronger and harder, which
improves fatigue properties.
 The fatigue performance is improved when notched fatigue specimens are
nitrided.
 Electroplating of surface generally decreases the fatigue limit of steel.
 Surface residual stress:
 Residual stresses are stresses that exist in a part independent of any external
force.
 Nearly every manufacturing operation will result in residual stresses in varying
degrees.
 Residual stresses are beneficial when they are opposite to the applied load.

 The simultaneous action of cyclic stress and chemical attack is known
as corrosion fatigue.
 Corrosion attack without superimposed stress often produces pitting
of metal surfaces.
 The pits acts as notches and produce a reduction in fatigue strength.
 When corrosion and fatigue occur simultaneously, the chemical attack
greatly accelerates the rate at which fatigue cracks propagate.
 Material which shows a definite fatigue limit when tested in air at
room temperature shows no indication of a fatigue limit when the test
is carried out in a corrosive environment.

 When fatigue test is carried out in air not affected by the speed of testing, over a
range from about 10 to 200 Hz, when test is carried out in corrosive environment
there is a definite dependence on testing speed.
 Since corrosive attack is a time-dependent phenomenon, the higher the testing speed,
the smaller the damage due to corrosion.
 In usual method, corrosion-fatigue test is carried out by continuously subjecting
specimen in combined influences of corrosion and cyclic stress until failure occurs.
 A number of method are available for minimizing corrosion-fatigue damage.
 In general material is protected from corrosive environment by metallic and non-
metallic coating is successful method.
 Nitriding is effective in minimizing corrosion fatigue.

 Pre-stressing is the process of loading an engineering component
under controlled conditions to a cyclic stress for a fixed number of
cycles prior to any possibility of fatigue failure.
 When magnitude of pre-stressing is lower than the operating stress
level then it is known as under-stressing.
 Under controlled conditions when the magnitude of the stress is higher
than operating stress level the condition is called overstressing.
 The number of pre-stressing cycles is always lesser than the number
of cycles needed to cause fatigue failure.
 Pre-stressing is desirable under conditions of under-stressing.

 An under stressed components always exhibits best fatigue resistance.
 For an over-stressed component the failure resistance is lesser
compared to a components that has been pre-stresses.
 Thus under stressing cause significant improvement in fatigue
behaviour by strengthening the weak regions and enhancing their
dynamic response to operating stress.

Failure mechanics: Fatigue Failure

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