[基础知识] Mechant Ship Construction Second Edition D A Taylor

The various forcesacting on a ship are constantly varying as to their degree and frequency. Forsimplicity, however, they will be considered individually and the particularmeasures adopted to counter each type of force will be outlined.

The forces mayinitially be classified as static and dynamic. Static forces are due to thedifferences in weight and buoyancy which occur at various points along thelength of the ship. Dynamic forces result from the ship's motion in the sea andthe action of the wind and waves. A ship is free to move with six degrees offreedom - three linear and three rotational. These motions are described by theterms shown in Figure 2.1.

These static anddynamic forces create longitudinal, transverse and local stresses in the ship'sstructure. Longitudinal stresses are greatest in magnitude and result in bendingof the ship along its length.

Figure 2.1 Shipmovement – the six degrees of freedom

If the ship is consideredfloating in still water, two different forces will be acting upon it along itslength. The weight of the ship and its contents will be acting vertically downwards.The buoyancy or vertical component of hydrostatic pressure will be actingupwards. in total, the two forces exactly equal and balance one another suchthat the ship floats at some particular draught. The centre of the buoyancyforce and the centre of the weight will be vertically in line. However, atparticular points along the ship's length the net effect may be an excess ofbuoyancy or an excess of weight. This net effect produces a loading of thestructure, as with a beam. This loading results in shearing forces and bending momentsbeing set up in the ship's structure which tend to bend it. The static forcesacting on a ship's structure are shown in Figure 2.2(a). This distribution ofweight and buoyancy will also result in a variation of load, shear forces andbending moments along the length of the ship, as shown in Figures 2.2(b)-(d).Depending upon the direction in which the bending moment acts, the ship willbend in a longitudinal vertical plane. This bending moment is known as thestill water bending moment (SWBM). Special terms are used to describe the twoextreme cases: where the buoyancy amidships exceeds the weight, the ship is saidto 'hog', and this condition is shown in Figure 2.3; where the weight amidshipsexceeds the buoyancy, the ship is said to 'sag', and this condition is shown inFigure 2.4.

Figure 2.2 Static loadingof a ship’s structure

Figure 2.3 Hoggingcondition

Figure 2.4 Saggingcondition

If the ship is nowconsidered to be moving among waves, the distribution of weight will still bethe same. The distribution of buoyancy, however, will vary as a result of thewaves. the movement of the ship will also introduce dynamic forces.

The traditionalapproach to solving this problem is to convert this dynamic situation into anequivalent static one. To do this, the ship is assumed to be balanced on astatic wave of trochoidal form and length equalto the ship. The profile of a wave at sea is considered to be a trochoid. Thisgives waves where the crests are sharper than the troughs. The wave crest is consideredinitially at midships and then at the ends of the ship. The maximum hogging andsagging moments will thus occur in the structure far the particular loadedcondition considered, as shown in Figure 2.5.

The total shearforce and bending moment are thus obtained and these will include the stillwater bending moment considered previously. If actual loading conditions forthe ship are considered which will make the above conditions worse, e.g. heavyloads amidships when the wave trough is amidships, then the maximum bending momentsin normal operating service can be found.

The ship'sstructure will thus be subjected to constantly fluctuatingstresses resulting from these shear forces and bending moments as the wavesmove along the ship's length.

Figure 2.5 Dynamicloading of a ship’s structure: (a) still water condition; (b) saggingcondition; (c) hogging condition

The bending of aship causes stresses to be set up within its structure. When a ship sags,tensile stresses are set up in the bottom shell plating and compressive tressesare set up in the deck. When the ship hogs, tensile stresses occur in the decksand compressive stresses in the bottom shell. This stressing, whethercompressive or tensile, reduces in magnitude towards a position known as theneutral axis. The neutral axis in a ship is somewhere below half the depth andis, in effect, a horizontal line drawn through the centre of gravity of the ship'ssection.

The fundamentalbending equation for a beam is

M / I = σ / y

This equation hasbeen proved in full-scale tests to be applicable to the longitudinal bending ofa ship. From the equation the expression

σ = M / I/y

The structuralmaterial included in the calculation for the second moment I will be all thelongitudinal material which extends for a considerable proportion of the ship'slength. This material will include side and bottom shell plating, inner bottomplating (where fitted), centre girders and decks. The material forms what is knownas the hull girder, whose dimensions are very large compared to its thickness.

A transversesection of a ship is subjected to static pressure from the surrounding water inaddition to the loading resulting from the weight of the structure, cargo, etc.Although transverse stresses are of lesser magnitude than longitudinalstresses, considerable distortion of the structure could occur, in the absenceof adequate stiffening (Figure 2.6).

The parts of thestructure which resist transverse stresses are transverse bulkheads, floors in thedouble bottom (where fitted), deck beams, side frames and the brackets betweenthem and adjacent structure such as tank top flooring or margin plates

Figure 2.6 Staticwater pressure loading of a ship’s structure

When a ship is rollingit is accelerated and decelerated, resulting in forces in the structure tendingto distort it. This condition is known as racking and its greatest effect is feltwhen the ship is in the light or ballast condition (Figure 2.7). The bracketsand beam knees joining horizontal and vertical items of structure are used toresist this distortion.

Figure 2.7 Racking

The movement of aship in a seaway results in forces being generated which are largely of a localnature. These forces are, however, liable to cause the structure to vibrate andthus transmit stresses to other pans of the structure.

In heavy weather,when the ship is heaving and pitching, the forward end leaves and re-enters thewater with a slamming effect (Figure 2.8). This slamming down of the forwardregion on to the water is known as pounding. Additional stiffening must befitted in the pounding region to reduce the possibility of damage to thestructure. This is discussed further in Section A of Chapter 5.

Figure 2.8 Pounding

The movement ofwaves along a ship causes fluctuations in waterpressure on the plating. The tends to create an in-and-out movement of theshell plating, known as panting. The effect is particularly evident at the bowsas the ship pushes its way through the water.

The pitching motionof the ship produces additional variations in water pressure, particularly atthe bow and stem, which also cause panting of the plating. Additionalstiffening is provided in the form of panting beams and stringers. This is discussedfurther in Section D of Chapter 5.

Heavy weights, suchas equipment in the machinery spaces or particular items of general cargo, cangive rise to localised distortion of the transverse section (Figure 2.9).Arrangements for spreading the load, additional stiffening and thicker platingare methods used in dealing with this problem.

Figure 2.9 Localisedloads tending to distort the ship’s structure

The ends of superstructuresrepresent major discontinuities in the ship's structure where a considerable changein section modulus occurs. Localised stresses will occur which may result in crackingof adjacent structure. Sharp discontinuities, are therefore to be avoided bygradual tapers being introduced. Thicker strakes of deck and shell plating may also be fittedat these points.

Any holes oropenings cut in decks create similar areas of high local stress. well-roundedcomers must be used where openings are necessary, and doubling plates may alsobe filled. in the case of hatchways the bulk of the longitudinal strength materialis concentrated outboard of the hatch openings on either side to reduce thechange in section modulus at the openings. This is discussed further in SectionsB and F of Chapter 5.

Vibrations set upin a ship due to reciprocating machinery, propellers, etc., can result in thesetting up of stresses in the structure. These are cyclic stresses which couldresult in fatigue failure of local items of structure leading to more generalcollapse. Balancing of machinery and adequate propeller tip clearances can reducethe effects of vibration to acceptable proportions. Apart from possible damageto equipment and structure, the presence of vibration can be most uncomfortableto any passengers and the crew.

The design of the structureis outside the scope of this book. The various shipbuilding materials used to providethe structure will now be considered.

Steel is the basicshipbuilding material in use today. Steel may be regarded as an iron-carbonalloy, usually containing other elements, the carbon content not usually exceedingabout 2%. Special steels of high tensile strength are used on certain highlystressed parts of the ship's structure. Aluminium alloys have particularapplications in the construction of superstructures, especially on passengerships.

'Acid' or 'basic'are terms often used when referring to steels. The reference is to theproduction process and the type of furnace lining, e.g. an alkaline or basiclining is used to produce basic steel. The choice of furnace lining is dictatedby the raw materials used in the manufacture of the steel. There are three particularprocesses currently used for the manufacture of carbon steel namely the open hearthprocess, the oxygen or basic oxygen steel process and the electric furnace process.in all these processes the hot molten metal is exposed to air or oxygen whichoxidises the impurities to refine the pig iron into high quality steel.

In the open hearthprocess a long shallow furnace is used which is fired from both ends. A highproportion of steel scrap may be used in this process. High quality steel is producedwhose properties can be controlled by the addition of suitable alloyingelements.

In the oxygen orbasic oxygen steel process the molten metal is contained in a basic linedfurnace. A jet of oxygen is injected into the molten metal by an overheadlance. Alloying elements can be introduced into the molten metal and a highquality steel is produced.

In the electricfurnace process, an electric arc is struck between carbon electrodes and thesteel charge in the furnace. Accurate control of the final composition of thesteel and a high standard of purity are possible with this process.

Steels from theabove-mentioned processes will all contain an excess of oxygen, usually in theform of iron oxide. Several finishing treatments are possible in the finalcasting of the steel.

Rimmed steel is producedas result of little or no treatment to remove Oxygen. in the molten state theoxygen combines with the carbon in the steel, releasing carbon monoxide gas. Onsolidifying, an almost pure iron outer surface is formed. The central core ofthe ingot is, however, a mass of blow holes. Hot rolling of the ingot usually 'weldsup' these holes but thick plates of this material are prone to laminations.

Killed steel is producedby fixing the Oxygen by the addition of aluminium or silicon before pouring thesteel into the mould. The aluminium or silicon produces oxides reducing theiron oxides to iron. A homogeneous material of superior quality to rimmed steelis thus produced.

Balanced or semi-killedsteels are an intermediate form of steel. This results from the beginning ofthe rimming process in the mould and its termination by the use of deoxidisers.

Vacuum degassedsteels are produced by reducing the atmospheric pressure when the steel is illthe molten state. The equilibrium between carbon and oxygen is thus obtained ata much lower level and the oxygen content becomes very small. Final residualdeoxidation can be achieved with the minimum additions of aluminium or silicon.A very 'clean' steel is produced with good notch toughness properties andfreedom from lamellar tearing problems (lamellartearing is explained in Chapter 4).

The composition ofsteel has a major influence on its properties and this will be discussed in thenext subsection. The properties of steel are further improved by various formsof heat treatment which will now be outlined. in simplified terms the heattreatment of steels results in a change in the grain structure which alters themechanical properties of the material.

Various terms areused with reference to steel and other materials to describe their properties.These terms will now be explained in more detail

A variety ofstandard sections are produced with varying scantlings to suit theirapplication. The stiffening of plates and sections utilises one or more ofthese sections. which are shown in Figure 2.10.

Figure 2.10 Standardsteel sections: (a) flat plate; (b) offset bulb plate; (c) equal angle; (d)unequal angle; (e) channel; (f) tee

The steel used in shipconstruction is mild steel with a 0.15-0.23% carbon content. The propertiesrequired of a good shipbuilding steel are:

(1) Reasonablecost.

(2) Easily weldedwith simple techniques and equipment.

(3) Ductility andhomogeneity.

(4) Yield point tobe a high proportion of ultimate tensile strength.

(5) Chemicalcomposition suitable for flame cutting without hardening.

(6) Resistance to corrosion.

These features areprovided by the five grades of mild steel (A-E) designated by the classificationsocieties (see Chapter 10). To be classed, the steel for ship construction mustbe manufactured under approved conditions, and inspected, and prescribed testsmust be carried out on selected specimens. Finished material is stamped withthe society's brand, a symbol with L superimposed on R being of Lloyd’s Register.The chemical composition and mechanical properties of a selection of mild steelgrades are given in Table 2.1.

Table 2.1PROPERTIES AND COMPOSITION OF SOME MILD STEELS

Table 2.2PROPERTIES AND COMPOSITION OF SOME HIGH TENSILE STEELS

Table 2.3PROPERTIES AND COMPOSITION OF LOW TEMPERATURE CONSTRUCTIONAL MATERIALS

Developments in steelproduction and alloying techniques have resulted in the availability of higherstrength steels for ship construction. These higher tensile strength (HTS)steels, as they are called, have adequate notch toughness, ductility andweldability, in addition to their increased strength. The increased strengthresults from the addition of alloying elements such as vanadium, chromium,nickel and niobium. Niobium in particular improves the mechanical properties oftensile strength and notch ductility. Particular care must be taken in thechoice of electrodes and welding processes for these steels. Low hydrogenelectrodes and welding processes must be used. Table 2.2 indicates the chemicalcomposition and mechanical properties of several high tensile steel grades. Aspecial grade mark, H, is used by the classification societies to denote highertensile steel.

Benefits arisingfrom the use of these steels in ship construction include reduced structuralweight, since smaller sections may be used; larger unit fabrications arepossible for the same weight and less welding time, although a more specialisedprocess, is needed for the reduced material scantlings.

Cryogenic or lowtemperature materials are being increasingly used as a consequence of thecarriage of liquefied gases in bulk tankers. Table 2.3 details the propertiesand composition of several of these cryogenic materials. The main criterion ofselection is an adequate amount of notch toughness at the operating temperatureto be encountered. Various alloys are principally used for the very lowtemperature situations, although special quality carbon/manganese steels havebeen used satisfactorily down to -50°C.

The larger castingsused in ship construction are usually manufactured from carbon or carbonmanganese steels. Table 2.4 details the composition and properties of thesematerials. Examples of large castings are the stern frame, bossings, A-bracketsand parts of the rudder. The examples mentioned may also be manufactured as forgings.Table 2.4 also details the composition and properties of materials used forforgings.

Table 2.4PROPERTIES AND COMPOSITION OF CASTING AND FORGING MATERIALS

Table 2.5PROPERTIES AND COMPOSITION OF ALUMINIUM ALLOY CONSTRUCTIONAL MATERIALS

The increasing useof aluminium alloy has resulted from its several advantages over steel.Aluminium is about one-third the weight of steel for equivalent volume ofmaterial. The use of aluminium alloys in a structure can result in reductionsof 60% of the weight of an equivalent steel structure. This reduction in weight,particularly in the upper regions of the structure, can improve the stability ofthe vessel. This follows from the lowering of the vessel's centre of gravity,resulting in an increased metacentric height. Stability is discussed in detailin Muckle's Naval Architecture for Marine Engineering. The corrosion resistanceof aluminium is very good but careful maintenance and insulation from theadjoining steel structure are necessary. The properties required of analuminium alloy to be used in ship construction are much the same as for steel,namely strength, resistance to corrosion, workability and weldabilily. Theserequirements are adequately met, the main disadvantage being the high cost ofaluminium.

The chemicalcomposition and mechanical properties of the common shipbuilding alloys areshown in Table 2.5. Again these are classification society gradings where thematerial must be manufactured and tested to the satisfaction of the society.

Aluminium alloysare available as plate and section, and a selection of aluminium alloy sectionsis shown in Figure 2.11. These sections are formed by extrusion, which is theforcing of a billet of the hot material through a suitably shaped die.Intricate or unusual shapes to suit particular applications are thereforepossible.

Where aluminiumalloys join the steel structure, special insulating arrangements must be employedto avoid galvanic corrosion where the metals meet (see Chapter II). Whererivets are used, they should be manufactured from a corrosion-resistant alloy(see Table 2.5).

Figure 2.11 Aluminiumalloy sections

Various qualitiesof the materials discussed so far have been mentioned. These qualities aredetermined by a variety of tests which are carried out on samples of the metal.

The terms 'stress'and 'strain' are used most frequently. Stress or intensity of stress, itscorrect name, is the force acting on a unit area of the material. Strain is thedeforming of a material due to stress. When the force applied to a materialtends to shorten or compress the material, the stress is termed 'compressivestresses’. When the force applied to a material tends to lengthen the material,the stress is termed 'tensile stresses’. When the force tends to cause thevarious parts of the material to slide over one another the stress is termed'shear stress'.

The tensile test isused to determine the behaviour of a material up to its breaking point. A speciallyshaped specimen piece (Figure 2.12) of standard size is gripped in the jaws ofa testing machine. A load is gradually applied to draw the ends of the barapart such that it is subject to a tensile stress. The original test length L1of the specimen is known and for each applied load the new length L2can be measured. The specimen will be found to have extended by some smallamount L2 -L1. This deformation, expressed as

Extension /Original length

Figure 2.12 Tensiletest specimens: (a) for plates, strips and sections (a = thickness ofmaterial); (b) for hot-rolled bar

Additional loadingof the specimen will produce results which show a uniform increase of extensionuntil the yield point is reached. Up to the yield point the removal of load wouldhave resulted in the specimen returning to its original size. Stress and strainare therefore proportional up to the yield point, or elastic limit as it is alsoknown. The stress and strain values for various loads can be shown on a graphsuch as Figure 2.13.

Figure 2.13 Stress/ Strain graph for mild steel

Figure 2.14 Stress/ Strain graph for higher tensile steel

If the testing werecontinued beyond the yield point the specimen would 'neck' or reduce in cross-section.The load values divided by the original specimen cross-sectional area wouldgive the sharp shown in Figure 2.14. The highest value of stress is known asthe ultimate tensile stress (UTS) of the material.

Within the elasticlimit, stress is proportional to strain, and therefore

Stress / Strain =constant

The bend test is usedto determine the ductility of a material. A piece of material is bent over aradiused former, sometimes through 180 degrees. No cracks or surface laminations should appear in the material.

Impact tests canhave a number of forms but the Charpy vee-notch test is usually specified. Thetest specimen is a 10 mm square cross-section, 55 mm in Length. A vee-notch is cutin the centre of one face, as shown in Figure 2.15. The specimen is mountedhorizontally with the notch axis vertical. The test involves the specimen beingstruck opposite the notch and fractured. A striker or hammer on the end of aswinging pendulum provides the blow which breaks the specimen. The energyabsorbed by the material in fracturing is measured by the machine. A particularvalue of average impact energy must be obtained for the material at the testtemperature. This test is particularly important for materials to be used in lowtemperature regions. For low temperature testing the specimen is cooled byimmersion in a bath of liquid nitrogen or dry ice and acetone for about 15minutes. The specimen is then handled and tested rapidly to minimise anytemperature changes. The impact test, in effect, measures a material'sresistance to fracture when shock loaded.

Figure 2.15 Charpyimpact test

A dump test is usedon a specimen length of bar from which rivets are to be made. The bar is compressedto half its original length and no surface cracks must appear. Other rivetmaterial tests include bending the shank until the two ends touch without anycracks or fractures appearing. The head must also accept flattening until itreaches two and a half times the shank diameter.

船舶应力和造船材料

在海上或静水中的船舶不断受到各种应力和应变的作用，这些应力和应变来自船舶内外的力的作用。船内的力来自结构重量、货物、机器重量和操作机器的影响。外力包括水对船体的静水压力以及风和波浪的作用。船舶必须始终能够抵抗和承受整个结构中的这些应力和应变。因此，它必须以提供必要强度的方式和材料来构造。船还必须能够有效地作为载货船只。

作用在船上的各种力在程度和频率上不断变化。然而，为了简单起见，它们将被单独考虑，并且将概述为对抗每种类型的力而采取的特定措施。

力初步可以分为静态力和动态力。静力是由于重量和浮力的差异而产生的，这种差异出现在沿船舶长度方向的不同点上。动力来自船在海里的运动以及风和波浪的作用。一艘船可以自由移动，有六个自由度——三个线性自由度和三个旋转自由度。这些运动用图2.1所示的术语来描述。

图2.1船的运动-6个自由度

这些静态和动态力在船舶结构中产生纵向、横向和局部应力。纵向应力最大，导致船舶沿其长度弯曲。

图2.2船体结构的静态载荷

纵向应力

静态载荷

如果船被认为是漂浮在静止的水中，两种不同的力将沿着它的长度方向作用于它。船的重量及其内容物将垂直向下作用。静水压力的浮力或垂直分量将向上作用。总的来说，这两个力正好相等并相互平衡，这样船就能在特定的吃水深度下漂浮。浮力的中心和重量的中心将垂直地在一条线上。然而，在沿船舶长度的特定点上，净效应可能是浮力过剩或重量过剩。这种净效应产生了结构的载荷，就像梁一样。这种负载导致船舶结构中产生剪切力和弯矩，从而使其弯曲。作用在船舶结构上的静力如图2.2(a)所示。如图2.2(b)-(d)所示，重量和浮力的这种分布也会导致载荷、剪切力和弯矩沿船舶长度的变化。根据弯矩作用的方向，船将在纵向垂直平面内弯曲。这个弯矩被称为静水弯矩(SWBM)。特殊术语用于描述两种极端情况:当船中部的浮力超过重量时，船舶被称为“中拱”，这种情况如图2.3所示；当船中部的重量超过浮力时，船舶被称为“中垂”，这种情况如图2.4所示。

图2.3中拱状态

图2.4中垂状态

动态载荷

如果船现在被认为是在波浪中运动，重量的分布仍然是一样的。然而，浮力的分布会因波浪而变化。船的运动也将引入动态力。

解决这个问题的传统方法是将这种动态情况转换成等效的静态情况。为了做到这一点，假设船舶在与船舶长度相等的余摆线形式的静态波浪上保持平衡。海上波浪的轮廓被认为是次摆线。这就产生了波峰比波谷更尖锐的波浪。波峰最初被认为是在船的中部，然后是在船的两端。如图2.5所示，在所考虑的特定载荷条件下，最大中拱和中垂力矩将出现在结构中。

图2.5船体结构的动态负载：（a）静水状态；（b）中垂状态；（c）中拱状态

这样就得到总剪力和弯矩，其中包括前面考虑的静水弯矩。如果考虑到船舶的实际装载工况，这将使上述状态变得更糟，例如，当波谷位于船中部时，船中部的重负载，则可以找到正常运行服务中的最大弯矩。

因此，当波浪沿着船的长度移动时，船的结构将受到由这些剪切力和弯矩产生的不断波动的应力。

结构的应力

船的弯曲使其结构内部产生应力。当船中垂时，船底外板会产生拉应力，甲板会产生压应力。当船中拱时，甲板上会产生拉应力，船底会产生压应力。这种应力，无论是压缩应力还是拉伸应力，都朝着称为中性轴的位置减小。船的中性轴在一半深度以下的某个地方，实际上是一条穿过船身重心的水平线。

梁的基本弯曲方程为

M / I = σ / y

其中M是弯矩，I是截面绕中性轴的二阶面积矩，σ是外层纤维的应力，y是中性轴到外层纤维的距离。

该方程在实船试验中已被证明适用于船舶的纵向弯曲。从方程中，得到表达式

σ = M / (I/y)

在计算二阶面积矩I时，包括的结构材料将是延伸相当一部分船舶长度的所有纵向材料。这种材料将包括船舷和船底外板、内底板(如有安装)、中纵桁和甲板。这种材料形成了所谓的船体梁，与厚度相比，它的尺寸非常大。

横向应力

静态载荷

除了由结构、货物等的重量产生的载荷之外，船的横截面还受到来自周围水的静压力。尽管横向应力比纵向应力小，但如果没有足够的加强，结构可能会发生相当大的变形(图2.6)。

抵抗横向应力的结构部分是横向舱壁、双层底的肋板(如安装)、甲板横梁、舷侧肋骨和它们之间的肘板以及相邻结构，如舱顶肋板或边缘板。

图2.6横向船体结构的静水压力载荷

动态应力

当船横摇时，它被加速和减速，导致结构中的力倾向于扭曲它。这种情况被称为倾侧，当船舶处于空载或压载状态时，其影响最大(图2.7)。连接水平和垂直结构的肘板和梁弯头用来抵抗这种变形。

图2.7倾侧

局部应力

船舶在海上的运动导致产生的力主要是局部性质的。然而，这些力容易导致结构振动，从而将应力传递到结构的其他部分。

砰击或冲击

在恶劣天气下，当船舶发生垂荡和纵摇时，艏端离开并重新进入水中，产生砰击效应(图2.8)。这种艏部区域向水中的猛烈撞击被称为砰击（冲击）。必须在砰击区域安装额外的加强件，以降低损坏结构的可能性。这将在第5章A节中进一步讨论。

图2.8砰击

拍击

波浪沿着船的运动导致了板上水压的波动。倾向于产生外板的进出运动，称为拍击（振颤）。当船在水中前进时，这种效应在船头尤其明显。

船舶的纵摇运动会产生额外的水压变化，尤其是在船头和船尾，这也会导致船板拍击。额外的加强以抗震横梁和纵材的形式提供。这将在第5章D节中进一步讨论。

局部负载

较重的重量，如机舱中的设备或某些杂货，会引起横截面的局部变形(图2.9)。处理这一问题的方法是分散载荷、额外的加强和加厚板。

上层建筑和不连续性

上层建筑的末端代表了船舶结构中的主要不连续性，在此处，截面模量发生了相当大的变化。将出现局部应力，这可能导致相邻结构开裂。因此，通过引入渐变来避免尖锐的不连续性。在这些点上也可以安装较厚的甲板列板和船外板。

甲板上的任何孔洞或开口都会产生类似的高局部应力区域。在需要开口的地方必须使用圆角，也可以安装复板。对于舱口的情况，大部分纵向强度材料集中在舱口开口两侧的舷外侧，以减少开口处截面模量的变化。这将在第5章B和F节中进一步讨论。

图2.9趋使船体结构扭转变形的局部载荷

振动

由于往复机器、螺旋桨等在船上产生的振动。会导致结构中产生应力。这些是循环应力，可能导致结构局部的疲劳破坏，进而导致更普遍的倒塌。机器平衡和适当的螺旋桨桨尖间隙可以将振动的影响降低到可接受的程度。除了可能损坏设备和结构之外，振动的存在可能会使乘客和船员感到非常不舒服。

结构的设计超出了本书的范围。现在将考虑用于提供该结构的各种造船材料。

钢铁

钢是今天使用的基本造船材料。钢可以被认为是一种铁碳合金，通常含有其他元素，碳含量通常不超过2%。高抗拉强度的特种钢被用在船体结构的某些高应力部位。铝合金在建造上层建筑，尤其是客轮上有特殊的用途。

生产

“酸性”或“碱性”是指钢时经常使用的术语。参考生产工艺和炉衬类型，例如碱性炉衬用于生产碱性钢。炉衬的选择取决于炼钢所用的原材料。目前有三种用于制造碳钢的特殊方法，即平炉法、氧气或碱性氧气炼钢法和电炉法。在所有这些过程中，熔融金属暴露在空气或氧气中，氧化杂质，将生铁炼成高质量的钢。

在平炉工艺中，使用一个从两端燃烧的长而浅的炉子。在这个过程中可能会用到大量的废钢。生产高质量的钢，其性能可以通过添加合适的合金元素来控制。

在氧气或碱性氧气炼钢法中，熔融金属装在碱性炉衬炉中。用头顶的喷枪将氧气喷入熔融金属中。合金元素可以被引入到熔融金属中，从而生产出高质量的钢。

在电炉法中，电弧在碳电极和炉中的钢料之间产生。用这种方法可以精确控制钢的最终成分和高标准的纯度。

整理处理

上述过程产生的钢都含有过量的氧，通常以氧化铁的形式存在。在钢的最终铸造中，可以进行几种精整处理。

沸腾钢是由于很少或根本没有脱氧处理而产生的。在熔融状态下，氧与钢中的碳结合，释放出一氧化碳气体。凝固时，形成几乎纯铁的外表面。然而，钢锭的中心是一大堆气孔。钢锭的热轧通常会“焊住”这些孔，但这种材料的厚板容易分层。

镇静钢是通过在将钢倒入模具之前添加铝或硅来固定氧而生产的。铝或硅产生氧化物，将氧化铁还原成铁。这样就生产出了质量优于沸腾钢的同质材料。

平衡钢或半镇静钢是钢的一种中间形态。这是由于模具中沸腾过程的开始和终止阶段使用脱氧剂。

当钢处于熔融状态时，通过降低大气压力来生产真空脱气钢。碳和氧之间的平衡因此在低得多的水平上获得，并且氧含量变得非常小。添加最少量的铝或硅就能达到最终的残余脱氧。一种非常“干净”的钢被生产出来，具有良好的缺口韧性，并且没有层状撕裂问题(层状撕裂在第4章中解释)。

钢的成分对其性能有重大影响，这将在下一小节中讨论。钢的性能通过各种形式的热处理得到进一步改善，下面将对此进行概述。简而言之，钢的热处理会导致晶粒结构的变化，从而改变材料的机械性能。

正常化。根据钢的含碳量，将钢加热到850-950 ℃,然后在空气中冷却。生产出具有精细晶粒结构的高强度钢。

退火。钢再一次被加热到850-950℃左右，但是在炉子里或者在一个隔热的空间里慢慢冷却。生产出比正常状态下更软、更有韧性的钢。

硬化。钢被加热到850-950 ℃,然后通过在油或水中淬火来快速冷却。因此产生了特定钢的最坚硬的可能条件，并增加了抗拉强度。

回火。这一过程在钢淬火后进行，包括加热到680℃左右。回火温度越高，钢的拉伸性能越低。一旦回火，金属通过淬火迅速冷却。

成分和性质

在描述钢和其他材料的特性时，使用了不同的术语。现在将更详细地解释这些术语。

拉伸强度。这是关于金属的主要单一标准。这是衡量材料在使用中承受负荷的能力。诸如应力、应变、极限拉伸强度、屈服应力和验证应力等术语都是量化材料拉伸强度的不同方法。影响抗拉强度的两个主要因素是钢的碳含量及其制造后的热处理。

延展性。这是一种材料在不破裂或不损失强度的情况下永久改变形状的能力。在制造过程中金属经受成形过程的地方，这一点尤其重要。

硬度。这是对材料可加工性的一种度量。它用于评估材料的可加工性及其耐磨性。

韧性。这是介于脆性和柔软性之间的一种状态。它通常由缺口棒试验中获得的值来量化。

标准型钢

各种各样的标准节生产与各种构件尺寸，以适应他们的应用。板和型材的加强利用了这些型材中的一个或多个。如图2.10所示。

图2.10标准型钢：（a）扁钢；（b）球扁钢；（c）等边角钢；（d）不等边角钢；（e）槽钢；（f）T型材

造船用钢

用于造船的钢是含碳量为0.15-0.23%的低碳钢。一种好的造船用钢所需的性能是:

(1)成本合理。

(2)易于焊接，工艺和设备简单。

(3)延展性和均匀性。

(4)屈服点是极限抗拉强度的一个高比例。

(5)化学成分适合火焰切割，无需硬化。

(6)抗腐蚀性。

这些特性由船级社指定的五个等级的低碳钢(A-E)提供(见第10章)。要进行入级，造船用钢必须在批准的条件下生产，并进行检验，而且必须对选定的样品进行规定的试验。成品材料上印有该协会的品牌，这是一个L叠加在R上的劳氏船级社标志。表2.1给出了一系列低碳钢的化学成分和机械性能。

钢铁生产和合金化技术的发展使得更高强度的钢可用于造船。这些所谓的高抗拉强度(HTS)钢，除了增加强度之外，还具有足够的缺口韧性、延展性和可焊性。强度的增加是由于添加了合金元素，如钒、铬、镍和铌。铌尤其提高了拉伸强度和缺口延展性的机械性能。对于这些钢，在选择电极和焊接工艺时必须特别小心。必须使用低氢焊条和焊接工艺。表2.2显示了几种高强度钢的化学成分和机械性能。船级社使用特殊的等级标记H来表示高强度钢。

在船舶建造中使用这些钢的好处包括减少结构重量，因为可以使用更小的截面；对于相同的重量和更少的焊接时间，更大的单元制造是可能的，尽管减少材料尺寸需要更专业的工艺。

由于用散装油轮运输液化气体，低温材料正越来越多地被使用。表2.3详细列出了几种低温材料的特性和成分。选择的主要标准是在工作温度下要有足够的缺口韧性。各种合金主要用于非常低的温度环境，尽管特殊质量的碳/锰钢已经在低至-50℃的情况下得到令人满意的应用。

表2.1某些低碳钢的化学成分和机械性能

表2.3某些高碳钢的化学成分和机械性能

表2.3某些低温材料的化学成分和机械性能

铸件和锻件

用于造船的大型铸件通常由碳钢或碳锰钢制成。表2.4详细说明了这些材料的成分和特性。大型铸件的例子有船尾肋骨、桨毂、A字支架和舵的零件。提到的例子也可以制造成锻件。表2.4还详细说明了锻件所用材料的成分和性能。

表2.4铸件和锻件的化学成分和机械性能

表2.5铝合金材料的化学成分和机械性能

铝合金

铝合金的使用越来越广泛，这是因为它比钢有几个优点。对于同等体积的材料，铝的重量大约是钢的三分之一。在结构中使用铝合金可以使同等钢结构的重量减少60%。这种重量的减少，特别是在结构的上部区域，可以提高船的稳性。这是由于船只重心降低，导致稳心高度增加。稳性在Muckle的《海洋工程船舶建筑》中有详细讨论。铝的耐腐蚀性非常好，但小心维护和与相邻钢结构的绝缘是必要的。用于造船的铝合金所要求的性能与钢的性能大致相同，即强度、耐腐蚀性、可加工性和可焊性。这些要求已充分满足，主要缺点是铝的成本高。

常用造船合金的化学成分和机械性能见表2.5。同样，这些是船级社分级，材料必须按照船级社的要求进行制造和测试。

图2.11铝合金型材

铝合金可用于板材和型材，图2.11显示了铝合金型材的选择。这些部分通过挤压形成，挤压是将热材料的坯料挤压通过合适形状的模具。因此，适合特定应用的复杂或不寻常的形状是可能的。

在铝合金连接钢结构的地方，必须采用特殊的绝缘装置，以避免金属接触处的电化腐蚀(见第二章)。如果使用铆钉，它们应该由耐腐蚀合金制成(见表2.5)。

材料试验

到目前为止所讨论的各种质量的材料已经提到。这些质量是由对金属样品进行的各种试验来确定的。

“应力”和“应变”这两个词用得最多。应力或应力强度，它的正确名称是作用在材料单位面积上的力。应变是材料因应力而产生的变形。当施加在材料上的力趋向于缩短或压缩材料时，这种应力称为“压应力”。当施加在材料上的力趋向于拉长材料时，这种应力称为“拉应力”。当力趋向于使材料的不同部分相互滑动时，这种应力称为“剪切应力”。

图2.12拉伸试验试样：（a）板、条、型材（a = 材料厚度）；（b）热轧棒材

拉伸试验用于确定材料在断裂点之前的行为。标准尺寸的特殊形状试样(图2.12)被夹在试验机的钳口中。逐渐施加载荷以拉开杆的端部，使其承受拉伸应力。本的原始测试长度L1是已知的，并且对于每个施加的载荷，新的长度L2可以被测量。通过将L2 - L1人们将会发现，这个样本已经延伸了某个很小的量。这种变形表现为

延伸量/原始长度

对试样加额外的载荷将产生结果，该结果显示延伸均匀增加，直到达到屈服点。在屈服点之前，移除载荷会导致试样恢复到其原始尺寸。因此，直到屈服点或弹性极限，应力与应变成正比。各种载荷下的应力和应变值可以显示在图2.13中。

图2.13低碳钢的应力-应变图

图2.13高拉力钢的应力-应变图

如果试验持续到超过屈服点，试样的横截面就会“收缩”或缩小。载荷值除以原始试样的横截面积，就得到图2.14所示的曲线。应力的最高值称为材料的极限拉伸应力(UTS)。

在弹性极限内，应力与应变成正比，因此

应力/应变=常数

弯曲试验用于确定材料的延展性。一块材料在一个弧形样板上弯曲，有时弯曲180度。材料中不应出现裂纹或表面分层。

图2.15夏比冲击试验

冲击试验可以有多种形式，但夏比v形缺口试验通常是指定的。试样是一个10毫米见方的横截面，长55毫米。在一个面的中心切出一个v形切口，如图2.15所示。试样水平安装，缺口轴垂直。该试验包括在缺口对面撞击样品并使其断裂。在摆动的钟摆末端有一个撞针或锤子，它提供了击碎样品的力量。破裂时材料吸收的能量由机器测量。必须获得材料在试验温度下的特定平均冲击能量值。该测试对于在低温区域使用的材料尤为重要。对于低温试验，将样品浸入液氮或干冰和丙酮浴中冷却约15分钟。然后快速处理和测试样本，以最大限度地减少任何温度变化。冲击试验实际上是测量材料在冲击载荷下的抗断裂能力。

倾倒试验用于制作铆钉的一段棒材样品。棒材被压缩至其原始长度的一半，并且不得出现表面裂纹。其他铆钉材料测试包括弯曲铆钉杆，直到两端接触而不出现任何裂纹或断裂。头部也必须接受展平，直到它达到柄直径的两倍半。