1. Chapter 2 – Materials in Mechanical Design
MET 210W
Chapter 2 – Materials in Mechanical Design
Discussion of:
material mechanical properties
manufacturing processes
material characteristics
heat treating
materials

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 Example: stress-strain curves:
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:

5. Figure: 03-04

6. Stress-Strain Curve for Steel
Yield Point, Sy Sy
Tensile Strength, Su
Elastic Limit
Proportional Limit
Stress, s
Modulus of Elasticity
Modulus of elasticity – stiffness – describes a materials resistance to deforming
Yield Strength – point at which material permanently elongates or yields. Associated with large increase in strain with little or no stress increase
Proportional Limit – point at which stress and strain are no longer proportional. Hard to find. Materials in mechanical design are rarely used above the proportional limit
Elastic Limit – point at which the material still returns to original size if unloaded. Hard to find so it is rarely reported.
Modulus of Elasticity – stiffness or resistance to deformation
Strain, e

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

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%
A material’s ductility describes its tendency to exhibit permanent deformation or plastic behavior under applied forces before breading. Such deformation can be important as a warning signal that the material is about to break. Conversely, a brittle material such as glass will shatter without providing any such warning signal.

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:
Figure: 03-16a

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

12. 0.30 – 0.33 for Aluminum and Titanium
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
For most metals, G is about 3/8E.

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 stress strain 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 stress strain 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:
Figure: 03-06

17. Example: find yield strength and ultimate for material that does not exhibit knee behavior
Figure: 03-07

18. Example – DATA generated on MTS machine:

19. EX: Su = ultimate Strength = 47,820 psi
Sy = Yield Strength = 44,200 psi
E = Young’s Modulus = (34,640 – 10,597)/( ) = 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 = psi + (46,000)( ) = 5,190 psi

22. Hardness Resistance of a material to be indented by an indenter.
BRINELL 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)
Term used to denote a material’s ability to resist abrasion or scratching. Several standardized tests have been developed to measure this resistance. These include Brinell, Vickers, Rockwell, and Moh.

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
A material’s toughness is related to its ductility and yield stress. It describes the material’s ability to withstand sudden impact by absorbing the applied energy without fracturing.
Toughness can be measured several ways. The area under the stress-strain curve for a given material corresponds to the work or energy per unit volume needed to stretch a material to its breaking point. Materials with high yield stress values remain elastic even when stretched over large distances, eventually becoming permanently deformed and finally breaking. Therefore, such materials also exhibit high values of toughness
Toughness can be measured via the Charpy and Izod tests in which sudden impacts are applied to a test piece until it fractures; the amount of applied energy necessary to break the material corresponds to its toughness. Since ductile materials deform plastically before finally breaking under an applied force, they also exhibit high values of toughness.
Fracture toughness denotes a material’s ability to resist crack propagation, given that a crack is already present in the material.

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
Interpretation
Common or Related Measure
Strength
Ability to resist breaking
Yield stress
Stiffness
Ability to resist deformation
Modulus of elasticity
Ductility
Permanent deformation before breaking
%Elongation
Toughness
Ability to withstand impact or resist breaking
Energy or work necessary to fracture material
Hardness
Ability to resist abrasion/scratching
Scores on hardness tests
Creep
Gradual, continuing deformation under an applied constant stress
Creep strength
Table from: Voland, Engineering by Design, Addison-Wesley, 1999, Page 405

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
Metals: iron, steel, aluminum, copper, magnesium, nickel, titanium, zinc
iron: wrought iron (0 to .05% carbon) is soft, ductile and corrosion-resisting alloy
steel (0.05 to 2% CARBON)
cast iron (2 to 4.5% carbon) hard, brittle alloy, malleable, durable and strong
Alum – naturally soft and relatively weak. Alloyed to increase strength and hardness corrosion resistant and lightweight
Copper – high electrical and thermal conductivity, ductile and corrosion resistant
Copper & zinc make BRASS
Copper & tin make BRONZE
Magnesium – fairly expensive, corrodes w/o surface protection, tough and durable. Formable and has low weigh
Nickel = base metal for many non-ferrous corrosion resistant alloys (nickel-copper, nickel-chromium-iron)
Titanium – a corrosion resistant material which strength to weight ratio. Ductile and tough, difficult to fabricate, expensive
Zinc – used to galvanize steel,

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?
Figure: 03-17

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 AISI 1020 AISI 4340
Carbon Content in Hundredths of a percent
Specific alloy in the group
Alloy group; indicates major alloying elements
1020 –
1 = carbon steel
0 = no other major alloying material except carbon
20 = .20% carbon
4340
4 = molybdenum alloy steel
3 = nickel and chromium added in specified concentrations
40 = .40% carbon
AISI 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
*Note, some texts including CES state med carbon as .3% to .5%

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?
Figure: 03-13

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
Done after welding, machining or cold forming to relieve residual stresses minimizing distortions
Steel is heated to 1000F to 1200F until a uniform temperature
Then slow cooled in air to room temperature
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.
Similar to annealing except at a higher temperature (abt. 1600F) above transformation range where austenite is formed.
Results in uniform structure and higher strength than from annealing
Machinability and toughness are improved.
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
Through Hardening – quenching and tempering:
Heat steel above transformation range where austenite forms, then RAPIDLY cool in a quenching medium
Rapid cooling causes MARTENSITE, which is a hard strong form of steel. The alloy composition of the material dictates how much martensite forms.
Alloy w/ >= 80% martensite has “high hardenability” which is important when selecting steel requiring high strength and hardness.
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 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
Flame hardening and induction hardening:
rapidly heat surface for a limited time so that a small, controlled depth of the material reaches the transformation range
The part is quenched and that portion which reached the transformation range produces martensite which provides the required hardness
Induction heating – electric current
Goal of case hardening: Rockwell C 55 to 60, Brinell 550 to 650
Carbon and alloy steels with fewer than 30 points of carbon cannot be hardened to that level. Generally, alloy steels with >= 40 points of carbon are flame or induction hardened.
Carboriziing, nitriding, cyaniding and carbonitriding:
Methods alter the composition of the surface of the material by exposing it to carbon-bearing gasses, liquids or solids at high temperatures that produce carbon and diffuse it into the surface of the part.
Nitriding and cyaniding results in very hard, thin cases:
Carburizing – exposure to carbon atmosphere at about 1700F for 8 hours. Immediate quenching achieves highest strength but the case is brittle. Normal to cool slowly, then reheat to about 1500F and then quenched. Temperaing at F follows to relieve stress
Target: HRC 55 to 64, BHN 550 to 700

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.
10% chromium is minimum for this classification
AISI 200, 300 and 400 series are stainless steel
Three main groups:
Austenitic SS falls into AISI 200 and 300 series, General purpose grades with moderate strength. Most are NOT heat treatable. Non-magnetic and generally used in food processing equipment.
Ferritic SS falls into AISI 400 series, it is magnetic, performs well at high temperatures (1300 to 1900F), isn’t heat treatable, can be cold worked, applications include heat exchanger tubes, petroleum refining equipment, automotive trim, furnace parts, chemical equipment.
Martensitic SS also falls into the AISI 400 series. Its is magnetic, CAN be heat treated, have higher strength than 200 and 300 series, good toughness, applications include turbine engine parts, cutlery, scissors, pump parts, valve parts, surgical instruments, aircraft fittings, and marine hardware.

49. Structural Steels High strength, low carbon alloy steel
Preferred material specification for W-shapes is ASTM A992 (Fy = 50 ksi and Fu = 65 ksi). The availability of W-shapes in grades other than ASTM A992 should be confirmed prior to their specification. W-shapes with higher yield and tensile strength can be obtained by specifying ASTM A572 grade 60 or 65 or ASTM A913 grades 60, 65 or 70. W-shapes with atmospheric corrosion resistance (weathering) characteristics can be obtained by specifying ASTM A588
grade 50 or ASTM A242 grade 42, 46, or 50. Other material specifications applicable to W-shapes include ASTM A36; ASTM A529 grade 50 and 55, ASTM A572 grade 42 and 50, and ASTM A913 grade 50.
The preferred material specification for rectangular HSS is ASTM A500 grade B (Fy = 46 ksi; Fu = 58 ksi), although ASTM A500 grade C (Fy = 50 ksi; Fu = 62 ksi) is increasingly very common. The availability of rectangular HSS in grades other than ASTM A500 grade B should be confirmed prior to their specification.
Rectangular HSS with atmospheric corrosion resistance (weathering) characteristics can be obtained by specifying ASTM A847. Other material specifications applicable to rectangular HSS include ASTM A501 and ASTM A618.
High strength, low carbon alloy steel

50. Structural Plates and Bars
b = bars over 1 inch

51. Gray Iron Brittle material, Su from 20 to 60 ksi
Compressive stress  5X Su
Excellent wear resistance
Easy to machine
Good vibration dampening ability
Classes: 20, 25, 30, 40, 50, 60
Minimum Su

52. Ductile Iron GRADE 80-55-06 Higher strength than gray iron
More ductile
Grade designation:
GRADE
Tensile strength in ksi
% elongation in a 2” gage length
Yield strength in ksi

53. Malleable Iron GRADE 40010 Heat treatable cast iron
Moderate to high strength
High modulus of elasticity
Good machineability
Good wear resistance
Grade designation:
GRADE 40010
Yield strength
% elongation

54. Powdered Metals
Metal powders are placed into a die and compacted under high pressure.
Sintering at high temperatures fuses the powder into a uniform mass.
Usually brittle – not good for impact
Sintered bearings – porous and can be saturated with lubricant
Sintering is a method for making objects from powder, by heating the material (below its melting point) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.

55. Aluminum
Lightweight material, good corrosion resistance, relative ease of forming & machining.
Good appearance.
Generally tempered
O = annealed
H = strain-hardened
T = heat treated
6061-T6
Aluminum Association designation system: xxxx. First number is alloy type by major alloy element see Table 2-6 page 57. Second number is modifications to alloy. Last two numbers are specific other alloy additives
1xxx: % or greater aluminum content
2xxx copper
3xxx manganese
4xxx silicon
5xxx magnesium
6xxx magnesium and silicon
7xxx zinc
Specification must include a temper reference:
F – as fabricated – no special control of properties – limits unknown – test part thoroughly before using
O – annealed – thermal treatment provides softest and lowest strength condition
H – strain hardening – process of cold-working under controlled conditions that produces improved, predictable properties for alloys in 1xxx, 3xxx and 5xxx groups. MORE cold working, strength and hardness increases while ductility decreases
T – heat treated – heating and cooling process for 2xxx, 4xxx, 6xxx and 7xxx groups
Alloy 6061 is one of the most versatile alloys. Its available in virtually all forms, has good strength, is corrosion resistant, is heat treatable to obtain various properties, in softer forms, its easily formed and worked, good weldability but generally low machinability
Strain-hardening: controlled cold working of the alloy – increases hardness and strength, reduces ductility.

57. Titanium Good corrosion resistance High strength to weight ratio
Modulus of Elasticity  16 x 106 psi
Specific weight = .160 #/in3
Strength 25 to 75 ksi
High cost
Difficult to machine
E is somewhere between aluminum and steel. Very strong and very light.
Pretty expensive material
Not easy to maching
Designation:
Ti-50A
Yield strength expected in ksi

58. Plastics
Thermoplastic – can be repeatedly formed by heating or molding – properties not changed. CAN BE RECYLCED!
Nylon
ABS
Polycarbonate
Acrylic
Commodity plastics: Polypropylene (P), Polyethylene (PE), Polyvinyl Chloride (PVC), Polystyrene (PS)
Thermoset – undergoes a chemical change during forming. It can’t be reshaped. CAN NOT BE RECYCLED!
Phenolic
Polyester
Epoxy
Nylon has good strength, wear resistance, toughness; wide range of possible properties depending on fillers and formulations. Used for structural parts, mechanical devices such as gears and bearings, and parts needing wear resistance
ABS good impact resistance, rigidity, moderate strength. Used for housings, helmets, cases, appliance parts, pipes and pipe fittings
Polycarbonate – excellent toughness, impact resistance and dimensional stability. Used for cams, gears, housings, electrical connectors, food processing products, helmets, and pump and meter parts.
Acrylic – good weather resistance and impact resistance; can be made with excellent transparency or translucent or opaque with color. Used for glazing, lenses, signs and housings
PVC – good strength, weather resistance and rigidity. Used for pipe, electrical conduit, small housings, ductwork, and moldings.
Thermosets:
Phenolic – high rigidity, good moldability and dimensional stability, very good electrical properties. Used for load-carrying parts in electrical equipment, switchgear, terminal strips, small housings, handles for appliances, and cooking utensils, gears, and structural and mechanical parts.
Polyester – known as fiberglass when reinforced with glass fibers; high strength and stiffness. Good weather resistance. Used for housings, structural shapes and panels.

59. Ceramics
Formed by applying high temperatures to inorganic, nonmetallic, and generally inexpensive material, especially clay.
Strong, nonconductive and weather resistant.
Brittle

60. Composites
Two or more materials acting together to provide material properties that can be tailored to specific conditions.
Often glass or carbon fibers bonded together with a matrix material – epoxy, polyester, others.
2 or more different materials acting together to produce properties that are different from and generally superior to the individual components.
Low weight application
Suggestions
align fibers with load
avoid shear loading if possible
integrate components
use light core – place strength of surfaces
avoid high temperatures
must design with concurrent engineering – must consider manufacturing during design

61. Material Selection
A good material is one that works in the given application cheaply.
If wt & size not important  use cheap matl
Size no problem, wt is  use hollow matl
Wt & size important  use $$$ material