20-256-695:Introduction to Polymer Composite Materials
Instructor:Prof. J. O. Iroh - Winter, 2002
Textbook: "Analysis and Performance of Fiber Composites" by B. D. Agarwal and L. J. Broutman
Objective:Introduction of the concept of composite materials with particular focus on fiber reinforced polymers.
Chapter 1] Introduction
An overview of the different classes of materials and their characteristics
Chapter 2] An insight into the nature and characteristics of the constituents of polymeric composites
2.IMatrix materials
2.IIThe reinforcements
2.IIIThe interface
Chapter 3] Composites technology
Exploration of the different techniques for processing polymeric composites;
Sheet molding compounds, compression molding, reaction injection molding, thermoplastic composites
Chapter 4.Unidirectional composites/Micromechanics
4.ILongitudinal properties
4.IITransverse properties
4.IIIFailure modes
Chapter 5.Short fiber composites/Micromechanics
5.IStress transfer analysis
5.IIStiffness and strength of short fiber composites
5.IIIFatigue, impact and fracture toughness of short fiber composites
Chapter 6.Deformation of materials/Macromechanics
6.IOrthotropic materials
6.IIIsotropic materials
6.IIIUnidirectional lamina
Chapter 7.Test methods/Special topics /Special projects
Evaluation/Grading
Points
Attendance 5
Home work (4)20
Midterm exam20
Final exam60
1.IComposites
Composites constitute two or more chemically distinct parts combined macroscopically.
The properties of composites are superior to those of their constituents.
Examples of composites include; leaf, bones (naturally occurring composite).
Most synthetic composites may be classified as; polymer, metal and ceramic matrix composites.
A 2-phase metal alloy or 2-phase polymer alloy are composites.
The addition of additives in plastics does not result in a composite.
Why not?.
Types of Composites
Class / Fiber/MatrixMetal-matrix / B/Al, Al2O3/Al
Al2O3/Mg, SiC/Al
SiC/Ti (alloys)
Ceramic-matrix / C/C, C/SiC
SiC/Al2O3, SiC/SiC
SiC/Si3N4
Polymer-matrix / Kevlar/epoxy
Kevlar/polyester
Graphite/PEEK
Graphite/PPS
Carbon/polyimide
Other examples
Wood composed of elongated biological cells (fibers) and lignin (matrix).
Bambo stick
Composed of hard outer phase, continuous unidirectional fibers and soft foamy matrix.
Concrete
Composed of rocks and sands in a matrix of calcium aluminosilicate.
The formation of a composite must result in significant property changes.
Lets say that one of the phases is fibrous or platelet and has Vf ≥ 10%;
The expected property change should be ≥ 5 times that of the components.
Example I
An E-glass composite contain 75 vol % of fibers in epoxy matrix.
Calculate the weight % of glass fibers in the composite.
What is the density of the composite ?.
Hints:density of fibers ~ 2.54 Mg/m3, density of epoxy = 1.1 Mg/m3
Solution to example II
Continuous/Long Fiber Composites
Compare the mechanical performance of I, II & II in terms of the geometry of the preform.
Short Fiber/Discontinuous Composites
Particulate Composites
1. IICharacteristics of Composites
Composites are made up of one or more discontinuous phases embeded in a continuous phase.
The discontinuous component which is usually the harder and stronger phase is the reinforcement.
The continuous and ductile/viscoelast- -ic phase is the matrix.
The reinforcements are usually seperated from the matrix by the interface.
Some fillers have unusually lower stiffness than the matrix.
Example: Rubber-modified polymers made up of rigid polymer matrix and rubbery particles.
The properties of composites depend on the properties of the components, their amount, distribution and their interaction.
The properties of composites can be predicted by the rule of mixture.
Where P = material prop
f = composition
f, m = fiber, matrix
Properties of composites often don't obey the simple rule of mixture.
To fully describe a composite:
You must specify;
athe constituent materials
bthe properties of components
cgeometry of the reinforcement with reference to the system
The shape, size and size distribution of the components define the geometry.
The volume fraction:
I Determines the contribution of a single component
IIIs a very important parameter that influences composite properties
IIIControlled during manufacture
Redo example II
Concentration distribution:
Measure of homogeneity and uniformity of the system
Non-uniformity: area/zone weakness, crack initiation/propagation
Orientation of Components;
Affect the isotropy of the system
Particulate (equiaxied) fillers form isotropic composites and their properties are independent of direction.
Isotropic composites are also formed when fillers are randomly oriented.
An example is short fiber composites.
Orientation may be induced during manufacture.
E.g. Injection molding of short fiber composites may induce orientation of the fibers and hence induce anisotropy.
Continuous fiber composites;
IUnidirectional
IICross-ply
Anisotropy may be desirable.
In these composites the ability to control anisotropy by design and fabrication is a major advantage.
The strengthening mechanism of composites depends strongly on the geometry of reinforcement.
1.IIIClassification of Composites
Composites may be classified on the basis of geometry of representative unit reinforcement.
Fibers;Length are longer than diameter/width.
Particulate; mostly equiaxied
Particulate Composites
The fillers are non-fibrous particles possessing no long dimensions.
Reinforcements with long dimensions terminate crack propagation;
i.e. are toughening
Particles are not as effective as fibers in improving fracture resistance,Why?.
Rubber-like particles improve fracture resistance in brittle matrices. How?
Ceramics, metals and inorganic particles produce reinforcing effects in metallic matrices by different strengthening mechanism.
Particulate fillers are harder than the matrix. They therefore constrain deformation of the composite.
Particles share the applied load with the matrix to a smaller extent than unidirectional fibrous composites.
They enhance the stiffness composites but do not strengthen the composites.
A hard particles placed in a brittle matrix reduce the strength due to stress concentration in the adjacent matrix material.
Role of Particulate Fillers;
amodify the thermal and electrical conductivity
bimprove performance at elevated temperature
cthey are also used to reduce cost
dimprove the stiffness
Eg. Combination of metallic and non-metallic materials.
Choice depends on the desired end-use.
Hard particles mixed with copper alloys and steel to improve their mechineability.
Lead is used as a lubricant in bearings made of copper alloys.
Reinforcement of Cu/Ag matrices with tungsten/Cr/Mo/other carbides for electrical contact applications
Most commercial elastomers are filled with carbon-black or silica to improve their strength or abrasion resistance while maintaining their extensibility.
Cold solder constituting metal powder in thermoset matrix are hard strong and conduct heat/electricity.
Cu/epoxy increased conductivity
Pb/plastic for sound proof, shield radiation
Plastic fluorocarbon bearings; metals are added to improve thermal conductivity and lower the coefficient of thermal expansion.
Thin flakes having 2-D geometry reduce wear, impart equal strength in all direction in their plane. Note that fibers are unidirectional.
Flakes can be packed more closely than fibers and particles.
E.g. Mica is used for electrical and heat insulating applications.
Mica/Al used in paints and coatings.
Summary:Types of Composites
Nylon/Thermoset
Composite / Rubber/Ps / Metal/Metal
Composite
Glass/ Boron/ Graphite-Thermoset/Ceramics / Rubber-/Glass / Glass/
Thermoplastic,
Boron/
Graphite-Metals
E.g.
Al (m)
Steel (R)
Generally matrix and fillers are brittle. But resulting composite is ductile.
1.IVFibrous Composites
For most materials;
Measured strength < theoretical strength by about one half-order.
Why so ?.
Ipresence of flaws
IIimperfection
Elimination of flaws in materials improves their strength.
Flaws and cracks perpendicular to the direction of applied load lower the strength of materials.
Synthetic polymeric fibers have small cross-sectional dimensions.
Fiber properties (E) in fiber axis are optimized by;
Ielimination of large flaws and
IIinduced molecular orientation during manufacture
Table1.1.Characteristics of some fibers
Glass fibers have defect free surface and high .
Graphite and Kevlar fibers are very highly oriented
E-glass are the most important reinforcement fibers because of their high and low cost.
Boron, graphite, Kevlar (aramid) fibers have exceptional E values.
Graphite fibers offer the greatest variety of properties because of the controllability of their structure.
Fibers are embedded in the matrix to form fibrous composites.
The matrix:
1Holds the fibers together
2Transfers the applied load to fibers
3Protect fibers from environmental and mechanical damage
Fibrous composites may be classified as either single layer or multiple layer (angle ply) composites.
Single layer fibrous composites are made up of several layers, each layer having the same orientation and properties.
Short fiber composites show no distinct layers. They are therefore single layer composites.
In non-woven composites the random orientation is constant in each layer and the resulting composite is a single layer composite, though resin "rivers/islands" may be present.
Most composites used in structural applications are multi-layered constituting several layers of fibrous composites.
Each layer or lamina is a single-layer composite and each orientation is varied according to design.
Each layer of the composite is about 0.1 mm thin and cannot be used directly.
Several identical or different layers are bonded together to form a multilayered composite for engineering applications.
When the constituents are the same they are called laminates.
Hybrid laminates are multi-layered composites consisting of layers made up of different constituent materials.
Example: One layer of a hybrid laminate may be glass filled epoxy, while another layer may be graphite fiber filled epoxy.
A single layer of composite is the basic building block.
Types of Fibrous Composites
There are two types of fibrous composites;
IShort/discontinuous fiber composites
IILong/continuous fiber composites
The length of fibers is very important in short fiber composites because it affects their properties.
How ?
In long fiber composites the load is directly applied to the fibers.
The fibers aligned in the direction of the applied load are the major load bearing components.
The fiber volume fraction (%) must be ≥ 10% to impart high modulus to the composite.
The fibers control the failure mode of composites.
When long fibers in a single composite are aligned in one direction, unidirectionalcomposites are formed.
Unidirectional composites are very strong in the fiber direction but very weak in the direction perpendicular to the fiber direction.
Fibers in transverse direction act as stress concentrators.
They cause composite to fail prematurely.
Transverse modulus
When the fibers are layed parallel to one-another and saturated or impregnated with resin (polyester, epoxy) a prepreg is formed.
Pre-impregnated fibers are called prepregs.
Unidirectional prepregs are stacked together in various orientation to form laminates for engineering applications.
Applications
Unidirectional glass reinforced adhesive tapes are used in heavy duty sealing applications.
Unidirectional fiber composites are also used in fishing poles and other rod-like structures.
Bi-directional Composite
Continuous reinforcement in single layer may also be provided in a second direction to give more balanced properties.
In woven fabrics the composite has the same strength in the perpendicular and parallel direction.
Orientation of short fibers is not easily controlled. The fibers are assumed to randomly distributed in the matrix.
Injection molded fibrous composites show considerable orientation in the flow direction (fig 4.11).
Short fibers may be sprayed simultaneously with liquid resin against a mold to form a composite.
They can be converted into a lightly bonded preform or mat that is later impregnated with resin to fabricate single lay composites.
S.F.C are said to be isotropic. i.e. their properties do not change with direction within the plane of the sheet.
S.F. may be blended with resin to form reinforced molding compound.
Fibrous composites are characterized by:
1High Strength/stiffness
2Controlled anisotropy (table 1.2)
3 Superior specific properties than metals.
Low weight/volume ratio make composites attractive to aerospace industries.
E/ used as design parameters
Cross-ply laminates resemble bulk isotropic materials.
In unidirectional composites; longitudinal strength can be changed by changing the volume fraction of fibers.
Other Characteristics of Fibrous Composites
IProperties can be altered by changing material and manufacturing variables
IIProperties can be altered by changing the volume fraction of fibers
IIIIt is possible to form intricate shape
Applications:
Aircraft, space, land transportation, sport, construction industries.
Disadvantages
Fibers have diameter ~ 7-10m.
Summary
Composites:
ILighter than metals
IIEasy to fabricate
IIICheaper than metals
Presence of glass fibers
IIncrease notch impact strength
II Increases use temp {HDT, (Tg)}
Nylon 6,6 deflects at 66˚C.
Nylon 6,6 composites deflects at 260˚C
Used in automobile, gear pieces and are often subjected to temperature rise.
Plastic-fiber composites are more dimensionally stable than un-reinforced plastics.
Polymer Matrix Materials
Polymer / E (MPa) / (MPa) / Use Temp (˚C)PC / 2345 / 62 / 120
Polyestr / 2415 / 76 / 125
Phenolic / 3100 / 62 / 160
Epoxy / 2480 / 83 / 145
Density of Fibers and Metals
Mat / E-glass / Cfiber / K-49 / Ti / Al / steel
(g/
cm3) / 2.54 / 1.90 / 1.50 / - / 2.7 / 7.8
Properties of Fibers and Metals
Fiber / E(GPa) / (GPa / E(MPa/Kg/m3) / (MPa/Kg
/m3)
E-glass / 72.4 / 3.5 / 28.5 / 1.38
carbon
(HM) / 390.0 / 2.1 / 205.0 / 1.3
Kevlar-49 / 130.0 / 2.8 / 87.0 / 1.87
Ti / - / - / - / -
Al / 70 / .14-.62 / 25.9 / 0.052-0.23
Steel / 210 / .34-2.1 / 26.9 / 0.043-0.27
Response Material in a Structural Element
where E is the modulus, M is the moment and b is the breaking strength.
Estimate the size of beam needed to sustain a given load.
Estimate the size of beam needed to sustain a given load.
Assume beams are equivalent (EI ~ constant).
Material / EI constantheight
(h mm) / (Wt beam) /
(Wt. Steel beam)
Steel / 6.0 / 1.00
E-glass-epoxy / 12.84 / 0.54
Kevlar-epoxy / 10.44 / 0.31
Carbon fiber-epoxy / 8.19 / 0.27
Calculation using the flexural strength:
Assume M = constant
Material / b constantheight
(h mm) / (Wt beam) /
(Wt. Steel beam)
Steel / 6.0 / 1.00
E-glass-epoxy / 6.36 / 0.27
Kevlar-epoxy / 5.95 / 0.18
Carbon fiber-epoxy / 7.79 / 0.26
Material / h (mm) / (EI)beam (EI)steel / (M)beam
(M)steel
Steel / 6.00 / 1.00 / 1.00
E-glass-epoxy / 23.76 / 6.36 / 13.97
Kevlar-epoxy / 33.43 / 32.95 / 31.53
Carbon-epoxy / 30.39 / 51.36 / 15.23
The cost of the weight advantage is the volume of material used.
2.Constituent of Polymeric Composites
2.1Fibers
Fibers have high aspect ratio (l/d) >1. This allows the transfer of load through the matrix.
2.1.1Carbon Fibers
They are usually about 7-8 m in diameter.
Graphite fibers have high strength and high modulus.
Graphite fibers contain about 99-100% carbon.
Carbon fibers contain about 80-95% carbon.
The heat treatment temperature determines the carbon content.
Graphite fibers are the product of thermal decomposition of the organic precursors such as PAN, rayon and pitch.
Carbon fibers are made up of repeating units of graphitic (an allotrope of carbon) layers. A single crystal of graphite is made up of single crystals of carbon atoms arranged in hexagonal arrays and stacked on top of each other in a regular ABAB sequence. Carbon atoms in each layer or basal plane are held together by strong covalent bonds.
Adjacent layers are held together by weak van der waals forces.
The basic crystal unit is therefore highly anisotropic.
For example in-plane Youngs modulus parallel to a-axis is 910 GPa while that parallel to the c-axis (normal to the basal plane is 30 GPa. The spacing between layers is about 0.335 nm.
To form high modulus and high strength fibers the graphite layer planes must be aligned parallel to the fiber axis.
In practice graphite fiber units contain defects and imperfection. Voids and flaws act as points of stress concentration and weaken the fibers.
The properties of the fibers depend on the extent of alignment and orientation.
Different degrees of imperfection result from the different manufacturing techniques.
2.1.1iPAN Fibers
PAN is converted into carbon fibers in five distinct steps:
aspinning of PAN into precursor fiber
bstretching of the precursor fiber
cstabilization of the precursor fiber;
Fiber is held under tension at about 205-240 ˚C for 24 h in oxidizing atmosphere
dcarbonization at T~1500˚C in inert atmosphere
egraphitization at about 3000˚C in inert atmosphere
Graphitization/heat treatment is carried out at T>1800˚C. This improves the crystalline structure and forces the preferred orientation and results in improved modulus.
The PAN process gives low cost graphite fibers with good properties.
Pitch based carbon fibers are currently the least cost fibers.
Rayon based fibers are very expensive because of the extreme high temperatures required for their stretch graphitization.
Graphite fibers are available as continuous, chopped, woven fabrics or mat.
Tows, yarns, rovings and tape are the common continuous graphite fibers.
A tow consists of numerous filaments in a straight laid bundle and is specified by their number~ 400-10,000.
A yarn is a twisted tow.
A roven is a number of ends. Strands are collected in a parallel bundle with no twist and is specified by the no of ends.
A tape consists of numerous tows or yarns (300) laid side-by -side on a backing.
2.1.1iiAramid fibers
There are two types; Kevlar 29 ( high , moderate E) for tire cord reinforcement and Kevlar 49 (high and high E ~ 130 GPa) for high performance composites.
Polymer aramid fibers (Kevlar) are produced form aromatic polyamides by dry-jet wet spinning process.
Polyamides are synthesized by solution condensation polymerization of diamines and diacid halides at low temperature.
The diacid chloride are rapidly added to a cool (5-10˚C) amine solution with stirring.
The polymer is then mixed with a strong (sulfuric) acid and extracted from spinnerets at elevated temperature (51-100˚C) into cold water.
The fibers initially of about 40 MPa strength and modulus of 3.6 GPa is wound on a drum and subsequently stretched and drawn to increase the strength and modulus (El~ 130 GPa) (increased degree of alignment).
Fiber properties are altered by:
Ivarying spinning conditions