Early structural concepts

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• Early structural concepts
• Some of the structures in earlier have endured
• for ages.
• Materials used were brittle type like bricks,
  stones,
• mortar poor to carry tensile loads.
• Avoided fracture possibilities by selecting
• appropriate geometric shapes like arches,
  domes
• The structure were designed to carry load by
• compression
• New structural concepts
• Availability of metals lead to change in
  structural
• concepts allowed tension in structure.
  (this invited
• additional problems like fracture)
• Designs based on strength allowed a factor of
  safety
• ranging from 2 to 10, but still structures
  failed by
• sudden brittle fracture

When ever there is new material or new design
concepts produces unexpected results leading to
catastrophic failure
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• 1943, Liberty ship a cargo ship
• Prior to II world war liberty ships were riveted
  (very slow process) having no fracture problems
• During war, to accelerate ship building, England
  sought help from USA. USA companies offered to
  build ship faster, by welding joints.
• They maintained same geometric shape, ship hull
  turned out to be a single envelope of steel.
• Ships were sailing across Atlantic and Artic
  ocean. (cold temperatures). During which two
  ships fractured suddenly in to two halves (
  brittle fracture). Out of 2700 ships built, 400
  ships suffered fractures of various degree.

• Analysis
• Unequal distribution of cargo and ballast was
• causing hogging bending moment
• Wave motion also caused hogging BM, resulting in
  tensile stress on the deck.
• Welds were produced by semi skilled work force,
  which contained crack like flaws

Negligence during construction or operation some
times results in catastrophic failures
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Flaw
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• Analysis (contd.)
• cracks were found to initiate at square hatch
  which induced stress concentration due tensile
  stress
• The high strength steel used for the ship had
  poor toughness (Charpy impact test).
• Heat Affected Zone (HAZ) will have low
  ductility, behaving like a hardened material.
• Due to rapid cooling, tensile residual stress
  are induced. This is equivalent to crack like
  defect.

Riveted joints act as crack arrester
welded joints produce continuous crack
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• Conclusion on liberty ship failure
• Steel-BCC crystal.
• They can fracture by extended slip in some
  preferred planes producing plasticity or
• Fracture by cleavage under different plane under
  tensile stress without plastic deformation, at a
  stress level below yield strength
• Cleavage fracture are predominant at lower
  temperatures ( at lower temperature yield
  strength is higher than fracture strength)
• The combined effect of low ductile steel,
  freezing temperature, presence of crack like
  defect (residual tensile stress), crack like
  defect in the weld lead to sudden brittle
  fracture, which initiated at the hatch on the
  deck due to tensile service load, crack
  propagated at fast rate (crack velocity
  velocity of sound) through the entire cross
  section of the hull breaking ship into two
  halves.

• Points to be noted
• At service load tensile stress is induced in the
  deck due to which crack is initiates/grows.
• Presence of microcrack leading to stress
  concentration

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• Conventional Design Method
• Conventional method ensures safety of structure
  based on strength characteristics
• often structure may have a FS varying from 2 to
  10
• Design does not safeguard against possible
  failure by fracture (brittle, ductile, fatigue,
  dynamic)

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Fracture Mechanics Design approach

• Fracture mechanics approaches require that an
  initial crack size be known or assumed. For
  components with imperfections or defects (such as
  welding porosities, inclusions and casting
  defects, etc.) an initial crack size may be
  known.
• Fracture Mechanics ensures safety against
  fracture failure
• Evaluation of fracture parameter may be
  required
• In presence of visible crack for ductile or
  fatigue loading condition, FM can predict safety
  and life of the structure

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Ductile Fracture

• Ductile fracture is preceded by extensive
  plastic deformation
• Ductile fracture is caused due to growth and
  coalescence of voids (at the sites of inclusion)
• Ductile fracture is a slow process , gives
  enough precaution before catastrophic failure
• Ductile fracture usually follows transgranular
  path
• If the density of inclusion are more along grain
  boundary, crack grows along boundaries leading to
  fibrous or ductile intergranular fracture
• If inclusions are not present, voids are formed
  at severely deformed regions leading to localized
  slip bands and macroscopic instability resulting
  in necking or shear fracture

Plasticity retards crack growth and it provides a
factor of safety against over loading or
oversight in design.
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Voids formed (at particle sites) during plastic
deformation and ductile fracture
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Voids formed (at non-particle sites) during
plastic deformation and ductile fracture
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Brittle fracture

• Fast crack growth without excessive or no
  plastic deformation.
• Fracture stress will be lower than yield
  strength
• Brittle fracture may be transgranular (cleavage)
  or intergranular
• Brittle fracture are mostly predominant in metals
  with bcc crystal at cryogenic temperature or at
  high strain rate.
• Micro cracks initiated by fatigue loading may
  lead to brittle fracture
• HAZ induces high tensile residual stress
• HAZ also reduces the ductility
• Shrinkage tears in weld may also cause brittle
  fracture

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• What are the general characteristics of brittle
  fracture?
• Very little general plasticity - broken pieces
  can be fitted together with no obvious plastic
  deformation
• Rapid crack propagation (one third the speed of
  sound), eg 1 km/s for steel
• Low energy absorption
• Low failure load relative to load for general
  yield
• Usually fractures are flat and perpendicular to
  the maximum principal stress
• Fracture always initiates at a flaw or a site of
  stress concentration.
• Examples
• Mild steel at low temperature
• high strength Fe, Al and Ti alloys
• glass perspex
• ceramics
• concrete
• carrots (particularly fresh ones)