Transcript COMPOSITES - University of Victoria

MECH 473 ~ LECTURE 15:
COMPOSITES
LECTURE 15:
COMPOSITE
MATERIALS
By: Casey Keulen
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
-Composite materials are engineered materials made
from two or more constituent materials that remain
separate and distinct while forming a single component
-Generally, one material forms a continuous matrix while
the other provides the reinforcement
- The two materials must be chemically inert with respect
to each other so no interaction occurs upon heating until
one of the components melts, an exception to this
condition is a small degree of interdiffusion at the
reinforcement-matrix interface to increase bonding
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
Can you think of any examples
of where composites are used?
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
Composites can be found in:
-Boat hulls
-The aerospace industry (structural components as well
as engines and motors)
-Automotive parts (panels, frames, dashboards, body
repairs)
-Sinks, bathtubs, hot tubs, swimming pools
-Cement buildings, bridges
-Surfboards, snowboards, skis
-Golf clubs, fishing poles, hockey sticks
-Trees are technically composite materials, plywood
-Electrical boxes, circuit boards, contacts
-Everywhere
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
Composites can be classified by their
matrix material which include:
-Metal matrix composites (MMC’s)
-Ceramic matrix composites (CMC’s)
-Polymer matrix composites (PMC’s) or
sometimes referred to as organic matrix
composites (OMC’s)
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
MMC - Metal Matrix Composites
-The matrix is relatively soft and flexible.
-The reinforcement must have high strength and stiffness
-Since the load must be transferred from the matrix to the
reinforcement, the reinforcement-matrix bond must be strong.
MMC use:
-Two types of particulates ( dispersion strengthened alloys
and regular particulate composites)
-Or long fiber reinforcements
MECH 473 ~ LECTURE 15:
COMPOSITES
COMPOSITES
PMC - Polymer Matrix Composites
-The matrix is relatively soft and flexible
-The reinforcement must have high strength and stiffness
-Since the load must be transferred from matrix to
reinforcement, the reinforcement-matrix bond must be strong
CMC – Ceramic Matrix Composites
-The matrix is relatively hard and brittle
-The reinforcement must have high tensile strength to arrest
crack growth
-The reinforcement must be free to pull out as a crack extends,
so the reinforcement-matrix bond must be relatively weak
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COMPOSITES
DISPERSION STRENGTHENED
MMC’S
-Dispersion strengthened alloys can be considered as composites
because there is little or no interaction between the two components
and the reinforcement is not soluble in the metal matrix.
-The dispersoids are usually 10-250 nm diameter oxide particles and
are introduced by physical means rather than chemical precipitation.
-They are located within the grains and at grain boundaries but are not
coherent with the matrix as in precipitation hardening
-The dispersed particles are sufficiently small in size to impede
dislocation movement and thus improve yield strength as well as
stiffness.
-Dispersion strengthened alloys are somewhat weaker than precipitation
hardened alloys at room temperature but since overaging, tempering,
grain growth or particle coarsening do not occur on heating, they are
stronger and more creep resistant at high temperatures.
MECH 473 ~ LECTURE 15:
COMPOSITES
SINTERED ALUMINUM
POWDER (SAP) COMPOSITES
-SAPs have an aluminum matrix with aluminum oxide
(Al2O3) particulate
-The matrix can be strengthened by %14
SAPs are produced using different methods, two examples
are as follows:
-Al and Al2O3 powders are blended then compacted at high
pressure then sintered like a ceramic.
-Al powder is heated in air to form a thick film of Al2O3 on
each particle, when the powder is compacted the Al2O3
film fractures into tiny particles and becomes surrounded
by the Al during sintering
MECH 473 ~ LECTURE 15:
COMPOSITES
SINTERED ALUMINUM
POWDER (SAP) COMPOSITES
Properties of SAP
compared to 2024-T8,
7075-T6 and a boron
fiber strengthened
1100 alloy.
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COMPOSITES
THORIA-DISPERSED
COMPOSTIES
-An important group of dispersion-strengthened composites is
thoria-dispersed (TD) metals
-Thorium is an element on the periodic table (atomic number 90)
-A common example is TD-nickel
TD-nickel composites roduced by:
-Powders of metallic Th and Ni are ball
milled, compacted at high pressure and
then sintered.
-The compact is then heated in air and
oxygen diffuses in to react with Th metal
to form a fine dispersion of ThO2.
-This method, internal oxidation is also
Electron micrograph of TD-Ni with 300
used for fabricating the W-ThO2
nm diameter ThO2 particles (X2000)
composites.
MECH 473 ~ LECTURE 15:
COMPOSITES
CEMENTED CARBIDES
(CERMETS)
-Cemented carbides are an example of regular particulate MMC’s
(as opposed to dispersion strengthened MMC’s)
-Carbides such as WC (tungsten-carbide) are used for cutting tool
inserts but this hard ceramic is very brittle so it cracks or chips
under impact loads, to remedy this cobalt is used as a matrix
-Co-WC (cobalt tungsten-carbide) cermets are produced by
pressing Co and W powders into compacts, which are heated
above the melting point of Co
-On cooling the carbide particles become embedded in the
solidified Co, which act as a tough matrix for the WC particles
-In addition to its strength and toughness, Co is also selected
because it wets the carbide particles to give a strong bond
MECH 473 ~ LECTURE 15:
COMPOSITES
CEMENTED CARBIDES
(CERMETS)
-Cemented carbides are commonly used as inserts for
cutting tools
Figure (from left to right):
Cutting tool inserts, a
milling tool and a lathe tool
-I’m sure you’ve seen these in the machine shop
MECH 473 ~ LECTURE 15:
COMPOSITES
PARTICULATE MMC’S FOR
ELECTRICAL CONTACTS
-Electrical contacts used in switches, relays and motors
must be quite wear resistant to stand up in service
-Highly conductive metals such as Cu and Ag are relatively
soft and thus show excessive wear when used as contacts
resulting in arcing and poor electrical conduction.
-The goal is to produce a contact that is both a good
conductor and has excellent wear properties
-This is done by using silver reinforced with tungsten
particles, the Ag is a terrific conductor while the W provides
good wear properties
MECH 473 ~ LECTURE 15:
COMPOSITES
PARTICULATE MMC’S FOR
ELECTRICAL CONTACTS
-The composite is made in two stages:
-First a low density compact with interconnected
pores is produced by pressing and firing tungsten
powders (figure a and b)
-Liquid silver is then infiltrated into the connected
voids under vacuum (figure c)
b
c
-The final product has a
continuous Ag and W
structure which provides
good electrical
conductivity and wear
resistance
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COMPOSITES
CAST METAL
PARTICULATE MMC’S
-Al alloys for automotive connecting rods and pistons can be
strengthened and hardened by the addition of SiC (silicon
carbide) particles.
-The SiC particles are introduced at a temperature at which the
alloy is in the solid plus liquid state, ie., by “compocasting”.
MECH 473 ~ LECTURE 15:
COMPOSITES
CAST METAL
PARTICULATE MMC’S
Compocasting of Al-SiC:
-Partially solidified alloy is stirred to break up dendrites (fig. a)
-Particles of SiC are added at this temperature (fig. b)
-In a pressure die casting machine, the solid mixture becomes
thixotropic to form a high density casting (fig. c)
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COMPOSITES
CAST METAL
PARTICULATE MMC’S
Microstructure of cast Al Alloy reinforced with particles of SiC
magnified X125
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COMPOSITES
RULE OF MIXTURES
For particulate composites, the rule of mixtures predicts the
density of the composite as well as other properties
(although other properties may vary depending on how the
dispersed phase is arranged)
Density, r, is given as a fraction, f, as:
rc  f m rm  f f r f
Notethat f m 1  f f
Where the subscripts m and f refer to the matrix and fiber.
MECH 473 ~ LECTURE 15:
COMPOSITES
RULE OF MIXTURES
For fiber reinforced composites, the rule of mixtures predicts
the density of the composite as well as electrical and
thermal conductivity along the direction of the fibers if they
are continuous and unidirectional.
Density, r, is given as a fraction, f, as:
rc  f m rm  f f r f
For thermal conductivity:
Kc  f m K m  f f K f
Notethat f m 1  f f
For electrical conductivity:
 c  f m m  f f  f
Thermal and electrical energy can be transferred through the
composite at a rate that is proportional to the volume fraction,
f of the conductive material
MECH 473 ~ LECTURE 15:
COMPOSITES
RULE OF MIXTURES
In a composite material with a metal matrix and ceramic
fibers, the bulk of the energy would be transferred
through the matrix.
In a composite consisting of a polymer matrix containing
metallic fibers, the energy would be transferred
through the fibers.
When the fibers are not continuous or unidirectional, the
simple rule of mixtures may not apply.
For example, in a metal fiber-polymer matrix composite,
electrical conductivity would be low and would
depend on the length of the fibers, the volume
fraction of fibers and how often the fibers touch one
another.
MECH 473 ~ LECTURE 15:
COMPOSITES
RULE OF MIXTURES
The rule of mixtures can also be used to predict the modulus of
elasticity when the fibers are continuous and unidirectional.
Parallel to the fibers, the modulus of elasticity may be as high as:
Ec  f m Em  f f E f
However, when the applied load is
very large, the matrix begins to deform
and the stress-strain curve is no
longer linear. Since the matrix now
contributes little to the stiffness, the
modulus is approximated by:
Ec  f f E f
MECH 473 ~ LECTURE 15:
COMPOSITES
RULE OF MIXTURES
Perpendicular to the fibers, the modulus of elasticity may be as
high as:
ff
fm
1


Ec E f Em
There are many good examples provided in your text by
Askland and Phule in the chapter, “Composites: Teamwork
and Synergy in Materials”. Review this chapter and the
examples provided.
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER REINFORCED
COMPOSITES
-Fiber reinforced composites provide improved strength,
fatigue resistance, Young’s modulus and strength to
weight ratio over the constituent materials.
-This is achieved by incorporating strong, stiff, yet brittle
fibers into a more ductile matrix.
-Generally speaking the fiber supplies the strength and
stiffness while the matrix binds the fibers together and
provides a means of transferring the load between fibers
-The matrix also provides protection for the fibers
MECH 473 ~ LECTURE 15:
COMPOSITES
CHARACTERISTICS OF FIBER
REINFORCED COMPOSITES
-Many factors must be considered when designing a fiberreinforced composite including the length, diameter, orientation,
amount and properties of the constituents, and the bonding
between them.
-The method used to produce the final product is also very
important as it dictates the type of properties just mentioned as
well as the quality of the product.
MECH 473 ~ LECTURE 15:
COMPOSITES
CHARACTERISTICS OF FIBER
REINFORCED COMPOSITES
Fiber length and diameter: Fiber dimensions are characterized by
their aspect ratio l/d where l is the fiber length and d is the
diameter.
The strength improves when the aspect ratio is large.
Typical fiber diameters are from 10 mm to 150 mm.
Fibers often fracture because of surface imperfections. Making
the diameter small reduces its surface area, which has fewer
flaws.
Long fibers are preferred because the ends of the fiber carry less
of the load. Thus the longer the fiber, the fewer the ends and the
higher the load carrying capacity of the fibers.
MECH 473 ~ LECTURE 15:
COMPOSITES
CHARACTERISTICS OF FIBER
REINFORCED COMPOSITES
-As can be seen
from this plot, the
strength of the
composite
increases as the
fiber length
increases (this is a
chopped E-glassepoxy composite)
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER ORIENTATION
-Maximum strength is
obtained when long fibers are
oriented parallel to the
applied load
-The effect of fiber orientation
and strength can be seen in
the plot
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER ORIENTATION
-The properties of fiber
composites can be
tailored to meet different
loading requirements
-By using combinations of
different fiber orientation
quasi-isotropic materials
may be produced
Figure (a) shows a unidirectional arrangement
Figure (b) shows a quasi-isotropic arrangement
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER ORIENTATION
-A three dimensional
weave is also possible
-This could be found
when fabrics are knitted
or weaved together
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER PROPERTIES
In most fiber-reinforced composites, the fibers are strong, stiff
and lightweight.
If the composite is to used at elevated temperatures, the fiber
should also have a high melting temperature.
The specific strength and specific modulus of fibers are
important characteristics given by:
Specific Strength
TS
r
Specific modulus 
E
r
Where TS is the tensile strength, E is the elastic modulus
and r is the density.
MECH 473 ~ LECTURE 15:
COMPOSITES
FIBER PROPERTIES
-On the left is a graph
showing specific strength
vs. specific modulus for
different types of fibers
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COMPOSITES
TYPES OF FIBERS
Some commonly used fibers for polymer matrix composites:
-Glass fibers
-Carbon fibers
-Aramid fibers
Some commonly used fibers for metal matrix composites:
-Boron fibers
-Carbon fibers
-Oxide ceramic and non-oxide ceramic fibers
MECH 473 ~ LECTURE 15:
COMPOSITES
GLASS FIBERS
-Due to the relatively inexpensive cost glass fibers are the
most commonly used reinforcement
-There are a variety of types of glass, they are all compounds
of silica with a variety of metallic oxides
Designation:
Property or Characteristic:
E, electrical
low electrical conductivity
S, strength
high strength
C, chemical
high chemical durability
M, modulus
high stiffness
A, alkali
high alkali or soda lime glass
D, dielectric
low dielectric constant
-The most commonly used glass is E-glass, this is the most
popular because of it’s cost
MECH 473 ~ LECTURE 15:
COMPOSITES
CARBON FIBERS
-Carbon fibers have gained a lot of popularity in the last two
decades due to the price reduction
“Carbon fiber composites are five times stronger than 1020
steel yet five times lighter. In comparison to 6061 aluminum,
carbon fiber composites are seven times stronger and two
times stiffer yet still 1.5 times lighter”
-Initially used exclusively by the aerospace industry they are
becoming more and more common in fields such as
automotive, civil infrastructure, and paper production
MECH 473 ~ LECTURE 15:
COMPOSITES
ARAMID FIBERS
-Aramid fibers are also becoming more and more
common
-They have the highest level of specific strength of all
the common fibers
-They are commonly used when a degree of impact
resistance is required such as in ballistic armour
-The most common type of aramid is Kevlar
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COMPOSITES
COMPARATIVE COST OF FIBER
REINFORCEMENT
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COMPOSITES
COMMERCIALLY AVAILABLE
FORMS OF REINFORCEMENT
-Filament: a single thread like fiber
-Roving: a bundle of filaments wound to form a large
strand
-Chopped strand mat: assembled from chopped filaments
bound with a binder
-Continuous filament random mat: assembled from
continuous filaments bound with a binder
-Many varieties of woven fabrics: woven from rovings
MECH 473 ~ LECTURE 15:
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COMMERCIALLY AVAILABLE
FORMS OF REINFORCEMENT
Above Left: Roving
Above Right: Filaments
Right: Close up of a roving
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COMPOSITES
COMMERCIALLY AVAILABLE
FORMS OF REINFORCEMENT
Random mat and woven fabric
(glass fibers)
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COMPOSITES
COMMERCIALLY AVAILABLE
FORMS OF REINFORCEMENT
Carbon fiber woven fabric
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COMPOSITES
MATRIX MATERIALS
MECH 473 ~ LECTURE 15:
COMPOSITES
POLYMER MATRIX MATERIAL
-There are two basic categories of polymer matrices:
-Thermoplastics
-Thermoset plastics
-Roughly 95% of the composite market uses thermosetting
plastics
-Thermoseting plastics are polymerized in two ways:
-By adding a catalyst to the resin causing the resin to ‘cure’,
basically one must measure and mix two parts of the resin
and apply it before the resin cures
-By heating the resin to it’s cure temperature
MECH 473 ~ LECTURE 15:
COMPOSITES
POLYMER MATRIX MATERIAL
Common thermosetting plastics:
-Phenolics: good electrical properties, often used in
circuit board applications
-Epoxies: low solvent emission (fumes) upon curing, low
shrink rate upon polymerization which produces a
relatively residual stress-free bond with the reinforcement,
it is the matrix material that produces the highest strength
and stiffness, often used in aerospace applications
-Polyester: most commonly used resin, slightly weaker
than epoxy but about half the price, produces emission
when curing, used in everything from boats to RVs to
piping to Corvette bodies
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COMPOSITES
METAL MATRIX MATERIAL
Common Metal Matrices:
-Metal martices include aluminum, magnesium, copper,
nickel, and intermetallic compound alloys
-MMCs are better at higher temperatures than PMCs
although production is much more difficult and expensive
-MMCs can have applications such as fan blades in
engines, clutch and brake linings, engine cylinder liners,
etc.
MECH 473 ~ LECTURE 15:
COMPOSITES
MANUFACTURING WITH
POLYMER MATRIX MATERIALS
MECH 473 ~ LECTURE 15:
COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
-The method of manufacturing composites is very
important to the design and outcome of the product
-With traditional materials one starts out with a blank piece
of material ie: rod, ingot, sheet, etc and works it to produce
the desired part.
-However, this is not the case with polymer-matrix
composites.
-With these composites the material and the component
are being produced at the same time, therefore we aim for
the product to be a net or near net shape with little to no
post processing
MECH 473 ~ LECTURE 15:
COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Hand Lay-Up/Spray-Up
-Oldest and most commonly used manufacturing method
-Usually used to produce polyester or epoxy resin parts such as boat
hulls, tanks and vessels, pick-up truck canopies
-The method is quite simple, the resin and reinforcement is placed
against the surface of an open (one sided) mold and allowed to cure or
in the case of spray-up the resin/reinforcement is sprayed onto the
mold with a spray gun
-Often a gel coat is applied to the mold prior to
produce a better surface quality and protect the
composite from the elements
-A gel coat is a resin usually 0.4 to 0.7 mm
thick, commonly seen on the outer surface of
smaller boats
MECH 473 ~ LECTURE 15:
COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Hand Lay-Up/Spray-Up
-The pros of this process include: low initial start up cost, easy to
change mold/design, on-site production possible (ie portable process)
-The cons include: labor intensive, the quality of parts depends on
operator’s skill and therefore inconsistent, only one good side to the part
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Prepreg
-A pregreg (short for preimpregnated) is a composite that
comes with the resin already added to the reinforcement
-This means that the only concern when working with
prepreg is shaping the part
-Since the resin is already mixed (resin and catalyst) there
is a limited shelf life
-For the same reason prepreg must be cured in an oven or
autoclave
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Prepreg
-Prepreg can be used in a few different ways
-It can be placed against a mold similar to the hand lay-up method
-Once placed in the mold the material must be compressed and cured
according to a specific pressure/temperature cycle
-This is often done by means of a vacuum bag where a thin plastic cover is
secured overtop of the composite and the air is vacuumed out
-This process can reduce manufacturing time and produce a stronger part
(if a knitted preform is used)
-Another process used is ‘automated tape lay-up’
-This process uses a large automated roller similar to a packing tape roller
-The roller applies the tape with pressure which eliminates the need for a
vacuum bag
- Automated tape lay-up is used to produce large parts, generally in
aerospace applications and is also capable of 3-d parts
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Pultrusion
-Similar to extrusion of metal parts
-Pultrusion involves pulling resin-impregnated glass strands through a die
-Standard extruded shapes can easily be produced such as pipes,
channels, I-beams, etc.
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Pultrusion
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Filament Winding
-A continuous reinforcement, either previously impregnated or impregnated
during winding is wound around a rotating mandrel to form a composite part
-Pros: fast lay-up speed, very accurate and repeatable product, possibility to
use continuous fiber
-Cons: expensive equipment, high cost for mandrel, poor surface finish,
some shapes not possible
-Examples: oxygen bottles for firemen, rocket motors, tennis rackets, shafts
MECH 473 ~ LECTURE 15:
COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Filament Winding
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Filament Winding
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MANUFACTURING OF POLYMERMATRIX COMPOSITES
Resin Transfer Molding
-Resin transfer molding is a manufacturing method that is
quite similar to injection molding where plastic is injected
into a closed mold
-In the RTM process the preform (precut piece(s) of
reinforcement) is placed in the mold, the mold is closed
and the thermoset plastic matrix is injected into the mold,
once the matrix is cured the part is ejected
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Resin Transfer Molding
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Resin Transfer Molding
Pros:
-Complex components can be produced
-Components have two good surfaces
-Component can be created within a fairly tight tolerance
-High level of repeatability
-Process can be automated and repeated
-Process can be numerically modeled and analyzed
Cons:
-Molds often need to be designed using trial and error methods
-‘race tracking’ may occur
-‘wash out’ may occur
-Air voids are easily formed with poor process parameters
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COMPOSITES
MANUFACTURING OF POLYMERMATRIX COMPOSITES
Resin Transfer Molding
Researcher from Aerospace Manufacturing Technology Center
in Montreal molding members for a helicopter
MECH 473 ~ LECTURE 15:
COMPOSITES
THE END
ANY QUESTIONS?