Chapter 8: Mechanical Failure ISSUES TO ADDRESS...

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1 Chapter 8: Mechanical Failure ISSUES TO ADDRESS... What are the common modes of mechanical failures? How do micro-cracks lead to failure? How do the fracture resistances of the different materials compare? How do loading rate, loading history, and temperature affect the failure behavior of materials? Ship-cyclic loading from waves. Adapted from chapter-opening photograph, Chapter 8, Callister & Rethwisch 8e. (by Neil Boenzi, The New York Times.) Computer chip-cyclic thermal loading. Adapted from Fig (b), Callister 7e. (Fig (b) is courtesy of National Semiconductor Corporation.) Hip implant-cyclic loading from walking. Adapted from Fig (b), Callister 7e. Chapter 8-1

2 The First (#1) Type of Mechanical Failure - Fracture Ductile fracture Accompanied by significant plastic deformation More predictable Brittle fracture Little or no plastic deformation Often sudden & catastrophic Chapter 8-2

3 Ductile vs Brittle Fracture Morphology difference: Fracture behavior: Very Ductile Moderately Ductile Brittle Adapted from Fig. 8.1, Callister & Rethwisch 8e. %AR or %EL Ductile fracture is usually more desirable than brittle fracture! Large Ductile: Warning before fracture Moderate Small Brittle: No warning Chapter 8-3

4 Example: Pipe Fractures Ductile fracture: -- large plastic deformation Brittle fracture: -- many pieces -- almost no plastic deformations Figures 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. Chapter 8-4

5 Failure process: necking s Ductile Fracture void nucleation void growth and coalescence shearing at surface fracture Resulting fracture surfaces (steel) particles serve as void nucleation sites mm 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 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. Chapter 8-5

6 Ductile vs. Brittle Fracture cup-and-cone for ductile fracture flat cross-section for brittle fracture Adapted from Fig. 8.3, Callister & Rethwisch 8e. Chapter 8-6

7 Intergranular (between grains) 4 mm Fracture Surfaces 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.) Transgranular (through grains) 160 mm 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., mm (Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p ) Al2O3 (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 mm Chapter 8-7

8 Fracture (Tensile) Strength of Ideal (Perfect) Material vs Real Materials Stress-strain behavior (Room T): E/10 s Perfect/ideal materials (i.e., no flaws) carefully produced glass fiber TS engineering materials << TS perfect materials E/100 typical ceramic 0.1 typical strengthened metal typical polymer Material (3D) defects degrades strength -- Example: the longer the wire, the smaller the load for failure. Reasons: -- flaws cause premature failure. -- larger samples contain more & larger flaws/defects (especially micro-cracks)! e Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig John Wiley and Sons, Inc., Chapter 8-8

9 t Flaws are Stress Concentrators! Stress at the tip of a micro-crack is many times higher than the average applied stress σ 0 Micro-cracks are examples of common flaws or 3D defects Stress concentration at tip of a micro-crack 1/ 2 s m 2s o a t where t = radius of curvature at the crack tip s o = (average) applied stress s m = stress at crack tip a = crack width K s Adapted from Fig. 8.8(a), Callister & Rethwisch 8e. t o Chapter 8-9

10 Concentration of Stress at Crack Tip Adapted from Fig. 8.8(b), Callister & Rethwisch 8e. Chapter 8-10

11 Crack Propagation Cracks having sharp tips propagate easier than cracks having blunt tips A material with better toughness plastically deforms before cracks grows at its tip, which need more energy for the crack to grow (or propagate) compared with brittle material. deformed region brittle ductile Chapter 8-11

12 Criterion for Crack Propagation Crack propagates when crack-tip concentrated stress (s m ) exceeds the critical stress (s c ), which can be theoretically estimated i.e., s m > s c s c 2E a s 1/ 2 where E = modulus of elasticity s = specific surface energy a = one half length of internal crack Chapter 8-12

13 KIc(MPa m 0.5 ) Fracture Toughness: Measure of Materials Property as Resistance to Brittle Fracture Metals/ Alloys Steels Ti alloys Al alloys Mg alloys Graphite/ Ceramics/ Semicond Diamond Si carbide Al oxide Si nitride <100> Si crystal <111> Glass -soda Concrete Polymers PET PP PC PS PVC Polyester Composites/ fibers C-C( fibers) 1 Al/Al oxide(sf) 2 Y 2 O 3 /ZrO 2 (p) 4 C/C( fibers) 1 Al oxid/sic(w) 3 Si nitr/sic(w) 5 Al oxid/zro 2 (p) 4 Glass/SiC(w) 6 Glass 6 Based on data in Table B.5, Callister & Rethwisch 8e. 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 (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). pp 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, (20vol%) F.D. Gace et al., Ceram. Eng. Sci. Proc., Vol. 7 (1986) pp Chapter 8-13

14 Impact Testing for Evaluating Fracture Toughness Impact loading: -- severe testing case (Charpy) Adapted from Fig. 8.12(b), Callister & Rethwisch 8e. (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.) final height initial height Chapter 8-14

15 Impact Energy Influence of Temperature on Materials Fracture Toughness Ductile-to-Brittle Transition Temperature (DBTT) For certain metals and polymers, they are ductile above DBTT, but suddenly become brittle below DBTT FCC metals (e.g., Cu, Ni), no obvious DBTT BCC metals (e.g., iron) & some polymers High strength materials (s y > E/150), no obvious DBTT Ductile-to-brittle transition temperature (DBTT) Temperature Adapted from Fig. 8.15, Callister & Rethwisch 8e. Chapter 8-15

16 Design Strategy: Use Materials with DBTT Much Lower than Lowest Usage Temperature Pre-WWII: The Titanic WWII: Liberty ships 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.) Problem: Steels used had DBTT s just below room temperature in those cases 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.) Chapter 8-16

17 A Second (#2) Type of Mechanical Failure - Fatigue Fatigue = failure under repeated applied or cyclic stress. bearing specimen compression on top bearing Stress varies with time. -- key parameters are S, s m, and cycling frequency motor flex coupling tension on bottom s max s min s m counter s Adapted from Fig. 8.18, Callister & Rethwisch 8e. (Fig is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.) S time Fatigue --can cause part failure, even though s max < s y. --responsible for ~ 90% of mechanical engineering failures. Chapter 8-17

18 Types of Fatigue Behavior Fatigue limit, S fat : --no fatigue if S < S fat i.e., material can be used indefinitely as long as applied stress stays below the fatigue limit value S = stress amplitude S fat safe unsafe N = Cycles to failure Example: steel Adapted from Fig. 8.19(a), Callister & Rethwisch 8e. For some materials, there is no apparent fatigue limit! i.e., to use it for longer/more cycles, the maximum stress applied has to be reduced further S = stress amplitude safe unsafe N = Cycles to failure Example: Al Adapted from Fig. 8.19(b), Callister & Rethwisch 8e. Chapter 8-18

19 Fatigue Crack Growth Failed rotating shaft -- Crack grew with relatively low stress level -- Crack grows faster as Ds or S increases crack gets longer loading freq. increases. Adapted from Fig. 8.21, Callister & Rethwisch 8e. (Fig is from D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.) crack origin Chapter 8-19

20 S = stress amplitude 1. Impose compressive surface stresses (to suppress surface cracks from growing) Improving Fatigue Life Increasing mean (tensile) stress s m leads to shorter cycle life N = Cycles to failure Adapted from Fig. 8.24, Callister & Rethwisch 8e. near zero or compressive s m moderate tensile s m Larger tensile s m --Method 1: shot peening shot put surface into compression --Method 2: carburizing C-rich gas 2. Remove stress concentrators. bad bad better better Adapted from Fig. 8.25, Callister & Rethwisch 8e. Chapter 8-20

21 A Third (#3) Type of Mechanical Failure - Creep Sample deforms more and more at a constant stress (s) over time - Creep s s,e 0 t Primary Creep: slope (creep rate) decreases with time. Secondary Creep: steady-state i.e., constant slope (De/Dt). Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate. Adapted from Fig. 8.28, Callister & Rethwisch 8e. Chapter 8-21

22 Creep - Temperature Dependence Occurs at elevated or higher temperature: T > 0.4 T m (in K) Higher temperature faster creep! tertiary primary secondary elastic Adapted from Fig. 8.29, Callister & Rethwisch 8e. Chapter 8-22

23 Creep Failure Failure: often along grain boundaries. g.b. cavities applied stress From 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.) Chapter 8-23

24 Creep for Al 2 O 3 Ceramics Al 2 O 3 : Melting point: Tm = 2025 o C As received Al 2 O 3 tube After running at >1400 o C for hours without proper mechanical support Curved Al 2 O 3 tube after creep Chapter 8-24

25 SUMMARY Engineering materials not as strong as predicted by theory Flaws/microcracks act as stress concentrators that cause failure, fracture in particular, at stress level much lower than theoretical values. Sharp corners produce large stress concentrations and premature failure. Failure type depends on T and s : -For simple fracture (noncyclic s and T < 0.4T m), failure stress decreases with: - increased maximum flaw size - For fatigue (cyclic s): - cycles to fail decreases as Ds or s m increases or increase T. - For creep (T > 0.4T m ): - time to rupture decreases as s or T increases. Chapter 8-25

26 Homework Read chapter 8 and give a statement confirm reading Explain in your own words the following concepts: Brittle fracture Ductile fracture Ductile to brittle transition temperature (DBTT) Fatigue Creep Chapter 8-26