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1、4StressLimitsinDesign第第4课课设计中的应力极限设计中的应力极限Howlargecanwepermitthestressestobe?Orconversely: How large must a part be towithstandagivensetofloads?Whataretheoverallconditionsorlimitsthatwilldeterminethesizeandmaterialforapart?我我们们可可以以允允许许多多大大应应力力?或或者者反反过过来来说说,构构件件尺尺寸寸该该多多大大才才能能抵抵抗抗给给定定的的一一组组荷荷载载?决决定定构构
2、件件尺尺寸寸和和材材料料的的总总体体条条件件和和限值是什么?限值是什么?Designlimitsarebasedonavoidingfailureoftheparttoperformitsdesiredfunction.设设计计限限值值是是以以构构件件不不发发生生破破坏坏而而执执行行其其要求的功能为基础的。要求的功能为基础的。Becausedifferentpartsmustsatisfydifferentfunctionalrequirements,theconditionswhichlimit load-carrying ability may be quitedifferentfordi
3、fferentelements.因因为为不不同同构构件件必必须须满满足足不不同同的的功功能能要要求求,不不同同构构件件界界定定承承载载能能力力的的条条件件可可能能差差别别很大。很大。Asanexample,comparethedesignlimitsforthefloorofahousewiththoseforthewingofanairplane.举举例例来来说说,可可以以将将住住宅宅楼楼板板和和机机翼翼的的设设计限值比较一下。计限值比较一下。Ifweweretodeterminethesizeofthewoodenbeamsinahomesuchthattheysimplydidnotbr
4、eak,wewouldnotbeveryhappywiththem;theywouldbetoo“springy”.Walkingacrosstheroomwouldbelikewalkingoutonadivingboard.如如果果我我们们要要这这样样来来确确定定木木梁梁的的尺尺寸寸,只只要要它它不不断断裂裂就就行行,那那我我们们可可能能就就不不会会很很喜喜欢欢这这些些木木梁梁了了,这这些些木木梁梁可可能能弹弹性性变变形形太太大大了了。人人们们穿穿过过房房间间就就会会像像从从跳跳水水板下来一样。板下来一样。Obviously, we should be concerned with the
5、maximum“deflection”thatwe,asindividuals,find acceptable. This level will be rathersubjective, and different people will givedifferentanswers.显显然然,我我们们应应该该关关心心我我们们作作为为个个体体能能够够接接受受的的最最大大挠挠度度,这这个个水水平平的的主主观观性性相相当大,不同人将给出不同的答案。当大,不同人将给出不同的答案。In fact, the same people may give differentanswersdependingonwh
6、ethertheyarepayingforthefloorornot!事事实实上上,同同一一群群人人也也会会给给出出不不同同的的回回答答,这取决于他们是否支付楼板的费用。这取决于他们是否支付楼板的费用。Anairplanewingstructureisclearlydifferent.Ifyoulookoutanairplanewindowandwatchthewingduringturbulentweather,youwillseelarge deflections; in fact you may wish thattheyweresmaller.However,youknowthatth
7、eimportantissueisthatof“structural integrity”,notdeflection.一一个个机机翼翼结结构构显显然然不不同同,你你如如从从机机窗窗向向外外观观看看恶恶劣劣天天气气中中的的机机翼翼,你你就就会会看看到到很很大大的的变变形形,而而事事实实上上你你会会希希望望这这种种变变形形更更小小些些才才好好。然然而而,你你知知道道重重要要的的是是结构的完整性,而不是变形。结构的完整性,而不是变形。We want to be assured that the wing willremainintact.Wewanttobeassuredthatnomatterw
8、hatthepilotandtheweatherdo,thatwing will continue to act like a good andproperwing.我我们们要要确确保保机机翼翼完完好好无无损损,我我们们要要确确保保机机翼翼一一直直处处于于良良好好的的工工作作状状态态,而而不不管管飞飞行行员员做做什什么么或者天气怎么样。或者天气怎么样。Infact,wereallywanttobeassuredthatthewingwillneverfailunderanyconditions.Nowthatisaprettytall order;whoknowswhatthe“worst”c
9、onditionsmightbe?事事实实上上,我我们们确确实实想想要要确确保保机机翼翼在在任任何何条条件件下下都都不不会会失失效效。现现在在这这是是不不可可能能实实现现的的事事,谁谁又又知知道道最最恶恶劣劣的的条条件件是是什什么么呢?呢?Engineerswhoareresponsibleforthedesignof airplane wing structures must know, withsome degree of certainty, what the “worst”conditionsarelikelytobe.负负责责机机翼翼结结构构设设计计的的工工程程师师们们必必须须在在某
10、某种种确确定定的的程程度度上上了了解解最最恶恶劣劣的的条条件件该该是是什么样的。什么样的。It takes great patience and dedication formanyyearstoassembleenoughtestdataandfailure analyses to be able to predict the“worst”case.多多年年来来,人人们们花花费费很很多多耐耐心心和和辛辛勤勤工工作作,收收集集了了充充足足的的试试验验数数据据,进进行行了了大大量量的的失败分析,能够预测最坏的情况。失败分析,能够预测最坏的情况。Thegeneralprocedureistodev
11、elopstatisticaldatawhichallowustosayhowfrequentlyagivenconditionislikelytobeencounteredonceevery1000hours,oronceevery10000hours,etc.通通常常的的程程序序是是统统计计数数据据,以以此此分分析析特特定定条条件件发发生生的的频频率率,是是1000小小时时一一次次,还还是是10000小时一次,等等。小时一次,等等。As we said earlier, our object is to avoidfailure. Suppose, however, that a part
12、 hasfailed in service, and we are asked: Why?“Error”assuchcancomefromthreedistinctlydifferentsources,anyorallofwhichcancausefailure:如如前前所所述述,我我们们的的目目的的是是不不让让失失效效发发生生。但但是是,如如果果假假设设一一个个部部件件在在使使用用中中失失效效了了,我我们们会会被被问问道道:为为什什么么?这这种种“失失误误”可可能能来来源源于于下下列列三三种种情情况况,其其中中一一种或全部都可能引起失效:种或全部都可能引起失效:1.Error in desi
13、gn: We the designers or thedesign analysts may have been a bit toooptimistic: Maybe we ignored some loads;maybeourequationsdidnotapplyorwerenotproperlyapplied;maybeweoverestimated the intelligence of the user;maybeweslippedadecimalpoint.设设计计失失误误:我我们们这这些些设设计计者者或或设设计计分分析析者者可可能能都都有有一一点点过过于于乐乐观观了了。也也许许我
14、我们们忽忽略略某某些些荷荷载载,也也许许我我们们的的公公式式不不合合适适或或者者被被不不恰恰当当地地使使用用了了。也也许许我我们们过过高高估估计计了了使使用用者者的的智智力力;也也许许我我们们点点错错了了一个小数点。一个小数点。2.Error in manufacture: When a deviceinvolves heavily stressed members, theeffective strength of the members can begreatlyreducedthroughimpropermanufactureandassembly:制制造造失失误误:当当一一个个设设备备
15、的的构构件件需需要要承承受受非非常常大大的的应应力力时时,可可能能通通过过不不恰恰当当的的制制造和装配而使构件的有效强度大大降低:造和装配而使构件的有效强度大大降低:Maybethewrongmaterialwasused;maybethe heat treatment was not as specified;maybethesurfacefinishwasnotasgoodascalled for; maybe a part was “out oftolerance”;maybethesurfacewasdamagedduring machining; maybe the threads
16、werenot lubricated at assembly; or perhaps theboltswerenotproperlytightened.也也许许采采用用了了错错误误的的材材料料,不不是是按按要要求求进进行行的的热热处处理理,表表面面喷喷漆漆也也不不如如所所要要求求的的那那样样好好,某某个个构构件件的的偏偏差差超超出出了了限限值值,加加工工时时表表面面被被损损坏坏,安安装装时时没没有有给给螺螺纹纹润润滑滑,或或者者螺螺栓栓没没有恰当地旋紧。有恰当地旋紧。3.Errorinuse:Asweallknow,wecandamagealmostanythingifwetryhardeno
17、ugh,andsometimeswedosoaccidentally:使使用用失失误误:正正如如我我们们所所知知,如如果果我我们们尽尽力力的的话话,可可以以将将任任何何东东西西损损坏坏,有有时时这这样的事意外发生了:样的事意外发生了:We went too fast; we lost control; we fellasleep;wewerenotwatchingthegages;thepower went off; the computer crashed; hewastakingacoffeebreak;sheforgottoturnthemachineoff;youfailedtolub
18、ricateit,etc.我我们们飞飞得得过过快快,失失去去了了控控制制,睡睡着着了了,没没有有注注意意仪仪表表,断断电电了了,计计算算机机坏坏了了,他他正正去去喝喝咖咖啡啡了了,她她忘忘了了关关掉掉机机器器了了,你没有好好做润滑,等等,等等。你没有好好做润滑,等等,等等。Any of the above can happen:Nothing isdesignedperfectly;nothingismadeperfectly;andnothingisusedperfectly.上上面面提提到到的的任任何何失失误误都都可可能能发发生生,任任何何产产品品都都不不能能设设计计、制制造造、使使用用得
19、得完完美美无无缺。缺。When failure does occur, and we try todeterminethecause,wecanusuallyexaminethedesign;wecanusuallyexaminethefailedpartsformanufacturingdeficiencies;butwecannotusuallydeterminehowthedevicewasused(ormisused).失失效效发发生生时时,我我们们力力图图确确定定失失败败的的原原因因。我我们们通通常常可可以以考考察察设设计计,检检验验失失效效的的部部件件,找找出出制制造造缺缺陷陷;但
20、但我我们们往往往往不不能能确确定设备是如何使用(或者错误使用)的。定设备是如何使用(或者错误使用)的。In serious cases, this can give rise toconsiderabledifferencesofopinion,differenceswhichfrequentlyendincourt.在在很很严严重重的的情情况况下下,这这会会造造成成很很大大的的意意见见分分歧歧,这这些些分分歧歧往往往往只只能能在在法法庭庭上上解解决。决。In an effort to account for all the abovepossibilities,wedesigneverypa
21、rtwithasafetyfactor.Simplyput,thesafetyfactor(SF)isthe ratio of the load that we think the part canwithstand to the load we expect it toexperience.在在我我们们尽尽力力考考虑虑上上面面提提及及的的所所有有可可能能之之后后,我我们们用用一一个个安安全全系系数数设设计计每每个个部部件件。简简单单地地说说,安安全全系系数数SF是是我我们们认认为为部部件件能能承承受受的的荷荷载载与与其其可可能能经经历历的的荷荷载载的的比比值。值。TheThe safetys
22、afety factorfactor cancan bebe appliedapplied byby increasingincreasing thethedesigndesign loadsloads beyondbeyond thosethose actuallyactually expected,expected, oror bybydesigningdesigning toto stressstress levelslevels belowbelow thosethose thatthat thethematerialmaterial actuallyactually cancan w
23、ithstandwithstand (frequently(frequently calledcalled“designstresses”).“designstresses”).Safetyfactor=SF=failureload/designloadSafetyfactor=SF=failureload/designload=failurestress/designstress=failurestress/designstress当当当当增增增增大大大大设设设设计计计计荷荷荷荷载载载载使使使使其其其其超超超超过过过过实实实实际际际际荷荷荷荷载载载载时时时时,或或或或者者者者当当当当设设设设
24、计计计计比比比比材材材材料料料料实实实实际际际际可可可可承承承承受受受受的的的的应应应应力力力力低低低低的的的的应应应应力力力力(通通通通常常常常称称称称为为为为设设设设计计计计应力)时,就使用安全系数。应力)时,就使用安全系数。应力)时,就使用安全系数。应力)时,就使用安全系数。 安全系数安全系数安全系数安全系数SF=SF= It is difficult to determine an appropriatevalue for the safety factor. In general, weshoulduselargervalueswhen:要要确确定定适适当当的的安安全全系系数数值值是
25、是困困难难的的。一一般般地地,在在下下列列情情况况下下我我们们应应该该采采用用较较大大的安全系数值:的安全系数值:1.The possible consequences of failure arehighintermsoflifeorcost.失失效效可可能能造造成成严严重重的的人人员员伤伤亡亡和和财财产产损失时;损失时;2.Therearelargeuncertaintiesinthedesignanalyses.设计分析中不确定因素多时。设计分析中不确定因素多时。Values of SF generally range from a low ofabout1.5to5ormore.Whe
26、ntheincentivestoreduce structural weight are great (as inaircraft and spacecraft), there is an obviousconflict.安安全全系系数数值值通通常常为为1.5到到5或或者者更更大大。当当减减小小结结构构重重量量意意义义重重大大时时,比比如如航航空空、航航天天设设备备,就就存在一个突出的矛盾。存在一个突出的矛盾。SafetydictatesalargeSF,whileperformancerequires a small value. The only resolutioninvolvesred
27、uctionofuncertainty.安安全全要要求求大大安安全全系系数数,而而机机器器工工作作性性能能又又要要求求较较小小的的安安全全系系数数值值。只只有有降降低低不不确定性才是唯一的解决办法。确定性才是唯一的解决办法。Because of extreme care and diligence indesign, test, manufacture, and use, theaircraft industry is able to maintain veryenviable safety records while using safetyfactorsaslowas1.5.由由于于人人们
28、们在在设设计计、试试验验、制制造造和和使使用用过过程程中中具具有有最最大大程程度度的的细细心心和和勤勤勉勉,才才使使得得航航空空工工业业保保持持着着非非常常骄骄人人的的安安全全记记录录,而这里使用的安全系数却仅仅是而这里使用的安全系数却仅仅是1.5。We might note that the safety factor isfrequentlycalledthe“ignorancefactor”.Thisisnottoimplythatengineersareignorant,butto help instill in them humility, caution, andcare.我我们们
29、可可能能注注意意到到,安安全全系系数数还还常常被被称称为为“无无知知系系数数”。这这并并不不是是说说工工程程师师无无知知,而是要求他们谦卑、谨慎和细心。而是要求他们谦卑、谨慎和细心。Anengineerisresponsibleforhisorherdesigndecisions, both ethically and legally. Try tolearnfromthemistakesofothersratherthanmakingyourown.一一个个工工程程师师要要为为他他或或她她的的设设计计负负责责,在在道道德德上上,也也在在法法律律上上负负责责。努努力力从从他他人人而不是自己的教
30、训中学习吧!而不是自己的教训中学习吧!Excisecriticallycritically 1.批批判判性性地地;苛苛求求地地。2.危危急急地地;严重地严重地forbiddingforbidding冷峻的冷峻的;令人生畏的令人生畏的contemplatecontemplate1.深深思思,细细想想,仔仔细细考考虑虑。1.注注视视,凝凝视视。2.考考虑虑接接受受(发发生生某某事事的的可可能能性)性)susceptiblesusceptible1.易易受受影影响响的的;易易动动感感情情的的。2.过过敏敏的的;易易受受感感染染的的。3.能能经经受受的的。4.好好动动感感情情的的;感感情情丰丰富富的的
31、;善善感感的的。5.容容许许的的;可能可能的;可以的;可以的。的。LookedLookedatatcritically,critically,allallstructuresstructuresareareassembliesassembliesofofthreethree dimensionaldimensional elements,elements, thethe exactexact analysisanalysis ofofwhichwhich isis a a forbiddingforbidding tasktask eveneven underunder idealidealc
32、onditionsconditions andand impossibleimpossible toto contemplatecontemplate underunderconditionsconditions ofof professionalprofessional practice.practice. ForFor thisthis reason,reason,anan importantimportant partpart ofof thethe analystsanalysts workwork isis thethesimplificationsimplification ofo
33、f thethe actualactual structurestructure andand loadingloadingconditionsconditions toto a a modelmodel thatthat isis susceptiblesusceptible toto rationalrationalanalysis.analysis.严严严严格格格格看看看看来来来来,所所所所有有有有的的的的结结结结构构构构都都都都是是是是由由由由三三三三维维维维构构构构件件件件组组组组成成成成的的的的,这这这这些些些些构构构构件件件件的的的的精精精精确确确确分分分分析析析析,即即即即使使使使在在在在理理理理想想想想条条条条件件件件下下下下也也也也是是是是很很很很困困困困难难难难的的的的,考考考考虑虑虑虑专专专专业业业业实实实实践践践践条条条条件件件件更更更更是是是是不不不不可可可可能能能能的的的的。因因因因此此此此,分分分分析析析析者者者者所所所所做做做做工工工工作作作作的的的的一一一一个个个个重重重重要要要要部部部部分分分分,就就就就是是是是将将将将实实实实际际际际结结结结构构构构与与与与荷荷荷荷载条件简化成一个适于示性分析的模型。载条件简化成一个适于示性分析的模型。载条件简化成一个适于示性分析的模型。载条件简化成一个适于示性分析的模型。