Materials Lab Procedures
Materials Lab Procedures MSE 300
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This 7 page Class Notes was uploaded by Eudora Blick on Monday October 26, 2015. The Class Notes belongs to MSE 300 at University of Tennessee - Knoxville taught by Philip Rack in Fall. Since its upload, it has received 23 views. For similar materials see /class/229812/mse-300-university-of-tennessee-knoxville in Materials Science Engineering at University of Tennessee - Knoxville.
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Date Created: 10/26/15
I I i l Shipcyclicloading ComputerchIPcycllc from wave thermal loading University ni TEnnzsu Den nfMaveria s Science and Engm39 eeriig 1 MsE 3m Chapter Outline Failure How do Materials Break I Ductile vs brittle fracture I Principles offracture mechanics Stress concentmtion I Impact fracture testing University ni TEnnzssm Den nfMaverials Science and Engineering 2 IDEAL VS REAL MATERIALS 0 Stressstrain behavior Room T perfect mat lno aws arefully produced glass ber typical ceramic EH 0 pical strengthened metal Ical polymer 01 8 Tsengineerin lt TS perfect materials materials DaVinci 500 yrs ago observed the longer the Wire the smaller the load to fail it Reasons flaws cause premature failure Larger samples are more flawed University of Tennessee Dept of Materials Science and Engineering 3 300 Fracture Fracture separation of a body into pieces due to stress at temperatures below the melting point Steps in fracture gt crack formation gt crack propagation Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be de ned ductile 0r brittle Ductile fracture most metals not too cold gt Extensive plastic deformation ahead of crack gt Crack is stable resists further extension unless applied stress is increased Brittle fracture ceramics ice cold metals gt Relatively little plastic deformation gt Crack is unstable propagates rapidly without increase in applied stress Ductile fracture is preferred in most applications University of Tennessee Dept of Materials Science and Engineering 4 D ctile u ene Brittle materials MsE inn Brittle vs Ductile Fracture ma energy absorption before fracture Stress Ductlle Stra m torials extensive plastic deformation and rgy absorption toughness before fracture little plastic defamation and low 9w Very ductile so metals temp temp Msmnn Brittle vs Ductile Fracture B U W C A e g Pb Au erature other metals polymers glasses erature Moderately ductile fracture typical for ductile metals Brittle fracture cold metals ceramics at room at high Mxmnn Ductile Fracture Dislocation Mediated I 04 s I l m I ibvnus 35mm a ii a Necking b Cavity Formation c Cavity coalescence to form a crack d Crack propagation e Fracture Mxmnn Dudzile Fracture is Scanning He high resolution Sp cricai 39crocavi 39es at initiate crack formation ction Microscopy chtagmphlc studies at dimples c sp same end 2M mm Brink Fracture Limited Dislocation Mobility gt No appreciable plastic deformation gt Crack propagation is Very fast gt Crack propagates nearly perpendicular to the direction 0 the app ie stres gt Crack otter propagates by cleavage breaking of atomic bonds along speci c crystallographic planes cleawge planes Bn39 le actum in amild steel mm In Brittle Fracture A Transgranular fracture Fracture cracks pass through grains Fracture surface have faceted texture because ofdiffermt orientation ofcleawge planes in grains Interganular fracture Fracture crack propagation is along grain boundaries grain boundaries are weakened or embrimed by impurities segregation etc mum Stress Concentration forces betwem atoms 0 e theoretical cohesive stnngth ofa brittle material should he N 0 But experimental fracture strength is normally F100 E10000 This much lower fracture strength is explained by the effect of stress concentration at microscopic aws The applied stress is amplified at the tips of micro cracks voids notches surface scratches comers etc that are The magnitude ofthis ampli cation depends on microcrack orientations geometxy and dimensions V Figure by N Einsteins D HessJWL Stress Concentration n For a long crack oriented perpendicular to the applied stress the maximum stress near the crack tip is in cm us 26 1W Pl where an is the applied cxtemal stress a is the halflength of the crack and p the radius of curvature ofthe crack tip note that a is halflmgth ofthe intemal aw but the full lmgth for a sur ce flaw 12 0 a The suess concmtratim factor is K s 2 6o P l2 mmn SIMULATION DISLOCATION MOTIONGENERATION Tensile loading nonmmal arota rcc meta Wm notches m the top and bonom sunaoe Over1 mmon atoms nmeled m an block Note me large Increme m asl density I mmn oomssy rmAbraham Ussdwtmpsrmtumn om ntsrnatmna Eusmsu Macmns aratmn Emu om U u wolim mnot Mm1 samemum 13 Impact raeture Testin testing fracture chataeten39stjes under high strain rams Two standard tests the Charpy and Izod measure the 39139 a under an impact load also called the hoteh toughness Charpy
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