土木專業(yè)英語翻譯

1、Ch 2 Overview of Engineering MechanicsAs we look around us we see a world full of things: machines, devices, tools; things that we have designed, 當(dāng)我們環(huán)顧四周,我們可以看到充滿物質(zhì)的世界:機(jī)器,儀器,工具:這些被我設(shè)計(jì)、建造和使用過built and used; things made of wood, metals, ceramics, and plastics. We know from experience that some 的東西:這些東

2、西是由木頭,金屬,陶瓷和塑料做成。我們從經(jīng)驗(yàn)可以知道一些東西比別的東things are better than others; they last longer, cost less, are quieter, look better, or are easier to use.比別的東西好:他們比較耐用,低成本,安靜,看起來好點(diǎn),或者使用比較簡單。Ideally, however, every such item has been designed according to some set of functional requirements; as 不管怎么樣,最理想的是這樣一個(gè)東西都

3、是被設(shè)計(jì)者所認(rèn)為的那樣根據(jù)某些“使用要求”設(shè)計(jì)出來的。perceived by the designersthat is, it has been designed so as to answer the question, Exactly what function也就是說他的設(shè)計(jì)是為了回答這些問題,確切的說它應(yīng)該執(zhí)行怎么樣的功should it perform? In the world of engineering, the major function frequently is to support some type of 能?在工程的領(lǐng)域里,最主要的功能是經(jīng)常支持一些由于重力,慣

4、性,壓力等作用產(chǎn)loading due to weight, inertia, pressure etc. From the beams in our homes to the wings of an airplane, there 生的荷載。從我們家里的梁柱要飛機(jī)的雙翼,這些都是由材料尺寸和must be an appropriate melding of materials, dimensions, and fastenings to produce structures that will緊固件適當(dāng)?shù)慕M合,從而生產(chǎn)的結(jié)構(gòu)在合理成本和使用周期的基礎(chǔ)上可靠的履行它的職責(zé)。perform th

5、eir functions reliably for a reasonably cost over a reasonable lifetime.In practice, engineering mechanics methods are used in two quite different ways:在實(shí)踐中,工程力學(xué)方法用于兩種完全不同的方式。1. The development of any new device requires an interactive, iterative consideration of form, size, materials,任何新設(shè)計(jì)的發(fā)展要求都在尺寸

6、,材料,荷載,耐久度,安全性和成本等方面進(jìn)行交互、反復(fù)的考慮。loads, durability, safety, and cost.。2. When a device fails (unexpectedly it is often necessary to carry out a study to pinpoint the cause of failure當(dāng)一個(gè)設(shè)計(jì)以外的失敗的時(shí)候,必須經(jīng)常的展開研究去查明失敗的原因和確定可能的校正措施。and to identify potential corrective measures. Our best designs often evolve t

7、hrough a successive elimination of我們最好的設(shè)計(jì)是經(jīng)常通過逐漸的消除缺點(diǎn)來實(shí)現(xiàn)的。weak points.To many engineers, both of the above processes can prove to be absolutely fascinating and enjoyable, not to 對(duì)于許多工程師,上述的過程被證明它是極具吸引力和令人愉悅的,更不用說(有時(shí)是有利可圖的。mention (at times lucrative.In any real problem there is never sufficient good

8、, useful information; we seldom know the actual loads and 在任何真正的問題當(dāng)中從未有足夠好的,有用的信息,我們很少知道實(shí)際的荷載和任何精確的工作條operating conditions with any precision, and the analyses are seldom exact. While our mathematics may件。并且分析的結(jié)構(gòu)很少是精確的。雖然我們的數(shù)學(xué)可以很精確, precise, the overall analysis is generally only approximate, and di

9、fferent skilled people can obtain different 所有的分析結(jié)果一般僅僅是近似的,和不同掌握技術(shù)的人可以獲得不同的解決方案。solutions. In the study of engineering mechanics most of the problems will be sufficiently idealized to 在工程力學(xué)的研究當(dāng)中,大多數(shù)的問題都將被足夠的理想化,以便允許得到唯一的答案。permit unique solutions, but it should be clear that the real world is far l

10、ess idealized, and that you usually但是我們應(yīng)該清楚意識(shí)到“真實(shí)的世界”遠(yuǎn)不是理想化的,你通常不得不執(zhí)行will have to perform some idealization in order to obtain a solution.一些理想化以便獲得答案。The technical areas we will consider are frequently called statics and strength of materials, statics我們將考慮的技術(shù)領(lǐng)域通常叫做“靜力學(xué)”和“材料力學(xué)”。靜力學(xué)是指力作用于靜止物體上的一種refer

11、ring to the study of forces acting on stationary devices, and strength of materials referring to the effects 研究,“材料力學(xué)”是指那些力作用在結(jié)構(gòu)上的of those forces on the structure (deformation, loading limits, etc.影響(變形,荷載極限等。While a great many devices are not, in fact, static, the methods developed here are perfect

12、ly applicable to然而在實(shí)際上許多的設(shè)備并不是靜力學(xué)的問題,如果額外的荷載被考慮,這里的研究方法完全適用于dynamic situations if the extra loadings associated with the dynamics are taken into account (we shall briefly 動(dòng)力學(xué)情形,以至于在動(dòng)力的情況下也被考慮在內(nèi)(我們應(yīng)該簡單的提一下是怎么樣被做到的 mention how this is done. Whenever the dynamic forces are small relative to the static

13、loadings, the system只要?jiǎng)恿W(xué)的力相對(duì)于靜力荷載來說很小,這個(gè)系統(tǒng)通?？梢钥紤]為靜力學(xué)is usually considered to be static.的問題。In engineering mechanics, we will begin to appreciate the various types of approximations that are inherent在工程力學(xué)里,我們將開始正確評(píng)價(jià)各種類型在實(shí)際問題中本身就存在的近似方法。in any real problem:Primarily, we will be discussing things whic

14、h are in equilibrium, i.e., not accelerating. However, if we look 首先,我們將會(huì)討論處于平衡及不產(chǎn)生加速度的物體。然而如果我們看的夠仔細(xì),所有東西是運(yùn)動(dòng)的。closely enough, everything is accelerating.We will consider many structural members to weightless but they never are.我們會(huì)把一些結(jié)構(gòu)構(gòu)件考慮成失重,但他們從來都不是。We will deal with forces that act at a pointbut

15、 all forces act over an area.我們要處理的力作用在一個(gè)點(diǎn)上但是所有的力作用在一個(gè)區(qū)域上。We will consider some parts to be rigidbut all bodies will deform under load.我們會(huì)認(rèn)為一些結(jié)構(gòu)是剛性的但是所有的構(gòu)件都在何在下變形。We will make many assumptions that clearly are false. But these assumptions should always render the我們將做很多顯然錯(cuò)誤的假設(shè)。但是這些假設(shè)應(yīng)該總是使得問題更加容易處理。pr

16、oblem easier, more tractable. You will discover that the goal is to made as many simplifying assumptions你將發(fā)覺目標(biāo)是盡可能的做簡單的假設(shè),而沒有嚴(yán)重降低結(jié)果。as possible without seriously degrading the result.Generally there is no clear method to determine how completely, or how precisely, to treat a problem. If our 通常沒有明確的方

17、法去可以完美的確定或者精確的處理這個(gè)問題。如果我analysis is too simple, we may not get a pertinent answer; if our analysis is too detailed, we may not be able 們的分析結(jié)構(gòu)太過于簡單,我們可能得不到正確的答案,如果我們的分析太過于詳細(xì),我們可能不能to obtain any answer. It is usually preferable to start with a relatively simple analysis and then add more或者任何的答案。通常我們更喜

18、歡的是從相對(duì)簡單的分析開始,然后增加更多的細(xì)節(jié)以便于得到detail as required to obtain a practical solution.必須的實(shí)際結(jié)果。During the past two decades, there has been a tremendous growth in the availability of computerized在過去二十年中,在計(jì)算機(jī)解決問題方法的實(shí)用性有了相當(dāng)大的發(fā)展,methods for solving problems that previously were beyond solution because the tie re

19、quired to solve them這些問題以前是不能解決的,因?yàn)樾枰鉀Q他們時(shí)間不允許。would have been prohibitive. At the same time the cost of computer capability and use has decreased by同時(shí)計(jì)算機(jī)的內(nèi)存能力和使用成本已經(jīng)降下了許多個(gè)數(shù)量級(jí)。orders of magnitude. We are experiencing an influx of personal computers; on campus, in the home, and in 我們正在經(jīng)歷個(gè)人計(jì)算機(jī)進(jìn)入校園,家庭和

20、商場的時(shí)代。business.Ch 4 Stress Limits in DesignHow large can we permit the stresses to be? Or conversely: How large must a part be to withstand a given set 我們能夠允許的最大應(yīng)力是多少?反過來,構(gòu)件必須要多少才能夠承受一組給定的荷載呢?of loads?what are the overall conditions or limits that will determine the size and material for a part?將決定構(gòu)

21、件的尺寸和材料的整體狀況或限制條件是什么?Design limits are based on avoiding failure of the part to perform its desired function. Because different parts 設(shè)計(jì)極限是基于構(gòu)件執(zhí)行設(shè)定功能避免遭受失敗。因?yàn)椴煌臉?gòu)件must satisfy different functional requirements, the conditions which limit load-carrying ability may be quite 滿足不同的功能要求,極限承載能力的條件在不同的原理可能

22、完全的不同。different for different elements. As an example, compare the design limits for the floor of a house with those例如,比較房屋樓板的設(shè)計(jì)極限和那些飛機(jī)的雙翼。for the wing of an airplane.If we were to determine the size of the wooden beams in a home such that they simply did not break, we如果我們確定了房屋木梁的尺寸,以致他們僅僅不會(huì)斷掉,我們不會(huì)因

23、為他們這樣而非常開心; would not be very happy with them; they would be too springy. Walking across the room would be like他們太過于有彈性,穿過房間就行走在潛艇夾板上。walking out on a diving board.Obviously, we should be concerned with the maximum deflection that we, as individuals, find acceptable. 顯然我們應(yīng)該關(guān)心我們每個(gè)人所能接受的最大撓度。This leve

24、l will be rather subjective, and different people will give different answers. In fact, the same people 這個(gè)標(biāo)準(zhǔn)是非常主觀的,不同的人給出不同的答案。事實(shí)上,相同的人給出不同的答案取決于他們may give different answers depending on whether they are paying for the floor or not!是否會(huì)為這個(gè)樓板付錢。An airplane wing structure is clearly different. If you l

25、ook out an airplane window and watch the wing during 飛機(jī)機(jī)翼的結(jié)構(gòu)式顯然不同。如果你從飛機(jī)窗戶往外看,你會(huì)看到機(jī)翼在強(qiáng)對(duì)流天氣中,你將看到很turbulent weather, you will see large deflections; in fact you may wish that they were smaller. However, you 的撓度。事實(shí)上,你會(huì)希望他們更小一點(diǎn)。然而你知道最重要的know that the important issue is that of structural integrity, not

26、 deflection.是結(jié)構(gòu)的整體性而不是撓度。We want to be assured that the wing will remain intact. We want to be assured that no matter what the pilot 我們想要確保機(jī)翼保持完好無缺,我們想要確保無論飛行員做了什么和無論天氣是怎么樣,雙翼也將and the weather do, that wing will continue to act like a good and proper wing. In fact, we really want to be 繼續(xù)表現(xiàn)得良好和合適的雙翼

27、事實(shí)上,我們真的想要assured that the wing will never fail under any conditions. Now that is a pretty tall order; who knows what 確保機(jī)翼在任何情況下沒有失敗。由于這是非常艱難的任務(wù)。誰知道最壞的條the worst conditions might be?件是怎樣的呢?Engineers who are responsible for the design of airplane wing structures must know, with some degree of 那些負(fù)責(zé)飛機(jī)機(jī)

28、翼結(jié)構(gòu)設(shè)計(jì)的工程師一定懂得,一定程度的把握最有certainty, what the worst conditions are likely to be. It takes great patience and dedication for many years to 可能的最壞條件是什么。他需要花費(fèi)許多年的極大耐心和貢獻(xiàn)去收集足夠assemble enough test data and failure analyses to be able to predict the worst case. The general procedure is 的實(shí)驗(yàn)數(shù)據(jù)和失敗失敗分析,才能夠預(yù)知“最壞”的

29、情況。一般的程序是研究to develop statistical data which allow us to say how frequently a given condition is likely to be統(tǒng)計(jì)數(shù)據(jù)讓我們解釋在特定情況下可能會(huì)遇到的頻率每1000小時(shí)一次,或者10000小時(shí)一次, encounteredonce every 1000 hours, or once every 10000 hours, etc.等等。As we said earlier, our object is to avoid failure. Suppose, however, that a

30、part has failed in service, and we 就像我們?cè)缙谒f的,我們的目標(biāo)是避免失敗。假設(shè),然而一部分在使用中失敗了,我們就會(huì)被問到are asked; Why? Error as such can come from three distinctly different sources, any or all of which can “為什么?”“誤差”類似的,就會(huì)從來自三個(gè)截然不同的來源出發(fā),部分或者全部都能導(dǎo)致失敗。cause failure:1. Error in design: We the designers or the design analysts

31、 may have been a bit too optimistic: Maybe we ignored 設(shè)計(jì)的誤差:我們的設(shè)計(jì)師或者設(shè)計(jì)分析者有那么一點(diǎn)過于樂觀了:也許我們忽視了一些荷載;some loads; maybe our equations did not apply or were not properly applied; maybe we overestimated the 也許我們的公式不適用或者利用得不當(dāng);也許我們高估了使用者的智慧。intelligence of the user; may we slipped a decimal point.也許我們忽略了一個(gè)小數(shù)點(diǎn)

32、。2. Error in manufacture: When a device involves heavily stressed members, the effective strength of the members 制造的誤差:當(dāng)設(shè)計(jì)涉及到重壓力的一部分,這部分的有效應(yīng)力通過不適當(dāng)?shù)弥圃旌徒M裝而大大減弱。can be greatly reduced through improper manufacture and assembly: May the wrong material was used; maybe也許使用了錯(cuò)誤的材料;也許the heat treatment was no

33、t as specified; maybe the surface finish was not as good as called for; may a part was熱處理不是和指定的一樣;也許表面的完成沒有說的那么好;也許有一部分out of tolerance; may be surface was damaged during machining; maybe the threads were not lubricated at超出了可承受的范圍;也許在制造中損壞了建筑表面;也許螺紋在組裝中不夠潤滑。assembly; or perhaps the bolts were not p

34、roperly tightened.或者螺栓沒有適當(dāng)?shù)睦o。3. Error in use: As we all know, we can damage almost anything if we try hard enough, and sometimes we do so 使用中的誤差:我們都知道,如果我們用力過度的話會(huì)損壞所有的東西,和我們有時(shí)候是意外的; accidentally: We went too fast; we lost control; we fell asleep; we were not watching the gages; the power went 我們走的太

35、快;我們會(huì)失去控制;我們睡著了;我們沒有看著計(jì)量器;我們精力逐漸減弱, off; the computer crashed; he was taking a coffee break; she forgot to turn the machine off; you failed to lubricate 電腦故障;他真的在喝咖啡休息;他忘記了關(guān)掉儀器;你沒有給它加潤滑it, etc.劑等。Any of the above can happen: Nothing is designed perfectly; nothing is made perfectly; and nothing is us

36、ed 上面提到的都有可能發(fā)生:,沒有東西設(shè)計(jì)得是完美的;沒有東西被制造的很完美,和沒有東西會(huì)被perfectly. When failure does occur, and we try to determine the cause, we can usually examine the design; we 使用得很完美,當(dāng)失敗發(fā)生的時(shí)候,我們我們嘗試去確定原因,我們通常做的是檢驗(yàn)設(shè)計(jì);我們通常can usually examine the failed parts for manufacturing deficiencies; but we cannot usually determine

37、 how 檢查失靈的部分,為了彌補(bǔ)制造時(shí)的缺失,但是我們通常不能決定這些設(shè)備怎么樣式正確使用(或the device was used (or misused. In serious cases, this can give rise to considerable differences of opinion, 誤用. 。在一些嚴(yán)重的示例中,這會(huì)引起相當(dāng)不同的觀點(diǎn),differences which frequently end in court.這些不同的觀點(diǎn)通常只能在法庭上解決。In an effort to account for all the above possibilities

38、, we design every part with a safety factor. Simply put, 在努力解決上述可能性的時(shí)候,我們要用安全系數(shù)來設(shè)計(jì)每一部分。簡單的說,the safety factor (SF is the ratio of the load that we think the part can withstand to the load we expect it to 在荷載中安全系數(shù)的比例我們認(rèn)為這個(gè)結(jié)構(gòu)可以承受以我們經(jīng)驗(yàn)預(yù)期的荷載。experience. The safety factor can be applied by increasing the

39、 design loads beyond those actually expected, 安全系數(shù)應(yīng)用于超越實(shí)際預(yù)期而逐漸增加的設(shè)計(jì)荷載。or by designing to stress levels below those that the material actually can withstand (frequently called design 或者設(shè)計(jì)的應(yīng)力水平低于那些材料實(shí)際上可以承受的標(biāo)準(zhǔn)(通常稱為“設(shè)計(jì)應(yīng)力”stresses.Safety factor=SF=failure load/design load安全系數(shù)=SF=破壞荷載/設(shè)計(jì)荷載=failure stress

40、/design stress=破壞應(yīng)力/設(shè)計(jì)應(yīng)力It is difficult to determine an appropriate value for the safety factor. In general, we should use larger values 很難確定安全系數(shù)的適當(dāng)值。一般而言,我們應(yīng)該用更大的值當(dāng): when:1. The possible consequences of failure are high in terms of life or cost.1.失敗的可能結(jié)果是高周期和成本為條件。2. There are large uncertainties i

41、n the design analyses.2.在設(shè)計(jì)分析里面有很大的不確定性。Values of SF generally range from a low of about 1.5 to 5 or more. When the incentives to reduce structural 安全系數(shù)的數(shù)值一般在低約1.5到5或更多之間變動(dòng),減少結(jié)構(gòu)重量的獎(jiǎng)勵(lì)措施是很重要的(例如飛weight are great (as in aircraft and spacecraft, there is an obvious conflict. Safety dictates a large SF,

42、while 和太空船,有著很沖突。安全條例規(guī)定最大的安全系數(shù),當(dāng)執(zhí)行要求最小的數(shù)值。performance requires a small value. The only resolution involves reduction of uncertainty. Because of唯一的解決方法是減少不確定性。由于特別小心,并在extreme care and diligence in design, test, manufacture, and use, the aircraft industry is able to maintain very 設(shè)計(jì)、測(cè)試、制造和使用中的特別注意,航空

43、工業(yè)能夠保持非常令人羨慕的安全紀(jì)錄,然而使用enviable safety records while using safety factors as low as 1.5.安全因素低至1.5。We might not that the safety factor is frequently called the ignorance factor. This is not to imply that我們常常不把安全系數(shù)稱作未知因素。但這并不是說工程師無知engineers are ignorant, but to help instill in them humility, caution,

44、and care. An engineer is responsible for而是幫助他們慢慢灌輸謙虛,謹(jǐn)慎和注意。工程師為他的或她的設(shè)計(jì)決策h(yuǎn)is or her design decisions, both ethically and legally. Try to learn from the mistakes of others rather than 負(fù)責(zé)任,在法律和倫理兩方面。嘗試從別人的錯(cuò)誤中學(xué)習(xí)而不是你自己本人制造making your own.錯(cuò)誤。Ch 6 Testing of MaterialsThe most common test of building materi

45、als is the strength test to destruction. This is partly because strength 大部分建筑材料的測(cè)試都是強(qiáng)度破壞的測(cè)試。部分是因?yàn)閺?qiáng)度是建筑材is a very important property of a building material, even a material in a non-load-bearing part of the料很重要的性能,甚至是建筑物中非承重部分的材料,building; partly because strength tests are comparatively simple to ca

46、rry out; and partly because they offer a 部分是因?yàn)閺?qiáng)度測(cè)試比較簡單的完成,和部分是因?yàn)樗麄兲峁┝似渌阅艿闹笇?dǎo),guide to other properties, such as durability.例如耐久性。The strength of a ductile material such as steel, aluminium, or plastics is usually determined by applying a延展性材料的強(qiáng)度例如鋼筋,鋁合金,或者塑料通常是通過拉力荷載說決定的。tensile load. A compression

47、test is used for brittle materials such as concrete, stone, and brick because their 壓應(yīng)力的測(cè)試用來測(cè)試脆性材料例如混凝土,石頭,和磚,因?yàn)樗麄兊目估瓘?qiáng)度很低tensile strength is low and thus harder to measure accurately.和因而很難準(zhǔn)確測(cè)量。The method of testing and the dimensions of the test pieces are laid down in the appropriate standards測(cè)試的方法

48、和試樣的尺寸要符合美國材料實(shí)驗(yàn)學(xué)會(huì)、英國標(biāo)準(zhǔn)學(xué)會(huì)、澳大利亞標(biāo)準(zhǔn)學(xué)會(huì)等公布的published by the American Society for Testing Materials (ASTM, the British Standards Institution (BSI, the 適當(dāng)標(biāo)準(zhǔn)。Standards Association of Australia (SAA, etc.The size and shape of the test specimen are particularly important for brittle materials because they infl

49、uence 試驗(yàn)的尺寸和形狀對(duì)于脆性材料尤其重要,因?yàn)樗麄冇绊懥藢?shí)驗(yàn)可能會(huì)發(fā)生的裂縫數(shù)量。the number of flaws that are likely to occur in the test specimen. For concrete tests, the standard American對(duì)于混凝土測(cè)試,美國和澳大利亞的and Australian test specimena cylinder 150mm (6 in. in diameter and 300 mm (12 in. longgives a標(biāo)準(zhǔn)試驗(yàn)是150mm(6in直徑的圓柱和300mm(12in長給一個(gè)比英國

50、標(biāo)注更低的試驗(yàn)lower result than the standard British test specimena 150-mm cubebecause the former contains more 150mm立方體因?yàn)榍罢呖梢匀菁{更多的混凝土。concrete.The speed of testing is also specified. A passage of time is required for both plastic deformation and the測(cè)試的速度也是被指定的。塑性變形和裂縫形成所通過的時(shí)間也是被要求的,formation of cracks, an

51、d a faster rate of testing thus gives a higher result.更快的測(cè)試速度從而得出更高的結(jié)構(gòu)。For tests on concrete and timber, it is necessary to specify moisture content because this affects the strength. 混凝土和木材的測(cè)試,指定水的含量是非常有必要的,因?yàn)檫@個(gè)影響到他的強(qiáng)度。A test on a single specimen is unreliable because we do not know whether it is a

52、n average test specimen or 單一構(gòu)件的測(cè)試是不可靠的,因?yàn)槲覀儾恢浪欠袷且粋€(gè)均勻的式樣或者它是否低于或高于細(xì)小whether it has fewer or more than the average number of minute flaws. Standard specifications lay down裂縫的平均數(shù)量。標(biāo)準(zhǔn)規(guī)范規(guī)定多少構(gòu)件應(yīng)該被測(cè)how many specimens shall be tested and how they are to be selected.試和他們要怎么樣被選擇。Tests of factory-made mater

53、ials carried out by the manufacturer are usually accepted by the user of the在建造商下完成的由工廠制造的材料測(cè)試通常被建筑材料使用者所接受除非他們有理由去懷疑他們building material unless he has reason to doubt their veracity. Since concrete is made on the building site or 的準(zhǔn)確性。自從混凝土在建筑工地生產(chǎn)或者從預(yù)拌制brought from a ready-mix concrete plant, its te

54、sting becomes the responsibility of the building contractor.混凝土工廠帶來,它的測(cè)試成為了承包商的責(zé)任。This is therefore a more frequent testing activity than that for other materials. Concrete cylinders or cubes are 因此這是比其他材料更常做的測(cè)試項(xiàng)目。圓柱形混凝土或立方體混normally tested in a hydraulic press, which may be used exclusively for thi

55、s purpose. A universal machine凝土通常用液壓機(jī)進(jìn)行測(cè)試,這是使用它的唯一目的。基于相同原理的普通for tension, compression, and bending is a more expensive machine based on the same principle.拉力機(jī),壓力機(jī)和彎筋機(jī)時(shí)更加昂貴的機(jī)器。;Timber differs from other building materials in that it is produced from growing trees and is thus more 木材不同于其他建筑材料,因?yàn)樗a(chǎn)至于

56、生長的樹木和因此更加的不確定性。variable. Cut timber form virgin forests may consist of a variety of different species. Even timber cut from a 從由多種不同物種組成的原始森林砍下的木材,即使砍下的木材是來自植林所包含的相同品planted forest containing trees of the same species all planted at the same time may show appreciable種的樹木,所有的植物在相同的時(shí)間會(huì)因?yàn)榧藿踊蛘咂渌毕荻诩?xì)節(jié)上

57、表現(xiàn)出明顯的變化。variation between pieces because of knots, or other flaws.A substantial proportion of timber is used in domestic construction where it is not highly stressed; in such大部分木材用于沒有太大壓力的民用建筑上,在這種情cases, visual grading (that is, merely looking at it may be sufficient. Because of the imperfections

58、in行下,“視覺分類”(也就是說,僅僅是看著它就足夠了。因?yàn)槊恳粋€(gè)構(gòu)件都是不完美的, individual pieces, stress grading is usually more reliable than even accurate testing of selected test pieces. A 應(yīng)力的分類通常比經(jīng)由選擇的樣品精確的測(cè)試更加可靠。stress grading machine tests every individual piece of timber by a method that is very fast and relatively應(yīng)力分類機(jī)對(duì)每塊木材試樣的測(cè)試的方法非?？煲埠鼙阋恕heap. The machine is based on an empirical relation between the strength and the deflection of timber. Each 機(jī)器是基于木材強(qiáng)度和撓度兩者的經(jīng)驗(yàn)關(guān)系來進(jìn)行測(cè)驗(yàn)的。每piece of timber is deflected (but not stressed to its limit at several points a