Chapter 16: Composites - Armstrongengineering.armstrong.edu/cameron/composites.pdf1 ISSUES TO...
Transcript of Chapter 16: Composites - Armstrongengineering.armstrong.edu/cameron/composites.pdf1 ISSUES TO...
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ISSUES TO ADDRESS...
• What are the classes and types of composites?
• What are the advantages of using composite
materials?
• How do we predict the stiffness and strength of the
various types of composites?
Chapter 16: Composites
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Composite • Combination of two or more individual materials
• Design goal: obtain a more desirable combination
of properties (principle of combined action)
– e.g., low density and high strength
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• Composite: -- Multiphase material that is artificially
made.
• Phase types: -- Matrix - is continuous
-- Dispersed - is discontinuous and
surrounded by matrix
Terminology/Classification
Adapted from Fig. 16.1(a),
Callister & Rethwisch 8e.
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• Dispersed phase: -- Purpose:
MMC: increase sy, TS, creep resist.
CMC: increase KIc
PMC: increase E, sy, TS, creep resist.
-- Types: particle, fiber, structural
• Matrix phase: -- Purposes are to: - transfer stress to dispersed phase
- protect dispersed phase from
environment
-- Types: MMC, CMC, PMC
metal ceramic polymer
Terminology/Classification
Reprinted with permission from
D. Hull and T.W. Clyne, An
Introduction to Composite Materials,
2nd ed., Cambridge University Press,
New York, 1996, Fig. 3.6, p. 47.
woven fibers
cross section view
0.5 mm
0.5 mm
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Classification of Composites
Adapted from Fig. 16.2,
Callister & Rethwisch 8e.
Small dispersed particles
impede motion of
dislocations
Stiffer particulate bears
fraction of load, restrain
matrix movement in vicinity
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• For which particle reinforced composite would
bond strength be more important?
• What geometric/spatial properties of particles in
dispersed phase would influence performance?
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Classification: Particle-Reinforced (i)
• Examples: Adapted from Fig.
10.19, Callister &
Rethwisch 8e. (Fig.
10.19 is copyright
United States Steel
Corporation, 1971.)
- Spheroidite
steel
matrix: ferrite (a)
(ductile)
particles: cementite ( Fe
3 C )
(brittle) 60 mm
Adapted from Fig.
16.4, Callister &
Rethwisch 8e. (Fig.
16.4 is courtesy
Carboloy Systems,
Department, General
Electric Company.)
- WC/Co
cemented
carbide
matrix: cobalt (ductile,
tough)
particles: WC (brittle, hard) :
600 mm
Adapted from Fig.
16.5, Callister &
Rethwisch 8e. (Fig.
16.5 is courtesy
Goodyear Tire and
Rubber Company.)
- Automobile
tire rubber
matrix: rubber (compliant)
particles: carbon
black
(stiff) 0.75 mm
Particle-reinforced Fiber-reinforced Structural
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Classification: Particle-Reinforced (ii)
Concrete – gravel + sand + cement + water
- Why sand and gravel? Sand fills voids between gravel particles
Reinforced concrete – Reinforce with steel rebar or remesh
- increases strength - even if cement matrix is cracked
Prestressed concrete - Rebar/remesh placed under tension during setting of concrete
- Release of tension after setting places concrete in a state of compression
- To fracture concrete, applied tensile stress must exceed this
compressive stress
Particle-reinforced Fiber-reinforced Structural
threaded
rod
nut
Posttensioning – tighten nuts to place concrete under compression
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• Elastic modulus, Ec, of composites: -- two “rule of mixture” extremes:
• Application to other properties: -- Electrical conductivity, se: Replace E’s in equations with se’s.
-- Thermal conductivity, k: Replace E’s in equations with k’s.
Adapted from Fig. 16.3,
Callister & Rethwisch 8e.
(Fig. 16.3 is from R.H.
Krock, ASTM Proc, Vol.
63, 1963.)
Classification: Particle-Reinforced (iii)
lower limit:
1
E c
= V m
E m
+ V p
E p
upper limit: c m m E = V E + V p E p
Particle-reinforced Fiber-reinforced Structural
Data:
Cu matrix
w/tungsten
particles
0 20 4 0 6 0 8 0 10 0
150
20 0
250
30 0
350
vol% tungsten
E(GPa)
(Cu) ( W)
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Classification: Fiber-Reinforced (i)
• Fibers very strong in tension
– Provide significant strength improvement to the
composite
– Ex: fiber-glass - continuous glass filaments in a
polymer matrix
• Glass fibers
– strength and stiffness
• Polymer matrix
– holds fibers in place
– protects fiber surfaces
– transfers load to fibers
Particle-reinforced Fiber-reinforced Structural
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Classification: Fiber-Reinforced (ii)
• Fiber Types – Whiskers - thin single crystals - large length to diameter ratios
• graphite, silicon nitride, silicon carbide
• high crystal perfection – extremely strong, strongest known
• very expensive and difficult to disperse
Particle-reinforced Fiber-reinforced Structural
– Fibers
• polycrystalline or amorphous
• generally polymers or ceramics
• Ex: alumina, aramid, E-glass, boron, UHMWPE
– Wires
• metals – steel, molybdenum, tungsten
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Fiber Alignment
aligned
continuous
aligned random
discontinuous
Adapted from Fig. 16.8,
Callister & Rethwisch 8e.
Transverse
direction
Longitudinal
direction
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• Critical fiber length for effective stiffening & strengthening:
• Ex: For fiberglass, common fiber length > 15 mm needed
Classification: Fiber-Reinforced (v)
Particle-reinforced Fiber-reinforced Structural
c
f d
s
2length fiber
fiber diameter
shear strength of
fiber-matrix interface
fiber ultimate tensile strength
• For longer fibers, stress transference from matrix is more efficient Short, thick fibers:
c
f d
s
2length fiber
Long, thin fibers:
Low fiber efficiency
c
f d
s
2length fiber
High fiber efficiency
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16
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Composite Stiffness:
Longitudinal Loading
Continuous fibers - Estimate fiber-reinforced composite modulus of
elasticity for continuous fibers
• Longitudinal deformation
sc = smVm + sfVf and c = m = f
volume fraction isostrain
Ecl = EmVm + Ef Vf Ecl = longitudinal modulus
c = composite
f = fiber m = matrix
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Composite Stiffness:
Transverse Loading
• In transverse loading the fibers carry less of the load
c= mVm + fVf and sc = sm = sf = s
f
f
m
m
ct E
V
E
V
E
1
Ect = transverse modulus
c = composite
f = fiber m = matrix
isostress
Ect EmEf
VmEf VfEm
Example 16.1
• A continuous and aligned glass fiber-reinforced
composite consists of 40 % vol of glass fibers having an
elastic modulus of 69 Gpa and 60 % vol of a polyester
resin that, when hardened, displays a modulus of 3.4 Gpa.
a. Compute the elastic modulus of the composite in the longitudinal
direction
b. If the cross-sectional area is 250 mm2, and a stress of 50 Mpa is
applied in this longitudinal direction, compute the magnitude of
the load carried by each of the fiber and matrix phases.
c. Determine the strain that is sustained by each phase when the
stress in part (b) is applied.
?
?
?
?
50
250
?
4.3
69
6.0
4.0
composite reinforced-fiber glass aligned and continuous -
:statement Problem
2
f
m
m
f
allongitudin
c
cl
m
f
m
f
F
F
Mpa
mmA
E
GpaE
GpaE
V
V
s
kNF
kNF
FF
GpaE
AFFF
VE
VE
F
F
VEVEE
m
f
mf
cl
cmfc
mm
ff
m
f
mmffcl
860.0
640.11
: and in equations two theUsing
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:Solution
:Theory
s
3
3
1069.1
1069.1
:Solution
//
//
:Theory
m
f
m
mmm
m
mm
m
mm
f
cff
f
ff
f
f
f
mfc
E
AVF
E
AF
E
E
AVF
E
AF
E
s
s
Example 16.2
• Compute the elastic modulus of the composite
material in example 16.1, but assume that the
stress is applied perpendicular to the direction of
fiber alignment.
?
4.3
69
6.0
4.0
composite reinforced-fiber glass aligned and continuous -
:statement Problem
ct
m
f
m
f
E
GpaE
GpaE
V
V
GpaE
EVEV
EEE
ct
mffm
fm
ct
5.5
:Solution
:Theory
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• Estimate of Ecd for discontinuous fibers:
-- valid when fiber length <
-- Elastic modulus in fiber direction:
efficiency factor: -- aligned: K = 1 (aligned parallel)
-- aligned: K = 0 (aligned perpendicular)
-- random 2D: K = 3/8 (2D isotropy)
-- random 3D: K = 1/5 (3D isotropy)
Values from Table 16.3, Callister &
Rethwisch 8e. (Source for Table
16.3 is H. Krenchel, Fibre
Reinforcement, Copenhagen:
Akademisk Forlag, 1964.)
Composite Stiffness
Particle-reinforced Fiber-reinforced Structural
Ecd = EmVm + KEfVf
15s fd
c
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Composite Strength
Particle-reinforced Fiber-reinforced Structural
• Estimate of for discontinuous fibers:
scd*
2. When l < lc
sc d *
c
dVf sm(1Vf )
l
sc d * s f
*Vf 1 c
2
sm (1Vf )
1. When l > lc
l
l
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• Laminates -
-- stacked and bonded fiber-reinforced sheets
- stacking sequence: e.g., 0º/90º
- benefit: balanced in-plane stiffness Adapted from
Fig. 16.16,
Callister &
Rethwisch 8e.
Classification: Structural
Particle-reinforced Fiber-reinforced Structural
• Sandwich panels -- honeycomb core between two facing sheets
- benefits: low density, large bending stiffness
honeycomb
adhesive layer face sheet
Adapted from Fig. 16.18,
Callister & Rethwisch 8e.
(Fig. 16.18 is from Engineered Materials
Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.)
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Composite Benefits
• PMCs: Increased E/r
E(GPa)
Density, r [mg/m3]
0.1 0.3 1 3 10 30 0.01
0.1
1
10
10 2
10 3
metal/ metal alloys
polymers
PMCs
ceramics
Adapted from T.G. Nieh, "Creep rupture of a
silicon-carbide reinforced aluminum
composite", Metall. Trans. A Vol. 15(1), pp.
139-146, 1984. Used with permission.
• MMCs: Increased
creep
resistance
20 30 50 100 200 10
-10
10 -8
10 -6
10 -4
6061 Al
6061 Al w/SiC whiskers s (MPa)
ss (s-1)
• CMCs: Increased toughness
fiber-reinf
un-reinf
particle-reinf Force
Bend displacement
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• Composites types are designated by: -- the matrix material (CMC, MMC, PMC)
-- the reinforcement (particles, fibers, structural)
• Composite property benefits: -- MMC: enhanced E, s, creep performance
-- CMC: enhanced KIc
-- PMC: enhanced E/r, sy, TS/r
• Particulate-reinforced: -- Types: large-particle and dispersion-strengthened
-- Properties are isotropic
• Fiber-reinforced: -- Types: continuous (aligned)
discontinuous (aligned or random)
-- Properties can be isotropic or anisotropic
• Structural: -- Laminates and sandwich panels
Summary
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Core Problems:
ANNOUNCEMENTS
Reading: 16.1-16.5
16.10, 16.11,
16.13