Mechanical Properties
Ceramic materials are more brittle than metals.
Why is this so?
• Consider mechanism of deformation
– In crystalline, by dislocation motion
– In highly ionic solids, dislocation motion is difficult
• few slip systems
• resistance to motion of ions of like charge (e.g., anions)
past one another
1
•
Flexural Tests – Measurement of Elastic
Modulus
Room T behavior is usually elastic, with brittle failure.
• 3-Point Bend Testing often used.
-- tensile tests are difficult for brittle materials.
cross section
d
b
rect.
F
L/2
L/2
Adapted from Fig. 12.32,
Callister & Rethwisch 8e.
R
d = midpoint
circ.
deflection
• Determine elastic modulus according to:
F
x
F
slope =
d
d
linear-elastic behavior
F L3
E
d 4bd 3
(rect. cross section)
F L3
(circ. cross section)
E
4
d 12R
2
•
Flexural Tests – Measurement of Flexural
Strength
3-point bend test to measure room-T flexural strength.
cross section
d
b
rect.
L/2
F
L/2
Adapted from Fig. 12.32,
Callister & Rethwisch 8e.
R
d = midpoint
circ.
deflection
location of max tension
• Typical values:
• Flexural strength:
sfs 
sfs 
3Ff L
2bd
2
Ff L
R
3
sfs (MPa) E(GPa)
Si nitride
250-1000 304
Si carbide
100-820 345
Al oxide
275-700 393
glass (soda-lime) 69
69
Material
(rect. cross section)
(circ. cross section)
Data from Table 12.5, Callister & Rethwisch 8e.
3
• Room-temperature mechanical behavior – flexural tests
-- linear-elastic; measurement of elastic modulus
-- brittle fracture; measurement of flexural modulus
4
Mechanical Properties of Polymers –
Stress-Strain Behavior
brittle polymer
plastic
elastomer
elastic moduli
– less than for metals
Adapted from Fig. 15.1,
Callister & Rethwisch 8e.
• Fracture strengths of polymers ~ 10% of those for metals
• Deformation strains for polymers > 1000%
– for most metals, deformation strains < 10%
5
5
Mechanisms of Deformation—Brittle Crosslinked
and Network Polymers
Initial
Near
Failure
s (MPa)
Initial
x brittle failure
Near
Failure
x plastic failure
aligned, crosslinked
polymer
e
network polymer
Stress-strain curves adapted from Fig. 15.1,
Callister & Rethwisch 8e.
9
9
Semicrystalline polymers
Mechanisms of Deformation — Semicrystalline
(Plastic) Polymers
s (MPa)
Stress-strain curves adapted
from Fig. 15.1, Callister &
Rethwisch 8e. Inset figures
along plastic response curve
adapted from Figs. 15.12 &
15.13, Callister & Rethwisch
8e. (15.12 & 15.13 are from
J.M. Schultz, Polymer
Materials Science, PrenticeHall, Inc., 1974, pp. 500-501.)
fibrillar
structure
x brittle failure
onset of
necking
plastic failure
near
failure
x
unload/reload
e
crystalline
block segments
separate
undeformed
structure
amorphous
regions
elongate
crystalline
regions align
11 11
Predeformation by Drawing
• Drawing…(ex: monofilament fishline)
-- stretches the polymer prior to use
-- aligns chains in the stretching direction
• Results of drawing:
-- increases the elastic modulus (E) in the
stretching direction
-- increases the tensile strength (TS) in the
stretching direction
Adapted from Fig. 15.13, Callister
-- decreases ductility (%EL)
& Rethwisch 8e. (Fig. 15.13 is
from J.M. Schultz, Polymer
• Annealing after drawing...
Materials Science, Prentice-Hall,
Inc., 1974, pp. 500-501.)
-- decreases chain alignment
-- reverses effects of drawing (reduces E and
TS, enhances %EL)
• Contrast to effects of cold working in metals!
12 12
Mechanisms of Deformation—Elastomers
s(MPa)
x brittle failure
x
plastic failure
elastomer
x
e
initial: amorphous chains are
kinked, cross-linked.
final: chains
are straighter,
still
cross-linked
Stress-strain curves
adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Inset figures along
elastomer curve (green)
adapted from Fig. 15.15,
Callister & Rethwisch 8e.
(Fig. 15.15 is from Z.D.
Jastrzebski, The Nature
and Properties of
Engineering Materials,
3rd ed., John Wiley and
Sons, 1987.)
deformation
is reversible (elastic)!
• Compare elastic behavior of elastomers with the:
-- brittle behavior (of aligned, crosslinked & network polymers), and
-- plastic behavior (of semicrystalline polymers)
(as shown on previous slides)
13 13
Why are fibers stronger than the bulk material?
How does molecular weight affect tensile strength?
How is the affect of % crystallization on tensile
strength?
What are the effects of temperature and strain rate
on tensile strength?
7. (12 points) Compare the following
for amorphous rubber, Fe2O3, and Cu.
i)
Density
Rubber is composed of light atoms (C,
H, N, and O)
Fe2O3 is a mixture of light(O) and
heavy (Fe) atoms.
Cu is composed of a heavy atom.
Density; Cu> Fe2O3 > Rubber
ii)
Dislocation density
Since dislocation density is present
when there are crystallographic
planes,
dislocation
density
of
AMORPHOUS rubber is equal to zero.
Dislocation density of Cu is higher
than that of Fe2O3 since the formation
of dislocations is more difficult in
ceramic material due to strong
electrostatic interactions.
Dislocation density; Cu> Fe2O3 >
Rubber=0
iii)
Yielding strength
Plastic deformation requires higher
Rubber=0
iii)
Yielding strength
Plastic deformation requires higher
stress for Fe2O3 due to STRONG
electrostatic interactions.
Yielding Strength; Fe2O3 > Cu >
Rubber
iv)
Ductility
The origin of ductility is plastic
deformation which is present in
crystalline materials. Slip planes are
more accesible in metal compared to
ceramic materials.
v)
Elastic strain when 15
MPa is applied
Young Modulus; polymers < Metals <
ceramic material since ionic bond is
the stronger than metallic bond and
Van der Waals bond. So when 15 MPa
is applied the highest strain is
observed for rubber.
Rubber > Cu > Fe2O3.
vi)
Young’s Modulus
Rubber<Cu< Fe2O3