Phy351 ch 6


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Phy351 ch 6

  1. 1. PHY351 CHAPTER 6 1 Metals
  2. 2. METALLIC MATERIAL Metals and Alloys Ferrous Eg: Steel, Cast Iron   Nonferrous Eg:Copper Aluminum Ferrous Metals and alloys - Metals and alloys that contain a large percentage of iron such as steels and cast irons Nonferrous metals and alloys - Metals and alloys that do not contain iron. - If they do contain iron, it is only in a relatively small percentage. 2
  3. 3. QUESTION 1 1. Define the following materials in terms of their properties and give an example each of their application. a. b. c. Stainless steel Brass Cast iron 3
  4. 4. PROCESSING OF METAL - CASTING Most metals are first melted in a furnace.  Alloying is done if required.  Large ingots are then cast.  Sheets and plates are then produced from ingots by rolling (wrought alloy products).  Channels and other shapes are produced by extrusion.  Some small parts can be cast as final product. Example :- Automobile Piston.  4
  5. 5. Casting Process Casting mold 5
  6. 6. HOT ROLLING OF STEEL Hot rolling : Greater reduction of thickneess in a single pass.  Rolling carried out at above recrystallization temperature.  Ingots preheated to about 12000C.  Ingots reheated between passes if required.  Usually, series of 4 high rolling mills are used.  6
  7. 7. COLD ROLLING OF METAL SHEET Cold rolling is rolling performed below recrystallization temperature.  This results in strain hardening.  Hot rolled slabs have to be annealed before cold rolling.  Series of 4 high rolling mills are usually used.  Less reduction of thickness.  Needs high power.  7
  8. 8. % Cold work = Initial metal thickness – Final metal thickness x 100 Initial metal thickness 8
  9. 9. EXTRUSION Metal under high pressure is forced through opening in a die.  Common Products are cylindrical bar, hollow tubes from copper, aluminum etc.  Normally done at high temperature.  Indirect extrusion needs less power however has limit on load applied  9
  10. 10. Direct Extrusion Die Container Metal Indirect Extrusion Container 10 Metal 10
  11. 11. FORGING Metal, usually hot, is hammered or pressed into desired shape.  Types:    Open die: Dies are flat and simple in shape. (Example products: Steel shafts) Closed die: Dies have upper and lower impresion. (Example products: Automobile engine connection rod) Forging increases structural properties, removes porosity and increases homogeneity. 11
  12. 12. Direct Forging Indirect Forging Metal Die 12
  13. 13. DRAWING  Wire drawing :- Starting rod or wire is drawn through several drawing dies to reduce diameter. % cold work = Change in cross-sectional area X 100 Original area 13
  14. 14.  Deep drawing:- Used to shape cup like articles from flats and sheets of metals 14 14
  15. 15. MECHANICAL PROPERTIES OF METAL Stress  Strain  Hardness  Impact Energy  Fracture  Toughness  Fatigue  Creep  15
  16. 16. STRESS  Metals undergo deformation under uniaxial tensile force.  Elastic deformation: Metal returns to its original dimension after tensile force is removed.  Plastic deformation: The metal is deformed to such an extent such that it cannot return to its original dimension 16
  17. 17. F Engineering stress , σ = A0 Units of Stress are PSI or N/M2 (Pascals) 1 PSI = 6.89 x 103 Pa 17
  18. 18. Engineering strain , ε =  Change in length Original length   0 0    Units of strain are in/in or m/m. 18
  19. 19. SHEAR STRESS AND SHEAR STRAIN τ = Shear stress = S A Shear strain γ = Amount of shear displacement Distance ‘h’ over which shear acts. 19
  20. 20.  Modulus of elasticity (E) or Young’s modulus : Stress and strain are linearly related in elastic region. (Hooks law) σ (Stress) E= ε (Strain) Strain Δσ Δσ E= Δε Δε Stress Linear portion of the stress strain curve 20
  21. 21.  Higher the bonding strength, higher is the modulus of elasticity. Examples: Modulus of Elasticity of steel is 207 Gpa. Modulus of elasticity of Aluminum is 76 Gpa 21
  22. 22. YIELD STRENGTH  Yield strength is strength at which metal or alloy show significant amount of plastic deformation.  0.2% offset yield strength is that strength at which 0.2% plastic deformation takes place.  Construction line, starting at 0.2% strain and parallel to elastic region is drawn to 0.2% offset yield strength. 22
  23. 23. ULTIMATE TENSILE STRENGTH   Ultimate tensile strength (UTS) is the maximum strength reached by the engineering stress strain curve. Necking starts after UTS is reached. Al 2024-Tempered S T R E S S Mpa Necking Point Al 2024-Annealed Strain Stress strain curves of Al 2024 With two different heat treatments. Ductile annealed sample necks more 23
  24. 24.  More ductile the metal is, more is the necking before failure.  Stress increases till failure. Drop in stress strain curve is due to stress calculation based on original area. 24
  25. 25. PERCENT ELONGATION  Percent elongation is a measure of ductility of a material.  It is the elongation of the metal before fracture expressed as percentage of original length. % Elongation = Final length* – initial Length* Initial Length 25
  26. 26. PERCENT REDUCTION IN AREA  Percent reduction area is also a measure of ductility.  The diameter of fractured end of specimen is measured using caliper. % Reduction = Area Initial area – Final area Initial area 26
  27. 27.  Percent reduction in area in metals decreases in case of presence of porosity. 27
  28. 28. TRUE STRESS – TRUE STRAIN  True stress and true strain are based upon instantaneous cross-sectional area and length.  True stress is always greater than engineering stress. 28
  29. 29.  True Stress = σt = F Ai (instantaneous area) i  True Strain = εt =  0 d   Ln li l0  Ln A0 Ai 29
  30. 30. QUESTION 2 1. A 0.5cm diameter aluminium bar is subjected to a force of 500N. Calculate the engineering stress in MPa on the bar. (Answer: 25.5 MPa) 2. A 1.25cm diameter bar is subjected to a load of 2500 kg. Calculate the engineering stress on the bar in MPa. (Answer: 200 MPa) 3. A sample of commercially pure aluminium 1.27cm wide, 0.1cm thick and 20.3cm long that has gage markings 5.1cm apart in the middle of the sample is strained so that the gage markings are 6.7cm apart. Calculate the engineering strain and the percent engineering strain elongation that the sample undergoes. (Answer: 0.31, 31%) 30
  31. 31. 4. A 12.7mm diameter round sample of a 1030 carbon steel is pulled to failure in a tensile testing machine. The diameter of the sample was 8.7mm at the fracture surface. Calculate the percent reduction in area of the sample. (Answer: 53%) 5. A 70% Cu-30% Zn brass sheet is 0.12cm thick and is cold-rolled with a 20 percent reduction in thickness. What must be the final thickness of the sheet? (Answer: 0.096cm) 6. Calculate the percent cold reduction when an aluminium wire is cold-drawn from a diameter of 6.5mm to a diameter of 4.25mm. (Answer: 57.2%) 7. A tensile specimen of cartridge brass sheet has a cross section of 10 mm x 4mm and a gage length of 51mm. Calculate the engineering strain that occurred during a test if the distance between gage markings is 63 mm after the test. (Answer: 0.235) 31
  32. 32. 8. Compare the engineering stress and strain with the true test and strain for the tensile test of low-carbon steel that has the following test values. Load applied to specimen = 69 000N Initial specimen diameter = 1.27 cm Diameter specimen under 69 200 N load = 1.2 cm (Answer: 544.6 MPa, 610 Mpa, 0.12, 0.117) 9. A 20cm long rod with a diameter of 0.25cm is loaded on an FCC single crystal. Calculate a. The engineering stress and strain at this load (Answer: 1019 MPa, 0.147) b. The trues tress and strain at this load. (Answer: 1443 MPa, 0.349) 32
  33. 33. HARDNESS Hardness is a measure of the resistance of a metal to permanent (plastic) deformation.  General procedure:  Press the indenter that is harder than the metal Into metal surface. Withdraw the indenter Measure hardness by measuring depth or width of indentation. 33
  34. 34. Rockwell hardness tester 34
  35. 35. FRACTURE  Fracture results in separation of stressed solid into two or more parts.  There are two types of fracture:  Ductile fracture  Brittle Fracture 35
  36. 36. DUCTILE FRACTURE  Ductile fracture : High plastic deformation & slow crack propagation (fracture due to 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 450 resulting in cup-cone fracture. 36
  37. 37. Ductile fracture 37
  38. 38. BRITTLE FRACTURE  No significant plastic deformation before fracture (fracture due to rapid crack propagation).  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. 38
  39. 39. Example: HCP Zinc ingle crystal under high stress along {0001} plane undergoes brittle fracture. SEM of ductile fracture SEM of brittle fracture 39
  40. 40. Brittle Fracture 40
  41. 41.  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 41
  42. 42. TOUGHNESS 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  42
  43. 43. FRACTURE TOUGHNESS  Cracks and flaws cause stress concentration. K1  Y a where K1 = Stress intensity factor. σ = Applied stress. a = edge crack length Y = geometric constant. 43
  44. 44.  KIc = critical value of stress intensity factor (fracture toughness) Example: Al 2024 T851 26.2MPam1/2 4340 alloy steel 60.4MPam1/2 44
  45. 45. QUESTION 3 1. A structural plate component of an engineering design must support 207 MPa in tension. If aluminium alloy 2024-T851 is used for this application, what is the largest internal flaw size that this material can support? (Use Y = 1, KIc =26.4 MPa) (Answer: 5.18 mm) 45
  46. 46. FATIGUE  The phenomenon leading to fracture under repealed stresses having a maximum value less than the ultimate strength of the material.  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. 46
  47. 47. Fracture started here Final rupture Fatigue fractured surface of keyed shaft 47
  48. 48.  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. 48
  49. 49. CREEP  Creep is a time-dependent plastic deformation when subjected to a constant stress or load.  Important in high temperature applications: i. Primary creep: creep rate decreases with time due to strain hardening. ii. Secondary creep: Creep rate is constant due to simultaneous strain hard- ening and recovery process. iii. Tertiary creep: Creep rate increases with time leading to necking and fracture. 49
  50. 50. Creep curve. The slope of the linear part of the curve is the steady-state creep rate. 50
  51. 51. 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 51
  52. 52.  Creep strength: Stress to produce minimum creep rate of 10-5%/h at a given temperature. 52
  53. 53.  Creep rupture test is same as creep test but aimed at failing the specimen.  Plotted as log stress versus log rupture time. 53
  54. 54.  Time for stress rupture decreases with increased and temperature stress 54
  55. 55. REFERENCES     A.G. Guy (1972) Introduction to Material Science, McGraw Hill. J.F. Shackelford (2000). Introduction to Material Science for Engineers, (5th Edition), Prentice Hall. W.F. Smith (1996). Principle to Material Science and Engineering, (3rd Edition), McGraw Hill. W.D. Callister Jr. (1997) Material Science and Engineering: An Introduction, (4th Edition) John Wiley. 55