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To provide the heat for smelting, coke is used and limestone is also added. This makes the slag formed by the incombustible impurities in the iron ore fluid, so that it can be drawn off. Air necessary for combustion is blown in through a ring of holes near the bottom, and the coke, ore, and limestone are charged into the top of the furnace in rotation.
Molten metal may be drawn off at intervals from a hole or spout at the bottom of the furnace and run into molds formed in a bed of sand or into metal molds.
In the subsequent manufacture of steels the pig- iron is refined; in other words the impurities are reduced. These may be produced by one of four different processes: the open hearth process, the Bessemer converter process, the electric furnace process, or an oxygen process. Processes may be either an acid or basic process according to the chemical nature of the slag produced.
Acid processes are used to refine pig-iron low in phosphorus and sulfur that are rich in silicon and therefore produce an acid slag.
The furnace lining is constructed of an acid material so that it will prevent a reaction with the slag. A basic process is used to refine pig-iron that is rich in phosphorus and low in silicon. Phosphorus can be removed only by introducing a large amount of lime, which produces a basic slag. The furnace lining must then be of a basic refractory to prevent a reaction with the slag.
Only the open hearth, electric furnace, and oxygen processes are described here as the Bessemer converter process is not used for shipbuilding steels. Open hearth process The open hearth furnace is capable of producing large quantities of steel, handling � tonnes in a single melt.
It consists of a shallow bath, roofed in, and set above two brick-lined heating chambers. At the ends are openings for heated air and fuel gas or oil to be introduced into the furnace. Also, these permit the escape of the burned gas, which is used for heating the air and fuel. Every 20 minutes or so the flow of air and fuel is reversed. In this process a mixture of pig-iron and steel scrap is melted in the furnace, carbon and the impurities being oxidized.
Oxidization is produced by the oxygen present in the iron oxide of the pig-iron. Subsequently carbon, manganese, and other elements are added to eliminate iron oxides and give the required chemical composition. Electric furnaces Electric furnaces are generally of two types: the arc furnace and the high-frequency induction furnace.
The former is used for refining a charge to give the required composition, whereas the latter may only be used for melting down a charge whose composition is similar to that finally required. For this reason only the arc furnace is considered in any detail.
In an arc furnace melting is produced by striking an arc between electrodes suspended from the roof of the furnace and the charge itself in the hearth of the furnace.
A charge consists of pig-iron and steel scrap, and the process enables consistent results to be obtained and the final composition of the steel can be accurately controlled.
Electric furnace processes are often used for the production of high-grade alloy steels. Steels 47 Oxygen process This is a modern steelmaking process by which a molten charge of pig-iron and steel scrap with alloying elements is contained in a basic lined converter.
A jet of high- purity gaseous oxygen is then directed onto the surface of the liquid metal in order to refine it. Steel from the open hearth or electric furnace is tapped into large ladles and poured into ingot molds.
Chemical additions to steels Additions of chemical elements to steels during the above processes serve several purposes. They may be used to deoxidize the metal, to remove impurities and bring them out into the slag, and finally to bring about the desired composition.
Rimmed steels are produced when only small additions of deoxidizing material are added to the molten metal. Only those steels having less than 0. Owing to the absence of deoxidizing material, the oxygen in the steel combines with the carbon and other gases present and a large volume of gas is liberated. So long as the metal is molten, the gas passes upwards through the molten metal.
When solidifi- cation takes place in ingot form, initially from the sides and bottom and then across the top, the gases can no longer leave the metal. In the central portion of the ingot a large quantity of gas is trapped, with the result that the core of the rimmed ingot is a mass of blow holes.
Normally the hot rolling of the ingot into thin sheet is sufficient to weld the surfaces of the blow holes together, but this material is unsuitable for thicker plate. This has been prevented by the addition of sufficient quantities of deoxidizing material, normally silicon or aluminum. Steel of this type has a high degree of chemical homogeneity, and killed steels are superior to rimmed steels.
In the ingot mold the steel gradually solidifies from the sides and base, as mentioned previously. The melting points of impurities like sulfides and phos- phides in the steel are lower than that of the pure metal and these will tend to separate out and collect towards the center and top of the ingot, which is the last to solidify.
Owing to the high concentration of impurities at this point, this portion of the ingot is often discarded prior to rolling plate and sections. Those heat treatments that concern shipbuilding materials are described. The objects of annealing are to relieve any internal stresses, to soften the steel, or to bring the steel to a condition suitable for a subsequent heat treatment. Normalizing This is carried out by heating the steel slowly to a temperature similar to that for annealing and allowing it to cool in air.
The resulting faster cooling rate produces a harder, stronger steel than annealing, and also refines the grain size. Quenching or hardening Steel is heated to temperatures similar to that for annealing and normalizing, and then quenched in water or oil.
The fast cooling rate produces a very hard structure with a higher tensile strength. The object of this treatment is to relieve the severe internal stresses produced by the original hardening process and to make the material less brittle but retain the higher tensile stress. Stress relieving To relieve internal stresses the temperature of the steel may be raised so that no structural change of the material occurs and then it may be slowly cooled.
Steel sections A range of steel sections are rolled hot from ingots. The more common types asso- ciated with shipbuilding are shown in Figure 5.
It is preferable to limit the sections required for shipbuilding to those readily available, i. Shipbuilding steels Steel for hull construction purposes is usually mild steel containing 0. Both sulfur and phosphorus in the mild steel are kept to a minimum less than 0. Higher concentrations of both are detrimental to the welding properties of the steel, and cracks can develop during the rolling process if the sulfur content is high. Ship classification societies originally had varying specifications for steel.
However, in , the major societies agreed to standardize their requirements in order to reduce the required grades of steel to a minimum. There are now five different qualities of steel employed in merchant ship construction and now often referred to as IACS steels. Grade B is a better quality mild steel than Grade A and specified where thicker plates are required in the more critical regions. Use of higher strength steels allows reductions in thickness of deck, bottom shell, and framing where fitted in the midships portion of larger vessels; it does, however, lead to larger deflections.
The weldability of higher tensile steels is an important consideration in their application in ship structures and the question of reduced fatigue life with these steels has been suggested. Also, the effects of corrosion with lesser thicknesses of plate and section may require more vigilant inspection. Corrosion-resistant steels Steels with alloying elements that give them good corrosion resistance and colloquially referred to as stainless steels are not commonly used in ship struc- tures, primarily because of their higher initial and fabrication costs.
Only in the fabrication of cargo tanks containing highly corrosive cargoes might such steels be found. For oil tankers the inner surfaces, particularly the deckhead and bottom, are generally protected by high-cost corrosion-resistant coatings that require vigilant inspection and maintenance see Chapter Steel sandwich panels As an alternative to conventional shipyard-fabricated stiffened steel plate structures, proprietary manufactured steel sandwich panels have become available and used on ships where their lighter weight was important.
Early use of these bought-in steel sandwich panels was primarily for nonhull structures in naval construction, where their light weight was important. Such panels have also been used for the superstructures of passenger ships, where lightness can allow additional decks and hence increased passenger accom- modation.
Also, when fabricated using stainless steel their corrosion resistance and low maintenance properties have been utilized. A proprietary steel sandwich plate system SPS has been developed that consists of an elastomer core between steel face plates. Elastomers are a specific class of polyurethane that has a high tolerance to mechanical stress, i. The SPS elastomer also has a high resistance to most common chemical species.
Initial application of SPS in shipbuilding has been in passenger ship superstructures, where the absence of stiffening has increased the space available and provided factory-finished surfaces with built-in vibration damping, acoustic insu- lation, and fire protection. SPS structures have been approved with an A 60 fire- resistance rating see Chapter The main use of this system has been for repair of ships, especially decks.
A single steel panel is used to secure the elastomer core to the existing deck. This creates a sandwich panel that is structurally acceptable. The major benefit is in the reduced time required to complete a major steelwork repair, compared to removing and replacing existing, corroded structure. SPS structures can be fabri- cated using joining technologies presently used in the shipbuilding industry, but the design of all joints must take into account the structural and material characteristics of the metal�elastomer composite.
The manufacturer envisages the use of SPS panels throughout the hull and superstructure of ships, providing a simpler construction with greater carrying capacity and less corrosion, maintenance, and inspection.
The rules cover construction procedures, scantling determination for primary sup- porting structures, framing arrangements, and methods of scantling determination for steel sandwich panels. This sandwich would be much narrower than for a comparable steel- only double-skin bulk carrier, thus increasing the potential carrying capacity, although water ballast may have to be carried in some designated holds as the double skin would not be available for this purpose.
The conclusions from the study, a report on which is available for download, are that the sandwich has potential applications for small vessels, for short sea or rivers, and for some offshore structures. It does not appear viable for larger, ocean-going ships at present. Similar panels have been adopted in some offshore applications. After removal from the mold, a heat treatment is required, for example annealing, or normalizing and tempering to reduce brittleness.
Stern frames, rudder frames, spec- tacle frames for bossings, and other structural components may be produced as castings. Steel forgings Forging is simply a method of shaping a metal by heating it to a temperature where it becomes more or less plastic and then hammering or squeezing it to the required form. Forgings are manufactured from killed steel made by the open hearth, electric furnace, or oxygen process, the steel being in the form of ingots cast in chill molds.
Adequate top and bottom discards are made to ensure no harmful segregations in the finished forgings and the sound ingot is gradually and uniformly hot worked. Where possible the working of the metal is such that metal flow is in the most favorable direction with regard to the mode of stressing in service.
Subsequent heat treatment is required, preferably annealing or normalizing and tempering to remove effects of working and non-uniform cooling. Firstly, aluminum is lighter than mild steel approximate weights being 2. This is in fact the principal advantage as far as merchant ships are concerned, the other two advantages of aluminum being a high resistance to corrosion and its nonmagnetic properties.
The nonmagnetic properties can have advantages in warships and locally in the way of the magnetic compass, but they are generally of little importance in merchant vessels. Good corrosion properties can be utilized, but correct maintenance procedures and careful insulation from the adjoining steel structure are necessary. A major disadvantage of the use of aluminum alloys is their higher initial and fabrication costs.
The higher costs must be offset by an increased earning capacity of the vessel, resulting from a reduced lightship weight or increased passenger accommodation on the same ship dimensions. Experience with large passenger liners on the North Atlantic service has indicated that maintenance costs of aluminum alloy structures can be higher for this type of ship and service.
Although aluminum was first used for small craft in and for experimental naval vessels in , it has not been a significant material for ships until compar- atively recently. A significant number of larger ships have been fitted with superstructures of aluminum alloy and, apart from the resulting reduction in displacement, benefits have been obtained in improving the transverse stability. Since the reduced weight of Ship Construction.
For example, on the Queen Elizabeth 2 with a limited beam to transit the Panama Canal, the top five decks constructed of aluminum alloy enabled the ship to support one more deck than would have been possible with an all-steel construction.
Such ships are moderate- and high-speed passenger liners having a low deadweight. It is interesting to note, however, that for the Queen Mary 2, not having a beam limitation, the owners decided to avoid aluminum alloy as far as possible to ensure ease of maintenance over a life cycle of 40 years.
A very small number of cargo liners have been fitted with an aluminum alloy superstructure, principally to clear a fixed draft over a river bar with maximum cargo.
Smaller naval vessels have often had aluminum superstructures on a steel hull as a weight saving measure. A difficulty is the joining of the two metals, in such a way as to avoid a corrosion cell being set up.
In the explosively bonded material the interface between the metals remains corrosion free. The total construction in aluminum alloy of a large ship is not considered an economic proposition and it is only in the construction of smaller multi-hull and other high-speed craft where aluminum alloys of higher strength-to-weight ratio are fully used to good advantage.
Aluminum has been used for specialized craft, including hydrofoils, and is currently particularly used for high-speed ferries where weight is critical. The material is also used for small, high-speed military vessels. One advantage of aluminum is that the material can be extruded to produce a very wide range of profiles. These can have benefits in design of efficient structures. An example is a plate that is extruded with stiffeners as part of the profile.
The plates can be joined to assemble deck panels, with minimum welding and hence a lower risk of distortion. The production process is also faster. Specialized extrusions can be expensive so the economic benefits have to be considered carefully. The actual extraction of the aluminum from the ore is a complicated and costly process involving two distinct stages.
Firstly, the bauxite is purified to obtain pure aluminum oxide, known as alumina; the alumina is then reduced to metallic aluminum.
The metal is cast in pig or ingot forms and alloys are added where required before the metal is cast into billets or slabs for subsequent rolling, extrusion, or other forming operations. Sectional material is mostly produced by the extrusion process. This involves forcing a billet of the hot material through a die of the desired shape. However, the range of thickness of section may be limited since each thickness requires a different die.
Typical sections are shown in Figure 6. Aluminum alloys Pure aluminum has a low tensile strength and is of little use for structural purposes; therefore, the pure metal is alloyed with small percentages of other materials to give greater tensile strengths see Table 6. There are a number of aluminum alloys in use, but these may be separated into two distinct groups, nonheat-treated alloys and heat-treated alloys.
The latter, as implied, are subjected to a carefully controlled heating and cooling cycle in order to improve the tensile strength. Cold working of the nonheat-treated plate has the effect of strengthening the material and this can be employed to advantage. However, at the same time the plate becomes less ductile, and if cold working is considerable the material may crack; this places a limit on the amount of cold forming possible in ship building.
Cold-worked alloys may be subsequently subjected to a slow heating and cooling annealing or stabilizing process to improve their ductility. With aluminum alloys a suitable heat treatment is necessary to obtain a high tensile strength. A heat-treated aluminum alloy that is suitable for shipbuilding purposes is one having as its main alloying constituents magnesium and silicon. These form a compound Mg2Si and the resulting alloy has very good resistance to corrosion and a higher ultimate tensile strength than that of the nonheat-treated alloys.
Since the material is heat treated to achieve this increased strength, subsequent heating, for example welding or hot forming, may destroy the improved properties locally. Aluminum alloys are generally identified by their Aluminum Association numeric designation, the alloys being nonheat treated and the alloys being heat treated.
The nature of any treatment is indicated by additional lettering and numbering. Other shipbuilding materials 57 Table 6.
Nonheat- treated alloy rivets may be driven cold or hot. In driving the rivets cold relatively few heavy blows are applied and the rivet is quickly closed to avoid too much cold work, i. Where rivets are driven hot the temperature must be carefully controlled to avoid metallurgical damage. The shear strength of hot driven rivets is slightly less than that of cold driven rivets.
Aluminum alloy sandwich panels As with steel construction, proprietary aluminum alloy honeycomb sandwich panels are now available to replace fabricated plate and stiffener structures and can offer extremely low weight options for the superstructures of high-speed craft. Fire protection It was considered necessary to mention when discussing aluminum alloys that fire protection is more critical in ships in which this material is used because of the low melting point of aluminum alloys.
During a fire the temperatures reached may be sufficient to cause a collapse of the structure unless protection is provided. The insulation on the main bulkheads in passenger ships will have to be sufficient to make the aluminum bulkhead equivalent to a steel bulkhead for fire purposes.
For the same reason it is general practice to fit steel machinery casings through an aluminum superstructure on cargo ships. The fiber provides the strength and the matrix in which it is contained, usually a plastic, holds the fiber in place.
The fiber can be arranged to provide directional strength so the composite can be tailored to very specific structural requirements. Composites have a lengthy history, for example mud bricks reinforced with straw are a composite, as is reinforced concrete. For marine applications glass fiber- reinforced plastic GRP , first introduced in the s, is the main material for small boats.
It can be considered to be the most forgiving of all construction media. There can be problems with poor construction technique, and this can result in expensive repairs, for example due to hull and deck cores becoming water saturated and laminates blistered.
However, well-built GRP boats can still be operational after 50 years. FRC is used extensively in leisure craft, where it provides advantages of lightness and durability. FRC has also been used for naval vessels, in particular mine coun- termeasures vessels, where the attraction of the material is that it is nonmagnetic.
The vessels are relatively large, typically around 50 meters loa. The full load displacement is tonnes. For both a mine countermeasures ship and a luxury yacht the proportion of cost represented by the hull is small.
Such large vessels would probably not be economic for a commercial application, due to the labor-intensive nature of FRP construction.
Major advantages of FRP for small vessels include low weight, combined with high strength and stiffness. The material is resistant to harsh marine environments. The hull is laid up as a continuous structure, with no seams, which avoids any possible leakage problems. FRP is durable and has low maintenance requirements. For specialist applications the ability of FRP to absorb high energy can be valu- able, and the material can be tailored for specific loadings.
This gives excellent design flexibility and leads to efficient structures. The material consists of fibers, usually glass but including Kevlar and carbon fibers, for very high strength and low weight applications, which are bound into a resin. The higher strength fibers are used in such applications as masts, where strength and light weight are important considerations.
For most FRP applications the fibers may be randomly arranged in a chopped strand mat or woven into a specific material to provide additional strength. The hull is laid up in a mold. Once the mold is made it can be used many times, which is one reason for the popularity of the material for small boats. The mold has to be carefully supported to ensure it does not deform under the loads imposed by the FRP and the production activities. The surface finish of the mold is important as this will determine the smoothness of the finished hull.
Other shipbuilding materials 59 A coating of PV a clear odorless liquid , known as the releasing agent, is applied to the mold to prevent the mold from sticking to the plug or finished hull. Then a gel coat is applied, which once cured will form the smooth, water-resistant outer surface of the hull. At this stage the gel coat can be colored. The glass mat or woven roving is then laid onto the mold and the resin applied.
The mat can be cut to match the hull shape. It is important that the fibers are thoroughly wetted by the resin and that all air is excluded. The hull is built up to the required thickness with several layers of glass. Additional glass is used to build up thicker material where additional strength is required.
Stiffeners can be incorporated in the hull. These are usually a solid or hollow core former. These are positioned on the hull in the mold and then covered with the glass and resin. When covered with several layers of fiberglass mat they create a closed box or semicircular section.
Internal fittings, machinery mountings, and locations for bulkheads are also built into the hull as it progresses. Once the hull, and fittings incorporated into it, are complete the hull is left to cure until the resin is fully set.
The mold is often in two halves for small vessels and this allows it to be split to release the hull. Material properties are important to the capability of the structure. The properties outlined below are appropriate in determining the suitability of a material for ship construction.
The strength of the material is its ability to resist deformation. Yield stress and ultimate tensile strength measure the ability to resist forces on the structure. Hardness of a material describes its ability to resist abrasion. Hardness is impor- tant, for example, in bulk carriers where the cargo handling produces abrasive action on the cargo hold structures. Hardness is usually measured on a scale Rockwell or Brinell , based on test results.
Ductility is the ability of a material to be deformed before it fails. Brittleness is the opposite of ductility and describes a material that fails under stress because it cannot deform.
Softer metals, such as aluminum, are ductile. Hard materials such as cast iron are strong but brittle. Toughness is the ability of a material to absorb energy. In comparing the strengths of various metals, stresses and strains are often referred to and require to be defined.
Stress is a measure of the ability of a material to transmit a load, and the intensity of stress in the material, which is the load per unit area, is often stated. The load per unit area is simply obtained by dividing the applied load by the cross-sectional area of the material, e. Total strain is defined as the total deformation that a body undergoes when subjected to an applied load. The strain is the deformation per unit length or unit volume, e.
Extension dl or Original length l It can be shown that the load on the rod may be increased uniformly and the resulting extension will also increase uniformly until a certain load is reached. This indicates that the load is proportional to extension, and hence stress and strain are proportional since the cross-sectional area and original length of the rod remain constant. If a mild steel bar is placed in a testing machine and the extensions are recorded for uniformly increasing loads, a graph of load against extension, or stress against strain, may be plotted as in Figure 7.
This shows the straight-line relationship i. Ultimate tensile stress is the maximum load to which the metal is subjected, divided by the original cross-sectional area. Beyond the yield point the metal behaves plastically, which means that the metal deforms at a greater, unproportional, rate when the yield stress is exceeded, and will not return to its original dimensions on removal of the load.
Many metals do not have a clearly defined yield point, for example aluminum, having a stress�strain curve over its lower range that is a straight line becoming gradually curved without any sharp transformation on yielding, as shown by mild steel see Figure 7. The intersection of this line with the actual stress�strain curve marks the proof stress. During production an analysis of the material is required and so are prescribed tests of the rolled metal.
Similar analyses and tests are required by the classification societies for steel forgings and steel castings, in order to maintain an approved quality. These tests usually take the form of a tensile test and impact test. Tensile test The basic principle of this test has already been described, a specimen of given dimensions being subject to an axial pull and a minimum specified yield stress, ultimate tensile stress, and elongation must be obtained.
In order to make compari- sons between the elongation of tensile test pieces of the same material the test pieces must have the same proportions of sectional area and gage length.
Therefore, a standard gage length equal to 5. Impact tests There are several forms of impact test, but the Charpy V-notch test or Charpy U-notch test is commonly specified and therefore described in this text.
Testing of materials 65 impact test is to determine the toughness of the material; that is, its ability to with- stand fracture under shock loading.
In Figure 7. This specimen is placed on an anvil and the pendulum is allowed to swing so that the striker hits the specimen opposite the notch and fractures it. Energy absorbed in fracturing the specimen is automatically recorded by the machine.
Basically, making allowances for friction, the energy absorbed in fracturing the specimen is the difference between the potential energy the pendulum possesses before being released, and that which it attains in swinging past the vertical after fracturing the specimen.
A specified average impact energy for the specimens tested must be obtained at the specified test temperature, fracture energy being dependent on temperature, as will be illustrated in Chapter 8. Aluminum alloy tests Aluminum alloy plate and section material is subject to specified tensile tests. Bar material for aluminum alloy rivets is subject to a tensile test and also a dump test.
The latter test requires compression of the bar until its diameter is increased to 1. Selected manufactured rivets are also subjected to the same dump test. Vertical shear and longitudinal bending in still water If a homogeneous body of uniform cross-section and weight is floating in still water, at any section the weight and buoyancy forces are equal and opposite. Therefore, there is no resultant force at a section and the body will not be stressed or deformed.
A ship floating in still water has an unevenly distributed weight owing to both cargo distribution and structural distribution. The buoyancy distribution is also non-uniform since the underwater sectional area is not constant along the length. Total weight and total buoyancy are of course balanced, but at each section there will be a resultant force or load, either an excess of buoyancy or excess of load.
Since the vessel remains intact there are vertical upward and downward forces tending to distort the vessel Ship Construction. The ship shown in Figure 8. It can be seen that the upper fibers of the beam will be in tension, as will the material forming the deck of the ship with this loading.
Conversely, the lower fibers of the beam, and likewise the material forming the bottom of the ship, will be in compression. When sagging the deck will be in compression and the bottom shell in tension.
Lying in still water the vessel is subjected to bending moments either hogging and sagging depending on the relative weight and buoyancy forces, and it will also be subjected to vertical shear forces. Bending moments in a seaway When a ship is in a seaway the waves with their troughs and crests produce a greater variation in the buoyant forces and therefore can increase the bending moment, vertical shear force, and stresses. Classically the extreme effects can be illustrated with the vessel balanced on a wave of length equal to that of the ship.
In a seaway the overall effect is an increase of bending moment from that in still water when the greater buoyancy variation is taken into account. Longitudinal shear forces When the vessel hogs and sags in still water and at sea, shear forces similar to the vertical shear forces will be present in the longitudinal plane see Figure 8. Vertical and longitudinal shear stresses are complementary and exist in conjunction with a change of bending moment between adjacent sections of the hull girder.
The magnitude of the longitudinal shear force is greatest at the neutral axis and decreases towards the top and bottom of the girder. Stresses to which a ship is subject Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes Tonnes Figure 8.
Stresses to which a ship is subject 71 When the beam bends it is seen that the extreme fibers are, say in the case of hogging, in tension at the top and in compression at the bottom. Somewhere between the two there is a position where the fibers are neither in tension nor compression. This position is called the neutral axis, and at the furthest fibers from the neutral axis the greatest stress occurs for plane bending. It should be noted that the neutral axis always contains the center of gravity of the cross-section.
In the equation the second moment of area I of the section is a divisor; therefore, the greater the value of the second moment of area, the less the bending stress will be. This second moment of area of section varies as the depth squared and therefore a small increase in depth of section can be very beneficial in reducing the bending stress.
Occasionally reference is made to the sectional modulus Z of a beam; this is simply the ratio between the second moment of area and the distance of the point considered from the neutral axis, i. The ship as a beam It was seen earlier that the ship bends like a beam, and in fact the hull can be considered as a box-shaped girder for which the position of the neutral axis and second moment of area may be calculated.
The deck and bottom shell form the flanges of the hull girder, and are far more important to longitudinal strength than the sides that form the web of the girder and carry the shear forces.
The box-shaped hull girder and a conventional I girder are compared in Figure 8. In a ship the neutral axis is generally nearer the bottom, since the bottom shell will be heavier than the deck, having to resist water pressure as well as the bending stresses.
In calculating the second moment of an area of the cross-section all longi- tudinal material is of greatest importance and the further the material is from the neutral axis, the greater will be its second moment of area about the neutral axis.
However, at greater distances from the neutral axis, the sectional modulus will be reduced and correspondingly higher stress may occur in extreme hull girder plates such as the deck stringer, sheer strake, and bilge. These strakes of plating are generally heavier than other plating. Other scantlings may taper towards the ends of the ship, apart from locally highly stressed regions where other forms of loading are encountered.
Strength deck The deck forming the uppermost flange of the main hull girder is often referred to as the strength deck. This is to some extent a misleading term since all continuous decks are in fact strength decks if properly constructed.
Along the length of the ship the top flange of the hull girder, i. Early vessels fitted with large superstructures of light construction demonstrated this to their cost.
Attempts to avoid fracture have been made by fitting expansion joints, which made the light structure discontinuous.
These were not entirely successful and the expansion joint may itself form a stress concentration at the strength deck, which one would wish to avoid. In modern construction the superstructure is usually made continuous and of such strength that its sectional modulus is equivalent to that which the strength deck would have if no superstructure were fitted see Chapter These forces may be produced by hydrostatic loads and impact of seas or cargo and structural weights, both directly and as the result of reactions due to change of ship motion.
Racking When a ship is rolling, the deck tends to move laterally relative to the bottom structure, and the shell on one side to move vertically relative to the other side. Where transverse bulkheads are widely spaced, deep web frames and beams may be introduced to compensate. In most ships these torsional moments and stresses are negligible but in ships with extremely wide and long deck openings they are significant.
A particular example is the larger container ship, where at the topsides a heavy torsion box girder structure including the upper deck is provided to accommodate the torsional stresses see Figures 8.
Strengthening to resist panting both forward and aft is covered in Chapter Pounding Severe local stresses occur in way of the bottom shell and framing forward when a vessel is driven into head seas.
These pounding stresses, as they are known, are likely to be most severe in a lightly ballasted condition, and occur over an area of the bottom shell aft of the collision bulkhead. Additional stiffening is required in this region; this is dealt with in Chapter Other local stresses Ship structural members are often subjected to high stresses in localized areas, and great care is required to ensure that these areas are correctly designed.
Another highly stressed area occurs where there is a discontinuity of the hull girder at ends of deck house structures, also at hatch and other opening corners, and where there are sudden breaks in the bulwarks.
Brittle fracture With the large-scale introduction of welding in ship construction, much consideration has been given to the correct selection of materials and structural design to prevent the possibility of brittle fracture occurring.
During the Second World War the incidence of this phenomenon was high amongst tonnage hastily constructed, whilst little was known about the mechanics of brittle fracture. Although instances of brittle fracture were recorded in riveted ships, the consequences were more disastrous in the welded vessels because of the continuity of metal provided by the welded joint as opposed to the riveted lap, which tended to limit the propagating crack.
Brittle fracture occurs when an otherwise elastic material fractures without any apparent sign or little evidence of material deformation prior to failure. Mild steel used extensively in ship construction is particularly prone to brittle fracture given the conditions necessary to trigger it off. The subject is too complex to be dealt with in detail in this text, but it is known that the following factors influence the possibility of brittle fracture and are taken into consideration in the design and material selection of modern ships: 1.
A sharp notch is present in the structure from which the fracture initiates. A tensile stress is present. There is a temperature above which brittle fracture will not occur. The metallurgical properties of the steel plate. Thick plate is more prone. A brittle fracture is distinguishable from a ductile failure by the lack of deformation at the edge of the tear, and its bright granular appearance.
A ductile failure has a dull gray appearance. The brittle fracture is also distinguished by the apparent chevron marking, which aids location of the fracture initiation point since these tend to point in that direction. Factors that are known to exist where a brittle fracture may occur must be considered if this is to be avoided.
Firstly, the design of individual items of ship structure must be such that sharp notches where cracks may be initiated are avoided. With welded structures as large as a ship the complete elimination of crack initiation is not entirely possible owing to the existence of small faults in the welds, a complete weld examination not being practicable.
Notch ductility is a measure of the relative toughness of the steel, which has already been seen to be determined by an impact test. Figure 8. Grade D and Grade E steels, which have higher notch ductility, are employed where thick plate is used and in way of higher stressed regions, as will be seen when the ship structural details are considered later.
The term related to the now outdated practice of introducing riveted seams in cargo ships to subdivide the vessel into welded substructures so that any possible crack propagation was limited to the substructure. Today strakes of higher notch tough- ness steel are required to be fitted in such areas.
Fatigue failures Unlike brittle fracture, fatigue fracture occurs very slowly and can in fact take years to propagate. The greatest danger with fatigue fractures is that they occur at low stresses that are applied to a structure repeatedly over a period of time Figure 8. A fatigue crack, once initiated, may grow unnoticed until the load-bearing member is reduced to a cross-sectional area that is insufficient to carry the applied load.
With the growth in size of oil tankers, bulk carriers, and container ships there has been increasing use of higher yield strength steels in their hull structures. The clas- sification societies have subsequently placed special emphasis on analysis of the fatigue performance of these larger structures, usually over a year life cycle, as part of their approval process. Buckling With the substantial increase in size of oil tankers, bulk carriers, and container ships in recent years, greater attention has had to be given to the buckling strength of the stiffened plate panels constituting the shell.
The most common example of buckling failure is the collapse of a pillar under a compressive load. Unlike the pillar, however, slightly exceeding this load will not necessarily result in collapse of the plate but only in elastic deflection of the center portion of the plate from its initial plane.
After removal of the load, the plate will return to its original undeformed state. The ultimate load that may be carried by a buckled plate is determined by the onset of yielding i. Once begun, this yielding may propagate rapidly throughout the stiffened panel with further increase in load until failure of the plate or stiffeners occurs. Where further buckling assessment is required a computer-based general and local stiffened plate panel ultimate buckling strength evaluation assessment procedure is used.
This entails the fitting of strain gages to the deck structure, an accelerometer, and a personal computer with software that displays ship stress and motion readings on the bridge. An alarm is activated if the safety limits are exceeded, enabling remedial action to be taken.
However, riveting remained the predominant method employed for joining ship plates and sections until the time of the Second World War. During and after this war the use and development of welding for shipbuilding purposes was widespread, and welding totally replaced riveting in the latter part of the twentieth century.
These may be considered as advantages in both building and in operating the ship. For the shipbuilder the advantages are: 1. Welding lends itself to the adoption of prefabrication techniques. It is easier to obtain watertightness and oiltightness with welded joints.
Joints are produced more quickly. Less skilled labor is required. For the shipowner the advantages are: 1. Reduced hull steel weight, therefore more deadweight. Less maintenance from slack rivets, etc. The smoother hull with the elimination of overlapping plate joints leads to reduced skin friction resistance, which can reduce fuel costs.
Other than some blacksmith work involving solid-phase welding, the welding processes employed in shipbuilding are of the fusion welding type. Fusion welding is achieved by means of a heat source that is intense enough to melt the edges of the material to be joined as it is traversed along the joint.
Gas welding, arc welding, laser welding, and resistance welding all provide heat sources of sufficient intensity to achieve fusion welds.
Gas welding A gas flame was probably the first form of heat source to be used for fusion welding, and a variety of fuel gases with oxygen have been used to produce a high-temperature flame. An oxyacetylene flame has two distinct regions: an inner cone, in which the oxygen for combustion is supplied via the torch; and a surrounding envelope, in which some or all the oxygen for combustion is drawn from the surrounding air.
By varying the ratio of oxygen to acetylene in the gas mixture supplied by the torch, it is possible to vary the efficiency of the combustion and alter the nature of the flame Figure 9. This type of flame may be used for welding materials of high thermal conductivity, e. These readily go into solution in the molten steel, and can produce metallurgical problems in service. The outer envelope of the oxyacetylene flame by consuming the surrounding oxygen to some extent protects the molten weld metal pool from the surrounding air.
With metals containing refractory oxides, such as stainless steels and aluminum, it is necessary to use an active flux to remove the oxides during the welding process.
Both oxygen and acetylene are supplied in cylinders, the oxygen under pressure and the acetylene dissolved in acetone since it cannot be compressed. Each cylinder, which is distinctly colored red�acetylene, black�oxygen , has a regulator for controlling the working gas pressures. The welding torch consists of a long thick copper nozzle, a gas mixer body, and valves for adjusting the oxygen and acetylene flow rates.
Usually a welding rod is used to provide filler metal for the joint, but in some cases the parts to be joined may be fused together without any filler metal. Gas welding techniques are shown in Figure 9. Oxyacetylene welding tends to be slower than other fusion welding processes because the process temperature is low in comparison with the melting temperature of the metal, and because the heat must be transferred from the flame to the plate.
The process is therefore only really applicable to thinner mild steel plate, thicknesses up to 7 mm being welded using this process with a speed of 3�4 meters per hour. In shipbuilding oxyacetylene welding has almost disappeared but can be employed in the fabrication of ventilation and air-conditioning trunking, cable trays, and light steel furniture; some plumbing and similar work may also make use of gas welding.
These trades may also employ the gas flame for brazing purposes, where joints are obtained without reaching the fusion temperature of the material being joined. Electric arc welding The basic principle of electric arc welding is that a wire or electrode is connected to a source of electrical supply with a return lead to the plates to be welded.
If the electrode is brought into contact with the plates an electric current flows in the circuit. By removing the electrode a short distance from the plate, so that the electric current is able to jump the gap, a high-temperature electrical arc is created.
This will melt the plate edges and the end of the electrode if this is of the consumable type. Electrical power sources vary, DC generators or rectifiers with variable or constant voltage characteristics being available, as well as AC transformers with variable voltage characteristics for single or multiple operation. The latter are most commonly used in shipbuilding. Illustrated in Figure 9. Each of these electric arc welding processes is discussed below with its application.
Slag-shielded processes Metal arc welding started as bare wire welding, the wire being attached to normal power lines.
This gave unsatisfactory welds, and subsequently it was discovered that by dipping the wire in lime a more stable arc was obtained. Other developments include a hollow wire for continuous welding with the flux within the hollow core. The flux melts, then solidifies during the welding process, forming a solid slag that protects the weld from atmospheric oxygen and nitrogen.
Manual welding electrodes The core wire normally used for mild steel electrodes is rimming steel. Coatings for the electrodes normally consist of a mixture of mineral silicates, oxides, fluorides, carbonates, hydro- carbons, and powdered metal alloys plus a liquid binder.
After mixing, the coating is then extruded onto the core wire and the finished electrodes are dried in batches in ovens. Electrode coatings should provide gas shielding for the arc, easy striking and arc stability, a protective slag, good weld shape, and most important of all a gas shield consuming the surrounding oxygen and protecting the molten weld metal. Various electrode types are available, the type often being defined by the nature of the coating.
The more important types are the rutile and basic or low-hydrogen electrodes. Rutile electrodes have coatings containing a high percentage of titania, and are general- purpose electrodes that are easily controlled and give a good weld finish with sound properties. Basic or low-hydrogen electrodes, the coating of which has a high lime content, are manufactured with the moisture content of the coating reduced to a minimum to ensure low-hydrogen properties.
The mechanical properties of weld metal deposited with this type of electrode are superior to those of other types, and basic electrodes are generally specified for welding the higher tensile strength steels.
Where high restraint occurs, for example at the final erection seam weld between two athwartships rings of unit structure, low-hydrogen electrodes may also be employed. An experienced welder is required where this type of electrode is used since it is less easily controlled.
Welding with manual electrodes may be accomplished in the downhand position, for example welding at the deck from above, also in the hori- zontal vertical, or vertical positions, for example across or up a bulkhead, and in the overhead position, for example welding at the deck from below Figure 9.
Welding in any of these positions requires selection of the correct electrode positional suit- ability stipulated by the manufacturer , correct current, correct technique, and inev- itably experience, particularly for the vertical and overhead positions. Automatic welding with cored wires Flux-cored wires FCAW are often used in mechanized welding, allowing higher deposition rates and improved quality of weld.
Basic or rutile flux-cored wires are commonly used for one-sided welding with a ceramic backing. Submerged arc welding This is an arc welding process in which the arc is maintained within a blanket of granulated flux see Figure 9. Around the arc the granulated flux breaks down and provides some gases, and a highly protective thermally insulating molten container for the arc. This allows a high concentration of heat, making the process very efficient and suitable for heavy deposits at fast speeds.
After welding the molten metal is protected by a layer of fused flux, which together with the unfused flux may be recovered before cooling. This is the most commonly used process for downhand mechanical welding in the shipbuilding industry, in particular for joining plates for ship shell, decks, and bulkheads.
Submerged arc multi-wire and twin- arc systems are also used to give high productivity. With shipyards worldwide adopting one-side welding in their ship panel lines for improved productivity, the submerged arc process is commonly used with a fusible backing, using either flux or glass fiber materials to contain and control the weld penetration bead. Stud welding Stud welding may be classed as a shielded arc process, the arc being drawn between the stud electrode and the plate to which the stud is to be attached.
Each stud is inserted into a stud welding gun chuck, and a ceramic ferrule is slipped over it before the stud is placed against the plate surface. When the arcing period is complete, the current is automatically shut off and the stud driven into a molten pool of weld metal, so attaching stud to plate.
Apart from the stud welding gun the equipment includes a control unit for timing the period of current flow. Granular flux is contained within the end of each stud to create a protective atmosphere during arcing. The ceramic ferrule that surrounds the weld area restricts the access of air to the weld zone; it also concentrates the heat of the arc and confines the molten metal to the weld area see Figure 9. Stud welding is often used in shipbuilding, generally for the fastening of stud bolts to secure supports for pipe hangars, electric cable trays and other fittings, also insulation to bulkheads and wood sheathing to decks, etc.
Apart from various forms of stud bolts, items like stud hooks and rings are also available. Gas-shielded arc welding processes The application of bare wire welding with gas shielding was developed in the s, and was quickly adopted for the welding of lighter steel structures in shipyards, as well as for welding aluminum alloys. Gas-shielded processes are principally of an automatic or semi-automatic nature.
Tungsten inert gas TIG welding In the TIG welding process the arc is drawn between a water-cooled nonconsumable tungsten electrode and the plate Figure 9. An inert gas shield is provided to protect the weld metal from the atmosphere, and filler metal may be added to the weld pool as required.
Ignition of the arc is obtained by means of a high-frequency discharge across the gap, since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally in Britain the inert gas shield used for welding aluminum and steel is argon.
Only plate thicknesses of less than 6 mm would normally be welded by this process, and in particular aluminum sheet, a skilled operator being required for manual work. This may also be referred to as TAGS welding, i.
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