den4001 - composites
TRANSCRIPT
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Dr Steve Dunn
SEMS
DEN4001
Engineering Materials in Design
Dr Haixue Yan
SEMS
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Composites – essential in aviation, automotive and health care...
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Have you met a composite today?
1 2
50%50%1. Yes
2. No
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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
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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
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What is important?
Pivotal Questions:- What are composite materials?- How are they made?
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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
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Composites
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• Composite:
-- Multiphase material that is artificially
made.
• Phase types:
-- Matrix - is continuous
-- Dispersed - is discontinuous and
surrounded by matrix
Terminology/Classification
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In a car tyre the matrix is the..
1 2
50%50%1. Elastomer
2. Carbon black
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• 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
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Classification of Composites
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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
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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
Particle-reinforced Fiber-reinforced Structural
threaded
rod
nut
Posttensioning – tighten nuts to place concrete under compression
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• 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
Particle-reinforced Fiber-reinforced Structural
Data:
Cu matrix
w/tungsten
particles
0 20 40 60 80 100
150
200
250
300
350
vol% tungsten
E(GPa)
(Cu) (W)
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• 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
Particle-reinforced Fiber-reinforced Structural
Data:
Cu matrix
w/tungsten
particles
0 20 40 60 80 100
150
200
250
300
350
vol% tungsten
E(GPa)
(Cu) (W)
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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
Particle-reinforced Fiber-reinforced Structural
– Fibers
• polycrystalline or amorphous
• generally polymers or ceramics
• Ex: alumina, aramid, E-glass, boron, UHMWPE
– Wires
• metals – steel, molybdenum, tungsten
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Fiber Alignment
aligned
continuous
aligned random
discontinuous
Adapted from Fig. 16.8,
Callister & Rethwisch 8e.
Transverse
direction
Longitudinal
direction
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• Aligned Continuous fibers• Examples:
-- Metal: g'(Ni3Al)-a(Mo)by eutectic solidification.
Classification: Fiber-Reinforced (iii)
Particle-reinforced Fiber-reinforced Structural
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
.
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• 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)
Particle-reinforced Fiber-reinforced Structural
(b)
fibers lie in plane
view onto plane
C fibers: very stiff very strong
C matrix: less stiff less strong
(a)
500 mm
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• Critical fiber length for effective stiffening & strengthening:
• Ex: For fiberglass, common fiber length > 15 mm needed
Classification: Fiber-Reinforced (v)
Particle-reinforced Fiber-reinforced Structural
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
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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
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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
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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
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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
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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
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Composites – manufacture
Commercial fiber-reinforced composites are produced through a variety of techniques including:
Resin FormulationPultrusionWet-Filament WindingResin Transfer MoldingPrepregging
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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
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Composites – pultrusion
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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
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Composites – wet filament winding
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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
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Composites – resin transfer moulding
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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
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Composites – pre-pegging
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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
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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
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Composites – selection of fibre material
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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
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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
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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)
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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)
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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
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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
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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
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Who is interested in Engineering?
50%
50% 1. Yes
2. No
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Who is interested in high tech or innovative Engineering?
50%
50% 1. Yes
2. No
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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
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What are the important things from today....
You decide...
Summary....
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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
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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
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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
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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
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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
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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
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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
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Composites – some properties apply well
Density: rc = rfff + rmfm
Thermal conductivity: kc = kfff + kmfm
Electrical conductivity: sc = sfff + smfm
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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
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Questions