MET 210W

1. MET 210W Chapter 2 – Materials in Mechanical Design

2. Properties of Materials: • Chemical – relate to structure of material, atomic bonds, etc. • Physical – response of a material due to interaction with various forms of energy (i.e. magnetic, thermal, etc). • Mechanical – response of a material due to an applied force. Main focus for Machine Design.

3. Important Mechanical Properties:

4. Tension Test • Most important and common material test for generating mechanical properties. • Can be load vs displacement or load versus strain. Always convert load to stress. Example: stress-strain curves:

6. Modulus of Elasticity Stress-Strain Curve for Steel Yield Point, Sy Sy Tensile Strength, Su Elastic Limit Proportional Limit Stress, s Strain, e

7. Proportional Limit Elastic Limit Sy Tensile Strength, Su Yield Strength, Sy Parallel Lines Stress, s Offset strain, usually 0.2% Strain, e Stress Strain Curve for Aluminum

8. Ductility • The degree to which a material will deform before ultimate fracture. • Ductile materials indicate impending failure. (%E ≥ 5%) • Brittle materials don’t (%E < 5%) • For machine members subject to repeated loads or shock or impact, use %E ≥ 12%

9. Ductile materials - extensive plastic deformation and energy absorption (toughness) before fracture Brittle materials - little plastic deformation and low energy absorption before failure

10. Other properties determined from stress strain curve:

11. Shear Strength Estimates Yield strength in shear Ultimate strength in shear

12. Poisson’s Ratio RANGES 0.25 – 0.27 for Cast Iron 0.27 – 0.30 for Steel 0.30 – 0.33 for Aluminum and Titanium

13. Modulus of Rigidity in Shear • Measure of resistance to shear deformation. • Valid within the ELASTIC range of the material

14. Summary: Key Material Properties: Percent Elongation: Yield Strength (psi) = onset of permanent deformation: Lo = original gauge length Lf = final gauge length Tensile Strength (psi) = max stress or peak stress sustainable: • >5% = ductile • <5% = brittle Percent Reduction of Area : Modulus of Elasticity aka Young’s Modulus (psi) – slope of linear region: Ao = original cross-sectional area Af = final cross-sectional area σ2-σ1 = difference in tensile stress between points 1 and 2 ε2-ε1 = difference in tensile strain between points 1 and 2 Modulus of Resilience (psi) = area under stressstrain curve up to elastic limit or yield strength Poison's Ratio (unit less) = ratio of transverse to longitudinal strain: Modulus of Toughness (psi) = total area under stressstrain curve up from 0 to fracture. Related to impact Strength: Misc: fracture stress, proportional limit, elastic limit, elastic strain, impact strength, fracture toughness, etc……

15. Summary: Key Material Properties: Yield Strength in shear: Note: Ultimate Strength in shear: Ultimate strength in compression: Other important material properties specific to Polymers: Also secant strengths, secant modulus, compression set, stress creep, relaxation, etc..

16. Example: find yield strength, ultimate strength and modulus of elasticity:

17. Example: find yield strength and ultimate for material that does not exhibit knee behavior

18. Example – DATA generated on MTS machine:

19. EX: Su = ultimateStrength = 47,820 psi Sy = YieldStrength = 44,200 psi E = Young’s Modulus = (34,640 – 10,597)/(.0036 - .0011) = 9.6 E6 % Elongation = 11.5% .002 = .2% offset

20. EX: Modulus of Resilience = area under stress-strain curve up to elastic limit Elastic strain approx: .005 in/in

21. Modulus of Toughness = UT = area under stress-strain curve from 0 to fracture strain. Approx = 96.8 psi + (46,000)(.115 - .0043) = 5,190 psi

22. Hardness • Resistance of a material to be indented by an indenter. • BRINELL 3000 kg load 10 mm ball  of hole = BHN • ROCKWELL 100 kg load (B Scale) 1/16” Ball (B Scale) B-Scale for soft materials C-Scale for harder metals (Heat treated) (Use 150 kg load with diamond cone indenter) Hardness calculated directly by machine (depth of indentation)

23. Hardness Comparison Hardness values in the ranges HRB >100 and HRC < 20 are not recommended

24. Ultimate Tensile Strength • Highest level of stress a material can develop. • FOR CARBON STEEL ONLY: Su ≈ 500 * BHN (in PSI, BHN = Brinell Hardness Number)

25. Toughness • Toughness is the ability of a material to absorb energy without failure. • Parts subjected to impact or shock loads need to be tough. • Testing: Charpy and Izod tests • Impact energy determined from the testing is used to compare materials

26. Fatigue • Failure mode of parts experiencing thousands or millions of repeated loads. • Endurance Strength - a materials resistance to fatigue. Determined by testing.

27. Creep • Progressive elongation of a part over time. • Metals – usually requires a large load • usually requires high temperature (> .3Tm) • Plastic – creep occurs at low temperatures Polymers: Creep vs Stress Relaxation vs. Compression Set – related but measured differently!!

28. Mechanical Property Summary

29. Material Selection • “The materials selected for a design often will determine the fabrication processes that can be used to manufacture the product, its performance characteristics, and its recyclability and environmental impact. As a result, engineers should acquire a robust understanding of material characteristics and the criteria that one should use in making material selections.” - Voland, Engineering by Design, Addison-Wesley, 1999, pg. 400

30. Material Categories • Metals – iron, steel, aluminum, copper, magnesium, nickel, titanium, zinc • Polymers – thermoplastics & thermosets • Ceramics • Composites – Carbon fiber, Kevlar & fiberglass, wood and reinforced concrete

31. Steel • Widely used for machine elements • High strength • High stiffness • Durable • Relative ease of fabrication • Alloy of Iron, Carbon, Manganese & 1 or more other significant elements. (Sulfur, Phosphorus, Silicon, Nickel, Chromium, Molydbenum and Vanadium)

32. Carbon • Carbon has huge effect on strength, hardness and ductility of steel. Carbon Content  Strength & Hardness  Ductility ↓

33. All these curves are steels. What do they have in common? What is different?

34. Steel Designation Systems • AISI – American Iron & Steel Institute • SAE – Society of Automobile Engineers • ASTM – American Society for Testing Materials

35. General Designation • General Form AISI: AISI XXXX Carbon Content in Hundredths of a percent Specific alloy in the group Alloy group; indicates major alloying elements AISI 1020 AISI 4340

36. Examples: 2350 2550 4140 1060

37. Plain Carbon Steel • Low Carbon (less than 0.3% carbon) • Low strength, good formability • If wear is a potential problem, can be carburized (diffusion hardening) • Most stampings made from these steels • AISI 1008, 1010, 1015, 1018, 1020, 1022, 1025 • 2. Med Carbon (0.3% to 0.6%) • Have moderate to high strength with fairly good ductility • Can be used in most machine elements • AISI 1030, 1040, 1050, 1060* • High Carbon (0.6% to 0.95%) • Have high strength, lower elongation • Can be quench hardened • Used in applications where surface subject to abrasion – tools, knives, chisels, ag implements. • AISI 1080, 1095

38. Steel Conditions • Steel properties vary depending on the manufacturing process • Steel is often rolled or drawn through a die • Hot-rolled – rolled at elevated temperature • Cold-rolled – improved strength & surface finish • Cold-drawn – highest strength with good surface finish

39. Heat Treating • Process for modifying the properties of steel by heating • Processes used most for machine steels: • Annealing • Normalizing • Through-hardening (quench & temper) • Case hardening

40. All these curves are steels. What do they have in common? What is different?

41. Annealing • Full-Annealing: creates uniform composition of the material. • Soft, low-strength material • No significant internal stress RT = Room Temperature LC = Lower Critical Temperature UC = Upper Critical Temperature

42. Stress Relief Annealing • Stress Relief Annealing • Done after welding, machining or cold forming to relieve residual stresses minimizing distortions RT = Room Temperature LC = Lower Critical Temperature UC = Upper Critical Temperature

43. Normalizing • Similar to annealing but at a higher temperature (about 1600°F) • Higher strength • Machinability and toughness are improved over as-rolled state. RT = Room Temperature LC = Lower Critical Temperature UC = Upper Critical Temperature Austenite: A nonmagnetic solid solution of ferric carbide or carbon in iron, used in making corrosion-resistant steel

44. Through-hardening • Heated quickly forming austenite then quickly cooling in a quenching medium. • Martensite – hard form of steel is formed • Quenching mediums: water, brine and special mineral oils. • Quenched steel that isn’t tempered is brittle RT = Room Temperature LC = Lower Critical Temperature UC = Upper Critical Temperature

45. Tempering • Reheat steel to 400°F – 1300°F immediately after quenching and allowing it to cool slowly. • As tempering temperature increases, ultimate and yield strengths decrease and ductility increases • Machine parts should be tempered at 700 °F minimum after quenching. Quenching leaves the material brittle.

46. AISI 1040 WQT Higher Tempering temps. decreases strength but increases ductility WQT = water quenched & tempered Fig. A4-1, Appendix 4, pg. A-8

47. Case Hardening • Surface of a part is hardened but core remains soft & ductile – think m&m’s. • Usually .010 to .040 thick • Methods: • Flame hardening and induction hardening • Carburizing, nitriding, cyaniding, and carbo-nitriding

48. Stainless Steel • Corrosion resistant steel – 12 to 18% chromium content • Types • Austenitic – moderate strength, nonmagnetic, tempering: 1/4 hard, 1/2 hard, 3/4 hard and full hard. (200 and 300 series) • Ferritic – magnetic, good for use at high temps. Can’t be heat-treated. (400 series) • Martensitic – magnetic, can be heat-treated. Good toughness and stronger than 200 and 300 series. Wide range of uses: scissors, pump arts, airplanes, marine hardware, medical equipment.

49. Structural Steels High strength, low carbon alloy steel

50. Structural Plates and Bars