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Chapter 9 Failure of Materials. Failure of Materials Why study materials failure? - design of structure needs the engineer to minimize (prevent)

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Presentation on theme: "Chapter 9 Failure of Materials. Failure of Materials Why study materials failure? - design of structure needs the engineer to minimize (prevent)"— Presentation transcript:

1 Chapter 9 Failure of Materials

2 Failure of Materials Why study materials failure? - design of structure needs the engineer to minimize (prevent) failure. - important to understand the concept of 3 failure modes; 1. Fracture. 2. Fatigue. 3. Creep. Ship-cyclic loading from waves. Computer chip- cyclic thermal loading. Hip implant- cyclic loading from walking. Adapted from Fig (b), Callister 7e. (Fig (b) is courtesy of National Semiconductor Corporation.) Adapted from Fig (b), Callister 7e. Adapted from chapter-opening photograph, Chapter 9, Callister & Rethwisch 3e. (by Neil Boenzi, The New York Times.) Other examples of failure Brittle fracture: -- many pieces -- small deformations Ductile fracture: -- one piece -- large deformation Failure 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 measurement to against failure. Fundamental concept - crack (defects). - characteristic & mechanisms. Testing techniques - modes of failure. - produce & prevent failure. mode Fracture Fatigue Creep Ductile fracture Brittle fracture mechanismtesting technique Impact test Initial crack Crack grow Topics covered in this chapter… Fatigue test Creep test Failure of Materials - fracture mechanics.

3 Failure of Materials brittle fracture mode of failure: Fracture ductile fracturebrittle fracture Very ductileModerately ductileBrittleFracture behavior LargeModerate%AR or %EL Small Ductile fracture is usually more desirable than brittle fracture! Ductile: Warning before fracture Brittle: No warning - accompanied by significant plastic deformation & slow crack propagation. - common at low strain rate & high temp. - three steps : 1. Specimen forms neck & cavities within neck. 2. Cavities form crack & crack propagates towards surface, perpendicular to stress. 3. Direction of crack changes to 45 o resulting in cup-cone fracture. - no significant plastic deformation before fracture & high crack propagation. - Catastrophic (rapid crack propagation). - common at high strain rates & low temp. - three stages: 1. Plastic deformation concentrates dislocation along slip planes. 2. Microcracks nucleate due to shear stress where dislocations are blocked. 3. Crack propagates to fracture. characteristic of fracture SEM micrograph ductile fracture At low operating temp., ductile to brittle transition (DBT) takes place Schematic diagram Fracture of materials results in separation of stressed solid into two or more parts.

4 Failure of Materials brittle fracture mode of failure: Fracture Moderately ductile fracture brittle fracture surfaces SEM micrograph ductile fracture Intergranular (between 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.) 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 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 Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.) 3 mm 160 mm 1 mm necking  void nucleation shearing at surface fracture void growth & linkage 100 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. evolution to failure V-shaped “chevron” origin of crack

5 Failure of Materials brittle fracture mode of failure: Fracture brittle fracture (in ceramics) ductile fracture (in thermoplastic polymer) SEM micrograph ductile fracture- characteristic of fracture behavior: -- origin point. -- initial region (mirror) is flat & smooth. -- after reaches critical velocity crack branches (mist & hackle). - characteristic of fracture behavior: -- craze formation prior to cracking. -- during crazing, plastic deformation of spherulites. -- and formation of micro voids and fibrillar bridges. fibrillar bridges microvoids crack aligned chains

6 Failure of Materials - engineering materials don't reach theoretical strength. - flaws produce stress concentrations that cause premature failure. tt Results from crack propagation. Griffith Crack where  t = radius of curvature s o = applied stress s m = stress at crack tip Stress concentration at crack tip Flaws are stress concentrators Engineering Fracture Design - avoid sharp corners., r fillet radius w h oo  max r/hr/h sharper fillet radius increasingw/hw/h Stress Conc. Factor, K t =  max 00 Crack Propagation - cracks propagate due to sharpness of crack tip. ductile brittle deformed region mode of failure: Fracture concept of fracture mechanics

7 Failure of Materials Crack Propagation - crack propagates if above critical stress. i.e.,  m >  c or K t > K c where E = modulus of elasticity  s = specific surface energy a = one half length of internal crack K c =  c /s o Design Against Crack Growth mode of failure: Fracture - crack growth condition: K ≥ K c = - engineering materials don't reach theoretical strength. - flaws produce stress concentrations that cause premature failure. concept of fracture mechanics - largest, most stressed cracks grow first! -- Result 1: Max. flaw size dictates design stress.  a max no fracture -- Result 2: Design stress dictates max. flaw size. a max  no fracture  TS <<  TS engineering materials perfect materials   E/10 E/ perfect mat’l-no flaws carefully produced glass fiber typical ceramic typical strengthened metal typical polymer Stress-strain curves (Room T) perfect & engineering materials

8 Design Example: Aircraft Wing Two designs to consider... Design A -- largest flaw is 9 mm -- failure stress = 112 MPa Design B -- use same material -- largest flaw is 4 mm -- failure stress = ? Key point: Y and K c are the same in both designs. Answer: Reducing flaw size pays off! Material has K c = 26 MPa-m 0.5 Use... 9 mm112 MPa 4 mm -- Result: Failure of Materials mode of failure: Fracture - engineering materials don't reach theoretical strength. - flaws produce stress concentrations that cause premature failure. concept of fracture mechanics Fracture Toughness, K IC for selected materials

9 Failure of Materials mode of failure: Fracture testing technique: Impact test final heightinitial height (Charpy) - classify into 3: 1. Charpy impact test. 2. Izod impact test. 3. Drop weight impact test. 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. 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.) 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(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.) Design Strategy: Stay above the DBT! BCC metals (e.g., iron at T < 914°C) Impact Energy Temperature High strength materials (( y > E/150) & polymers DBT temperature FCC metals (e.g., Cu, Ni) more brittlemore ductile 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.) - aim: to investigate the energy absorbed & fracture of materials during impact loading at various temperature. - fracture behavior can be shown in impact energy vs temperature curves. Sinking of Titanic: Titanic was made up of steel which has DBT temperature 32 o C. On the day of sinking, sea temperature was –2 o C which made the the structure highly brittle and susceptible to more damage.

10 Failure of Materials mode of failure: Fatigue - metals often fail at much lower stress at cyclic loading compared to static loading. - crack nucleates at region of stress concentration & propagates due to cyclic loading. - failure occurs when cross sectional area of the metal too small to withstand applied load. Fatigue fractured surface of keyed shaft Fracture started here Final rupture Key points: Fatigue... - failure under cyclic loading. - can cause part failure, even though  max <  c. - causes ~ 90% of mechanical engineering failures. - stress varies with time. -- key parameters are: 1. stress amplitude, S  a ). 2. mean stress,  m. 3. frequency, f (will convert to no. of cycle, N).  max  min  time  m S Mean stress Stress amplitude Stress range Stress amplitude, S = stress to cause failure Number of cycle, N = 1/f - fatigue behavior can be shown in SN curves. testing technique: Fatigue test Important point from fatigue test… 1. Fatigue limit. 2. Fatigue strength. 3. Fatigue life. tension on bottom compression on top counter motor flex coupling specimen bearing

11 Failure of Materials mode of failure: Fatigue Fatigue Design Parameters - Fatigue limit, S fat : -- no fatigue if S < S fat Adapted from Fig. 9.25(a), Callister & Rethwisch 3e. S fat N = Cycles to failure unsafe safe S = stress amplitude example: SN curves for typical steel - Sometimes, the fatigue limit is zero! -- fatigue occurs at any time. Adapted from Fig. 9.25(b), Callister & Rethwisch 3e. N = Cycles to failure unsafe safe S = stress amplitude Fatigue limit: - PMMA, PP, PE No fatigue limit: - PET, Nylon (dry) example: SN curves for typical polymers Improving Fatigue Life 1. Impose a compressive surface stress (to suppress surface cracks from growing) --Method 1: shot peening shot --Method 2: carburizing C-rich gas put surface into compression 2. Remove stress concentrators. bad better Adapted from Fig. 9.32, Callister & Rethwisch 3e.

12 Failure of Materials mode of failure: Creep - Creep is progressive deformation under constant stress. - Occurs at elevated temperature, T > 0.4 T m -- important in high temperature applications. Primary creep: - creep rate (slope) decreases with time due to strain hardening. Secondary creep: - creep rate is constant due to simultaneous strain hardening and recovery process. -- steady-state. Tertiary creep: - creep rate (slope) increases with time leading to necking & fracture. testing technique: Creep test - Creep test determines the effect of temperature & stress on creep rate. - Metals are tested at constant stress at different temperature & constant temperature with different stress.  0 t  Important information from  -t curve… Adapted from Fig. 9.35, Callister & Rethwisch 3e. Adapted from Figs. 9.36, Callister & Rethwisch 3e. elastic primary secondary tertiary Data from creep test… 1. creep strain,  2. temperature, T 3. time, t - creep behavior can be shown in  -t curves. t r = time to failure (rupture) L = function of applied stress T = temperature Time to rupture, t r Creep failure along grain boundaries. applied stress g.b. cavities creep calculation… Estimate rupture time for S-590 Iron, T = 800°C &  = 20 ksi example: 24x K Ans: t r = 233 hr L(10 3 K-log hr) Stress, ksi data for S-590 Iron 20 T(20 + log t r ) = L

13 Engineering materials don't reach theoretical strength. Flaws produce stress concentrations that cause premature failure. Sharp corners produce large stress concentrations and premature failure. Failure type depends on T and stress: - for noncyclic  and T < 0.4T m, failure stress decreases with: - increased maximum flaw size, - decreased T, - increased rate of loading. - for cyclic  : - cycles to fail decreases as  increases. - for higher T (T > 0.4T m ): - time to fail decreases as  or T increases. SUMMARY


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