Chapter 16 -

COMPOSITES

MSE 226-Engineering Materials

Lecture-13

Chapter 16 -

A Composite material is a material system composed of

two or more macro constituents that differ in shape and

chemical composition and which are insoluble in each

other.

Introduction

Combine materials with the objective of getting a more desirable

combination of properties

Ex: get flexibility & weight of a polymer plus the strength of a

ceramic

Principle of combined action

Mixture gives “averaged” properties

The history of composite materials dates back to early 20th

century. In 1940, fiber glass was first used to reinforce

epoxy.

Why composites?

Chapter 16 -

Why Composites? :

getting the best of all worlds

heavy Lower Tm

Chapter 16 -

Concrete Automobile tire

Boats

Cutting tools

Used for shaping of

hard materials

Structural Composites, i.e.

Honeycomb structure

Ceramic + ceramic

polymer + ceramic

Polymer + ceramic (glass)

metal+ ceramic

Ceramic + metal

Composites : Examples

Chapter 16 -

Talking about Bones

From: http://bioweb.uwlax.edu/aplab/Table_of_Contents/Lab_05/Bone_Model_1/bone_model_1.html

Bone is a composite material with various different components

Composition of Bone:

• ~45 wt% mineral salts (calcium phosphate and calcium carbonate)

• ~35 wt% organics (collagen protein)

• ~20 wt% water

Composites : Examples

Chapter 16 -

Composites

Multiphase material that contains matrix and dispersed phase

Purpose of disersed phase:

MMC: increase sy, TS, creep resist.

CMC: increase Kc

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

-- Classification: Particle, fiber, structural

-- Classification: MMC, CMC, PMC

metal ceramic polymer

Terminology/Classification

Matrix: Continuous and surrounds other phase

- transfer stress to other phases

- protect phases from environment

Dispersed phase:

-enhance matrix properties (reinforcement)

Epoxy (matrix)

Polymer matrix composite (PMC)

Glass fiber

(dispersed phase)

Chapter 16 -

Large-

particle

Dispersion-

strengthened

Particle-reinforced

Continuous

(aligned)

Aligned Randomly

oriented

Discontinuous

(short)

Fiber-reinforced

Laminates Sandwich

panels

Structural

Composites

Adapted from Fig.

16.2, Callister 7e.

Classification of Composites

Composite classification according to dispersed phase

Chapter 16 -

- Spheroidite

steel

matrix: ferrite (a)

(ductile)

particles: cementite ( Fe 3 C )

(brittle) 60 mm

- Cermets

WC/Co

cemented

carbide

matrix: cobalt (ductile)

particles: WC (brittle, hard) V m :

10-15 vol%! 600 mm

- Automobile

tires

matrix: rubber (compliant)

particles: C (stiffer)

0.75 mm

Particle-reinforced Fiber-reinforced Structural

Dispersed

Large-particle

nano-composite

Composite Survey: Particle-Reinforced

Chapter 16 -

Concrete – gravel + sand + cement LARGE particle!

- Why sand and gravel? Sand packs into gravel voids

Particle-reinforced Fiber-reinforced Structural

CMC

Dispersed phase Matrix phase

Disadvantage

Like ceramics concrete is weak

in tension (10-15 times smaller

than its compressive st

Composite Survey: Particle-Reinforced

Chapter 16 -

Reinforced concrete - Reinforce with steel rerod or remesh

- increases strength - even if cement matrix is cracked

Prestressed concrete - remesh under tension during setting of

concrete. Tension release puts concrete under compressive force

- Concrete much stronger under compression.

- Applied tension must exceed compressive force

threaded

rod

nut

Post tensioning – tighten nuts to put under tension

Steel rods are added to improve

tensile strength of the concrete.

(called reinforced concrete) Concrete

(matrix)

Steel rod

(dispersed phase)

(Fibers of glass, steel, nylon and PE are also used)

Composite Survey: Particle-Reinforced

Chapter 16 -

1-Elastic modulus, Ec, of composites:

Composite Survey: Particle-Reinforced

Mechanical Properties of Large Particle

Reinforced Composites

Ec: elastic modulus of composite

Em: elastic moduluc of matrix

Ep: elastic modulus of particles

Vm: volume fraction of matrix

Vp:volume fraction of particles

Chapter 16 -

Fibers provide significant strength improvement to material

Example: fiber-glass

• Continuous glass filaments in a polymer matrix

Composite Survey: Fiber-Reinforced

Glass fiber

Epoxy matrix

• Strength of the composite increases due to fibers

• Polymer simply holds them in place

Polymer matrix composite (PMC) /

Fiber reinforced composite

Fiber reinforced composites are characterized by high

strength/weight ratio

Usually used for polymers

Chapter 16 -

Whiskers - Thin single crystals - large length to diameter ratio

• graphite, SiN, SiC

• high crystal perfection – extremely strong, strongest known

• very expensive

Fibers

• polycrystalline or amorphous and have small diameters

• generally polymers or ceramics

• Ex: Carbon, Al2O3 , Aramid, E-glass, Boron, UHMWPE

Wires

• Large diameter

• Metal – steel, Mo, W

• i.e. Steel reinforcement in automobile tires

Composite Survey: Fiber-Reinforced

According to diameter and character fibers are groupped

into 3 different classifications: whiskers, fiber, wires

Chapter 16 -

Particle-reinforced Fiber-reinforced Structural

Composite Survey: Fiber-Reinforced

1-Continuous

(aligned)

2-Discontinuous

(aligned) (random)

Chapter 16 -

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.

-- Metal matrix

g'(Ni3Al)-a(Mo) composite by eutectic solidification.

matrix: a (Mo) (ductile)

fibers: g ’ (Ni3Al) (brittle)

2 mm

-- Ceramic matrix

: Glass w/SiC fibers (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.

Composite Survey: Fiber-Reinforced

1-Continuous aligned fibers: Examples

Chapter 16 -

Carbon-Carbon composite (ceramic matrix)

-Used in disk brakes,

gas turbine exhaust flaps, nose cones.

• Other variations: -- Discontinuous, random 3D

-- Discontinuous, 1D

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.

fibers lie in plane

view onto plane

C fibers: very stiff very strong

C matrix: less stiff less strong

2-Discontinuous fibers: Examples

Composite Survey: Fiber-Reinforced

Discontinuous in 2D

Chapter 16 - 17

How much load is being transferred to the fiber ?

depends on the bonding between fiber and matrix !

at the end of the fiber load is solely carried by matrix !

Mechanical Properties of Fiber-Reinforced

Composites

Mechanical properties depend on;

1-propeties of fiber

2-transmission of load to the fibers by the matrix phase

Chapter 16 -

How does fiber reinforcement work ?

Mechanical Properties of Fiber-Reinforced

Composites

Chapter 16 -

Critical fiber length (lc) for effective stiffening & strengthening:

fiber diameter

shear strength of

fiber-matrix interface

fiber strength in tension

c

f d

s

2l

*

c

Mechanical Properties of Fiber-Reinforced

Composites

l >> lc (when l>15lc): Continuous fibers

l< lc : discontinuous or short fibers

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

Longer fibers carry stress more efficiently!

1-Shorter, thicker fiber: 2-Longer, thinner fiber:

Poorer fiber efficiency Better fiber efficiency

s (x) s (x)

l>15lc l< lc

Chapter 16 -

Tensile Stress-Strain Behavior:

Continuous-aligned fibers,

Loading in longitudinal direction

Most load is

carried by

fibers

Fibers

start

fracturing

Mechanical Properties of Fiber-Reinforced

Composites

Chapter 16 -

Amount of load

carried by

fibers vs matrix

L>15Lc

ELASTIC BEHAVIOR in Longitudinal direction

Continuous-aligned fibers

Mechanical Properties of Fiber-Reinforced

Composites

Assume fiber/matrix bond is strong and they deform at the same time => isostrain condition ( εc = εm = εf)

Load carried by composite: Fc = Fm + Ff

Composite strength : sc = smVm + sfVf

Composite elastic modulus : Ecl = Em Vm + EfVf

mm

ff

m

f

VE

VE

F

F

f = fiber m = matrix

Chapter 16 -

Mechanical Properties of Fiber-Reinforced

Composites

TENSILE STRENGTH in Longitudinal direction

Continuous-aligned fibers

Tensile strength

Chapter 16 -

Example Problem 16.1

A continuous aligned glass fiber-reinforced composite consists of 40

vol% of glass fibers having a modulus of elasticity of 69 GPa and 60

vol% of a polyester resin that, when hardened, displays a modulus of

3.4 GPa.

(a) Compute the modulus of elasticity of this 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

Mechanical Properties of Fiber-Reinforced

Composites

Chapter 16 -

• Exposed stresses are same: isostress state

sc = sm = sf = s

c= mVm + fVf

f

f

m

m

ct E

V

E

V

E

1

transverse modulus of the composite

BUT

ELASTIC BEHAVIOR in Transverse direction

Continuous-aligned fibers

Mechanical Properties of Fiber-Reinforced

Composites

Since ε=s/E

Chapter 16 -

Mechanical Properties of Fiber-Reinforced

Composites

Example Problem 16.2

Compute the elastic modulus of the composite material

described in example problem 16.1, but assume that the

stress is applied perpendicular to the direction of fiber

alignment.

Chapter 16 -

Mechanical Properties of Fiber-Reinforced

Composites

Tensile strength in Longitudinal direction

Discontinuous and aligned fibers

When l>lc

When l<lc

Chapter 16 -

-- Elastic modulus in fiber direction:

-- TS in fiber direction:

efficiency factor, K: -- aligned 1D: K = 1 (aligned )

-- aligned 1D: K = 0 (aligned )

-- random 2D: K = 3/8 (2D isotropy)

-- random 3D: K = 1/5 (3D isotropy)

(aligned 1D) (TS)c = (TS)mVm + (TS)fVf

Ec = EmVm + KEfVf

Mechanical Properties of Fiber-Reinforced

Composites

ELASTIC BEHAVIOR and Tensile strength in

Longitudinal direction

Discontinuous and randomly oriented fibers

Chapter 16 -

• Stacked and bonded fiber-reinforced sheets

-- stacking sequence: e.g., 0º/90º -- benefit: balanced, in-plane stiffness

Structural Composites

Laminar composites and sandwich panels are two most

common structural composites

Example: snow-ski

Laminar Composites

Chapter 16 -

Contains honeycomb core, two outer stiff/strong sheets

(Sheet material: Al-alloy, fiber reinforced plastic, titanium

steel or plywood)

Core : low modulus, Outer: high stiffness

honeycomb

adhesive layer

face sheet

Structural Composites

Sandwich Composites

They are designed to be light-weight beams or panels having

relatively high stiffness and strength

Use in roofs, floors of buildings and aerospace and aircrafts (wings, fuselage)

Chapter 16 -

Composite Production Methods-

Fiber Reinforced Composites

• Pultrusion

– Continuous fibers pulled through resin tank, then

preforming die & oven to cure

Adapted from Fig.

16.13, Callister 7e.

Chapter 16 - 31

• Filament Winding

– Ex: pressure tanks

– Continuous filaments wound onto mandrel

Composite Production Methods-

Fiber Reinforced Composites

Chapter 16 -

• CMCs: Increased toughness

Composite Benefits

fiber-reinf

un-reinf

particle-reinf Force

Bend displacement

• PMCs: Increased E/r

E(GPa)

G=3E/8 K=E

Density, r [mg/m3] .1 .3 1 3 10 30

.01

.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)

Chapter 16 -

• 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:

-- Elastic modulus can be estimated.

-- Properties are isotropic.

• Fiber-reinforced:

-- Elastic modulus and TS can be estimated along fiber dir.

-- Properties can be isotropic or anisotropic.

• Structural: -- Based on build-up of sandwiches in layered form.

Summary