【正文】 我們可能會下一個明顯的定義，即當材料分成兩部分或更多時失效。當剪應(yīng)力與正應(yīng)力一起存在時，主應(yīng)力便被確定。我們也將發(fā)現(xiàn)每天在使用一個最好的理論太復(fù)雜了，多數(shù)設(shè)計者更喜歡用一個會產(chǎn)生小而安全的錯誤但較簡單的理論。盡管在我們的知識以外，縱向應(yīng)力可能會對強度有影響，但不被考慮。 To illustrate the need for a failure theory, let us consider a cylindrical pressure vessel. To avoid unnecessary plications, we will consider that all welds（焊縫）are 100% efficient and that the walls（容器壁）are thin. Under internal pressure the main stresses（主應(yīng)力） are circumferential and longitudinal, and it was implied（認為）in an earlier case that only the circumferential stress, because it is larger than the longitudinal stress, needs to be considered in judging the adequacy of the design. In this approach we tacitly（默認）assumed that the maximum stress could be treated as（看作為）a uniaxial stress and that it alone determined the safety of the design. The longitudinal stress was not considered although it may, without our knowledge（在我們的知識之外）, have had an influence on strength. It happens that our approach in this case is acceptable, but, in a biaxial state of stress, the second stress is not always inconsequential（不重要）and an understanding of failure theory is necessary in order to avoid making some serious errors. 為了舉例說明需要一個失效理論，讓我們考慮一個圓柱形的壓力容器。但是，對設(shè)計中出現(xiàn)的每組新的多軸應(yīng)力都將需要一組試驗。這是所有設(shè)計問題中最簡單的；該法是非常合適的，因為試驗和設(shè)計中的荷載和應(yīng)力性質(zhì)是完全相同的。 In the design of a member subjected to a uniaxial（單軸的） load, the stress was pared with the stress to cause failure in test specimens（試件）that had also been subjected to uniaxial load. This is the simplest of all design problems。 there is much more to learned by those who have a special interest. To illustrate a serious limitation（缺陷） at our present stage, we can express distributed loads （分布荷載） that are variable and are intermittent, but we cannot write a load function for concentrated loads. If we had taken the next step and dealt with the concentrated load, we would have encountered the source of the expression（表達式）“singularity function”, but having regard for（考慮）the scope of this book we have stopped short of（達不到）that step. 本節(jié)使你熟悉了奇異函數(shù)，并發(fā)現(xiàn)通過利用它們能大大地簡化某些問題。但當遇到更先進的結(jié)構(gòu)時，此法會變得非常復(fù)雜，需要進一步地展開。當遇到超靜定的情況時，就此類問題的難點和重點獲得一些認識。能接受的最小的S值不必是最經(jīng)濟的構(gòu)件。這樣花費的時間比實際需要的多，因為表格中在截面模量S的標題下也提供了每一個構(gòu)件的I/c的值。暫時我們將采用許用應(yīng)力法，似乎它只取決于材料的強度和安全系數(shù)。Design of Beams 梁的設(shè)計 Up to this point（至此）we have looked at（考慮）the beam problem as a problem in analysis。許用應(yīng)力 sa 為where sf is the stress at which the material fails (failure to be defined later) and n is the safety factor.這里，sf 為材料失效（失效在下文有定義）時的應(yīng)力，而n為安全系數(shù)。在此假定下，荷載強度或應(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）。 Design of Axially Loaded Members 軸向力構(gòu)件的設(shè)計 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（遠離）the point of application of the load, are being pulled apart with the same load intensity（荷載強度）. 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)計周期有一些了解，首先研究一個非常簡單的構(gòu)件。通常，根據(jù)經(jīng)驗，設(shè)計者選擇一個試算構(gòu)件，然后進行分析，看它是否滿足規(guī)定的要求。工程師利用材料力學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。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. 材料力學用以研究不同物體（通常稱為構(gòu)件）對施加力的響應(yīng)。 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。 (b) Aerated concretes (US gas concretes) foamed（起泡） by whisking（攪拌）or by some chemical process during casting。 The current tendency is to develop（采用） lighter materials, aluminum, for example, weighs much less than steel but has many of the same properties. Aluminum beams have already been used for bridge construction and for the framework of a few buildings. 目前的趨勢是采用較輕的材料。另一種（更常用的）方法是將預(yù)應(yīng)力鋼筋置于按成品結(jié)構(gòu)的形狀設(shè)置的模板的較低部位，然后將混凝土倒入（模板）而包圍著鋼筋。如今鉚接已大量地被焊接所替代，鋼構(gòu)件間的連接通過在高熱下熔化它們之間的鋼材料（即焊條）進行。 Another advance in steel construction（結(jié)構(gòu)） is the method of fastening together（連在一起） the beams. For many years the standard method was riveting. A rivet is a bolt with a head that looks like a blunt screw（圓頭螺絲釘） without threads（螺紋）. It is heated, placed in holes through the pieces of steel（鋼構(gòu)件）, and a second head is formed at the other end by hammering（錘擊）it to hold it in place（固定就位）. Riveting has now largely been replaced by welding, the joining together of pieces of steel by melting（熔化） a steel material between them under high heat. 鋼結(jié)構(gòu)中的另一個進步是梁的連接方式。 they had the solid look of bearing walls（承重墻）. Today, however, curtain walls are often made of lightweight materials such as glass, aluminum, or plastic, in various binations. 由于現(xiàn)代結(jié)構(gòu)的重量由鋼或混凝土框架承受，墻體不再支承建筑物。對多層建筑，再也沒必要采用厚的石墻或磚墻，且施工防火地面變?yōu)槿菀椎枚??；炷僚c鋼筋形成如此強大的結(jié)合力——這個力將它們粘合在一起——以致于鋼筋在混凝土中不會滑移。這樣，兩種材料可以互補。在或靠近施工現(xiàn)場，將水泥與砂、骨料（小石頭、壓碎的巖石或礫石）、水混合而制成混凝土。新的合金進一步提高了鋼材的強度，并消除了一些缺點，如疲勞，即在連續(xù)的應(yīng)力變化下導(dǎo)致強度減弱的趨勢。