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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?. Composite.

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chapter 16 composites
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?

composite
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
terminology classification
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.

terminology classification4
Terminology/Classification

• Matrix phase:

-- Purposes are to:

- transfer stress to dispersed phase

- protect dispersed phase from

environment

-- Types: MMC, CMC, PMC

0.5

mm

metal

polymer

ceramic

0.5

mm

woven

fibers

cross

section

view

• Dispersed phase:

-- Purpose:

MMC: increase sy, TS, creep resist.

CMC: increase KIc

PMC: increase E, sy, TS, creep resist.

-- Types:particle, fiber, structural

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.

classification of composites
Classification of Composites

Adapted from Fig. 16.2, Callister & Rethwisch 8e.

classification particle reinforced i
Classification: Particle-Reinforced (i)

Particle-reinforced

Fiber-reinforced

Structural

• Examples:

particles:

- Spheroidite

matrix:

Adapted from Fig. 10.19, Callister & Rethwisch 8e. (Fig. 10.19 is copyright United States Steel Corporation, 1971.)

cementite

ferrite (a)

steel

(

Fe

C

)

(ductile)

3

(brittle)

60mm

Adapted from Fig. 16.4, Callister & Rethwisch 8e. (Fig. 16.4 is courtesy Carboloy Systems, Department, General Electric Company.)

- WC/Co

matrix:

particles:

cobalt

WC

cemented

(ductile, tough)

(brittle,

carbide

hard)

:

600mm

Adapted from Fig. 16.5, Callister & Rethwisch 8e. (Fig. 16.5 is courtesy Goodyear Tire and Rubber Company.)

- Automobile

matrix:

particles:

rubber

tire rubber

carbon

black

(stiff)

(compliant)

0.75mm

classification particle reinforced ii
Classification: Particle-Reinforced (ii)

Particle-reinforced

Fiber-reinforced

Structural

Posttensioning– tighten nuts to place concrete under compression

threadedrod

nut

  • 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

classification particle reinforced iii
Classification: Particle-Reinforced (iii)

Particle-reinforced

Fiber-reinforced

Structural

+

E

=

V

E

V

E

upper limit:

c

m

m

p

p

E(GPa)

350

Data:

lower limit:

30

0

Cu matrix

V

1

V

m

p

w/tungsten

250

=

+

E

E

E

particles

20

0

c

m

p

150

vol% tungsten

0

20

4

0

6

0

8

0

10

0

(Cu)

(

W)

• Elastic modulus, Ec, of composites:

-- two “rule of mixture” extremes:

Adapted from Fig. 16.3, Callister & Rethwisch 8e. (Fig. 16.3 is from R.H. Krock, ASTM Proc, Vol. 63, 1963.)

• 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.

classification fiber reinforced i
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
classification fiber reinforced ii
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
fiber alignment
Fiber Alignment

Longitudinal direction

Adapted from Fig. 16.8, Callister & Rethwisch 8e.

Transverse direction

aligned

continuous

aligned random

discontinuous

classification fiber reinforced iii
Classification: Fiber-Reinforced (iii)

Particle-reinforced

Fiber-reinforced

Structural

-- Ceramic: Glass w/SiC fibers

formed by glass slurry

Eglass = 76 GPa; ESiC = 400 GPa.

(a)

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.

(b)

• Aligned Continuous fibers

• Examples:

-- Metal: g'(Ni3Al)-a(Mo)

by eutectic solidification.

matrix:

(Mo) (ductile)

a

2mm

g

fibers:

’ (Ni3Al) (brittle)

From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp. 987-998, 1988. Used with permission.

classification fiber reinforced iv
Classification: Fiber-Reinforced (iv)

Particle-reinforced

Fiber-reinforced

Structural

C fibers:

very stiff

very

strong

(b)

C matrix:

less stiff

view onto plane

less strong

fibers lie

in plane

(a)

• 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.

500 m

• 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.

classification fiber reinforced v
Classification: Fiber-Reinforced (v)

Particle-reinforced

Fiber-reinforced

Structural

Short, thick fibers:

Long, thin fibers:

Low fiber efficiency

High fiber efficiency

• Critical fiber length for effective stiffening & strengthening:

fiber ultimate tensile strength

fiber diameter

shear strength of

fiber-matrix interface

• Ex: For fiberglass, common fiber length > 15 mm needed

• For longer fibers, stress transference from matrix is more efficient

composite stiffness longitudinal loading
Composite Stiffness:Longitudinal Loading

Continuous fibers - Estimate fiber-reinforced composite modulus of elasticity for continuous fibers

  • Longitudinal deformation

c = mVm+ fVf and c= m= f

volume fraction isostrain

  • Ecl= EmVm + Ef VfEcl = longitudinal modulus

c = compositef = fiber

m = matrix

composite stiffness transverse loading
Composite Stiffness:Transverse Loading
  • In transverse loading the fibers carry less of the load

c= mVm+ fVf andc= m= f= 

isostress

Ect = transverse modulus

c = compositef = fiber

m = matrix

composite stiffness
Composite Stiffness

Particle-reinforced

Fiber-reinforced

Structural

Ecd= EmVm + KEfVf

• 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 strength
Composite Strength

Particle-reinforced

Fiber-reinforced

Structural

1. When l > lc

l

l

2. When l < lc

l

• Estimate of for discontinuous fibers:

composite production methods i
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.

composite production methods ii
Composite Production Methods (ii)
  • Filament Winding
    • Continuous reinforcing fibers are accurately positioned in a predetermined pattern to form a hollow (usually cylindrical) shape
    • 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.]

classification structural
Classification:Structural

Particle-reinforced

Fiber-reinforced

Structural

• Sandwich panels

-- honeycomb core between two facing sheets

- benefits: low density, large bending stiffness

face sheet

adhesive layer

honeycomb

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.)

• 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.

composite benefits
Composite Benefits

• CMCs: Increased toughness

• PMCs: Increased E/r

ceramics

Force

3

10

particle-reinf

E(GPa)

PMCs

2

10

10

metal/

fiber-reinf

metal alloys

1

un-reinf

0.1

polymers

0.01

Bend displacement

0.1

0.3

1

3

10

30

Density, r [mg/m3]

-4

10

6061 Al

e

(s-1)

ss

• MMCs:

Increased

creep

resistance

-6

10

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.

-8

6061 Al

10

w/SiC

whiskers

s

(MPa)

-10

10

20

30

50

100

200

summary
Summary

• 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/, sy, TS/

• 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

announcements
ANNOUNCEMENTS

Reading:

Core Problems:

Self-help Problems: