1. ME260 Mechanical Engineering Design II
Instructor notes

2. Mechanical Properties of Materials
Force is not an objective measure of loading
Stress = s = Force/Area (F/Ao) is
Why? To answer this answer first: If a force of 1 lb is applied to a rubber band and a force of 100 lb is applied to another, which rubber band will break first? Answer: depends on their cross-sectional area, i.e. the stress that they are subjected to F
Area = A l F
Area = Ao lo
(left) Before deformation, and (right) after deformation

3. Mechanical Properties of Materials
Deformation is simply change in dimensions or geometry/shape of a material under loading
The change in length, Dl =l – lo is not an objective measure of deformation. This is positive change if material is loaded in tension and negative change if loaded in compression.
Strain (the relative change in length) = e (or e) = Dl / lo is. Strain sometimes is expressed as a percentage, i.e. as 100×Dl / lo.
If a rubber band is extended by 1 cm and another by 1 m, which one will break first? Answer: depends on how much they stretched (Dl) relative/compared to their original length (lo ), i.e. depends on how much they strained. F
Area = A l F
Area = Ao lo
(left) Before deformation, and (right) after deformation

4. Mechanical Properties of Materials
Ultimate Tensile Strength(UTS)
Or simply Tensile Strength (TS)
Yield stress,
sometimes sy
Slope is Young’s Modulus, E,
indicates stiffness

5. Mechanical Properties of Materials
Compression is the opposite of tension, i.e. you put pressure on the surface or push on it as opposed to pull on it
The stress-strain curve/diagram of a material subjected to compression usually looks similar to a tension test F
Area = Ao lo F
Area = A l
(left) Before deformation, and (right) after deformation

6. PLASTIC (PERMANENT) DEFORMATION
• Simple tension test:
Stress
Loading/ Unloading
Loading
Unloading
Strain
Permanent
Strain

7. YOUNG’S MODULI: COMPARISON
Graphite
Ceramics
Semicond
Metals
Alloys
Composites
/fibers
Polymers
E(GPa)
Based on data in Table B2,
Callister 6e.
Composite data based on
reinforced epoxy with 60 vol%
of aligned
carbon (CFRE),
aramid (AFRE), or
glass (GFRE)
fibers.

8. YIELD STRENGTH: COMPARISON
Room T values
Based on data in Table B4,
Callister 6e.
a = annealed
hr = hot rolled
ag = aged
cd = cold drawn
cw = cold worked
qt = quenched & tempered

9. TENSILE STRENGTH: COMPARISON
Room T values
Based on data in Table B4,
Callister 6e.
a = annealed
hr = hot rolled
ag = aged
cd = cold drawn
cw = cold worked
qt = quenched & tempered
AFRE, GFRE, & CFRE =
aramid, glass, & carbon
fiber-reinforced epoxy
composites, with 60 vol%
fibers.

10. DUCTILITY, %EL • strain at failure: • Another ductility measure:
Adapted from Fig. 6.13, Callister 6e.
• Another ductility measure:
• Aluminum/structural steels are examples of ductile materials whereas glass/ceramics are not

11. Effect of Temperature on the Stress-Strain Diagram

12. TOUGHNESS • Energy to break a unit volume of material
• Approximate by the area under the stress-strain
curve.
Toughness can be measured with an impact test (Izod or Charpy)

13. TOUGHNESS
Toughness can be measured with an impact test (Izod or Charpy)

14. HARDNESS • Resistance to permanently indenting the surface.
• Large hardness means:
--resistance to plastic deformation or cracking in
compression.
--better wear properties.
Adapted from Fig. 6.18, Callister 6e. (Fig is adapted from G.F. Kinney, Engineering Properties
and Applications of Plastics, p. 202, John Wiley and Sons, 1957.)

15. CREEP • Occurs at elevated temperatures, normally T > 0.4 Tmelt
• Deformation changes with time
• Examples: turbine engine blades
Deformation with time

16. FATIGUE Danger! • Fatigue = failure under cyclic stress.
Ship-cyclic loading
from waves.
Adapted from Fig. 8.0, Callister 6e. (Fig. 8.0 is by Neil Boenzi, The New York Times.)
• Stress varies with time.
Hip implant-cyclic
loading from walking.
Adapted from Fig.
17.19(b), Callister 6e.
• Key points: Fatigue...
--can cause part failure, even though s < sy.
--causes ~ 90% of mechanical engineering failures.
Danger!

17. DUCTILE VS BRITTLE FAILURE
• Classification:
Adapted from Fig. 8.1, Callister 6e.
• Ductile
fracture is
desirable
for structural applications!
Brittle: No warning
Ductile:
warning before fracture,
i.e. changes geometry before failure

18. Material Types Ferrous metals/alloys Nonferrous metals/alloys Polymers
Ceramics
Glass
Diamond, Graphite
Wood
Composites
Iron-based materials, e.g. steels
e.g. nickel, silver, etc.
Plastics and rubber are prime examples
Compounds of metallic & nonmetallic elements, e.g. chinaware
Solid with a random atomic structure like a fluid
Both made from pure carbon but have different atomic structure
Natural and organic material
A combination of two/more material types

19. Atomic Structure of Materials
Ferrous metals/alloys
Nonferrous metals/alloys
Ceramics

20. POLYMER MICROSTRUCTURE
• Polymer = many mers
Adapted from Fig. 14.2, Callister 6e.
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e. 2

21. STEELS 10 for plain carbon steels, and 40 for 0.4 wt% C High Strength,
Low Alloy
Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e.

22. NONFERROUS ALLOYS
Based on discussion and data provided in Section 11.3, Callister 6e.

23. Material Selection
Strength
(MPa)
Density (Mg/m3)

24. Applications of Material Use
Ferrous metals/alloys
Iron-based materials, e.g. steels

25. Applications of Material Use
Nonferrous metals/alloys
e.g. nickel, silver, etc.

26. Applications of Material Use
Polymers
Plastics and rubber are prime examples
PVC pipes
Rubbermaid containers
Bakelite (plastic)
electric outlet cover

27. Applications of Material Use
Ceramics
Compounds of metallic & nonmetallic elements, e.g. chinaware
oil drill bits
blades

28. Applications of Material Use
Composites
A combination of two/more material types
Trailer
Blast resistant panel
Fiberglass tube