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Chapter 16: Composites
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 - 1
Composite
Chapter 16 - 2
Composite
• Combination of two or more individual distinct
materials phases into one engineering
material
• Design goal: obtain a more desirable
combination of properties (principle of
combined action)
– e.g., low density and high strength
Chapter 16 - 3
Terminology/Classification
• Composite:
-- Multiphase material that is artificially made.
-- two components : Marix & Reinforcement
• Phase types:
-- Matrix ; continuous
→ protects reinforcement
→ environmental tolerance
Adapted from Fig. 16.1(a),
Callister & Rethwisch 8e.
-- Dispersed(reinforcement)
→ discontinuous and surrounded by matrix
→ supports structural load
Chapter 16 - 4
Terminology/Classification
• Matrix phase:
woven
fibers
-- Purposes are to:
- transfer stress to dispersed phase
- protect dispersed phase from
environment
-- Types:
0.5 mm
MMC, CMC, PMC
metal
ceramic
polymer
• Dispersed phase:
cross
section
view
-- Purpose:
MMC: increase y, TS, creep resist.
CMC: increase fracture toughness
PMC: increase E, y, TS, creep resist.
-- Types: particle, fiber, structural
0.5 mm
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.
Chapter 16 - 5
Classification of Composites
Adapted from Fig. 16.2,
Callister & Rethwisch 8e.
Chapter 16 - 6
Classification: Particle-Reinforced (i)
Particle-reinforced
Fiber-reinforced
Structural
• Examples:
- WC/Co
cemented
carbide
matrix:
cobalt
(ductile,
tough)
:
Adapted from Fig.
16.4, Callister &
Rethwisch 8e. (Fig.
16.4 is courtesy
Carboloy Systems,
Department, General
Electric Company.)
particles:
WC
(brittle,
hard)
600 m
- Automobile matrix:
tire rubber rubber
(compliant)
0.75 m
particles:
carbon
black
(stiff)
Adapted from Fig.
16.5, Callister &
Rethwisch 8e. (Fig.
16.5 is courtesy
Goodyear Tire and
Rubber Company.)
Chapter 16 - 7
Classification: Particle-Reinforced (ii)
Particle-reinforced
Fiber-reinforced
Structural
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
Posttensioning – tighten nuts to place concrete under compression
threaded
rod
nut
Chapter 16 - 8
Classification: Particle-Reinforced (ii)
■ 복합재료
Without
steel fibers
Brittle
Dispersion of bundle type
steel fibers in concrete
Steel fibers
Ductile
Chapter 16 -13-9
Classification: Fiber-Reinforced (i)
Particle-reinforced
Fiber-reinforced
Structural
• 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
Chapter 16 - 10
Classification: Fiber-Reinforced (i)
Particle-reinforced
Fiber-reinforced
Structural
Ex. Brittle fiber + ductile matrix
- Stage I : both fibers and matrix deform
elastically; linear
- Stage II : the matrix yields and deforms
plastically (εym) while the fibers continue
to stretch elastically.
- Failure : fibers start to fracture ;
→ not all fibers fracture at the same time
→ Even after the failure, the matrix is still
intact ; the fractured fibers are still embedded
within the intact matrix and sustain a
diminished load as the matrix continues to
plastically deform.
Chapter 16 - 11
Classification: Fiber-Reinforced (ii)
Particle-reinforced
Fiber-reinforced
Structural
• 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
– Fibers
• polycrystalline or amorphous
• generally polymers or ceramics
• Ex: alumina, aramid, E-glass, boron, UHMWPE
– Wires
• metals – steel, molybdenum, tungsten
Chapter 16 - 12
Longitudinal
direction
Fiber Alignment
Adapted from Fig. 16.8,
Callister & Rethwisch 8e.
Transverse
direction
aligned
continuous
aligned
random
discontinuous
Chapter 16 - 13
Classification: Fiber-Reinforced (iii)
Particle-reinforced
Fiber-reinforced
• Aligned Continuous fibers
• Examples:
Structural
-- Ceramic: Glass w/SiC fibers formed by glass slurry
Eglass = 76 GPa; ESiC = 400 GPa.
(a)
(b)
fracture
surface
From F.L. Matthews and R.L. Rawlings, Composite Materials;
Engineering and Science, Reprint ed., CRC Press, Boca
Raton, FL, 2000. (a) Fig. 4.22, p. 145 (photo by J. Davies); (b)
Fig. 11.20, p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and
R.D. Rawlings). Used with permission of CRC
Press, Boca Raton, FL.
Chapter 16 - 14
Composite Stiffness:
Longitudinal Loading
Continuous fibers - Estimate fiber-reinforced composite
modulus of elasticity for continuous fibers
•
Longitudinal deformation
c = mVm + fVf
and
volume fraction

c = m = f
isostrain
Ecl = EmVm + Ef Vf
Ecl = longitudinal modulus
c = composite
f = fiber
m = matrix
Chapter 16 - 15
Classification: Fiber-Reinforced (iv)
Particle-reinforced
Fiber-reinforced
• Discontinuous fibers, random in 2 dimensions
• Example: Carbon-Carbon
-- fabrication process:
- carbon fibers embedded
in polymer resin matrix,
- polymer resin pyrolyzed
at up to 2500ºC.
-- uses: disk brakes, gas
turbine exhaust flaps,
missile nose cones.
(b)
(a)
Structural
C fibers:
very stiff
very strong
C matrix:
less stiff
view onto plane less strong
500 m
fibers lie
in plane
• Other possibilities:
-- Discontinuous, random 3D
-- Discontinuous, aligned
Adapted from F.L. Matthews and R.L. Rawlings,
Composite Materials; Engineering and Science,
Reprint ed., CRC Press, Boca Raton, FL, 2000.
(a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p. 151.
(Courtesy I.J. Davies) Reproduced with
permission of CRC Press, Boca Raton, FL.
Chapter 16 - 16
Classification: Fiber-Reinforced (v)
Particle-reinforced
Fiber-reinforced
Structural
• Critical fiber length for effective stiffening & strengthening:
ex: For fiberglass, common fiber length > 15 mm needed
• For longer fibers, stress transference from matrix is more efficient
Short, thick fibers:
Low fiber efficiency
Long, thin fibers:
High fiber efficiency
Chapter 16 - 17
Composite Production Methods (i)
Pultrusion
•
•
•
Continuous fibers pulled through resin tank to impregnate fibers with
thermosetting resin
Impregnated fibers pass through steel die that preforms to the desired shape
Preformed stock passes through a curing die that is
– precision machined to impart final shape
– heated to initiate curing of the resin matrix
Fig. 16.13, Callister & Rethwisch 8e.
Chapter 16 -
Composite Production Methods (ii)
• Filament Winding
– Fibers are fed through a resin bath to impregnate with thermosetting resin
– Impregnated fibers are continuously wound (typically automatically) onto a
mandrel
– After appropriate number of layers added, curing is carried out either in an
oven or at room temperature
– The mandrel is removed to give the final product
Adapted from Fig. 16.15, Callister & Rethwisch 8e.
[Fig. 16.15 is from N. L. Hancox, (Editor), Fibre
Composite Hybrid Materials, The Macmillan
Company, New York, 1981.]
Chapter 16 -
Composite Production Methods (iii)
• Prepreg Production Process
– Continuous-fiber reinforcement preimpregnated with a polymer resin
that is only partially cured.
– delivered in tape form to the manufacturer, which then directly molds
and fully cures the product without having to add any resin
– The most widely used for structural applications
Chapter 16 -
Preimpregnated Tape (Prepreg)
Fibers are introduced into resin in continuous resin
impregnation process – rolled up on paper backing –
ready for component manufacturing
Chapter 16 -
Preimpregnated Tape (Prepreg)
Unidirectional
Pre-impregnated
Tape (prepreg)
Fabrics
Pressure/Heat
Aircraft
Trailing
Edge
Chapter 16 -