Composites

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Chapter 16

• Composites

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Introduction

• Many of modern technologies require materials
  with unusual combination of properties that
  cannot be met with conventional metal alloys,
  ceramics, and polymeric materials (Aerospace,
  underwater, transportation).
• Composite Any multiphase material (artificially
  made) that exhibits a significant proportion of
  the properties of both constituent phases such
  that a better combination of properties is
  realized.
• Constituent materials must be chemically
  dissimilar and separated by a distinct interface
  (Metallic alloys and ceramics do not fit the
  definition).

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Introduction (Cont.)

• Many composite materials are composed of two
  phases Matrix (Continuous medium surrounding the
  other phase) Dispersed phase.
• Properties of composites are a function of
• Properties of constituent phases.
• Relative amounts of constituents.
• Geometry of dispersed phases (shape of particles
  and particle size, distribution, and
  orientation).

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Terminology/Classification
Composites --Multiphase material
w/significant proportions of each phase.
Matrix --The continuous phase
--Purpose is to transfer stress to
other phases protect phases from
environment --Classification MMC, CMC, PMC
metal
ceramic
polymer
Dispersed phase --Purpose enhance matrix
properties. MMC increase sy, TS, creep
resist. CMC increase Kc PMC
increase E, sy, TS, creep resist.
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(1) Particle-reinforced composites

• Types
• Large particle composites
• Concrete (cement matrix and sand and gravel
  particulates).
• Cermets (ceramic metal composites)
• Dispersion-strengthened composites (10-100 nm).
• Dispersed phase is harder and stiffer than the
  matrix (it restrains movement of the matrix phase
  in the vicinity of each particle.
• For dispersion strengthened composites, phase
  Strengthening is retained at elevated
  temperatures and for extended time periods
  (relative to precipitation hardened alloys).

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Particle-reinforced
Examples
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Particle-reinforced
Elastic modulus, Ec, of composites -- two
approaches.
Application to other properties --
Electrical conductivity, se Replace E by se.
-- Thermal conductivity, k Replace E by k.
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(2) Fiber reinforced composites

• The most important composites.
• Design goals High strength and/or stiffness on a
  weight basis (i.e., specific strength and
  specific modulus).
• Mechanical characteristics depend on
• Fiber and matrix properties
• Degree to which an applied load is transmitted to
  the fibers by the matrix phase (depending on
  fiber orientation and arrangement) ,
• Phase volume fractions
• Direction of load application.

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Various fiber orientation and arrangement
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Fiber-reinforced
Aligned Continuous fibers
Examples
--Metal g'(Ni3Al)-a(Mo) by eutectic
solidification.
--Glass w/SiC fibers formed by glass slurry
Eglass 76GPa ESiC 400GPa.
(a)
(b)
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Fiber-reinforced
Discontinuous, random 2D fibers
Example Carbon-Carbon --process
fiber/pitch, then burn out at up to
2500C. --uses disk brakes, gas
turbine exhaust flaps, nose cones.
(b)
(a)
Other variations --Discontinuous, random
3D --Discontinuous, 1D
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Fiber reinforced composites (Cont.)

• Some critical fiber length ( gt 1 mm) is needed
  for effective strengthening and stiffness of the
  composite material.
• Fibers classification (based on diameter and
  character)
• Whiskers (very thin single crystal, extremely
  large length-to-diameter ratios, virtually flaw
  free, very high strength, expensive).
• Fibers (polycrystalline or amorphous materials
  having small diameters).
• Wires (having large diameters).

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Structural
Stacked and bonded fiber-reinforced sheets
-- stacking sequence e.g., 0/90 -- benefit
balanced, in-plane stiffness
Sandwich panels -- low density, honeycomb
core -- benefit small weight, large bending
stiffness
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Composite benefits
CMCs Increased toughness
PMCs Increased E/r
MMCs Increased creep
resistance
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Summary
Composites are classified according to --
the matrix material (CMC, MMC, PMC) -- the
reinforcement geometry (particles, fibers,
layers). Composites enhance matrix
properties -- MMC enhance sy, TS, creep
performance -- CMC enhance Kc -- PMC
enhance E, sy, TS, creep performance
Particulate-reinforced -- Properties are
isotropic. Fiber-reinforced -- Properties
can be isotropic or anisotropic. Structural
-- Based on build-up of sandwiches in layered
form.