Direct roving processes: The reinforcement, in the form of a spool of continuous tow or roving,

is converted directly into the component or intermediate form Spray up Filament winding Pultrusion

Moulding Processes: The reinforcement is first converted into an intermediate sheet or web

form. This might be a woven cloth, a continuous or discontinuous mat, a prepreg or sheet moulding compound. Then the sheet or web is impregnated with resin either before or during the final moulding operation Open moulding (or contact moulding) Compression moulding Prepreg manufacturing Autoclave moulding Sheet moulding compound, bulk moulding compound Injection moulding Resin transfer moulding

From previous lecture

Sandwiches Sandwiches is a class of structural composites which consist of two strong

outer sheets (faces) separated by a layer of less-dense material (core) which has lower stiffness and strength.

The bending stiffness to weight ratio is one of the most attractive properties of sandwich materials.

Faces: bear the in-plane stress and transverse bending stresses. Core: separates the faces, and provides the bending and shear rigidity

perpendicular to the faces. Face materials: aluminium alloys, FRPs, titanium, steel, plywood. Core materials: honeycomb, foamed polymers (polyurethane, polystyrene,

PVC), balsa wood. Applications:

Aircraft wings, fuselage, tailplane and rotorblade; wind turbines, boat hulls

Trains and Buildings: roofs, floors and walls Construction: bridges

Metal Matrix Composites (MMCs)

Metal Matrices vs. Polymer Matrices

Advantages of MMCs Significantly higher strength and stiffness Higher potential service temperature (200o to

1200oC) Greater contribution to composite transverse

properties Higher ductility and extensive plastic deformation

Disadvantages Higher densities More complex fabrication processes

Important Metal Matrix Materials

Al and Al Alloy Matrix: Light; inexpensive; good strength; ductility and corrosion

resistance Alloying: Al-4%Cu-Mg (2014), Al-Zn-Mg-6%Cu (7075), Al-Li Application: aerospace, sporting goods, automotive

Ti and Ti Alloy Matrix: Heavier (4.5g/cm3) than aluminum (2.71g/cm3), excellent

strength/weight ratio, high melting point (~1670oC), high service temperature, good corrosion/oxidation resistance

Extreme reactivity with reinforcements during processing at high temperature (needs fibre coating)

Application: aerospace alloys (Ti-6%Al-4%V), airfoil surfaces

Mg and Mg Alloy Matrix: Lightest among structural metals, (1.74g/cm3), mechanical

properties comparable to aluminum, poor corrosion resistance Poor wettability with carbon fibres, high production cost

Intermetallic Matrix: Ni3Al, NiAl, Ti3Al, TiAl, MoSi2: high temperature stability, good

oxidation resistance, good mechanical properties Low ductility at room temperature: ductility can be improved by

alloying with B (boron) for Ni3Al and V (vanadium) for TiAl or allowing ultrafine grain growth via PVD technique

MMCs Processing Techniques Liquid State Processes

Casting or Liquid infiltration: Fibre bundles are wetted by molten metal by passing through a bath and the coated wires are consolidated by extrusion. To improve wetting and reaction, fibres are precoated before infiltration.

Squeeze casting or Pressure infiltration: A variant of casting process. Liquid metal is forced into a fibre preform, followed by the application of pressure to facilitate wetting and infiltration.

Solid State Processes Diffusion bonding

Solid state welding, hot pressing, roll diffusion bonding Most common: solid state welding for joining similar or dissimilar metals.

Interdiffusion of atoms from clean metal surface in contact at an elevated temperature leads to welding.

Advantages: ability to process a wide variety of metals; ability to control of fibre orientation and volume fraction

Disadvantages: Long processing time (several hours), High cost of processing due to high temp and pressure, limited size of products.

Deformation process: swaging, extrusion, rolling Powder Process: cold pressing followed by sintering to produce particle or

whisker reinforced MMCs.

Deposition Techniques Spray deposition: Fibres are wound onto a foil coated drum which is

sprayed with molten metal by flame, arc or plasma to form a mono-tape. Advantages are easy control of fibre alignment and rapid solidification of molten metal.

Immersion plating Electrodeposition: coating from a solution containing the ion of the

desired material in the presence of an electric current. Chemical vapour deposition (CVD) (e.g. B/W or SiC fiber production):

A vaporized component reacts with another vaporized chemical on the substrate to form a coating at a high temp.

Physical vapour deposition (PVD): Three basic processes are evaporation, ion plating, sputtering. Advantages are versatility in the composition and microstructure of the coating produced. Good bonding to the substrate and excellent surface finish.

Properties of MMCs The strength and stiffness of continuous fibre reinforced MMCs in

general obey the rule of mixture in the longitudinal and transverse directions. The properties depend on the direction of loading: the

longitudinal properties are dominated by the fibre and the transverse properties by the matrix.

The primary advantage of MMCs over PMCs containing the same types of fibre is the maximum operating temperature: e.g. a boron-Al system offer useful mechanical properties up to

510°C, whereas the equivalent boron-epoxy system is limited to about 190°C.

Failure Mechanisms in MMCs Cavitation (Voids Formation)

Particle reinforced MMCs: cavitation followed by particle fracture and interface debonding

Short fibre reinforced MMCs: void formation at the ends of fibre due to triaxial tension, and crack growth along the fibre leading to interface debonding

A strongly bonded interface and low matrix ductility favours void initiation

Interface Debonding Debonding takes place predominantly along the fibre length A weak interface bond and a thick brittle reaction compound favours

debonding Interface debonding is often beneficial to preventing catastrophic

failure of composites Matrix Plastic Deformation

Most significant contribution to fracture toughness of MMCs

Damage Mechanisms of MMCs in Service Environments Thermal Cycling (e.g. turbine blades, brake drums, etc.)

A major driving force is CTE mismatch between the fibre and matrix under thermal cyclic loading

Oxidation Ceramic reinforcements are not susceptible to oxidation, but most

metals and alloys are, because metals’ reactivity with oxygen is relatively strong at service temperature

Oxidation initiates at fibre ends, and penetrates along the fibre length