Research On Fracture Toughness Of Metallic Materials

In this paper,the factors affecting fracture toughness of metallic materials aresystematically analyzed.The effect of temperature on fracture toughness are studiedseperatelyunderotherfactorsaffectingfracturetoughnessremainconstant.Manyresultson fracture toughness of pressure vessel steel show that fracture toughness will begin todeclince with increasing temperature above the upper shelf.Researches on fracturetoughness in high temperature range is still insufficient as comparison with that in thelow temperature range.And there exits the security risk for some structures which areoperating under the high temperature conditions,thus research on the variation offracturetoughness withtemperaturebecomes veryimportant.Usingtheresults from thestudy on C-Mn steel ductile fracture mechanism and the rules of the stress-drivenmigrationofpointdefectstoacrack,therelationshipbetweenfracturetoughnessJICandtemperatureTisestablished.Finally,itisprovedthatthereisagoodagreementbetweenthe theoretical model and the experimental results.The numerical analysis about Jintegration with changing temperature, changing crack length, changing crack angle andspecimengeometryfactorsaregivedusingthefiniteelementsoftwareANSYS.Finally,a project example is gived to analyze the effect of fracture toughness degradation on thefailure assessment diagram of pressure vessels and pipeline in the intermediatetemperature.The results show that there exits a great difference between evaluationresults of taking no account of fracture toughness degradation and that of consideringfracture toughness degradation,the latter is much better.Through analysis and discussion,it shows that fracture toughness degradation should be considered in thesafetyassessmentprocess.There is a certain degree universality about conclusions which are obtained in thispaper,and there also exits a very important significance for safety assessment ofstructures.Some achievements are acquired both in theory and applications and a fewhighlightsareasfollows:(1) For the studied pressure vessels,theoretical and experimental study show thatfracture toughness will decrease with increasing temperature above the upper shelftemperature;(2)Relationshipbetweenfracturetoughnessandtemperaturematchwell,whichcanestimatevalueoffracturetoughnessofmaterialsrelativelyconvenientandaccurate;(3) The finite element calculations show that there is a rising trend for J integrationwith increasingtemperature. J integration increases relativelyslow at first,and then itincreases dramatically as temperature further increases. Both specimen geometry andcrack length have also a significant impact on J integration.The longer the crack, thegreater likelihood of fracture. In the initial phase of crack angle change,the speed of Jintegration decrease slightly with a rise in the crack angle,with the crack angle furtherincreasing,J integration show an apparent downward trend;The results obtained fromthecracktipopeningdisplacementδissimilarwithJintegration;(4) The analysis about the project example shows that the effect of fracturetoughness degradation on the failure assessment diagram is obvious.The results showthat evaluation results of taking account of fracture toughness degradation is muchbetter.Through analysis and discussion,it shows that fracture toughness degradationshouldbeconsideredinthesafetyassessmentprocessofstructures.