Introduction to composite materials, classification, applications.

Introduction to composite materials, classification, applications.
AComposite material is a material system composed of two or more macro constituents that differ in shape and chemical composition and which are insoluble in each other. The history of composite materials dates back to early 20th century. In 1940, fiber glass was first used to reinforce epoxy.
Applications:
Aerospace industry -
Sporting Goods Industry -
-Automotive Industry
Home Appliance Industry -
Terminology/Classification
• Matrix:
-- The continuous phase
-- Purpose is to:
- transfer stress to other phases
- protect phases from environment
-- Classification: MMC, CMC, PMC,(M-Metal, C-Ceramic, P-Polymer)
MMC: increase sy, TS, creep resist.
CMC: increase Kc(Toughness)
PMC: increase E, sy, TS, creep resist
-- Classification: Particle, fiber, structural
Dispersed phase: --
-- Purpose: enhance matrix properties.
Metal Matrix Composites (MMC)
- Metal matrix composites (MMCs), as the name implies, have a metal matrix. Examples of matrices in such composites include:
Metal matrix: Al, Ti, Mg, Fe, Cu, Ni-
Example: Al-SiC (silicon carbide)-
Example: Al-Al2O3 (aluminum oxide)-

Typical ?bers include carbon and silicon carbide.-
-Metals are mainly reinforced to increase or decrease their properties to suit the needs of design. For example, the elastic stiffness and strength of metals can be increased, and large coef?cients of thermal expansion and thermal and electric conductivities of metals can be reduced, by the addition of ?bers such as silicon carbide.
Advantages of MMC’s
-Metal matrix composites are mainly used to provide advantages over monolithic metals such as steel and aluminum. These advantages include higher speci?c strength and modulus by reinforcing low-density metals, such as aluminum and titanium; lower coef?cients of thermal expansion by reinforcing with ?bers with low coef?cients of thermal expansion, such as graphite; and maintaining properties such as strength at high temperatures.
-Advantages over polymer matrix composites. These include higher elastic properties; higher service temperature; insensitivity to moisture; higher electric and thermal conductivities; and better wear, fatigue, and ?aw resistances.
-The drawbacks of MMCs over PMCs include higher processing temperatures and higher densities.
Fabrication Method
Fabrication methods for MMCs are varied. One method of manufacturing them is diffusion bonding (Figure below), which is used in manufacturing boron/aluminum composite parts. A ?ber mat of boron is placed between two thin aluminum foils about 0.05 mm thick. A polymer binder or an acrylic adhesive holds the ?bers together in the mat. Layers of these metal foils are stacked at angles as required by the design. The laminate is ?rst heated in a vacuum bag to remove the binder. The laminate is then hot pressed with a temperature of about 500°C and pressure of about 35 MPa in a die to form the required machine element.




Applications of MMC’s
-Space: The space shuttle uses boron/aluminum tubes to support its fuselage frame. In addition to decreasing the mass of the space shuttle by more than 145 kg, boron/aluminum also reduced the thermal insulation requirements because of its low thermal conductivity. The mast of the Hubble Telescope uses carbon-reinforced aluminum.
-Military: Precision components of missile guidance systems demand dimensional stability — that is, the geometries of the components cannot change during use. Metal matrix composites such as SiC/aluminum composites satisfy this requirement because they have high microyield strength. In addition, the volume fraction of SiC can be varied to have a coef?cient of thermal expansion compatible with other parts of the system assembly.
-Transportation: Metal matrix composites are ?nding use now in automotive engines that are lighter than their metal counterparts. Also, because of their high strength and low weight, metal matrix composites are the material of choice for gas turbine engines.
Ceramic Matrix Composites (CMC)
Ceramic matrix composites (CMCs) have a ceramic matrix such as alumina calcium alumino silicate reinforced by ?bers such as carbon or silicon carbide.
-The strength of the composite depends primarily on the amount, arrangement and type of fiber (or particle) reinforcement in the resin.
-Typically, the higher the reinforcement content, the greater the strength. In some cases, glass fibers are combined with other fibers, such as carbon or aramid (Kevlar29 and Kevlar49), to create a "hybrid" composite that combines the properties of more than one reinforcing material.
Classification of Composite by Filler Type:
Particle-reinforced composites-
Fiber-reinforced composites -
Structural composites -
Particle Reinforced Composites:
Particles used for reinforcing include:-
ceramics and glasses such as small mineral particles, -
metal particles such as aluminum,
and amorphous materials, including polymers and carbon black.
-Particles are used to increase the modulus of the matrix, to decrease the permeability of the matrix, or to decrease the ductility of the matrix.
-Particle reinforced composites support higher tensile, compressive and shear stresses
Particles are also used to produce inexpensive composites. -
Examples:
-automobile tire which has carbon black particles in a matrix of elastomeric polymer.
-spheroidized steel where cementite is transformed into a spherical shape which improves the machinability of the material.
-concrete where the aggregtes ( sand and gravel) are the particles and cement is the matrix.

Fiber-reinforced Composites:
- Reinforcing fibers can be made of metals, ceramics, glasses, or polymers that have been turned into graphite and known as carbon fibers. Fibers increase the modulus of the matrix material. The strong covalent bonds along the fiber s length gives them a very high modulus in this direction because to break or extend the fiber the bonds must also be broken or moved. Fibers are difficult to process into composites which makes fiber-reinforced composites relatively expensive. Fiber-reinforced composites are used in some of the most advanced, and therefore most expensive, sports equipment, such as a time-trial racing bicycle frame which consists of carbon fibers in a thermoset polymer matrix. Body parts of race cars and some automobiles are composites made of glass fibers (or fiberglass) in a thermoset matrix.
- The arrangement or orientation of the fibers relative to one another, the fiber concentration, and the distribution all have a significant influence on the strength and other properties of fiber-reinforced composites. Applications involving totally multidirectional applied stresses normally use discontinuous fibers, which are randomly oriented in the matrix material. Consideration of orientation and fiber length for a particular composites depends on the level and nature of the applied stress as well as fabrication cost. Production rates for short-fiber composites (both aligned and randomly oriented) are rapid, and intricate shapes can be formed which are not possible with continuous fiber reinforcement.


Structural Composites:
The properties of structural composites depend on:
- Constituents
- Geometrical design

Common structural composite types are:
-Laminar: Is composed of two-dimensional sheets or panels that have a preferred high strength direction such as is found in wood and continuous and aligned fiber-reinforced plastics. The layers are stacked and cemented together such that the orientation of the high-strength direction varies with each successive layer. One example of a relatively complex structure is modern ski and another example is plywood.
-Sandwich Panels: Consist of two strong outer sheets which are called face sheets and may be made of aluminum alloys, fiber reinforced plastics, titanium alloys, steel. Face sheets carry most of the loading and stresses. Core may be a honeycomb structure which has less density than the face sheets and resists perpendicular stresses and provides shear rigidity. Sandwich panels can be used in variety of applications which include roofs, floors, walls of buildings and in aircraft, for wings, fuselage and tailplane skins.



- A lamina (also called a ply or layer) is a single ?at layer of unidirectional ?bers or woven ?bers arranged in a matrix.
- A laminate is a stack of plies of composites. Each layer can be laid at various orientations and can be made up of different material systems.




Hybrid composites contain more than one ?ber or one matrix system in a laminate.
Interply hybrid laminates: contain plies made of two or more different composite systems.
Intraply hybrid composites: consist of two or more different ?bers used in the same ply.
An interply–intraply hybrid : consists of plies that have two or more different ?bers in the same ply and distinct composite systems in more than one ply.
Resin hybrid laminates: combine two or more resins instead of combining two or more ?bers in a laminate.