LECTURER 4 Fundamental Mechanical Properties Fatigue Cree

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LECTURER 4

• Fundamental Mechanical Properties
• Fatigue
• Creep

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Fatigue

• Fatigue is caused by repeated application of
  stress to the metal. It is the failure of a
  material by fracture when subjected to a cyclic
  stress.
• Fatigue is distinguished by three main features.
• i) Loss of strength
• ii) Loss of ductility
• iii) Increased uncertainty in strength and
• service life

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Fatigue

• Fatigue is an important form of behaviour in all
  materials including metals, plastics, rubber and
  concrete.
• All rotating machine parts are subjected to
  alternating stresses.
• Example aircraft wings are subjected to repeated
  loads, oil and gas pipes are often subjected to
  static loads but the dynamic effect of
  temperature variation will cause fatigue.
• There are many other situations where fatigue
  failure will be very harmful.
• Because of the difficulty of recognizing fatigue
  conditions, fatigue failure comprises a large
  percentage of the failures occurring in
  engineering.
• To avoid stress concentrations, rough surfaces
  and tensile residual stresses, fatigue specimens
  must be carefully prepared.

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Fatigue

• The S-N Curve
• A very useful way to visual the failure for a
  specific material is with the S-N curve.
• The S-N means stress verse cycles to failure,
  which when plotted using the stress amplitude on
  the vertical axis and the number of cycle to
  failure on the horizontal axis.
• An important characteristic to this plot as seen
  is the fatigue limit.

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Fatigue

• The point at which the curve flatters out is
  termed as fatigue limit and is well below the
  normal yield stress.
• The significance of the fatigue limit is that if
  the material is loaded below this stress, then it
  will not fail, regardless of the number of times
  it is loaded.
• Materials such as aluminium, copper and magnesium
  do not show a fatigue limit therefore they will
  fail at any stress and number of cycles.
• Other important terms are fatigue strength and
  fatigue life.
• The fatigue strength can be defined as the stress
  that produces failure in a given number of cycles
  usually 107.
• The fatigue life can be defined as the number
  of cycles required for a material to fail at a
  certain stress.

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Factors affecting fatigue properties

• Surface finish
• Scratches dents identification marks can act as
  stress raisers and so reduce the fatigue
  properties.
• Electro-plating produces tensile residual
  stresses and have a deterimental effect on the
  fatigue properties.
• Temperature
• As a consequence of oxidation or corrosion of the
  metal surface increasing, increase in temperature
  can lead to a reduction in fatigue properties.

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Factors affecting fatigue properties

• Residual stresses
• Residual stresses are produced by fabrication
  and finishing
• processes.
• Residual stresses on the surface of the material
  will improve the fatigue properties.
• Heat treatment
• Hardening and heat treatments reduce the surface
  compressive stresses as a result the fatigue
  properties of the materials are getting
  affected.
• Stress concentrations
• These are caused by sudden changes in cross
  section holes or sharp corners can more easily
  lead to fatigue failure. Even a small hole
  lowers fatigue-limit by 30.

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Stress Cycles

• There are different arrangements of fatigue
  loading.
• The simplest type of load is the alternating
  stress where the stress amplitude is equal to the
  maximum stress and the mean or average stress is
  zero. The bending stress in a shaft varies in
  this way.

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Fatigue Failure

• Fatigue fracture results from the presence of
  fatigue cracks, usually initiated by cyclic
  stresses, at surface imperfections such as
  machine marking and slip steps.
• The initial stress concentration associated with
  these cracks are too low to cause brittle
  fracture they may be sufficient to cause slow
  growth of the cracks into the interior.
• Eventually the cracks may become sufficiently
  deep so that the stress concentration exceeds the
  fracture strength and sudden failure occurs.
• The extent of the crack propagation process
  depends upon the brittleness of the material
  under test.
• In brittle materials the crack grows to a
  critical size from which it propagates right
  through the structures in a fast manner, whereas
  with ductile materials the crack keeps growing
  until the remaining area cannot support the load
  and an almost ductile fracture suddenly occurs.

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Fatigue Failure

• Failure can be recognized by the appearance of
  fracture.
• For a typical fracture ,Two distinct zones can be
  distinguished a smooth zone near the fatigue
  crack itself which, has been smoothened by the
  continual rubbing together of the cracked
  surfaces, and a rough crystalline-looking zone
  which is the final fracture.
• Occasionally fatigue cracks show rough concentric
  rings which correspond to successive positions of
  the crack.

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Design for Fatigue

• To secure satisfactory fatigue life
• Modification of the design to avoid stress
  concentration eliminating sharp recesses and
  severe stress raisers.
• Precise control of the surface finish by avoiding
  damage to surface by rough machining, punching,
  stamping, shearing etc.
• Control of corrosion and erosion or chemical
  attack in service and to prevent of surface
  decarburization during processing of heat
  treatment.
• Surface treatment of the metal.

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Creep

• The creep is defined as the property of a
  material by virtue of which it deforms
  continuously under a steady load.
• Creep is the slow plastic deformation of
  materials under the application of a constant
  load even for stressed below the yield strength
  of the material.
• Usually creep occurs at high temperatures.
• Creep is an important property for designing I.C.
  engines, jet engines, boilers and turbines. Iron,
  nickel, copper and their alloys exhibited this
  property at elevated temperature.
• But zin, tin, lead and their alloys shows creep
  at room temperature.
• In metals creep is a plastic deformation caused
  by slip occurring along crystallographic
  directions in the individual crystals together
  with some deformation of the grain boundary
  materials.

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Creep

• The creep curve usually consists of three \
  stages of creep.
• Primary Stage
• In this stage the creep rate decreases with time,
  the effect of work hardening is more than that of
  recovery processes. The primary stage is of
  great interest to the designer since it forms an
  early part of the total extension reached in a
  given time and may affect clearness provided
  between components of a machine.

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Creep

• Secondary Stage
• In this stage, the creep rate is a minimum and
  is constant with time. The work hardening and
  recovery processes are exactly balanced. It is
  the important property of the curve which is used
  to estimate the service life of the alloy.
• Tertiary Stage
• In this stage, the creep rate increases with
  time until fracture occurs. Tertiary creep can
  occur due to necking of the specimen and other
  processes that ultimately result in failure.
• The Creep Limit is the stress at which a
  material can be formed by a definite magnitude
  during a given time at a given temperature. The
  calculation of creep limit includes the
  temperature, the deformation and the time in
  which this deformation appears.

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Types of Creep

• Creep are classified based on temperature
• Logarithmic Creep
• Recovery Creep
• Diffusion Creep
• At low temperature the creep rate decreases with
  time and the logarithmic creep curve is obtained.
• At high temperature, the influence of work
  hardening is weakened and there is a possibility
  of mechanical recovery. As a result, the creep
  rate does not decrease and the recovery creep
  curve is obtained.
• At very high temperature, the creep is primarily
  influenced by diffusion and load applied has
  little effect. This creep is termed as diffusion
  creep or plastic creep.

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Factors affecting Creep

• Heat Treatment
• Creep resistance of steel is affected by heat
  treatment.
• At temperatures of 300C or higher maximum creep
  resistance is usually produced. But the quacking
  and drawing decreases the creep resistance.
• Grain size
• The major factor in creep is grain size.
• Normally large grained materials exhibit better
  creep resistance than fine grained one based on
  the temperature.
• At temperatures below the lowest temperature of
  recrystallisation, a fine grained structure
  possesses the greater resistance whereas at
  temperature above this point a large grained
  structure possesses the greater resistance and we
  must select it for high temperature applications.

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Factors affecting Creep

• Strain Hardening
• Strain hardening of steel increases its creep
  resistance.
• Particularly below the equicohesive temperature
  at which the fracture changes from intra
  crystalline to inner-crystalline strain hardening
  increases the creep resistance and hence there is
  no measurable creep. So the second stage of
  creep curve is almost horizontal.
• At temperature above the equicohesive temperature
  yield rate exceeds the strain hardening rate and
  creep will proceed even under low stresses.
• Alloying additions
• At temperatures, below the lowest temperatures of
  recrystallation the creep resistance of steel may
  be improved by the finite forming elements like
  nickel, cobalt and manganese or by the carbide
  forming elements like chromium molybdenum,
  tungsten and vanadium.

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Mechanism of Creep

• Some mechanisms that play vital roles during the
  creep process are
• Dislocation climb
• Vacancy Diffusion
• Grain boundary sliding

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Mechanism of Creep

• At high temperature, the appreciate atomic
  movement causes the dislocation to climb up or
  down.
• By a simple climb of edge dislocation the
  diffusion rate of vacancies may produce a motion
  in response to the applied stress.
• Thus edge dislocations are piled up by the
  obstacles in the glide plane and the rate of
  creep is governed by the rate of escape of
  dislocation.
• Another mechanism of creep is called diffusion of
  vacancies.
• In this mechanism, the diffusion of vacancies
  controls the creep rate but does not involve the
  climb of edge dislocations.
• It depends on the migration of vacancies from one
  side of a grain to another. In response to the
  applied stress, the vacancies move from surfaces
  of the specimen transverse to the stress axis

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Mechanism of Creep

• The third mechanism of creep is sliding of grain
  boundaries.
• It means sliding of neighboring grains with
  respect to the boundary that separates them.
• Grain boundaries become soft at low temperature
  as compared to individual grains.
• Grain boundaries play a major role in the creep
  of polycrystals at high temperatures as they side
  past each other or create vacancies.
• At high temperature, ductile metals begin to lose
  their ability to strain harden and become
  viscous to facilitate the sliding of grain
  boundaries.
• As the temperature increases the grain
  boundaries facilitate the deformation process by
  sliding, whereas at low temperature, they
  increase the yield strength by stopping the
  dislocations.