Effect of lateral impact loads on failure of pressurized pip(11)

Effectoflateralimpactloads203

andCandhadnoregardtothesupportgivenbythefoundationsofpipespecimensAandB,whichmayhavereducedtheglobaldeformationofthepipeline.Thevariationsoflcversustheinternalpressure(usingsu=sy)forthecurrenttestresultsareillustratedinFigure8,togetherwiththetestresultsreportedinreference[5].Inadditiontotheexistenceofthefoun-dationsupporttothepresentspecimensAandB,anotherdifferencebetweenthecurrenttestswiththoseinreference[5]wasthelongitudinalrestrictionsatthesupports,asmentionedpreviously.Itwaspointedoutinreference[5]thatthefullylongitudinalrestrictionatthesupportwouldlowerthecriticalinitialimpactenergystrikeagainstthemid-spanoftheunpressurizedpipeline.Inordertoinvestigatetheirprediction,impacttestswerecarriedoutonthepresentspecimenthatwaswithoutanyfoundationsupportunderneathandhadthesamesupportcon-ditionsasthecurrentspecimensAandB.AsprovedinFig.8,alargedifferenceoflcappearedatthebegin-ningoftheplotsforthecurrentspecimenCandthespecimensreportedinreference[5]whensu=sy¼0.Furthermore,thevalueoflcforthepresentspecimenCwasmuchlowerthanpredictedandthespecimensreportedinreference[5].

Figure8showsthatthevariationcharacteristicsoflcwithsu=sywerequitedifferentforthepipeswithandwithoutafoundationsupportaswellasamem-braneforce,wherelcdecreasedwiththeincreaseintheinternalpressureforallthefourseriesoftestsexceptforgroupsAandBwithhigherinternalpressure.However,thecriticalinitialimpactenergywasmuchlesssensitivetotheinternalpressureforthepresentpipeswithoutmembraneforcethanthosewithmembraneforcereportedinreference[5].Itwasalsoshownthatthedifferenceofthelcbetweenthetwodifferentfoundationsupportsslightlyincreasedasthepressureincreased.

Thepipewouldbeweakerduetotheexistenceofthecircumferentialstresses,resultingfromtheinternalpressure,asshowninFig.8.Thecircumfer-entialstressesofthepipeneartheendsupportscouldcausethepipetofractureeasilyduringtheimpactloadingatthemid-spanposition.ChenandShen[6]discussedthatmostoftheimpactenergycouldbecontributedtotheglobaldeformationandcausespiperupturingnearthesupports,asthelocaldeformationofthepressurizedpipewassup-pressedbytheinternalpressure.Inotherwords,lesscriticalinitialimpactenergywouldberequiredtofracturethepressurizedpipeneartheendsup-portsthanthatfortheunpressurizedpipe.Theyalsopointedoutthatthiscouldbeanotherreasonwhythecriticalfailureimpactenergiesforthepress-urizedpipesliebelowthanthosefortheunpressur-izedpipes.

Experimentalresultsreportedinreference[5]revealedthatlcwasinverselyproportionaltotheinternalpressure,asshowninFig.8.Incontrast,itwasfoundinthecurrentresults,forthepipeswiththefoundationsupportandwithoutthemembraneforce,thatlcinitiallydecreasedslightlyasthepressureincreaseduptothepressurelevelofsu=sy¼0:21.Then,lcstartedtoincreaseslightlyasthepressureincreased.Inotherwords,thehigherinternalpressure(heresu=sy.0.21)wouldstrengthenthestructure.Although,lcwasinverselyproportionaltotheinternalpressureforthepresentspecimensC,thisrelationshipwasonlyuptothepressurelevelofsu=sy¼0:21,itthendecreasedmoresigni cantly.Asaresult,thecriticalpointforallthepresentpipes,withandwithoutafoundationsupport,wassu=sy¼0:21,atwhichthecharacter-isticsofthepipeswereunexpectedlychangedunderinternalhighpressure.Furtherresearchmightbedoneinorderto ndoutthereasonfortheseanomalousphenomena.

Thein uencesoftheinternalpressureloadingandthefoundationsupportonthecriticalinitialimpactenergytranspiredtobeverysigni cant.Hence,thepreviousresearchonthepipelineswithoutafoun-dationsupportmaynolongerbevalidfortheactualpipelineswithafoundationsupport,evenmoreseriously,thesecouldleadtoawrongesti-mationofthefailureofpipelines.

4.2FEAresults

Fig.8

JPME97#IMechE2006

lcversussu=sy

Thedynamicnon-linearresponseofthepipessub-jectedtoalateralimpactloadingatthemid-spanpositionwasobtained.ResultsobtainedfromtheFEAwerecomparedwiththeimpacttestresultsforassessingtheaccuracyofthenumericalanalysisas

follows.

Proc.IMechEVol.220PartE:J.ProcessMechanicalEngineering