2 Failure of Materials Why study materials failure? - design of structure needsthe engineer to minimize (prevent) failure.- important to understand the concept of3 failure modes;1. Fracture.2. Fatigue.3. Creep.Topics covered in this chapter…Fundamental concept- crack (defects).- modes of failure.- characteristic & mechanisms.- fracture mechanics.Testing techniquesFailure of materials…- failure of engineering materials is undesirable.- several cause of failure:-- improper materials selection, processing& design.-- defects (i.e: internal/external flaws/cracks).-- economic losses.-- interference with availability of products &services.need an appropriate preventive measurementto against failure.- produce & prevent failure.• Ductile fracture:-- one piece-- large deformationFailure of Materialsmodemechanismtesting techniqueFractureDuctile fractureImpact testBrittle fractureFatigueInitial crackFatigue testCrack growCreep test• Brittle fracture:-- many pieces-- small deformationsCreepOther examples of failureComputer chip-cyclic thermal loading.Hip implant-cyclic loading from walking.Ship-cyclic loading from waves.Adapted from Fig (b), Callister 7e. (Fig (b) is courtesy of National Semiconductor Corporation.)Adapted from chapter-opening photograph, Chapter 9, Callister & Rethwisch 3e. (by Neil Boenzi, The New York Times.)Adapted from Fig (b), Callister 7e.
3 Failure of Materials mode of failure: Fracture Fracture of materials results in separation of stressed solid into two or more parts.ductile fractureductile fracturebrittle fractureaccompanied by significant plasticdeformation & slow crack propagation.- common at low strain rate & high temp.- three steps :1. Specimen forms neck & cavitieswithin neck.2. Cavities form crack & crackpropagates towards surface,perpendicular to stress.3. Direction of crack changes to 45oresulting in cup-cone fracture.no significant plastic deformationbefore fracture & high crack propagation.- Catastrophic (rapid crack propagation).- common at high strain rates & low temp.- three stages:1. Plastic deformation concentratesdislocation along slip planes.2. Microcracks nucleate due to shearstress where dislocations are blocked.3. Crack propagates to fracture.characteristic of fractureSEM micrographAt low operating temp., ductile to brittle transition (DBT) takes placeFracture behaviorVery ductileModerately ductileBrittlebrittle fractureSchematic diagramSEM micrograph%AR or %ELLargeModerateSmallDuctile fracture isusually more desirablethan brittle fracture!Ductile:Warning before fractureBrittle:No warning
4 mode of failure: Fracture Failure of Materialsmode of failure: FractureModerately ductile fractureductile fractureneckingsvoid nucleationshearingat surfacefracturevoid growth& linkage100 mmFracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.SEM micrographevolution to failurebrittle fracture surfacesIntergranular (between grains)Intragranular (within grains)304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)V-shaped “chevron”316 S. Steel (metal)Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.)160 mmbrittle fracture4 mmPolypropylene(polymer)Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.Al Oxide(ceramic)Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78. Copyright 1990, The American Ceramic Society, Westerville, OH. (Micrograph by R.M. Gruver and H. Kirchner.)3 mmSEM micrographorigin of crack1 mm
5 mode of failure: Fracture Failure of Materialsmode of failure: Fracturebrittle fracture (in ceramics)ductile fracture- characteristic of fracture behavior:-- origin point.-- initial region (mirror) is flat & smooth.-- after reaches critical velocity crack branches(mist & hackle).SEM micrographductile fracture (in thermoplastic polymer)characteristic of fracture behavior:-- craze formation prior to cracking.-- during crazing, plastic deformation of spherulites and formation of micro voids and fibrillar bridges.brittle fracturealigned chainsSEM micrographfibrillar bridgesmicrovoidscrack
6 mode of failure: Fracture Failure of Materialsmode of failure: Fractureconcept of fracture mechanics- engineering materials don't reach theoretical strength.- flaws produce stress concentrations that causepremature failure.Flaws are stress concentratorsStress concentration at crack tipEngineering Fracture Design- avoid sharp corners.t, rfilletradiuswhosmaxsmaxStress Conc. Factor, Kt=s02.52.0increasingw/hResults from crack propagation.Griffith CrackCrack Propagation- cracks propagate due to sharpness of crack tip.1.5where t = radius of curvatureso = applied stresssm = stress at crack tipbrittle1.0r/hdeformed region0.51.0ductilesharper fillet radius
7 mode of failure: Fracture Failure of Materialsmode of failure: Fractureconcept of fracture mechanics- engineering materials don't reach theoretical strength.- flaws produce stress concentrations that causepremature failure.Crack PropagationTS << TSengineeringmaterialsperfectseE/10E/1000.1perfect mat’l-no flawscarefully produced glass fibertypical ceramictypical strengthened metaltypical polymerStress-strain curves (Room T) perfect & engineering materials- crack propagates if above critical stress.i.e., sm > scor Kt > KcwhereE = modulus of elasticitys = specific surface energya = one half length of internal crackKc = c/soDesign Against Crack Growth- crack growth condition:K ≥ Kc =- largest, most stressed cracks grow first!-- Result 1: Max. flaw sizedictates design stress.-- Result 2: Design stressdictates max. flaw size.samaxnofractureamaxsnofracture
8 Design Example: Aircraft Wing Failure of Materialsmode of failure: Fractureconcept of fracture mechanics- engineering materials don't reach theoretical strength.- flaws produce stress concentrations that causepremature failure.Design Example: Aircraft WingFracture Toughness, KIC for selected materials• Material has Kc = 26 MPa-m0.5Graphite/Ceramics/SemicondMetals/AlloysComposites/fibersPolymers5KIc(MPa·m0.5)1Mg alloysAl alloysTi alloysSteelsSi crystalGlass-sodaConcreteSi carbidePC60.724310<100><111>DiamondPVCPPPolyesterPSPETC-C(|| fibers)0.67100Al oxideSi nitrideC/C( fibers)Al/Al oxide(sf)Al oxid/SiC(w)Al oxid/ZrO(p)Si nitr/SiC(w)Glass/SiC(w)YO/ZrO• Two designs to consider...Design A-- largest flaw is 9 mm-- failure stress = 112 MPaDesign B-- use same material-- largest flaw is 4 mm-- failure stress = ?• Use...• Key point: Y and Kc are the same in both designs.9 mm112 MPa4 mm-- Result:Answer:• Reducing flaw size pays off!8
9 mode of failure: Fracture Failure of Materialsmode of failure: Fracturetesting technique: Impact testaim: to investigate the energy absorbed &fracture of materials during impact loadingat various temperature.Important information from impact test…1. Fracture ductile).2. Brittleness (impact % shear).3. Ductility (impact % shear).4. Toughness (energy absorbed).5. Ductile-to-Brittle Transition (DBT)temperature.Pre-WWII: The Titanic- classify into 3:1. Charpy impact test.2. Izod impact test.3. Drop weight impact test.fracture behavior can be shown in impactenergy vs temperature curves.Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and Sons, Inc., (Orig. source: Dr. Robert D. Ballard, The Discovery of the Titanic.)BCC metals (e.g., iron at T < 914°C)Impact EnergyTemperatureHigh strength materials(sy> E/150)& polymersDBT temperatureFCC metals (e.g., Cu, Ni)more brittlemore ductilefinal heightinitial height(Charpy)WWII: Liberty shipsDesign Strategy: Stay above the DBT!Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and Sons, Inc., (Orig. source: Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.)Adapted from Fig. 9.18(b), Callister & Rethwisch 3e. (Fig. 9.18(b) is adapted from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc. (1965) p. 13.)Sinking of Titanic: Titanic was made up of steel which has DBT temperature 32oC. On the day of sinking, sea temperature was –2oC which made the the structure highly brittle and susceptible to more damage.
10 mode of failure: Fatigue Failure of Materialsmode of failure: FatigueKey points: Fatigue...- failure under cyclic loading.can cause part failure, even thoughsmax < sc.causes ~ 90% of mechanicalengineering failures.metals often fail at much lower stress at cyclicloading compared to static loading.crack nucleates at region of stressconcentration & propagates due to cyclicloading.failure occurs when cross sectional area ofthe metal too small to withstand applied load.Fatigue fractured surface of keyed shaft- stress varies with time.-- key parameters are:1. stress amplitude, S a).2. mean stress, sm.3. frequency, f (will convert to no. of cycle, N).Fracture started hereStress amplitude, S = stress to cause failureNumber of cycle, N = 1/fFinal rupture- fatigue behavior can be shown in SN curves.Important point from fatigue test…1. Fatigue limit.2. Fatigue strength.3. Fatigue life.testing technique: Fatigue testsmaxmintimemSMean stressStress amplitudetension on bottomcompression on topcountermotorflex couplingspecimenbearingStress rangeStress range
11 mode of failure: Fatigue Failure of Materialsmode of failure: FatigueFatigue Design ParametersSometimes, the fatigue limit is zero!-- fatigue occurs at any time.Improving Fatigue Life- Fatigue limit, Sfat:-- no fatigue if S < Sfat1. Impose a compressivesurface stress (to suppresssurface cracks from growing)example: SN curves for typical steelAdapted from Fig. 9.25(a), Callister & Rethwisch 3e.SfatN = Cycles to failure103579unsafesafeS = stress amplitudeS = stress amplitude--Method 1: shot peeningshotunsafesafe103105107109put surface into compressionN = Cycles to failureAdapted from Fig. 9.25(b), Callister & Rethwisch 3e.--Method 2: carburizingC-rich gasexample: SN curves for typical polymers• Fatigue limit:- PMMA, PP, PE2. Remove stress concentrators.• No fatigue limit:- PET, Nylon (dry)badbetterAdapted from Fig. 9.32, Callister & Rethwisch 3e.
12 Failure of Materials T(20 + log tr) = L mode of failure: Creep Creep is progressive deformation underconstant stress.- Occurs at elevated temperature, T > 0.4 Tm-- important in high temperature applications.Creep failurealong grain boundaries.appliedstressg.b. cavitiescreep calculation…Time to rupture, trAdapted from Figs. 9.36, Callister & Rethwisch 3e.Data from creep test…1. creep strain, 2. temperature, T3. time, ts,etsT = temperaturetr = time to failure (rupture)L = function of applied stressexample:elasticprimarysecondarytertiaryEstimate rupture time for S-590 Iron, T = 800°C & s = 20 ksitesting technique: Creep testCreep test determines the effect oftemperature & stress on creep rate.Metals are tested at constant stress atdifferent temperature & constanttemperature with different stress.10020Stress, ksi10data for- creep behavior can be shown in -t curves.S-590 IronImportant information from -t curve…11220242816Primary creep:creep rate (slope) decreases with timedue to strain hardening.Secondary creep:creep rate is constant due to simultaneousstrain hardening and recovery process.-- steady-state.Tertiary creep:creep rate (slope) increases with timeleading to necking & fracture.L(103K-log hr)24x103T(20 + log tr) = L1073KAns: tr = 233 hrAdapted from Fig. 9.35, Callister & Rethwisch 3e.
13 SUMMARY • Engineering materials don't reach theoretical strength. • Flaws produce stress concentrations that causepremature failure.• Sharp corners produce large stress concentrationsand premature failure.• Failure type depends on T and stress:- for noncyclic s and T < 0.4Tm, failure stress decreases with:- increased maximum flaw size,- decreased T,- increased rate of loading.- for cyclic s:- cycles to fail decreases as Ds increases.- for higher T (T > 0.4Tm):- time to fail decreases as s or T increases.13