1. Composites Introduction:
Modern technology require materials with combinations of properties that can’t be met by conventional materials
Composite : any multiphase material that exhibits a significant proportion of properties of both constituent phases such that a better combination of properties is realized.
This can be made with two or more distinct materials
It is a multiphase material that is artificial made. The constituent phases must be chemically dissimilar & separated by a distinct interface
Many composites are composed of two phases, the matrix which is continuous & surrounds the other phase, the dispersed phase

2. Classification for various types of composites

3. Particle Reinforced Composites
Sub classified as large particle & dispersion strengthened composites
Large Particle Composites
The term large indicate that particle-matrix interactions can’t be treated on the atomic or molecular level
The particulate phase is stiffer & harder than the matrix
These particles tend to restrain movement of the matrix phase in the vicinity of each particle due to its small size & even distribution
The volume fraction of the two phases influences the behavior; mechanical properties are enhanced with particulate content increment
The rules of mixtures equations predict that elastic modulus should fall between upper bound :
Ec (u)=EmVm + EpVp
& lower bound,
E= elastic modulus, V= volume fraction, c=composite, m=matrix, p=particulate phases

5. Concrete
A common example of large particle composite in which both matrix & dispersed phases are ceramic materials
Concrete is a composite material consisting an aggregate of particles that are bound together in a solid body.
The binding medium is the cement
Example: Portland cement concrete
The ingredients-portland cement, fine aggregate (sand), coarse aggregate (gravel) & water
The aggregate particles act as filler material
The aggregates comprise 60%-80% of total volume
The amount of cement-water paste should be sufficient to coat sand & gravel particles, if not cementious bond will be incomplete
Addition of correct quantity of water will ensure a complete bonding & sufficient porosity
Another example :reinforced concrete
The concrete is reinforced with steel rods, wires or mesh
Capable to support greater tensile, compressive & shear stresses

6. Dispersion Strengthened Composites
Particles normally smaller. Diameters between 0.01 & 0.1 m
Particle matrix interactions that lead to strengthening occur on the atomic or molecular level
Whereas, the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations.
Plastic deformation is restricted. Yield, tensile strength, hardness improved
Metal & metal alloy can be strengthened by this way by involving interactions between particles & dislocations within the matrix
The strengthening is retained at elevated temperatures & for extended time periods due to the dispersed particles chose to be unreactive with the matrix phase

7. Fiber Reinforced Composites
The dispersed phase in fiber form
Designed to have high strength &/or stiffness on weight basis. Expressed in specific strength & modulus
These correspond to the ratios of tensile strength to specific gravity & modulus of elasticity to specific gravity
Short fiber don’t produce significant improvement
Influence of fiber length
Some critical fiber length is necessary for effective strengthening & stiffening
lc , critical length, dependent on
d , fiber diameter & tensile strength, & c, fiber –matrix bond strength

9. Influence of Fiber Concentration and Orientation
Fibers which l>> lc (normally l> 15 lc) are termed continuous. Shorter than this are discontinuous or short fibers
For discontinuous fibers of lengths significantly less than lc, the matrix deforms around fiber. This results in no stress transference & little reinforcement by the fiber.
Thus, for strength improvement of composites, the fibers must be continuous
Influence of Fiber Concentration and Orientation
Continuous fibers are normally aligned as in figure (a) !

10. Tensile Stress-Strain Behavior-Longitudinal Loading
Continuous & Aligned Fiber Composites
Tensile Stress-Strain Behavior-Longitudinal Loading
In this figure, fracture strength in tension for fiber & matrix & ,
& their corresponding fracture strains, & ; It is assume that 
However, a fiber reinforced composite consist of these fiber & matrix materials will exhibit the uniaxial stress strain response

11. In the initial Stage I region, both fibers & matrix deform elastically; normally this portion of the curve is linear.
Typically, for a composite of this type, the matrix yields & deforms plastically at while the fibers continue to stretch elastically, in as much as the tensile strength of the fibers is significantly higher than the yield strength of the matrix.
This process constitutes Stage II which stage is ordinarily very nearly linear, but of diminished slope relative to Stage II.
In passing from Stage I to Stage II, the proportion of the applied load that is borne by the fibers increases.
The onset of composite failure begins as the fibers starts to fracture, which corresponds to a strain approximately .
Composite failure is not catastrophic. The reason:
Not all fibers fracture at the same time
These fractured fibers are still embedded within the intact matrix, thus capable of sustaining diminishing load as the matrix continues to plastically deform

12. Elastic behavior-Longitudinal Loading
It is assumed that the fiber-matrix interfacial bond is very good, such that deformation of both matrix & fibers is the same
The total load sustained by the composite Fc is equal to the loads carried by the matrix phase Fm & the fiber phase Ff or
From the stress formula, F=A, the substitution of these into above formula yields:
Then, the equation is divided by Ac , where Am/Ac & Af/Ac are the area fractions of the matrix & fiber phases. If the composite, matrix, & fiber phase lengths are all equal, Am/Ac is equivalent to the volume fraction of the matrix Vm, & for the fibers, Vf=Af/Ac
If an isostrain state means: c=m=f, by dividing the above equation with its respective strain:

13. If composite, matrix, & fiber deformations are all elastic, then c/c=Ec, m/m=Em, f/f=Ef, the E’s being the moduli of elasticityfor respective phases.
Then we substitute these into the previous equation which yields an expression for the modulus of elasticity of a continuous & aligned fibrous composite in the direction of alignment, Ecl, as
Since the composite consists of only matrix & fiber phases; that is Vm+Vf=1
Ecl is equal to the volume-fraction weighted average of the moduli of elasticity of the fiber and matrix phases.
Other properties including density, also have this dependence on volume fractions.

14. Longitudinal Tensile Strength
Failure of this type of composite material is a relatively complex process. Several different failure modes are possible
The mode that operates for a specific composite will depend on fiber & matrix properties, & the nature & strength of the fiber-matrix interfacial bond
If , then fibers will fail before the matrix
Once the fibers have fractured, the majority of the load that was borne by the fibers is now transferred to the matrix
From equation: , we obtain the following expression for the longitudinal strength of composite, :

15. Transverse Tensile Strength
Transverse tensile load might be present during in service applications
Under these circumstances, premature failure may result inasmuch as transverse strength is usually extremely low
Whereas, longitudinal strength is dominated by fiber strength, a variety of factors will influence on the transverse strength:
Properties of both fiber & matrix
Fiber-matrix bond strength
Presence of voids

16. Discontinuous & Aligned Fiber Composites
Advantage : short fiber composites can be produced having moduli of elasticity & tensile strength that approach 90% & 50% of continuous fiber counterparts.
For a discontinuous & aligned fiber composite having a uniform distribution of fibers & in which l>lc.
The longitudinal strength ( ) is given by the relationship:
Where, & represents, the fracture strength of the fiber & the stress in the matrix when the composite fails.
If the fiber length is less than critical (l<lc), then the longitudinal strength is given by:
d=fiber diameter
c=the smaller of either fiber matrix bond strength or matrix shear yield strength

17. Discontinuous & Randomly Oriented Fiber Composites
For these kinds of fiber orientation, an expression for the elastic modulus similar to rule of mixture may be utilized:
K=fiber efficiency parameter that depends on Vf & Ef/Em ratio

18. The Fiber Phase
An important characteristic of most materials, i.e. brittle one, small diameter fiber is much stronger than the bulk material
On the basis of diameter & character, fibers are classified to:
Whiskers
Fibers
Wires
Whiskers-very thin single crystals that have extremely large length-to-diameter ratio
Due to their small size, they have high degree of crystalline perfection, flaw free, high strengths, known as strongest material
Not utilized as reinforcement medium because they are extremely expensive. Impractical to incorporate into matrix
Whisker materials include graphite, silicon carbide, silicon nitride & aluminum oxide
Fibers-are either polycrystalline or amorphous & have small diameter. Generally polymer or ceramics
Fine wires-relatively large diameters. Typical materials include steel,molybdenum & tungsten

19. The Matrix Phase
The matrix phase of fibrous composites can be a metal, polymer or ceramic
Metal & polymers are used as matrix materials due to their ductility
For ceramic-matrix composites, reinforcing component is added to improve fracture toughness
For fiber reinforced composites, matrix phase serves as:
To bind the fibers together & acts as the medium by which an externally applied stress is transmitted & distributed to the fibers; only a very small proportion of an applied load is sustained by the matrix
To protect individual fibers from surface damage as a result of mechanical abrasion or chemical reactions with the environment
Matrix phase serves as barrier to crack propagation. Although some individual fibers fail, total composite fracture will not occur until large numbers of adjacent fibers have failed

20. Polymer-Matrix Composites
Consist of a polymer resin as the matrix, with fibers as the reinforcement medium
Glass Fiber Reinforced Polymer Composites
Fiberglass is a composite consisting of glass fibers, either continuous or discontinuous, contained within a polymer matrix
Produced in largest quantities.Fiber diameters range between 3 & 20 m
Glass is popular because:
Easily drawn into high strength fibers from molten state
Readily available & maybe fabricated into glass-reinforced plastic economically using wide variety of composite manufacturing techniques
As a fiber, it is relatively strong & when embedded in a plastic matrix, it produces a composite having a very high specific strength
When coupled with various plastics, it possesses a chemical inertness that renders the composite useful in a variety of corrosive environments

21. Carbon Fiber Reinforced Polymer Composites
Newly drawn fibers are coated during drawing. This is made from a thin layer of substance that protects the fiber surface from damage & undesirable environmental interactions
Coatings are desirable to avoid surface flaws which are easily introduced by rubbing the surface with harder materials
Limitations:
Not very stiff, do not display rigidity that is necessary for some applications
Limited to applications below 200oC
Carbon Fiber Reinforced Polymer Composites
Commonly used for reinforced in advanced (i.e.nonfiberglass) polymer-matrix composites
Reasons of application:
Carbon fibers have the highest specific modulus & specific strength of all reinforcing fiber materials
Retain high tensile modulus & high strength at elevated temperatures; high temperature oxidation
At room temperature; not affected by moisture & wide variety of chemicals
Relatively inexpensive & cost effective

22. Aramid Fiber Reinforced Polymer Composites
The stable form of crystalline carbon at ambient temperature is graphite
Carbon fibers are not totally crystalline but are composed of graphitic & non crystalline region
The classification scheme is based on tensile modulus: standard, intermediate, high & ultrahigh moduli
Range of fiber diameters between 4-10 m. Available in continuous & chopped form
Usually coated with protective epoxy to improve adhesion with polymer matrix
Aramid Fiber Reinforced Polymer Composites
Known chemically as poly paraphenylene terephtalamide
High strength, high modulus, tough; resistance to impact, creep & fatigue failure. High longitudinal tensile strength & tensile moduli
Desirable for their outstanding strength to weight ratios (superior than metal)

23. Schematic representation of mer & chain structures for aramid
Schematic representation of mer & chain structures for aramid. Chain alignment with the fiber direction & hydrogen bonds between adjacent chains are shown
Aramid are thermoplastic, but combustion resistant & stable to relatively high temperature. They can retain their high mechanical properties between -200oC & 200oC.
Chemically susceptible to degradation by strong acids & bases, but relatively inert in other solvents & chemicals
Most often used in composites have polymer matrices; common matrix materials are epoxies, & polyesters.
The fibers are relatively flexible & somewhat ductile, they maybe processed by common textile operations

24. Other Fiber Reinforcement Materials
Common fiber reinforcements incorporated in polymer matrices:
Glass, carbon & aramids
Others (but not that common): boron, silicon carbide, aluminum oxide
Polymer Matrix Materials
The matrix determines the maximum service temperature, since it normally softens, melts, or degrades at much lower temperature than the fiber reinforcement
Polyesters & vinyl esters are widely utilized due to inexpensiveness
Epoxies are more expensive but have better mechanical properties & resistance to moisture
Polyimide resins can be used for high temperature application i.e. up to 230oC.
Other materials-polyetheretherketone (PEEK), polyphenylene sulfide (PPS), & poly etherimide (PEI).

25. Metal Matrix Composites
The matrix is a ductile metal
Maybe utilized at higher service temperatures than their base metal counterparts.
The reinforcement may improve specific stiffness, strength, abrasion resistance, creep resistance, thermal conductivity & dimensional stability
Advantage over polymer-matrix composites: higher operating temperature, non flammability, greater resistance to degradation by organic fluids. However, more expensive
Superalloy, as well as alloys of aluminum, magnesium, titanium, & copper, are employed as matrix materials
Reinforcement might be in particulates, continuous & discontinuous fibers, whiskers. Concentrations normally range between 10 & 60 vol%.
Continuous fiber materials: carbon, silicon carbide, boron, alumina & refractory metals. Discontinuous-silicon carbide whiskers, chopped fibers of alumina & carbon, & particulates of silicon carbide & alumina

26. Some matrix-reinforcement combinations are highly reactive at elevated temperatures
Composite degradation maybe caused by high temperature processing or by subjecting the metal-matrix composites to elevated temperature during service
This is resolved by applying protective surface coating to the reinforcement or by modifying the matrix alloy composition

27. Ceramic Matrix Composites
The fracture toughness of ceramics have been improved significantly by these composites where particulates, fibers, or whiskers of one ceramic materials have been embedded into a matrix of another ceramics
Ceramic matrix composite materials have extended fracture toughness between 6 & 20 MPam
The improvement in the fracture properties results from interactions between advancing cracks & dispersed phase particles
Crack initiation normally occurs with the matrix phase whereas crack propagation is impeded or hindered by the particles, fibers or whiskers

28. In general, increasing fiber content improves strength & fracture toughness
There is also a considerable reduction in the scatter of fracture strengths for whisker-reinforced ceramics relative to their unreinforced counterpart
In addition, these ceramic matrix composites exhibit improved high temperature creep behavior & resistance to thermal shock

29. Carbon-Carbon Composites
Both reinforcement & matrix are carbon
Newly developed & expensive. Not extensively utilized
Desirable properties include:
High tensile moduli & tensile strength that are retained up to 2000oC
Resistance to creep
Large fracture toughness value
Low coefficients of thermal expansion
High thermal conductivities
Low susceptibility to thermal shock
Hybrid Composite
Obtained by using two or more different kind of fibers in a single matrix
Hybrids have better all around combination of properties than composites containing only single fiber type
A variety of fiber combinations & matrix materials are used, usually are from carbon & glass fibers which incorporated into polymeric resin
The glass-carbon hybrid is stronger & tougher, has higher impact resistance, maybe produced at lower cost

30. Structural Composites
When hybrid composites are stressed in tension, failure is usually non catastrophic
The carbon fibers are the first to fail, at which time the load is transferred to the glass fibers
Upon failure of the glass fibers, the matrix phase must sustain the applied load
Eventual composite failure concurs with that of the matrix phase
Structural Composites
Laminar Composites
Composed of 2 dimensional sheets or panels that have a preferred high strength directions as in wood & continuous & aligned fiber reinforced plastics
The layers are stacked & subsequently cemented together such that the orientation of the high strength direction varies with each successive layer

31. Laminar composite has relatively high strength in a number of directions in 2D plane, but the strength in any given direction is lower than it would be if all the fibers were oriented in that direction

32. Sandwich Panel Considered to be a class of structural composites
Consist of 2 strong outer sheets or faces, separated by a layer of less dense material, core which has lower stiffness & lower strength
The faces bear most of the in-plane loading & also any transverse bending stresses
Typical face materials- aluminum alloys, fiber reinforced plastics, titanium, steel, plywood
Structurally, the core serves 2 functions.
Separates the faces & resists deformations perpendicular to the face plane
Provides a certain degree of shear rigidity along planes that are perpendicular to the faces
Core materials-foamed polymers, synthetic rubbers, inorganic cements
Another popular core consists of honeycomb structure-thin foils that have been formed into interlocking hexagonal cells
The material maybe the same as face material