Presentation on theme: "Chapter 8: Mechanical Failure"— Presentation transcript:
1 Chapter 8: Mechanical Failure ISSUES TO ADDRESS...• How do flaws in a material initiate failure?• How is fracture resistance quantified; how do differentmaterial classes compare?• How do we estimate the stress to fracture?• How do loading rate, loading history, and temperatureaffect the failure stress?Ship-cyclic loadingfrom waves.Computer chip-cyclicthermal loading.Hip implant-cyclicloading from walking.Adapted from chapter-opening photograph, Chapter 8, Callister 7e. (by Neil Boenzi, The New York Times.)Adapted from Fig (b), Callister 7e. (Fig (b) is courtesy of National Semiconductor Corporation.)Adapted from Fig (b), Callister 7e.
2 Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fractureLittle or no plastic deformationCatastrophic
3 Ductile vs Brittle Failure • Classification:VeryDuctileModeratelyBrittleFracturebehavior:LargeModerate%AR or %ELSmallAdapted from Fig. 8.1, Callister 7e.• Ductilefracture is usuallydesirable!Ductile:warning before fractureBrittle: No warning
4 Example: Failure of a Pipe • Ductile failure:--one piece--large deformation• Brittle failure:--many pieces--small deformationFigures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., Used with permission.
5 Moderately Ductile Failure • Evolution to failure:voidnucleationvoid growthand linkageshearingat surfaceneckingsfracture• Resultingfracturesurfaces(steel)50 mmparticlesserve as voidnucleationsites.From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig , p. 294, John Wiley and Sons, Inc., (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp )100 mmFracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.
6 Ductile vs. Brittle Failure cup-and-cone fracturebrittle fractureAdapted from Fig. 8.3, Callister 7e.
7 Brittle Failure Arrows indicate pt at which failure originated Adapted from Fig. 8.5(a), Callister 7e.
8 Brittle Fracture Surfaces • Intergranular(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.)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 mm4 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 mm1 mm(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p )
9 Ideal vs Real Materials • Stress-strain behavior (Room T):TS << TSengineeringmaterialsperfectseE/10E/1000.1perfect mat’l-no flawscarefully produced glass fibertypical ceramictypical strengthened metaltypical polymer• DaVinci (500 yrs ago!) observed...-- the longer the wire, thesmaller the load for failure.• Reasons:-- flaws cause premature failure.-- Larger samples contain more flaws!Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig John Wiley and Sons, Inc., 1996.
10 Flaws are Stress Concentrators! Results from crack propagationGriffith Crackwhere t = radius of curvatureso = applied stresssm = stress at crack tiptStress concentrated at crack tipAdapted from Fig. 8.8(a), Callister 7e.
11 Concentration of Stress at Crack Tip Adapted from Fig. 8.8(b), Callister 7e.
13 Crack Propagation Cracks propagate due to sharpness of crack tip A plastic material deforms at the tip, “blunting” the crack.deformed regionbrittleEnergy balance on the crackElastic strain energy-energy stored in material as it is elastically deformedthis energy is released when the crack propagatescreation of new surfaces requires energyplastic
14 When Does a Crack Propagate? Crack propagates if above critical stresswhereE = modulus of elasticitys = specific surface energya = one half length of internal crackKc = sc/s0For ductile => replace gs by gs + gpwhere gp is plastic deformation energyi.e., sm > scor Kt > Kc
15 Fracture Toughness ) (MPa · m K 0.5 Ic Graphite/ Ceramics/ Semicond Metals/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/ZrOBased on data in Table B5,Callister 7e.Composite reinforcement geometry is: f = fibers; sf = short fibers; w = whiskers; p = particles. Addition data as noted (vol. fraction of reinforcement):1. (55vol%) ASM Handbook, Vol. 21, ASM Int., Materials Park, OH (2001) p. 606.2. (55 vol%) Courtesy J. Cornie, MMC, Inc., Waltham, MA.3. (30 vol%) P.F. Becher et al., Fracture Mechanics of Ceramics, Vol. 7, Plenum Press (1986). pp4. Courtesy CoorsTek, Golden, CO.5. (30 vol%) S.T. Buljan et al., "Development of Ceramic Matrix Composites for Application in Technology for Advanced Engines Program", ORNL/Sub/ /2, ORNL, 1992.6. (20vol%) F.D. Gace et al., Ceram. Eng. Sci. Proc., Vol. 7 (1986) pp
17 Design Example: Aircraft Wing • Material has Kc = 26 MPa-m0.5• 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!
18 Loading Rate s sy e • Increased loading rate... -- increases sy and TS-- decreases %EL• Why? An increased rategives less time fordislocations to move pastobstacles.sesyTSlargersmaller
19 Impact Testing • Impact loading: -- severe testing case -- makes material more brittle-- decreases toughness(Charpy)final heightinitial heightAdapted from Fig. 8.12(b), Callister 7e. (Fig. 8.12(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.)
20 Temperature • Increasing temperature... --increases %EL and Kc• Ductile-to-Brittle Transition Temperature (DBTT)...FCC metals (e.g., Cu, Ni)BCC metals (e.g., iron at T < 914°C)polymersImpact EnergyBrittleMore DuctileHigh strength materials (sy> E/150)Adapted from Fig. 8.15, Callister 7e.TemperatureDuctile-to-brittletransition temperature
21 Design Strategy: Stay Above The DBTT! • Pre-WWII: The Titanic• WWII: Liberty shipsReprinted 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.)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.)• Problem: Used a type of steel with a DBTT ~ Room temp.
22 Fatigue • Fatigue = failure under cyclic stress. tension on bottomcompression on topcountermotorflex couplingspecimenbearingAdapted from Fig. 8.18, Callister 7e. (Fig is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.)smaxmintimemS• Stress varies with time.-- key parameters are S, sm, and frequency• Key points: Fatigue...--can cause part failure, even though smax < sc.--causes ~ 90% of mechanical engineering failures.
23 Fatigue Design Parameters • Fatigue limit, Sfat:--no fatigue if S < SfatSfatcase forsteel(typ.)N = Cycles to failure103579unsafesafeS = stress amplitudeAdapted from Fig. 8.19(a), Callister 7e.• Sometimes, thefatigue limit is zero!Adapted from Fig. 8.19(b), Callister 7e.case forAl(typ.)N = Cycles to failure103579unsafesafeS = stress amplitude
24 Fatigue Mechanism • Crack grows incrementally • Failed rotating shaft typ. 1 to 6increase in crack length per loading cyclecrack origin• Failed rotating shaft--crack grew even thoughKmax < Kc--crack grows faster as• Ds increases• crack gets longer• loading freq. increases.Adapted fromFig. 8.21, Callister 7e. (Fig is from D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.)
25 Improving Fatigue Life 1. Impose a compressivesurface stress(to suppress surfacecracks from growing)N = Cycles to failuremoderate tensilesmLarger tensileS = stress amplitudenear zero or compressiveIncreasing mAdapted fromFig. 8.24, Callister 7e.--Method 1: shot peeningputsurfaceintocompressionshot--Method 2: carburizingC-rich gas2. Remove stressconcentrators.Adapted fromFig. 8.25, Callister 7e.badbetter
26 Creep Sample deformation at a constant stress (s) vs. time s s,e tPrimary Creep: slope (creep rate) decreases with time.Secondary Creep: steady-state i.e., constant slope.Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate.Adapted fromFig. 8.28, Callister 7e.
27 Creep • Occurs at elevated temperature, T > 0.4 Tm tertiary primary secondaryelasticAdapted from Figs. 8.29, Callister 7e.
28 Secondary Creep • Strain rate is constant at a given T, s -- strain hardening is balanced by recoverystress exponent (material parameter)activation energy for creep(material parameter)strain ratematerial const.applied stress• Strain rateincreasesfor higher T, s1024-2-11Steady state creep rate (%/1000hr)esStress (MPa)427°C538°C649Adapted fromFig. 8.31, Callister 7e.(Fig is from Metals Handbook: Properties and Selection: Stainless Steels, Tool Materials, and Special Purpose Metals, Vol. 3, 9th ed., D. Benjamin (Senior Ed.), American Society for Metals, 1980, p. 131.)
29 Creep Failure • Failure: along grain boundaries. • Time to rupture, tr time to failure (rupture)function ofapplied stresstemperatureappliedstressg.b. cavities• Time to rupture, tr• Estimate rupture timeS-590 Iron, T = 800°C, s = 20 ksiAdapted fromFig. 8.32, Callister 7e.(Fig is from F.R. Larson and J. Miller, Trans. ASME, 74, 765 (1952).)L(103K-log hr)Stress, ksi1001011220242816data forS-590 IronFrom V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.32, p. 87, John Wiley and Sons, Inc., (Orig. source: Pergamon Press, Inc.)1073KAns: tr = 233 hr24x103 K-log hr
30 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.
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