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Chapter 2 Materials

Chapter 2 Materials. Chapter Outline. Material Strength and Stiffness Statistical Significance of Material Properties Strength and Cold Work Hardness Impact Properties Temperature Effects Numbering Systems Hot-Working Processes Cold-Working Processes Alloy Steels

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Chapter 2 Materials

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  1. Chapter 2 Materials Dr. Mohammad Suliman Abuhaiba, PE

  2. Chapter Outline • Material Strength and Stiffness • Statistical Significance of Material Properties • Strength and Cold Work • Hardness • Impact Properties • Temperature Effects • Numbering Systems • Hot-Working Processes • Cold-Working Processes • Alloy Steels • Corrosion-Resistant Steels • Casting Materials • Nonferrous Metals • Plastics • Composite Materials • Materials Selection Dr. Mohammad Suliman Abuhaiba, PE

  3. Standard Tensile Test • Used to obtain material characteristics and strengths • Loaded in tension with slowly increasing P • Load and deflection are recorded Dr. Mohammad Suliman Abuhaiba, PE

  4. Stress-Strain Diagram • Linear relation until proportional limit, pl • No permanent deformation until elastic limit, el • Yield strength, Sy • Ultimate strength, Su Ductile material Brittle material Dr. Mohammad Suliman Abuhaiba, PE

  5. Elastic Relationship of Stress and Strain • E = modulus of elasticity • E is relatively constant for a given type of material (steel, copper) • Table A-5: typical values • Usually independent of heat treatment, carbon content, or alloying Dr. Mohammad Suliman Abuhaiba, PE

  6. True Stress-Strain Diagram • Engineering stress-strain diagram. • True stress-strain diagram Engineeringstress-strain True Stress-strain Dr. Mohammad Suliman Abuhaiba, PE

  7. Compression Strength • For ductile materials, Suc ≈ Sut • For brittle materials, Suc >Sut Dr. Mohammad Suliman Abuhaiba, PE

  8. Torsional Strengths • Torsional strengths are found by twisting solid circular bars. • Max shear stress is related to angle of twist by • q = angle of twist (radians) • r = radius of bar • l0 = gauge length • G =shear modulus or modulus of rigidity. Dr. Mohammad Suliman Abuhaiba, PE

  9. Torsional Strengths • Max shear stress is related to applied torque J = polar second moment of area of cross section • Torsional yield strength, Ssy= max shear stress at the point where torque-twist diagram becomes significantly non-linear • Modulus of rupture, Ssu= max point on torque-twist diagram Dr. Mohammad Suliman Abuhaiba, PE

  10. Resilience • Resilience – Capacity of a material to absorb energy within its elastic range • Modulus of resilience, uR • Energy absorbed per unit volume without permanent deformation • Equals to area under stress-strain curve up to elastic limit • Elastic limit often approximated by yield point Dr. Mohammad Suliman Abuhaiba, PE

  11. Resilience • Area under curve to yield point gives approximation • If elastic region is linear, • For two materials with the same yield strength, the less stiff material (lower E) has greater resilience Dr. Mohammad Suliman Abuhaiba, PE

  12. Toughness • Toughness – capacity of a material to absorb energy without fracture • Modulus of toughness, uT • Energy absorbed per unit volume without fracture • Equals area under stress-strain curve up to fracture point Dr. Mohammad Suliman Abuhaiba, PE

  13. Toughness • Area under curve up to fracture point • Approximated by using average of yield & ultimate strengths and strain at fracture Dr. Mohammad Suliman Abuhaiba, PE

  14. Statistical Significance of Material Properties • Strength values are obtained from testing many nominally identical specimens • Strength, a material property, is distributional and thus statistical in nature Dr. Mohammad Suliman Abuhaiba, PE

  15. Example for Statistical Material Property • Histographic report for max stress of 1000 tensile tests on 1020 steel • Probability density: # of occurrences divided by total sample number Dr. Mohammad Suliman Abuhaiba, PE

  16. Example for Statistical Material Property • Probability density function (See Ex. 20-4) Dr. Mohammad Suliman Abuhaiba, PE

  17. Statistical Quantity • Statistical quantity described by mean, standard deviation, and distribution type • From 1020 steel example: • Mean stress = 63.62 kpsi • Standard deviation = 2.594 kpsi • Distribution is normal Dr. Mohammad Suliman Abuhaiba, PE

  18. Strengths from Tables • Property tables often only report a single value for a strength term • Important to check if it is mean, minimum, or some percentile • Common to use 99% min strength, indicating 99% of samples exceed the reported value Dr. Mohammad Suliman Abuhaiba, PE

  19. Cold Work • Cold work: Process of plastic straining below recrystallization temperature in the plastic region of stress-strain diagram • Loading to point i beyond yield point, then unloading, causes permanent plastic deformation, ϵp • Reloading to point i behaves elastically all the way to i, with additional elastic strain ϵe Dr. Mohammad Suliman Abuhaiba, PE

  20. Cold Work • Yield point is effectively increased to point i • Material is said to have been cold worked, or strain hardened • Material is less ductile (more brittle) since the plastic zone between yield strength & ultimate strength is reduced • Repeated strain hardening can lead to brittle failure Dr. Mohammad Suliman Abuhaiba, PE

  21. Reduction in Area • R is a measure of ductility • Ductility represents the ability of a material to absorb overloads and to be cold-worked Dr. Mohammad Suliman Abuhaiba, PE

  22. Cold-work Factor • Cold-work factor W: A measure of the quantity of cold work Dr. Mohammad Suliman Abuhaiba, PE

  23. Equations for Cold-worked Strengths Dr. Mohammad Suliman Abuhaiba, PE

  24. Example 2-1 Dr. Mohammad Suliman Abuhaiba, PE

  25. Example 2-1 (Continued) Dr. Mohammad Suliman Abuhaiba, PE

  26. Hardness • Hardness –resistance of a material to penetration by a pointed tool • Most common hardness-measuring systems • Rockwell • Brinell • Vickers Dr. Mohammad Suliman Abuhaiba, PE

  27. Strength and Hardness • For steels • For cast iron Dr. Mohammad Suliman Abuhaiba, PE

  28. Example 2-2 Dr. Mohammad Suliman Abuhaiba, PE

  29. Impact Properties • Charpy notched-bar test used to determine brittleness and impact strength • Specimen struck by pendulum • Energy absorbed, called impact value, is computed from height of swing after fracture Dr. Mohammad Suliman Abuhaiba, PE

  30. Effect of Temperature on Impact Dr. Mohammad Suliman Abuhaiba, PE

  31. Effect of Strain Rate on Impact • Average strain rate for stress-strain diagram is 0.001 in/(in·s) • Increasing strain rate increases strengths Dr. Mohammad Suliman Abuhaiba, PE

  32. Temperature Effects on Strengths • Strength vs. temperature for carbon & alloy steels • As temperature increases above RT: • Sutincrease slightly, then decreases significantly • Sy decreases continuously Dr. Mohammad Suliman Abuhaiba, PE

  33. Creep • Creep: continuous deformation under load for long periods of time at elevated temperatures Dr. Mohammad Suliman Abuhaiba, PE

  34. Creep • Three stages 1st stage: elastic & plastic deformation; decreasing creep rate due to strain hardening 2nd stage: constant min creep rate caused by annealing effect 3rd stage: considerable reduction in area; increased true stress; higher creep rate leading to fracture Dr. Mohammad Suliman Abuhaiba, PE

  35. Material Numbering Systems • Common numbering systems • Society of Automotive Engineers (SAE) • American Iron and Steel Institute (AISI) • Unified Numbering System (UNS) • American Society for Testing and Materials (ASTM) for cast irons Dr. Mohammad Suliman Abuhaiba, PE

  36. UNS Numbering System • Established by SAE in 1975 • Letter prefix followed by 5 digit number • Letter prefix designates material class • G – carbon and alloy steel • A – Aluminum alloy • C – Copper-based alloy • S – Stainless or corrosion-resistant steel Dr. Mohammad Suliman Abuhaiba, PE

  37. UNS for Steels, G • First two numbers indicate composition, excluding carbon content • Second pair of numbers indicates carbon content in hundredths of a percent by weight • Fifth number is used for special situations • Example: G52986 is chromium alloy with 0.98% carbon Dr. Mohammad Suliman Abuhaiba, PE

  38. Alloy Steels • Chromium • Nickel • Manganese • Silicon • Molybdenum • Vanadium • Tungsten Dr. Mohammad Suliman Abuhaiba, PE

  39. Corrosion-Resistant Steels • Stainless steels • Iron-base alloys with at least 12 % chromium • Resists many corrosive conditions • Four types of stainless steels • Ferritic chromium • Austenitic chromium-nickel • Martensitic • Precipitation-hardenable Dr. Mohammad Suliman Abuhaiba, PE

  40. Casting Materials • Gray Cast Iron • Ductile and Nodular Cast Iron • White Cast Iron • Malleable Cast Iron • Alloy Cast Iron • Cast Steel Dr. Mohammad Suliman Abuhaiba, PE

  41. Nonferrous Metals • Aluminum • Magnesium • Titanium Dr. Mohammad Suliman Abuhaiba, PE

  42. Nonferrous Metals • Copper-based alloys • Brass with 5 to 15 % zinc • Gilding brass, commercial bronze, red brass • Brass with 20 to 36 % zinc • Low brass, cartridge brass, yellow brass • Low-leaded brass, high-leaded brass (engraver’s brass), free-cutting brass • Admiralty metal • Aluminum brass • Brass with 36 to 40 % zinc • Muntz metal, naval brass • Bronze • Silcon bronze, phosphor bronze, aluminum bronze, beryllium bronze Dr. Mohammad Suliman Abuhaiba, PE

  43. Plastics • Thermoplastic – any plastic that flows or is moldable when heat is applied • Thermoset– a plastic for which the polymerization process is finished in a hot molding press where the plastic is liquefied under pressure Dr. Mohammad Suliman Abuhaiba, PE

  44. Table 2-2: Thermoplastic Properties Dr. Mohammad Suliman Abuhaiba, PE

  45. Table 2-3: Thermoset Properties Dr. Mohammad Suliman Abuhaiba, PE

  46. Composite Materials • Formed from two or more dissimilar materials, each of which contributes to the final properties • Materials remain distinct from each other at the macroscopic level • Usually amorphous & non-isotropic • Often consists of laminates of filler to provide stiffness and strength and a matrix to hold the material together Dr. Mohammad Suliman Abuhaiba, PE

  47. Composite Materials • Common filler types: Dr. Mohammad Suliman Abuhaiba, PE

  48. Table 2-4: Material Families and Classes Dr. Mohammad Suliman Abuhaiba, PE

  49. Table 2-4: Material Families and Classes Dr. Mohammad Suliman Abuhaiba, PE

  50. Table 2-4: Material Families and Classes Dr. Mohammad Suliman Abuhaiba, PE

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