Failure

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Chapter 8

• Failure

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Failure

• Failure is undesirable Putting human lives in
  jeopardy, economic losses, affecting availability
  of products and services.
• Causes
• Improper materials election and processing.
• Inadequate design of the component.
• Misuse

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Fundamentals of fracture

• Simple fracture Separation of body into two or
  more pieces in response to an imposed stress that
  is static (i.e., constant or slowly changing with
  time) and at temperatures that are low relative
  to the melting temperature.
• Fracture modes based on ability to experience
  plastic deformation
• Ductile.
• Brittle.

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Fundamentals of fracture (Cont.)

• For tensile stress, most metal alloys are
  ductile.
• Ceramics are brittle, polymers exhibit both types
  of fracture.
• See bend test for ceramics
• Fracture steps
• Crack formation.
• Crack propagation.
• Mode of fracture depends on mechanism of crack
  propagation.

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Highly ductile fracture Moderately ductile
fracture Brittle fracture
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Cup-and-cone fracture in aluminum Brittle
fracture in a mild steel
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Ductile fracture

• The material exhibits substantial plastic
  deformation in the vicinity of an advancing crack
  with high energy absorption before fracture.
  There is evidence of appreciable gross
  deformation at fracture surfaces (e.g., twining
  and tearing).
• It proceeds relatively slowly as the crack length
  is extended.
• Crack is stable, i.e., resists any further
  extension unless there is an increase in applied
  stress.
• Cup-and-cone facture type.

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Ductile fracture (Moderately)
Evolution to failure
50 mm
50 mm
Resulting fracture surfaces (steel)
100 mm
particles serve as void nucleation sites.
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Ductile fracture (Cont.)

• Ductile fracture preferred due to
• Ductile fracture gives warning (due to associated
  plastic deformation). This allows preventive
  measures to be taken.
• More strain energy is required to induce ductile
  facture (Ductile materials are tougher).

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Brittle fracture

• The material experiences little or no plastic
  deformation with low energy absorption.
• Cracks may spread extremely rapidly.
• Crack is unstable, i.e., crack propagation, once
  started, will continue spontaneously without an
  increase in applied stress.
• Direction of crack propagation is nearly
  perpendicular to direction of applied tensile
  stress and yields relatively flat fracture
  surface.

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Brittle fracture (Cont.)

• Cleavage In brittle fracture, crack propagation
  corresponds to successive and repeated breaking
  of atomic bonds along specific crystallographic
  planes.
• Cleavage is transgranular since cracks pass
  through the grains.
• Crack surface may have grainy or faceted texture
  due to changes in orientation of cleavage planes
  from one grain to another.

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DBTT
Ductile-to-brittle transition temperature
(DBTT)
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Design strategyStay above the DBTT!
Liberty ships
Problem Used a type of steel with a DBTT Room
temp.
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Fatigue

• A form of failure that occurs in structures
  subject to dynamic and fluctuating stresses.
• Failure occurs at stress levels lower than yield
  or tensile stresses for static loads.
• It occurs after a lengthy period of repeated
  stress of strain cycling.
• Comprise approximately 90 of metalic failures,
  Polymers and ceramics are also susceptible.
• See video on fatigue test

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Fatigue (Cont.)

• It is brittle-like failure even in normally
  ductile metals.
• It occurs by initiation and propagation of
  cracks.
• Ordinarily, fracture surface is perpendicular to
  direction of applied tensile stress.

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Creep

• Time dependent and permanent deformation of
  materials when subjected to a constant load or
  stress.
• It is often the limiting factor in lifetime of a
  part.
• It is observed in all materials type. Amorphous
  polymers (plastic and rubber) are especially
  sensitive to creep deformation.
• Important for metals at temperatures greater than
  0.4 Melting temperature (Tm).
• See creep test video

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Creep (Cont.)

• Primary (transient) creep Strain hardening.
• Secondary creep (steady state creep) Balance
  between strain hardening and recovery. It is the
  creep stage of longest duration.
• Tertiary creep Acceleration of rate and end with
  failure (called rupture). Results from
  microstructural and/or metallurgical changes
  (e.g., grain boundary separation, formation of
  internal cracks, cavities, and voids).

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Typical creep curve of strain versus time
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Influence of stress and temperature on creep
behavior
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Creep (Cont.)

• Creep is characterized by
• For long life applications by Steady state creep
  (slope of the secondary portion of creep curve).
• For relatively short life situations by Time to
  rupture.
• Proposed mechanisms (each leads to different
  stress exponent n)
• Stress-induced vacancy diffusion.
• Grain boundary diffusion.
• Dislocation motion.
• Grain boundary sliding.

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Stress vs. rupture lifetime for a low
carbon-nickel alloy
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Stress vs. steady-state creep rate for a low
carbon-nickel