STEEL PLATE AND SECTION

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STEEL PLATE AND SECTION

• Group C
• DMS(DO)

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QUESTION

• What are the requirements of ship hull material
  and what are the various methods for testing the
  quality of these materials?

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SHIP HULL MATERIAL

• Requirements of a good Ship Hull Material are the
  following
• Availability and Cost
• Uniformity
• Ease of Fabrication
• Ease of Maintenance
• Strength vs. Weight
• Fracture Toughness
• Resistance to Marine Corrosion
• Weldability

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AVAILABILITY AND COST

• Great quantities of material are required in
  construction of large ships.
• Therefore, material must be readily available and
  relatively inexpensive.

• Traditional steels and newer composite materials
  each offer specific advantages.
• As compared to composite materials, steels are
  less expensive to purchase and relatively less
  expensive to fabricate.

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UNIFORMITY

• Properties of the material must be uniform and
  dependable.
• This is possible only,
• When the material is subjected to careful quality
  control during its manufacture, and
• When the processes used in its manufacture are
  controllable and repeatable.

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EASE OF FABRICATION

• Easily and cheaply formed into different shapes
  (plates, rolled sections, castings, etc.) and
  easily cut to size and joined together to build
  large, sometimes complex structures.
• Fabrication processes should not significantly
  alter the properties of the material.
• Joints between structural members must be as
  strong as the materials being joined.
• Composites offer the ability to be formed into
  much more complex monolithic shapes, but are not
  as amenable to assembly of multiple subsections.
• Joining technology (including composite to
  composite and composite to steel) is currently
  the most limiting and potentially the most
  promising area of development.

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EASE OF MAINTENANCE

• All materials are subject over a period of time
  to deterioration in service because of their
  exposure to liquids, gases, chemical, radiation,
  or temperature changes.
• Choice of materials for particular engineering
  applications is often dictated by their
  resistance to oxidation, corrosion, dissolution,
  or thermal or radiation damage service.
• The required frequency and expense of painting
  the structure is an important consideration in
  the choice of materials.
• As compared to composite materials, steel is
  potentially more expensive to maintain.

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STRENGTH vs. WEIGHT

• Strength is an essential feature.
• A more important feature is the Strength vs.
  Weight ratio.
• A high strength-to-weight ration is the most
  desirable.
• The lighter metals such as aluminium, titanium,
  and magnesium have much higher strength-to-weight
  ratios than steel.

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FRACTURE TOUGHNESS

• A structural material should take large loads
  without permanent distortion, while remaining
  elastic over a large range of loading.
• Should have ability to withstand day-today rigors
  associated with thermal extremes, impacts from
  piers and other boats, maintenance procedures,
  sea conditions and other normal conditions is
  important.
• Steels used in ships are formulated and processed
  to have a greater ability to withstand fracture
  and greater flaw tolerance under shock conditions
  than more common structural grades.

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RESISTANCE TO MARINE CORROSION

• Salt water and sea spray are constantly attacking
  the ship structure.
• Galvanic corrosion is also a critical concern.
• There have been efforts to replace
  corrosion-prone steels with other metals like
  aluminium and titanium.
• Marine grade aluminium (5000 series) is the only
  non-ferrous corrosion resistant metal that has
  seen widespread use in Navy ships as topside
  structure over the past 30 years.

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WELDABILITY

• Am immense amount of welding is required to build
  a steel ship.
• Welds in a ship are critical to its overall
  strength, durability and toughness.
• In general, any steel grade meeting the strength,
  toughness, and other requirements, which is also
  simpler to weld with a lower predisposition to
  weld flaws is desirable.

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MATERIAL TESTING

• Various qualities desired of ship hull materials
  have been mentioned. These qualities are
  determined by a variety of tests, which are
  carried out on samples of the metal.

• Tensile Test
• Bend Test
• Impact Test

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TENSILE TEST

• Used to determine the behaviour of the material
  up to its breaking point.
• A specially shaped specimen is gripped in the
  jaws of a testing machine.
• A load is gradually applied to draw the ends of
  the bar apart such that it is subject to a
  tensile stress.

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TENSILE TEST OBSERVATIONS

• As the pull is continued on the material until it
  breaks, a good complete tensile profile is
  obtained. A curve will result showing how it
  reacted to the forces being applied.
• Up to the elastic limit the removal of the load
  will result in the specimen returning to original
  size.
• The point of failure is of much interest and is
  typically called its "Ultimate Tensile Strength"
  or UTS.

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BEND TEST

• Used to determine the ductility of a material.
• A piece of metal is bent over a rounded former,
  sometimes through 180 degrees.
• No cracks or surface laminations should appear in
  the material.

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IMPACT TEST

• To test an object's ability to resist high-rate
  loading .
• For determining the energy absorbed in fracturing
  a test piece at high velocity. Most of us think
  of it as one object striking another object at a
  relatively high speed.
• Izod and Charpy methods are used to investigate
  the behaviour of specified specimens under
  specified impact stresses, and to estimate the
  brittleness and ductility of specimens.

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IMPACT TEST PROCEDURE

• Both tests are performed on a pendulum impact
  machine.
• The specimen is clamped in a vice the pendulum
  hammer with a hardened steel striking edge with
  specified radius is released from a predefined
  height, causing the specimen to shear from the
  sudden load.
• The residual energy in the pendulum hammer
  carries it upwards the difference in the drop
  height and return height represents the energy to
  break the test bar.

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CONCLUSION

• Materials play a key role in many aspects of the
  construction and operation of modern ships.
• For construction of navy vessels, there are many
  standard and traditional procedures that
  determine most of these aspects.
• While steel is by far the most common, economical
  construction material, there is significant
  interest in aluminium, titanium, stainless steel,
  and composites for future, high speed ships.
• Future vessels will utilise new materials to
  enable capabilities that far out space their
  current sisters, just as the armoured steel,
  steam turbine powered ships of the early 20th
  century were revolutionary technological leaps
  ahead of their wooden wind powered predecessors.

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REFERENCES

• American Society of naval engineers,
  www.navalengineers.org
• National Shipbuilding Research Programme,
  www.nsrp.org
• Ship Powering and Construction Notes