1. ENS 205 Materials Science I
Chapter-14: Composites

3. Composites
Composites are composed of two or more different materials to obtain a combination of properties not available in either of the materials.
Usually 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 on 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.

4. Composites
The properties of a composite depend on the form of the reinforcement.

5. Composites -Examples
Concrete
Fiberglass
Plywood

6. MANUFACTURING COMPOSITES.

7. Rafele jet fighter
Airbus-320

8. Composite Combinations
Composite can be classified into three basic types. They are:
PMC - Polymer Matrix Composites
By far the most common type of composite material.
Matrix is relatively soft and flexible.
Reinforcement must have high strength and stiffness
As the load must be transferred from matrix to reinforcement, the reinforcement-matrix bond must be strong.
CMC – Ceramic Matrix Composites
Matrix is relatively hard and brittle
Reinforcement must have high tensile strength to arrest crack growth
MMC - Metal Matrix Composites
MMC composites use three basic types of particulates or fibers:
1) Dispersion strengthened alloys
2) Regular particulate composites
3) Long fiber reinforcements

9. Dispersioned Strengthened MMC’s
These composites have little or no interaction between the two components and the particulate reinforcement is not soluble in the metal matrix.
The dispersoids are usually 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.

10. Examples of Dispersioned Strengthened MMC’s
PbO

11. Thoria Dispersed Nickel (TD-Ni) Composites
Produced 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.
Electron micrograph of Th-Ni with 300 nm diameter ThO2 particles

12. Regular particulate MMC’s: Cemented Carbides (Cermets)
Cemented carbides are an example of regular particulate MMC’s.
Co-WC cermets are produced by pressing Co and W powders into compacts, which are heated above the melting point of Co.
On cooling the WC 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.
Other ceramics such as TaC and TiC are also used to make Cermets.
Microstructure of WC-20% Co cermet (x1000)

13. CEMENTED CARBIDES (CERMETS)
Cemented carbides are commonly used as inserts for cutting tool inserts
I’m sure you’ve seen these in the machine shop.
This hard ceramic is very brittle so cracks or chips under impact loads.
Cutting tool inserts
milling tool
lathe tool

14. Particulates MMC’s for Electical Contacts
The highly conductive metals such as Cu and Ag are relatively soft and thus show excessive wear when used in switches and relays resulting in arcing and poor contact.
Ag reinforced with W particles has reasonable conductivity with excellent wear properties.
The composite is made in two stages:
A low density W compact with interconnected pores is first produced by pressing and firing tungsten powders.
Liquid silver is then infiltrated into the connected voids under vacuum
The continuous matrix of solidified Ag provides good conductivity while the continuous particles of W gives the required wear resistance. (see next page)

15. Particulates MMC’s for Electrical Contacts
Production of Ag-W electrical contacts
Pressing of W powders to form compact
Sintering of pressed compact to form open pore structure
Infiltration of liquid silver into the open pore structure. c b

16. Boron fiber Reinforced Al Composites
Boron fibers are used to stiffen Al alloys for aircraft structures and fan blades because B has a density of 2.36 g/cm3 compared to 2.7 g/cm3 for Al and a Young’s modulus of 380 GPa compared to 69 GPa for Al. What is Young’s modulus?
To improve the bond with the Al matrix, the B fibers are coated with SiC, which is the origin of the trade name Borsic used for the fibers.

17. Characteristics of Fiber-Reinforced Composites
Many factors must be considered when designing a fiber-reinforced composite including the length, diameter, orientation, amount and properties of the fibers and matrix, and the bonding between the fibers and matrix.
Fiber length and diameter: Fiber dimensions are characterized by their aspect ratio l/d where l is the fiber length and d is its 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 load carrying capacity is less at the ends than the remainder. Thus the longer the rod, the fewer the ends, the higher the load carrying capacity.

19. Patterns of fiber reinforcement

20. Different Forms

21. 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-glass-epoxy composite)

22. Effect of fiber Orientation
Maximum strength is obtained when long fibers are oriented parallel to the applied load.
Effect of fiber orientation on tensile strength of E-glass fiber-reinforced epoxy composite.

23. Property Averaging: Isostrain

24. Isostrain
Due to the geometry, the area fraction is equal to the volume fraction.

26. Rule of Mixtures Load is transferred to fiber. Modulus of Elasticity:
The rule of mixtures is 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:
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:
Load is transferred to fiber.

27. Rule of Mixtures: Isostress
When the load is applied perpendicular to the fibers, each component of the composite acts independently of the other.

29. Effect of 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

30. Effect of Fiber Orientation.
A three-dimensional weave for fiber-reinforced composites.
This could be found when fabrics are knitted or weaved together

31. Specific 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: .
Where TS is the tensile strength, E is the elastic modulus and r is the density.

32. 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
-Oxide ceramic and non-oxide ceramic fibers .

33. Types of 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.

34. 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, and civil infrastructure. .

35. 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. .

36. Comparative Costs of fiber Reinforcements

37. Random mat and woven fabric (glass fibers)
Commercially Available Forms Of Reinforcement .
Random mat and woven fabric
(glass fibers)

38. Commercially Available Forms Of Reinforcement.
Carbon fiber woven fabric

39. Polymer Matrix Material
There are two basic categories of polymer matrices:
Thermoplastics
Thermoset plastics
Roughly 95% of the composite market uses thermosetting plastics
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 .

41. METAL MATRIX MATERIAL Common Metal Matrices:
Metal matrices 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. .

42. Sandwich Structures
a) A hexagonal cell honeycomb core, b) can be joined to two face sheets by means of adhesive sheets, c) producing an exceptionally lightweight yet stiff and strong honeycomb sandwich structure.

43. Sandwich Structures
In the corrugation method for producing a honeycomb core, the material such as Al is corrugated between two rolls, which are joined together with adhesive and then cut to the desired thickness. The plastic deformation will work harden the Al.

44. Manufacturing Techniques
Reinforcement and Matrix may be composed during manufacturing of the part
Reinforcement and Matrix may come composed as layers called Prepregs
Prepreg: an assembly of reinforcement impregnated with resin, prepared for preforming into a composite shape before the curing process used to set the resin.

45. Spray Lay-up (Wet)
Fiber is chopped in a hand-held gun and fed into a spray of catalysed resin directed at the mould.
Pros: well established, cheap and quick
Cons: Resin-rich and heavy, only short fibers so limited mechanical properties, limited mechanical/thermal properties of resin comes with lower viscosity sprayable resin, The high styrene contents potential to be more harmful
Applications: Simple enclosures, lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubs

46. Hand Lay-up (Wet)
Resins are impregnated by hand into fibers which are in the form of woven, knitted, stitched or bonded fabrics.
Pros: Well established, simple, low cost tooling, higher fiber contents, and longer fibers than with spray lay-up.
Cons: skill dependent, high resin content or excessive quantities of voids, Health and safety considerations of resins, need expensive extraction systems.
Applications: Standard wind-turbine blades, production boats, architectural moldings.

47. HAND and SPRAY LAY-UP
Hand and spray lay-up are often mixed : hand lay-up : fabric plies, spray lay-up : short fiber plies.

48. Vaccum Bagging
An extension of the wet lay-up process pressure up to one atmosphere is applied to improve its consolidation.
Pros: Higher fiber and lower void contents than with standard wet lay-up techniques. Better fiber wet-out less amount of volatiles emitted during cure.
Cons: The extra cost both in labour and in disposable bagging materials resin content still largely determined by operator skill
Applications: Large, one-off cruising boats, racecar components, core-bonding in production boats

49. Filament Winding
Fiber tows are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fiber feeding mechanism, and rate of rotation of the mandrel
Pros: This can be a very fast, Resin content can be controlled by metering the resin onto each fibre tow through nips or dies. Fiber cost is minimized (tows, not fabric) good structural properties
Cons: convex shaped only, Mandrel costs for large components can be high.
Applications: Chemical storage tanks and pipelines, gas cylinders, fire-fighters' breathing tanks

50. FILAMENT WINDING
Impregnated fibers are rolled up on a rotary mandrel, then cured in an oven

51. Fibers are pulled from a creel through a resin bath and then on through a heated die. The die completes the impregnation of the fiber, controls the resin content and cures the material into its final shape as it passes through the die. Fabrics may also be introduced into the die to provide fibre direction other than at 0°.
Pros: fast and economic, accurately controlled resin content, minimized fiber cost, superior structural properties
Cons: Limited to constant or near constant cross-section components, Heated die costs can be high.
Applications: Beams used in roof structures, bridges, ladders, frameworks
Pultrusion

52. PULTRUSION
Impregnated fibers are continuously pulled through a die in which the resin is cured

53. RTM- Resin Transfer Molding
Fabrics or preforms are laid up as a dry stack of materials. A second mould tool is then clamped over the first, and resin is injected into the cavity. Vacuum can also be applied to the mould cavity to assist resin in being drawn into the fabrics.
Pros: High fiber volume low void contents, ) Good environmental control due to enclosure of resin. Both sides of the component have a moulded surface.
Cons: Matched tooling is expensive, and heavy, Generally limited to smaller components. Unimpregnated areas can occur
Applications: Small complex aircraft and automotive components, train seats

54. VARTM- Vacuum Assisted RTM
Similar to RTM, vacuum bagged instead of closed mold
Pros: Much lower tooling cost, fabrication of large components, standard wet lay-up tools may be able to be modified for this process.
Cons: Resins must be very low in viscosity, thus comprising mechanical properties, potential for un-impregnated areas
Applications: Semi-production small yachts, train and truck body panels

55. Prepreg Autoclave Moulding
The prepregs are laid up by hand or machine onto a mould surface, vacuum bagged and then heated to typically °C. Additional pressure for the molding is usually provided by an autoclave
Pros: resin content accurately set by the materials manufacturer, excellent health and safety characteristics, very good quality, mechanical properties
Cons: High material and process cost, slow to operate and limited in size, tooling needs to be able to withstand the process temperatures involved
Applications: Semi-production small yachts, train and truck body panels