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CHAPTER 7 Mechanical Properties Of Metals - II 7-1.

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1 CHAPTER 7 Mechanical Properties Of Metals - II 7-1

2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Recovery and Recrystallization Cold worked metals become brittle. Reheating, which increases ductility results in recovery, recrystallization and grain growth. This is called annealing and changes material properties. (Adapted from Z.D. Jastrzebski, “The Nature and Properties of Engineering Materials,” 2d ed., Wiley, 1976, p.228.) 7-2

3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Structure of Cold Worked Metals Strain energy of cold work is stored as dislocations. Heating to recovery temperature relieves internal stresses (Recovery stage). Polygonization (formation of sub-grain structure) takes place. Dislocations are moved into lower energy configuration. Polyganization Figure 6.4 Dislocations Slip bands Grain Boundaries (After “Metals Handbook,” vol 7, 8 th ed., American Society of Metals, 1972, p.243) Structure of 85% Cold worked metal TEM of 85% Cold worked metal Structure of stress relived metal TEM of stress relived metal Figure 6.2 and

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Recrystallization If metal is held at recrystallization temperature long enough, cold worked structure is completely replaced with recrystallized grain structure. Two mechanisms of recrystallization  Expansion of nucleus  Migration of grains. Nucleus of recrystallized grain More deformed region Expansion Migration (After “Metals Handbook,” vol 7, 8 th ed., American Society of Metals, 1972, p.243) Figure 6.2 and 6.3 Figure 6.5 Structure and TEM of Recrystallized metal 7-4

5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Effects on Mechanical Properties Annealing decreases tensile strength, increases ductility. Example: Factors affecting recrystalization:  Amount of prior deformation  Temperature and time  Initial grain size  Composition of metal 85% Cu & 15% Zn Tensile strength 75 KSI Ductility 3% Tensile strength 45 KSI Ductility 38 % 50% cold rolled Annealed 1 h C (After “Metals Handbook,” vol 2, 9 th ed., American Society of Metals, 1979, p.320) Figure

6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Facts About Recrystallization A minimum amount of deformation is needed. Smaller the deformation, higher the recrystallization temperature. Higher the temperature, lesser is the time required. Greater the degree of deformation, smaller are the recrystallized grains. Larger the original grain size, greater amount of deformation is required to produce equivalent temperature. Recrystallization temperature increases with purity of metals. (After W.L. Roberts, “Flat Processing of steel,” Marcel Dekker, 1988.) Figure 6.7b Continuous annealing 7-6

7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fracture of Metals – Ductile Fracture Fracture results in separation of stressed solid into two or more parts. Ductile fracture : High plastic deformation & slow crack propagation. Three steps :  Specimen forms neck and cavities within neck.  Cavities form crack and crack propagates towards surface, perpendicular to stress.  Direction of crack changes to 45 0 resulting in cup-cone fracture. 7-7

8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Brittle Fracture No significant plastic deformation before fracture. Common at high strain rates and low temperature. Three stages:  Plastic deformation concentrates dislocation along slip planes.  Microcracks nucleate due to shear stress where dislocations are blocked.  Crack propagates to fracture. Example: HCP Zinc ingle crystal under high stress along {0001} plane undergoes brittle fracture. Figure 6.11 & 6.13 (From ASM handbook vol 12, page 12 and 14) SEM of ductile fracture SEM of brittle fracture 7-8

9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ductile and Brittle Fractures Ductile fracture Brittle Fracture

10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Brittle Fractures (cont..) Brittle fractures are due to defects like  Folds  Undesirable grain flow  Porosity  Tears and Cracks  Corrosion damage  Embrittlement due to atomic hydrogen At low operating temperature, ductile to brittle transition takes place

11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Toughness and Impact Testing Toughness is a measure of energy absorbed before failure. Impact test measures the ability of metal to absorb impact. Toughness is measured using impact testing machine (After H.W. Hayden, W.G. Moffatt, and J.Wulff, “The structure and Properties of Materials,” vol. III, Wiley, 1965, p.13.) Figure

12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Impact testing (Cont…) Also used to find the temperature range for ductile to brittle transition. Sinking of Titanic: Titanic was made up of steel which has ductile brittle transition temperature 32 0 C. On the day of sinking, sea temperature was –2 0 C which made the the structure highly brittle and susceptible to more damage. (After J.A.Rinebolt and W.H. Harris, Trans. ASM, 43: 1175(1951)) Figure 6.15 Figure

13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fracture Toughness Cracks and flaws cause stress concentration. K 1 = Stress intensity factor. σ = Applied stress. a = edge crack length Y = geometric constant. K Ic = critical value of stress intensity factor.(Fracture toughness) Example: Al 2024 T MPam 1/ alloy steel 60.4MPam 1/2 Figure

14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Measuring Fracture Toughness A notch is machined in a specimen of sufficient thickness B. B > > a plain strain condition. B = 2.5(K Ic /Yield strength) 2 Specimen is tensile tested. Higher the K Ic value, more ductile the metal is. Used in design to find allowable flaw size. Courtesy of White Shell research) Figure

15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigue of Metals Metals often fail at much lower stress at cyclic loading compared to static loading. Crack nucleates at region of stress concentration and 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 Figure 6.19 Fracture started here Final rupture (After “Metals Handbook,” vol 9, 8 th ed., American Society of Metals, 1974, p.389) 7-13

16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigues Testing Alternating compression and tension load is applied on metal piece tapered towards center. Stress to cause failure S and number of cycles required N are plotted to form SN curve. (After H.W. Hayden, W.G. Moffatt, and J.Wulff, “The structure and Properties of Materials,” vol. III, Wiley, 1965, p.15.) Figure 6.20 Figure 6.23 Figure

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cyclic Stresses Different types of stress cycles are possible (axial, torsional and flexural). Mean stress = Stress amplitude = Stress range = Figure

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Structural Changes in Fatigue Process Crack initiation first occurs. Reversed directions of crack initiation caused surface ridges and groves called slipband extrusion and intrusion. This is stage I and is very slow ( m/cycle). Crack growth changes direction to be perpendi- cular to maximum tensile stress (rate microns/sec). Sample ruptures by ductile failure when remaining cross-sectional area is small to withstand the stress. Persistent slip bands In copper crystal Figure 6.26 Courtesy of Windy C. Crone, University of Wisconsin 7-16

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Factors Affecting Fatigue Strength Stress concentration: Fatigue strength is reduced by stress concentration. Surface roughness: Smoother surface increases the fatigue strength. Surface condition: Surface treatments like carburizing and nitriding increases fatigue life. Environment: Chemically reactive environment, which might result in corrosion, decreases fatigue life. 7-17

20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigue Crack Propagation Rate Notched specimen used. Cyclic fatigue action is generated. Crack length is measured by change in potential produced by crack opening. Figure 6.27 (After “Metals Handbook,” Vol 8, 9 th ed., American Society of Metals, 1985, p.388.) 7-18

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stress & Crack Length Fatigue Crack Propagation. σ2σ2 σ1σ1 Δa ΔN Δa ΔN When ‘a’ is small, da/dN is also small. da/dN increases with inc- reasing crack length. Increase in σ increases crack growth rate. = fatigue crack growth rate. ΔK = K max -K min = stress intensity factor range. A,m = Constants depending on material, environment, frequency temperature and stress ratio. α f(σ,a) Figure

22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigue Crack Growth rate Versus ΔK Straight line with slope m Limiting value of ΔK below Which there is no measurable Crack growth is called stress intensity factor range threshold ΔK th (After P.C. Paris et al. Stress analysis and growth of cracks, STP 513 ASTM, Philadelphia, 1972, PP Figure

23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigue Life Calculation Integrating from initial crack size a 0 to final crack size a f at number of fatigue cycles N f Integrating and solving for N f (Assuming Y is independent of crack length) But Therefore 7-21

24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Creep in Metals Creep is progressive deformation under constant stress. Important in high temperature applications. Primary creep: creep rate decreases with time due to strain hardening. Secondary creep: Creep rate is constant due to simultaneous strain hard- ening and recovery process. Tertiary creep: Creep rate increases with time leading to necking and fracture. Figure

25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Creep Test Creep test determines the effect of temperature and stress on creep rate. Metals are tested at constant stress at different temperature & constant temperature with different stress. High temperature or stress Medium temperature or stress Low temperature or stress Creep strength: Stress to produce Minimum creep rate of %/h At a given temperature. Figure 6.32 Figure

26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Creep Test (Cont..) Creep rupture test is same as creep test but aimed at failing the specimen. Plotted as log stress versus log rupture time. Time for stress rupture decreases with increased stress and temperature. Figure 6.34 Figure 6.35 (After H.E. McGannon [ed]. “ The making, shaping and Treating of Steel,” 9 th ed., United States Steel, 1971, p

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Larsen Miller Parameter Larsen Miller parameter is used to represent creep- stress rupture data. P(Larsen-Miller) = T[log t r + C] T = temperature(K), tr = stress-rupture time h C = Constant (order of 20) Also, P(Larsen-Miller) = [T( 0 C) + 273(20+log t r ) or P(Larsen-Miller) = [T( 0 F) + 460(20+log t r ) At a given stress level, the log time to stress rupture plus constant multiplied by temperature remains constant for a given material. 7-25

28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Larsen Miller Parameter If two variables of time to rupture, temperature and stress are known, 3 rd parameter that fits L.M. parameter can be determined. Example: For alloy CM, at 207 MPa, LM parameter is 27.8 x 10 3 K Then if temperature is known, time to rupture can be found. Figure 6.36 (After “Metals Handbook,” vol 1, 10 th ed., ASM International, 1990, p.998.) 7-26

29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display L.M. Diagram of several alloys Example: Calculate time to cause 0.2% creep strain in gamma Titanium aluminide at 40 KSI and F From fig, p = = ( ) (log t 0.2% + 20) t=776 h Figure 6.37 After N.R. Osborne et. al., SAMPE Quart, (4)22;26(1992) 7-27

30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Case Study – Analysis of Failed Fan Shaft Requirements  Function – Fan drive support  Material 1045 cold drawn steel  Yield strength – 586 Mpa  Expected life – 6440 Km (failed at 3600 km) Visual examination (avoid additional damage)  Failure initiated at two points near fillet  Characteristic of reverse bending fracture

31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Failed Shaft – Further Analysis Tensile test proved yield strength to be 369 MPa (lower than specified 586 MPa). Metallographic examination revealed grain structure to be equiaxed ( cold drawn metal has elongated grains). Conclusion: Material is not cold drawn – it is hot rolled !.  Lower fatigue strength and stress raiser caused the failure of the shaft.

32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Recent Advances: Strength + Ductility Coarse grained – low strength, high ductility Nanocrystalline – High strength, low ductility (because of failure due to shear bands). Ductile nanocrystalline copper : Can be produced by  Cold rolling at liquid nitrogen temperature  Additional cooling after each pass  Controlled annealing Cold rolling creates dislocations and cooling stops recovery 25 % microcrystalline grains in a matrix of nanograins.

33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fatigue Behavior of Nanomaterials Nanomaterials and Ultrafine Ni are found to have higher endurance limit than microcrystalline Ni. Fatigue crack growth is increased in the intermediate regime with decreasing grain size. Lower fatigue crack growth threshold K th observed for nanocrystalline metal.


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