Chapter 16: Composites - Armstrong

30 Pages·2012·0.76 MB·English

by  Priya Thamburaj

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?
1
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

2
Terminology/Classification
• Composite:
-- Multiphase material that is artificially
made.
• Phase types:
-- Matrix - is continuous
-- Dispersed - is discontinuous and
surrounded by matrix

Adapted from Fig. 16.1(a),
Callister & Rethwisch 8e.
3
Terminology/Classification
• Matrix phase: woven
fibers
-- Purposes are to:
- transfer stress to dispersed phase
- protect dispersed phase from
environment
0.5 mm
-- Types: MMC, CMC, PMC
cross
metal ceramic polymer section
view
• Dispersed phase:
-- Purpose:
MMC: increase s , TS, creep resist. 0.5 mm
y

CMC: increase K
Reprinted with permission from
Ic
D. Hull and T.W. Clyne, An

PMC: increase E, s , TS, creep resist.
y Introduction to Composite Materials,
2nd ed., Cambridge University Press,

-- Types: particle, fiber, structural
New York, 1996, Fig. 3.6, p. 47.
4
Classification of Composites
Stiffer particulate bears
Adapted from Fig. 16.2,
fraction of load, restrain
Callister & Rethwisch 8e.
matrix movement in vicinity
Small dispersed particles
impede motion of
dislocations
5
• For which particle reinforced composite would
bond strength be more important?

• What geometric/spatial properties of particles in
dispersed phase would influence performance?
6
Particle-reinforced Fiber-reinforced Structural
Classification: Particle-Reinforced (i)
• Examples:
- Spheroidite matrix: particles:
Adapted from Fig.
ferrite (a) cementite 10.19, Callister &
steel
Rethwisch 8e. (Fig.
( Fe C )
(ductile)
10.19 is copyright
3

(brittle) United States Steel
Corporation, 1971.)
60mm

Adapted from Fig.
- WC/Co matrix: particles:
16.4, Callister &
cemented cobalt WC Rethwisch 8e. (Fig.
16.4 is courtesy
(ductile, (brittle,
carbide Carboloy Systems,
toug: h ) hard)
Department, General
Electric Company.)
600mm

Adapted from Fig.
- Automobile matrix: 16.5, Callister &
particles:
Rethwisch 8e. (Fig.
rubber
tire rubber carbon 16.5 is courtesy
(compliant) Goodyear Tire and
black
Rubber Company.)
(stiff)
0.75mm
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Particle-reinforced Fiber-reinforced Structural
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
Posttensioning
– tighten nuts to place concrete under compression

threaded
rod
nut
8
9
Particle-reinforced Fiber-reinforced Structural
Classification: Particle-Reinforced (iii)
• Elastic modulus, E , of composites:
c
-- two “rule of mixture” extremes:
upper limit: E = V E + V E
c m m p p
E(GPa)
350
Data:
lower limit: Adapted from Fig. 16.3,
Cu matrix 30 0 Callister & Rethwisch 8e.
1 V V
w/tungsten 250 = m + p (Fig. 16.3 is from R.H.
Krock, ASTM Proc, Vol.
particles E E E
20 0 c m p 63, 1963.)
150
0 20 4 0 6 0 8 0 10 0 vol% tungsten
(Cu) (W )
• Application to other properties:
-- Electrical conductivity, s : Replace E’s in equations with s ’s.
e e
-- Thermal conductivity, k: Replace E’s in equations with k’s.
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