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StressLimitsinDesignHowlargecanwepermitthestressestobe?Orconversely:Howlargemustapartbetowithstandagivensetofloadswhataretheoverallconditionsorlimitsthatwilldeterminethesizeandmaterialforapart?Designlimitsarebasedonavoidingfailureoftheparttoperformitsdesiredfunction.Becausedifferentpartsmustsatisfydifferentfunctionalrequirements,theconditionswhichlimitload-carryingabilitymaybequitedifferentfordifferentelements.Asanexample,comparethedesignlimitsforthefloorofahousewiththoseforthewingofanairplane.Ifweweretodeterminethesizeofthewoodenbeamsinahomesuchthattheysimplydidnotbreak,wewouldnotbeveryhappywiththem;theywouldbetoo‘springy’.Walkingacrosstheroomwouldbelikewalkingoutonadivingboard.Obviously,weshouldbeconcernedwiththemaximum‘deflection’thatwe,asindividuals,findacceptable.Thislevelwillberathersubjective,anddifferentpeoplewillgivedifferentanswers.Infact,thesamepeoplemaygivedifferentanswersdependingonwhethertheyarepayingforthefloorornot!Anairplanewingstructureisclearlydifferent.Ifyoulookoutanairplanewindowandwatchthewingduringturbulentweather,youwillseelargedeflections;infactyoumaywishthattheyweresmaller.However,youknowthattheimportantissueisthatof‘structuralintegrity’,notdeflection.Wewanttobeassuredthatthewingwillremainintact.Wewanttobeassuredthatnomatterwhatthepilotandtheweatherdo,thatwingwillcontinuetoactlikeagoodandproperwing.Infact,wereallywanttobeassuredthatthewingwillneverfailunderanyconditions.Nowthatisaprettytallorder;whoknowswhatthe‘worst’conditionsmightbe?Engineerswhoareresponsibleforthedesignofairplanewingstructuresmustknow,withsomedegreeofcertainty,whatthe‘worst’conditionsarelikelytobe.Ittakesgreatpatienceanddedicationformanyyearstoassembleenoughtestdataandfailureanalysestobeabletopredictthe‘worst’case.Thegeneralprocedureistodevelopstatisticaldatawhichallowustosayhowfrequentlyagivenconditionislikelytobeencountered—onceevery1000hours,oronceevery10000hours,etc.Aswesaidearlier,ourobjectistoavoidfailure.Suppose,however,thataparthasfailedinservice,andweareasked;Wh