DESIGNING AGAINST FATIGUE

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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 cross-section increase until it
  reaches a high enough level to cause catastrophic
  fracture of the part.

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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.

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DESIGNING AGAINST FATIGUE
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DESIGNING AGAINST FATIGUE

• Types of fatigue loading
• Alternating stress
• Alternating tension compression
• Stress ratio, R ?min / ?max -1
• Fluctuating stress
• Positive R value
• Greater tensile stress than compressive stress
• ?max ?m ?a
• ?max ?m - ?a

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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

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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

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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)

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DESIGNING AGAINST FATIGUE
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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.

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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.

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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

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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

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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

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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

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DESIGNING AGAINST FATIGUE

• kg Service environment factor
• Accounts for the reduced fatigue strength due to
  the action of a hostile environment.

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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.

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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

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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

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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.

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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

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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.

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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.

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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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DESIGNING AGAINST FATIGUE

• 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

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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.