【文章內(nèi)容簡介】 如，鋁的重量比鋼輕得多，但具有很多相同的性能。鋁材梁已經(jīng)用于橋梁建筑和一些建筑的框架。 Lightweight concretes, another example, are now rapidly developing（發(fā)展） throughout the world. They are used for their thermal insulation(絕熱性). The three types are illustrated below（舉例說明如下）: (a) Concretes made with lightweight aggregates。 (b) Aerated concretes (US gas concretes) foamed（起泡） by whisking（攪拌）or by some chemical process during casting。 (c) Nofines concretes. 另一個例子是輕質(zhì)混凝土，如今已在全世界快速地發(fā)展，因它們的絕熱性而被采用，其三種類型舉例說明如下：(a)輕質(zhì)骨料制成的混凝土；(b)通過澆筑時攪拌或一些化學(xué)方法起泡而成的加氣混凝土（US加氣混凝土）；(c)無細(xì)骨料混凝土。 All three types are used for their insulating properties（絕熱性）, mainly in housing, where they give high（非常） fort in cold climates and a low cost of cooling（降溫成本）in hot climates. In housing, the relative weakness of lightweight concrete walls is unimportant, but it matters（有重大關(guān)系） in roof slabs, floor slabs and beams. 這三種類型的混凝土都是由于它們的絕熱性而被使用，主要用于房屋，使其在寒冷的氣候中非常舒服，在炎熱的氣候中降溫的成本不高。在房屋中，墻采用較薄弱的輕質(zhì)混凝土不重要，但是屋面板、樓面板和梁（采用輕質(zhì)混凝土）則有重大關(guān)系。 In some locations, some lightweight aggregates cost little more than（幾乎等于） the best dense（致密） aggregates and a large number of （大量） floor slabs have therefore been built of lightweight aggregate concrete purely for its weight saving, with no thought of（沒考慮） its insulation value. 在某些地區(qū)，一些輕質(zhì)骨料的費(fèi)用幾乎等于最致密的骨料，因此大量的樓面板采用輕骨料混凝土制作純粹是節(jié)約重量，而沒考慮它的絕熱價值。 The lightweight aggregate reduces the floor dead load（恒載） by about 20 per cent resulting in（導(dǎo)致）considerable savings in the floor（樓蓋結(jié)構(gòu)） steel in every floor and the roof, as well as in the column steel and (less) in the foundations. One London contractor（承包商）prefers to use lightweight aggregate because it gives him the same weight reduction in the floor slab as the use of hollow tiles, with simpler organization and therefore higher speed and profit. The insulation value of the lightweight aggregate is only important in the roof insulation, which is greatly improved（改進(jìn)）. 輕質(zhì)骨料使樓面的恒載減少了約20%，因而大量的節(jié)約了每層樓面以及屋面的樓蓋結(jié)構(gòu)中的鋼材和柱子與基礎(chǔ)中（較少）的鋼材使用量。一位倫敦的承包商寧愿使用輕質(zhì)骨料，因?yàn)檫@使樓面板上減少的重量與用空心磚相同，且組織更簡單，因而速度和利潤更高。輕質(zhì)骨料的絕熱價值只在屋面絕熱時顯得重要，它已被大大地改進(jìn)了。UNIT 4 Mechanics of Materials deals with（研究）the response of various bodies, usually called members（構(gòu)件）, to applied forces（施加力）. In Mechanics of Engineering Materials the members have shapes that either exist in actual structures or are being considered for their suitability（根據(jù)其需要）as parts of proposed（擬建的）engineering structures. The materials in the members have properties that are characteristic of monly used（常用的）engineering materials such as steel, aluminum, concrete, and wood. 材料力學(xué)用以研究不同物體（通常稱為構(gòu)件）對施加力的響應(yīng)。在工程材料力學(xué)中，構(gòu)件的形狀可以是實(shí)際結(jié)構(gòu)中存在的，也可以根據(jù)其需要而進(jìn)行考慮（設(shè)計(jì)），作為擬建工程結(jié)構(gòu)的部件。構(gòu)件中材料的性能即是常用的工程材料如鋼材、鋁材、混凝土和木材的特性。 As you can see already from the variety of materials, forces, and shapes mentioned, Mechanics of Engineering Materials is of interest to（對..有價值）all fields of engineering. The engineer uses the principles of Mechanics of Materials to determine if the material properties and the dimensions of a member are adequate to（足以）ensure that it can carry its loads safely and without excessive distortion. In general（通常）, then, we are interested in both the safe load that a member can carry and the associated（相關(guān)的）deformation. Engineering design would be a simple process if the designer could take into consideration（考慮）the loads and the mechanical properties of the materials, manipulate（利用）an equation, and arrive at（得到）suitable dimensions. Design is seldom that simple. Usually（通常）, on the basis of（根據(jù)）experience, the designer selects a trial（試算） member and then does an analysis to see if that member meets the specified requirements. Frequently（常常）, it does not and then a new trial member is selected and the analysis repeated. This design cycle（設(shè)計(jì)周期） continues until a satisfactory solution is obtained. The number of cycles（循環(huán)次數(shù)） required to find an acceptable design diminishes as the designer gains experience. 正如你已經(jīng)從提到的各種各樣的材料、力和形狀所看到的，工程材料力學(xué)對所有的工程領(lǐng)域都有價值。工程師利用材料力學(xué)的原理來確定是否該材料的性能和構(gòu)件尺寸足以保證它能安全地承受荷載且沒有過多的變形。通常，我們關(guān)心的是構(gòu)件能承受的安全荷載及其相應(yīng)的變形。如果設(shè)計(jì)者能通過考慮荷載和材料的力學(xué)性能，并利用公式得到合適的構(gòu)件尺寸，那么工程設(shè)計(jì)將是一個簡單的過程。然而設(shè)計(jì)很少那么簡單。通常，根據(jù)經(jīng)驗(yàn)，設(shè)計(jì)者選擇一個試算構(gòu)件，然后進(jìn)行分析，看它是否滿足規(guī)定的要求。它常常不會滿足要求，則再選擇一個新的試算構(gòu)件，再進(jìn)行分析。這樣的設(shè)計(jì)不斷重復(fù)，直至得到一個滿意的結(jié)果。當(dāng)設(shè)計(jì)師擁有一定的經(jīng)驗(yàn)后，為得到一個可接受的設(shè)計(jì)所需要的循環(huán)次數(shù)會減少。 Design of Axially Loaded Members 軸向力構(gòu)件的設(shè)計(jì) To give you some insight into （使..有一些了解）the design cycle, an extremely simple member will be dealt with first. That member is a prismatic bar with a force, P, acting along its longitudinal axis in the direction（縱軸向）such that it tends to elongate the bar. Such a force is referred to as（稱為）an axial tensile load（軸向拉力）, and we can readily imagine it trying to（努力..）pull the fibers apart and to cause failure on a transverse plane（橫向平面）. It is safe to assume that all fibers of the bar, in regions remote from（遠(yuǎn)離）the point of application of the load, are being pulled apart with the same load intensity（荷載強(qiáng)度）. With this assumption, the load intensity or stress is uniform on a transverse plane and is given by 為了使你對設(shè)計(jì)周期有一些了解，首先研究一個非常簡單的構(gòu)件。構(gòu)件是個棱形的桿件，其上沿著它的縱軸向作用一個力P，這樣往往使桿件在該方向上伸長。這樣的力稱為軸向拉力，我們能容易地想象它在努力地將纖維拉開，導(dǎo)致橫向平面的破壞。安全地假定桿件的所有纖維在遠(yuǎn)離荷載施加點(diǎn)的區(qū)域以相同的荷載強(qiáng)度被拉開。在此假定下，荷載強(qiáng)度或應(yīng)力在橫向平面上是均勻的，為 when P is in（以..為單位）Newtons and A is in square metres, stress,s ,is in Newtons per square metre (N/m2), which is by definition（根據(jù)定義）Pascals (Pa).當(dāng)P的單位為牛頓、A的單位為平方米時，應(yīng)力σ的單位為牛頓每平方米（N/m2），根據(jù)定義為帕斯卡（Pa）。 For a given axial load and given dimensions, the stress can be calculated from (41) and pared with（與..相比）the stress that can be safely carried by the material. The safe stress, known as（稱為）the design stress or allowable stress（許用應(yīng)力）, is determined by tests performed on material made to（按照） the same specifications as the part being considered. A safety factor（安全系數(shù)）, frequently imposed by a legally established code（法規(guī)）, is applied to the strength, as determined by tests, to give the allowable stress. The allowable stress, sa , is given by