STEEL PLATE AND SECTION Group C DMSDO QUESTION
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STEEL PLATE AND SECTION Group C DMS(DO)
QUESTION What are the requirements of ship hull material and what are the various methods for testing the quality of these materials?
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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 20 th century were revolutionary technological leaps ahead of their wooden wind powered predecessors.
REFERENCES • American Society of naval engineers, www. navalengineers. org • National Shipbuilding Research Programme, www. nsrp. org • Ship Powering and Construction Notes