DEN4001 - Composites

Dr Steve Dunn

[email protected]

SEMS

DEN4001

Engineering Materials in Design

Dr Haixue Yan

[email protected]

SEMS

Composites – essential in aviation, automotive and health care...

Have you met a composite today?

1 2

50%50%1. Yes

2. No

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

Which of these is NOT a composite?

14%

14%

14%

14%

14%

14%

14% 1. Concrete

2. Hip replacement

3. Suspension arm from racing car

4. Tooth filling

5. Bone

6. Car tyre

7. Reinforced concrete

What is important?

Pivotal Questions:- What are composite materials?- How are they made?

Composites – historical to now

Bricks dating from antiquity are fiber-reinforced composites with straw fibers aligned in a clay matrix

Rebar-reinforced concrete used in modern bridges and other construction applications represent a medium technology application

Carbon fiber-reinforced composites used in military and aerospace applications represent the cutting edge

Ceramic composites in health care and bioengineering applications

Composites

• Composite:

-- Multiphase material that is artificially

made.

• Phase types:

-- Matrix - is continuous

-- Dispersed - is discontinuous and

surrounded by matrix

Terminology/Classification

In a car tyre the matrix is the..

1 2

50%50%1. Elastomer

2. Carbon black

• Dispersed phase:

-- Purpose:MMC: increase sy, TS, creep resist.

CMC: increase KIc

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

-- Types: particle, fiber, structural

• Matrix phase:

-- Purposes are to:- transfer stress to dispersed phase

- protect dispersed phase from

environment

-- Types: MMC, CMC, PMC

metal ceramic polymer

Terminology/Classification

0.5 mm

0.5 mm

Classification of Composites

Classification: Particle-Reinforced (i)

• Examples:

- Spheroidite

steel

matrix: ferrite (a)

(ductile)

particles: cementite(Fe

3C)

(brittle)60mm

- WC/Co

cemented

carbide

matrix: cobalt (ductile,

tough)

particles: WC (brittle, hard):

600mm

- Automobile

tire rubber

matrix: rubber (compliant)

particles: carbon

black

(stiff) 0.75mm

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


threaded

rod

nut

Posttensioning – tighten nuts to place concrete under compression

• Elastic modulus, Ec, of composites:

-- two “rule of mixture” extremes:

• 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.

Adapted from Fig. 16.3,

Callister & Rethwisch 8e.

(Fig. 16.3 is from R.H.

Krock, ASTM Proc, Vol.

63, 1963.)

Classification: Particle-Reinforced (iii)

lower limit:

1

Ec

=Vm

Em

+Vp

Ep

upper limit: c m mE = V E + VpEp


Data:

Cu matrix

w/tungsten

particles

0 20 40 60 80 100

150

200

250

300

350

vol% tungsten

E(GPa)

(Cu) (W)

Classification: Fiber-Reinforced (ii)

• 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

18

Fiber Alignment

aligned

continuous

aligned random

discontinuous

Adapted from Fig. 16.8,

Callister & Rethwisch 8e.

Transverse

direction

Longitudinal

direction

19

• Aligned Continuous fibers• Examples:

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

Classification: Fiber-Reinforced (iii)


matrix: a (Mo) (ductile)

fibers: g’ (Ni3Al) (brittle)

2mm

-- Ceramic: Glass w/SiC fibersformed by glass slurry

Eglass = 76 GPa; ESiC = 400 GPa.

(a)

(b)

fracture surface

.

20

• 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.

• Other possibilities:-- Discontinuous, random 3D

-- Discontinuous, aligned

Adapted from F.L. Matthews and R.L. Rawlings,

Classification: Fiber-Reinforced (iv)


(b)

fibers lie in plane

view onto plane

C fibers: very stiff very strong

C matrix: less stiff less strong

(a)

500 mm

21

• Critical fiber length for effective stiffening & strengthening:

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

Classification: Fiber-Reinforced (v)


c

f d

t

s

2length fiber

fiber diameter

shear strength of

fiber-matrix interface

fiber ultimate tensile strength

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

Short, thick fibers:

c

f d

t

s

2length fiber

Long, thin fibers:

Low fiber efficiency

c

f d

t

s

2length fiber

High fiber efficiency

Composites – fibre pull out

The previous analysis assumes high-quality bonding between the fibre and matrix

When the bonding is less strong, the bonds between the fibre and matrix break, resulting in fiber pull out

The load cannot be transferred and the matrix behaves as if it were not reinforced at all

Composites – amount of fibre

The amount of fibre impacts both performance and cost

More fibre leads to stronger composites, but the cost of fibre is usually much greater than the cost of the matrix

Above 80% fibre, there is not enough matrix material to completely surround and bond with the fibre to transfer the load

Most fibre reinforced composites contain between 35-50% fibre by volume

Composites – impact of orientation

Fibres aligned along a single axis in almost perfect alignment

Composite is much stronger in the longitudinal direction than the transverse

Composites – chopped fibres

The random orientation of small chopped fibres provides essentially isotropic properties in all directions

Because the fibres are smaller and only a fraction are oriented in any specific direction, they are much less strong than uniaxialcomposites

Composites – mats

Two- and three-dimensional woven mats allow for high strength in more than one direction

The more complicated weave pattern results in far more expensive composites

Composites – manufacture

Commercial fiber-reinforced composites are produced through a variety of techniques including:

Resin FormulationPultrusionWet-Filament WindingResin Transfer MoldingPrepregging

Composites – resin formulation

The simplest production strategy

Bits of chopped fiber and poured or blown into the matrix material (often already in the desired mould)

Curing agents, accelerators, diluents, fillers, and pigments are added as well

The matrix material hardens into the shape of the mold

Composites – pultrusion

Composites – pultrusion

Large numbers of single strands are wound in parallel to form a roving

Many rovings are connected into a device called a creel that lets fibers to be pulled together

Fibers are coated with matrix material and are cured in a heated die then cut into shape

Composites – wet filament winding

Composites – wet filament winding

Continuous fibres from rovings are pulled through a resin impregnation bath then wound into the desired shape

When enough wet filaments are wound around the part, it is taken to a curing oven to complete a composite of the desired shape

Composites – resin transfer moulding

Composites – resin transfer moulding

Woven fibre mats are placed into a space between the top and bottom mold

Resin is injected through the top cavity under sufficient pressure to ensure that it penetrates and surrounds the mat

The molds are cured using heat and pressure to create a composite part in the shape of the mold

Composites – pre-pegging

Composites – pre-pegging

Fibres are passed through an impregnation bath that coats them with small quantities of resin

They are then passed through a furnace and heated slightly to ensure that the resin sticks to the fibre

The resulting coated fibres (prepreg) can be used later to form composites

Composites – selection of fibre material

Most important properties are specific strength, cost, and ability to bond with the matrix

Specific strength is tensile strength divided by density

Ceramic fibers tend to be strong and stiff, but dense

Glass fibers offer chemical resistance and blend of properties but are easily broken during processing

Metals are strong but are also heavy

High performance polymers are strong and light, but have trouble withstanding compressive forces

Composites – selection of fibre material

Composites – selection of resin material

Polyester Resins - most common and economical resin

Isophthalic Polyester Resins – used when water resistance is needed

Epoxy Resins – more expensive but provide improved mechanical properties and exceptional environmental resistance

Vinyl Ester Resins – compromise between the economic advantages of polyester and the properties of epoxy

Phenolic Resins – Poor mechanical properties but excellent fire resistance

Polyimide Resins – expensive and used only for high-end applications such as missiles and military aircraft

Composites – alternative matrix materials

Metal Matrix Composites (often using aluminum) offer:

High strengthExcellent environmental resistance (including the fact that they do not burn)Greater thermal conductivity and abrasion resistance

Ceramic Matrix Composites offer:

Increased fracture toughness Ability to withstand extremely high temperaturesCorrosion resistance

Composites – particulate materials

Are less strong than fiber-reinforced but far less expensive and much easier to manufacture

Contain a large number of randomly oriented particles called aggregate that help the composite withstand compressive loads

Tend to be isotropic and are free from orientation issuesInclude the most important commercial composite – concrete (a blend of gravel and Portland Cement)

Composites - concrete

Technically, a generic term but now used almost exclusively for Portland Cement-based concrete

First use dates back to 1756

More than six billion tons produced each year

Contains Portland Cement, aggregate, water (to induce hydration reactions), and admixtures (additives designed to alter specific properties)

Composites - laminar

Consists of alternating layers of two-dimensional materials with an anisotropic orientation that are connected together by layers of matrix materials

Most common commercial laminar composite is Plywood, layers of wood veneers bonded together by adhesives

Plywood is more resistant to shrinking and warping than regular wood because of crossbanding, in which the grain of each layer is 90° offset from the previous layer

Composites – laminar, other commercial

Snow Skis – originally produced from layers of fiberglass and wood, but now made from a complex blend of layers of sintered polyethylene, steel, rubber, carbon, fiberglass, and wool)

Tire Rubber – 28% carbon black in a polyisobutylene matrix

Composites – sandwich type structures

Used to increase strength with little weight (often in aerospace applications)

Strong titanium or aluminum face sheets are separated by a low density material in the honeycomb structure shown on the right

In simple applications, the honeycomb can be made of cardboard, but high-performance polymers are used for higher-end applications

Who is interested in Engineering?

50%

50% 1. Yes

2. No

Who is interested in high tech or innovative Engineering?

50%

50% 1. Yes

2. No

Who has followed up on the space shuttle failure?

25%

25%

25%

25% 1. Yes

2. No

3. It is not important to me

4. Why would I do that

What are the important things from today....

You decide...

Summary....

Composites

Much like metal alloys, composites blend two or more material together to form a new material with different properties than either parent

Unlike alloys, each parent material in a composite continues to exist in a distinct phase

Four classes of composites:Fiber-reinforcedParticulateLaminarHybrid

Fibre reinforced - Composites

Consist of two phases: Fibres and MatrixFibres:

Strong and stiffDesigned to withstand the tensile loadCarbon, glass, steel, high-performance polymers, glass,

titanium and tungsten are common fiber typesMatrix

Surrounds and orients the fiberProtects fibers from environment and transfers loadPolyester and epoxy resins are common matrix materials

Composites – properties of fibre reinforced

Strong fibres are selected for tensile loads

The ability of the matrix to transfer load to the fibres is an essential factor in composite properties

The ability to transfer load is a function of the bonding between the fibre and matrix, which is impacted by:

Size of the fibreOrientation of the fibreSurface chemistry of the fibreAmount of voids presentLevel of curing

Composites – anisotropy

Composite properties are very different in the direction in which the fibres are aligned (the longitudinal direction) than they are in the direction perpendicular to the fibers (the transverse direction)

Aligned fibres all contribute to handling a longitudinal load, but contribute almost nothing to handling a transverse load

Composite with randomly oriented fibers are isotropic, but the majority of fibres provide little benefit to any specific load direction

Composites – length and diameter

Fibres can be any length from a few millimeters to several kilometers in the case of continuous monofilaments

Long fibres support loads more efficiently than shorter fibres because there are fewer ends

Typical diameters for reinforcing fibres range from 7 to 150 mm (human hair ~ 80 mm)

Thinner fibres tend to be stronger because their reduced surface area makes them less susceptible to surface defects

Composites – aspect ratio

Aspect Ratio is the ratio of fibre length to diameter (l/d)

Large aspect ratios result in stronger composites, but they are more difficult to orient and are often limited by the size of the composite itself

Designers define a critical length (lc) above which the fibre behaves roughly as if it were infinitely long

lc = sfd/2ti

where sf is the tensile strength of the fibre, d is the fibre diameter, and ti is an empirical constant called the wet out that relates to the quality of bonding between the fibre and matrix

Composites – rules of mixtures

For many properties, a simple mixing rule based on volume fractions (f) applies.

In such casesff + fm + fv = 1

whereff is the volume fraction of fibersfm is the volume fraction of matrixfv is the volume fraction on voids

Composites – some properties apply well

Density: rc = rfff + rmfm

Thermal conductivity: kc = kfff + kmfm

Electrical conductivity: sc = sfff + smfm

Composites – behaviour under stress

If the quality of bonding between the fibre and matrix is sufficient, they elongate at the same rate under stress and experience the same strain (an isostrain condition)

The strain behavior is often more complex because the fibres were selected because they have a much higher yield strength than the matrix

When the yield strength of the matrix is exceeded, it begins to experience plastic deformation while the fibres remain in elastic stretching

Load is passed to the fibres and the composite does not fail at stresses that would destroy the matrix

Questions

DEN4001 - Composites

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