Transcript ME260 Mechanical Engineering Design II

ME260
Mechanical Engineering Design II
Instructor notes
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
Mechanical Properties of Materials
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
Area =
Ao
F
(left) Before
deformation,
and (right)
after
deformation
lo
F
Area = A
F
l
F
PLASTIC (PERMANENT) DEFORMATION
• Simple tension test:
Stress
Loading/
Unloading
Unloading
Loading
Permanent
Strain
Strain
YOUNG’S MODULI: COMPARISON
Metals
Alloys
1200
1000
800
600
400
E(GPa)
200
100
80
60
40
109 Pa
Graphite
Composites
Ceramics Polymers
/fibers
Semicond
Diamond
Tungsten
Molybdenum
Steel, Ni
Tantalum
Platinum
Cu alloys
Zinc, Ti
Silver, Gold
Aluminum
Magnesium,
Tin
Si carbide
Al oxide
Si nitride
Carbon fibers only
CFRE(|| fibers)*
<111>
Si crystal
Aramid fibers only
<100>
AFRE(|| fibers)*
Glass-soda
Glass fibers only
GFRE(|| fibers)*
Concrete
GFRE*
20
10
8
6
4
2
1
0.8
0.6
0.4
0.2
CFRE*
GFRE( fibers)*
Graphite
Polyester
PET
PS
PC
CFRE( fibers)*
AFRE( fibers)*
Epoxy only
PP
HDPE
PTFE
LDPE
Wood(
grain)
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.
YIELD STRENGTH: COMPARISON
sy(ceramics)
>>sy(metals)
>> sy(polymers)
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
TENSILE STRENGTH: COMPARISON
TS(ceram)
~TS(met)
~ TS(comp)
>> TS(poly)
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.
DUCTILITY, %EL
• strain at failure: % EL  l f  lo 100
lo
Adapted from Fig. 6.13,
Callister 6e.
Ao  A f
• Another ductility measure: %AR 
x100
Ao
• Aluminum/structural steels are examples of
ductile materials whereas glass/ceramics are not
Effect of Temperature on the
Stress-Strain Diagram
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)
Engineering
tensile
stress, s
smaller toughness (ceramics)
larger toughness
(metals, PMCs)
smaller toughnessunreinforced
polymers
Engineering tensile strain, 
TOUGHNESS
Toughness can be measured with an impact test
(Izod or Charpy)
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. 6.18 is adapted from G.F. Kinney, Engineering Properties
and Applications of Plastics, p. 202, John Wiley and Sons, 1957.)
CREEP
• Occurs at elevated temperatures, normally T > 0.4 Tmelt
• Deformation changes with time
• Examples: turbine engine blades
Deformation with time
FATIGUE
• Fatigue = failure under cyclic stress.
Ship-cyclic loading
from waves.
• Stress varies with time.
Hip implant-cyclic
loading from walking.
Adapted from Fig. 8.0, Callister 6e.
(Fig. 8.0 is by Neil Boenzi, The New
York Times.)
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.
DUCTILE VS BRITTLE FAILURE
• Classification:
Adapted from Fig. 8.1,
Callister 6e.
• Ductile
fracture is
desirable
for structural
applications!
Ductile:
warning before
fracture,
i.e. changes geometry
before failure
Brittle:
No
warning
MECHANICAL PROPERTIES
The following associations normally
apply:
Strong
Hard
Brittle (not Tough)
Ductile (Tough)
Soft(er
)
Weak(er) (not
strong)
Material Types
Ferrous metals/alloys
Iron-based materials, e.g. steels
Nonferrous metals/alloys
e.g. nickel, silver, etc.
Plastics and rubber are prime examples
Polymers
Ceramics Compounds of metallic & nonmetallic elements,
e.g. chinaware
Glass Solid with a random atomic structure like a fluid
Both made from pure carbon
Diamond, Graphite
but have different atomic structure
Natural and organic material
Wood
A combination of two/more material types
Composites
Atomic Structure of Materials
Ferrous metals/alloys
Nonferrous metals/alloys
Ceramics
(metallic & non-metallic
elements)
POLYMER MICROSTRUCTURE
• Polymer = many mers
Adapted from Fig. 14.2, Callister 6e.
• Hydro-carbon based
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e.
2
10 for plain carbon steels, and 40 for 0.4 wt% C
STEELS
High Strength,
Low Alloy
Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e.
NONFERROUS ALLOYS
• Cu Alloys
• Al Alloys
Brass: Zn is subst. impurity -lower : 2.7g/cm3
(costume jewelry, coins,
-Cu, Mg, Si, Mn, Zn additions
corrosion resistant)
-solid sol. or precip.
Bronze: Sn, Al, Si, Ni are
strengthened (struct.
subst. impurity
aircraft parts
(bushings, landing
& packaging)
gear)
• Mg Alloys
NonFerrous
Cu-Be:
-very low : 1.7g/cm3
Alloys
precip. hardened
-ignites easily
for strength
-aircraft, missles
• Ti Alloys
-lower : 4.5g/cm3
• Refractory metals
-high melting T
vs 7.9 for steel
• Noble metals -Nb, Mo, W, Ta
-reactive at high T -Ag, Au, Pt
-oxid./corr. resistant
-space applic.
Based on discussion and data provided in Section 11.3, Callister 6e.
Material
Selection
Strength
(MPa)
Density (Mg/m3)
Applications of Material Use
Ferrous metals/alloys
Iron-based materials, e.g. steels
Applications of Material Use
Nonferrous metals/alloys
e.g. nickel, silver, etc.
Applications of Material Use
Polymers
Plastics and rubber are prime examples
PVC pipes
Bakelite (plastic)
electric outlet cover
Rubbermaid containers
Applications of Material Use
Ceramics
Compounds of metallic & nonmetallic elements,
e.g. chinaware
oil drill bits
blades
Applications of Material Use
Composites
A combination of two/more material types
Trailer
Blast resistant panel
Fiberglass tube