Chapter 8-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?1How 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 Fig. 8.0, Callister 6e.(Fig. 8.0 is by Neil Boenzi, The New York Times.)Adapted from Fig. 18.11W(b), Callister 6e. (Fig. 18.11W(b) is courtesy ofNational Semiconductor Corporation.)Adapted from Fig. 17.19(b), Callister 6e.CHAPTER 8:MECHANICAL FAILUREChapter 8-Mechanical Failure The usual causes for failure are: Improper materials selection and processing Inadequate design Misuse Cost of failure 1000 Billions of $ or YTL annually Loss of human life !Chapter 8 -Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Little or no plastic deformation CatastrophicChapter 8- 2 Ductilefracture isdesirable! Classification:Ductile:warning before fractureBrittle:No warningFailure is catastrophicAdapted from Fig. 8.1, Callister 6e.DUCTILE VS BRITTLE FAILURESubstantial plastic deformationAbsorb high amounts of energy before fractureWhat is the reduction in x-sectional area ?Chapter 8-Ductile vs. Brittle FailureAdapted from Fig. 8.3, Callister 7e.cup-and-cone fracture brittle fractureChapter 8-Fracture, in detail Two steps involved in fracture: Crack formation Crack growth Two fracture modes can be defined Ductile (preferred, most metals and some polymers) extensive plastic deformation in the vicinity of a crack Extension of crack length requires an increase in the applied load, hence crack is stable unless stress is increased. Crack propagation is therefore slow Brittle (undesired, ceramics, metals at low temperatures) Takes place w/o appreciable plastic deformation Crack is unstable, will propagate with high speed once formed and w/o increase in applied stressChapter 8-Energy to break a unit volume ofmaterialApproximate by the area under the stress-straincurve.20smaller toughness- unreinforced polymersEngineering tensile strain, cEngineering tensile stress, osmaller toughness (ceramics)larger toughness (metals, PMCs)TOUGHNESS & RESILIENCERESILIENCE is energy stored in the material w/o plastic deformation ! Ur= y2/ 2 ETOUGHNESS is total energy stored in the material upon fracture !Chapter 8-Heverill Fire Department aerial ladder failure from: http://www.engr.sjsu.edu/WofMatE/FailureAnaly.htmFrom http://plane-truth.com/comet.htmChapter 8- Ductile failure:--one piece--large deformationFigures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures(2nd ed.), Fig. 4.1(a) and (b), p. 66John Wiley and Sons, Inc., 1987.Used with permission.Example:Failure of a Pipe Brittle failure:--many pieces--small deformationObserve the variation in the amount of plastic deformationChapter 8-From http://plane-truth.com/comet.htmAnything peculiar in this image ! Chapter 8-Engineering Fracture Designr/hsharper fillet radiusincreasingw/h0 0.5 1.01.01.52.02.5Stress Conc. Factor, Ktomaxoo=Avoid sharp corners!oAdapted from Fig. 8.2W(c), Callister 6e.(Fig. 8.2W(c) is from G.H. Neugebauer, Prod.Eng.(NY), Vol. 14, pp. 82-87 1943.)r , filletradiuswhoomaxChapter 8- 7Elliptical hole ina plate:Stress distrib. in front ofa hole:Stress concentration factor:Large Ktpromotes failure:FLAWS ARE STRESS CONCENTRATORS!Sharper cracks amplify stress !More important for brittle materials as in ductile material Plas. Dfm takes place and stress is distributed more uniformly around a crack !omaxt~2ooa| \ | . |tNOT SO BADKt=2Chapter 8- Evolution to failure: Resultingfracturesurfaces(steel)50 mmparticlesserve as voidnucleationsites.50 mmFrom V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294,John Wiley and Sons, Inc., 1987.(Orig. source:P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)100 mmFracture surface of tire cord wireloaded in tension.Courtesy of F. Roehrig, CC Technologies, Dublin, OH.Used with permission.Moderately Ductile Failureneckingovoid nucleationvoid growth and linkageshearing at surfacefractureChapter 8-Ductile FractureCup and cone fracture in AluminumSEM of the fracture surface:Fractographic studiesDimples, micro-cavities that initiate crack formation, on the surface will be observed during high resolution investigation of the cup and cone type fracture surfaces.CupConeDimpleMicro-voidChapter 8- Intergranular(between grains) Intragranular(within grains)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.)316 S. Steel (metal)Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p. 357. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.)304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)Polypropylene(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., 1996.3mm4mm160mm1mm(Orig. source:K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)Brittle Fracture SurfacesChapter 8-Impact Testingfinal height initial heightImpact loading:-- severe testing case-- makes material more brittle-- decreases toughnessAdapted 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.)(Charpy)Chapter 8-Increasing temperature...--increases %EL and KcDuctile-to-Brittle Transition Temperature (DBTT)...TemperatureBCC metals (e.g., iron at T < 914C)Impact EnergyTemperatureHigh strength materials (oy> E/150)polymers More Ductile BrittleDuctile-to-brittle transition temperatureFCC metals (e.g., Cu, Ni)Adapted from Fig. 8.15, Callister 7e.Chapter 8-DBTT1. As temperature decreases a ductile material can become behave brittle - ductile-to-brittle transition2. FCC metals remain ductile down to very low temperatures.3. For ceramics, this type of transition occurs at much higher temperatures than for metals.4. The ductile-to-brittle transition can be measured by impact testing: the impact energy needed for fracture drops suddenly over a relatively narrow temperature range temperature of the ductile-to-brittle transition.Chapter 8-Pre-WWII:The Titanic WWII:Liberty shipsProblem:Used a type of steel with a DBTT ~ Room temp.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., 1996.(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., 1996.(Orig. source:Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.)Design Strategy:Stay Above The DBTT!Chapter 8 - Stress-strain behavior (Room T):Ideal vs Real MaterialsTS 105)Endurance LimitChapter 8-LOW cycle FATIGUEVs HIGH cycle FATIGUEChapter 8-Goodmans ruleChapter 8-Fatigue loading of cracked componentsChapter 8- 19Crack grows incrementally( )mK AdNdCA =typ. 1 to 6( ) C o A ~increase in crack length per loading cycleFailed rotating shaft--crack grew even thoughKmax

eFAo>oeM =@ 18000 rpmLife span30hNf3e7Chapter 8-Chapter 8-IMPROVING FATIGUE LIFEChapter 8-(a, b) Load-limited design.(c) Energy-limited design.(d) Displacement-limited design.Each must perform its function withoutfracturing.TYPES OF DESINGChapter 8-The key players in fracture-limited design: load and geometry, crack length and materialc YKCto1