Composites- Intro to Composites Flashcards
How do composites compare to polymers for tensile modulus and density?
Composites have similar densities but a greater YM
What is a composite material?
A material made from two or more constituent phases. These phases have different properties and remain separate within the composite
The idea behind composites
Take advantage of the properties of both phases. Specifically tailor properties to an application
What are the constituent phases in composites?
Reinforcements: typically fibres, add stiffness, strength, creep resistance, thermal stability, could be particulates.
Matrix: typically thermosetting polymer, add toughness, shear and compressive modulus and strength
One key advantage to composites
They are a lightweight alternative to traditional materials. Their specific stiffness and strength are better than other materials
What is a lamina?
Where many fibres are incorporated into a thin layer of matrix. Only one layer. Aka ply. Typically 0.1-1mm thick
Long fibres
Aka continuous fibres. If unidirectional the properties are mainly in that direction. Can be made bidirectional by weaving to reduce the anisotropy. Can be multidirectional so that the properties can be tailored to suit an application
Short fibres
Aka discontinuous fibres. If unidirectional the properties are poorer than for continuous fibres. Can be randomly oriented to give equal properties in all directions
Laminates
Consolidate several laminar into a laminate. Can be parallel laminate to give high anisotropy or cross-ply laminate to reduce anisotropy.
Hybrid laminates
Can be interply where each laminate only contains one type of fibre. Can be intraply where each laminate contains two or more kinds of interwoven fibres. Can make hybrids with other classes of materials like fibre-metal laminates (e.g GLARE which alternates glass fibres with Al)
Sandwich panels
A core material (foam or honeycomb) with two thin composite skins. Gives better flexures stiffness (bending) for little extra weight
What do fibre reinforcements do?
Bear most of the load in tension
General properties of fibre reinforcements
High tensile modulus and tensile strength. Lightweight. Low strain to failure. High dimensional stability. Expensive
Examples of fibres
E-glass, high strength carbon, high modulus carbon, Kevlar 49, spectra, flax, boron
Glass fibres
White. Most common fibre reinforcement. E-glass is cheapest but S-glass gives very high tensile strength for extra cost. Structure is random 3D network of tetrahedra linked by O atoms. They are amorphous and isotropic
Advantages of glass fibres
Low cost, high tensile strength, chemically resistant, electrically insulating, thermally insulating, radar transparent.
Disadvantages of glass fibres
Low tensile modulus, relatively high density, easily abraded, low fatigue resistance, high hardness, absorbs water
Common matrices and applications for glass fibres
Unsaturated polyesters, vinylesters, epoxies.
Automotive, military, construction, storage tanks and silos, electrical equipment, baths and shower enclosures, sporting goods.
All driven by low cost
Carbon fibres
Black. Very high performance and expensive. Interconnected carbon atoms hexagonally arranged in layers. Strong covalent bonds in basal plane but weak van der waals between basal planes meaning fibres are anisotropic.
Order within carbon fibres
Mixture of ordered graphitic carbon on the skin and less ordered carbon internally. Less ordered doesn’t take as much advantage of strong covalent bonds within planes
Advantages of carbon fibres
High tensile strength and modulus to weight ratios. Low linear thermal expansion. High thermal conductivity. Moisture resistant
Disadvantages of carbon fibres
Low strain to failure. Low impact resistance. High electrical conductivity. High cost
Why can high electrical conductivity also be an advantage to carbon fibres?
Means they can carry signals across parts
Common matrix and applications for carbon fibres
Epoxy
High performance automotive, aerospace, sporting goods.
All driven by low weight
Aramid fibres
Yellow. Highly crystalline aromatic polyamides. Famous is Kevlar. Structure is phenyl rings along backbone (enhance stiffness) between amide linkages that H-bond between chains.
Advantages of aramid fibres
Very high tensile strength to weight ratio, low linear thermal expansion, good impact resistance (toughness), no sizing needed, good temperature and corrosion resistance.
Disadvantages of aramid fibres
Hard to process, poor matrix compatibility, poor in compression, susceptible to moisture (water replaces H-bonds between chains), susceptible to UV radiation, high cost
Applications of aramid fibres
Aircraft (impact prone areas like leading edges), sporting goods, pure Kevlar used for ballistic armour.
All driven by impact tolerance
Polyethylene fibres
White. Highly crystalline (95-99%) polyolefins. UHMWPE. Highly aligned molecules provide high tensile strength. Used for ballistic and blast protection, sporting goods
Advantages of polyethylene fibres
Very high tensile strength to weight ratio. Low density. Low moisture absorption. High abrasion resistance. High impact resistance
Disadvantages of polyethylene fibres
Very low service temperature (loss of properties above 80-90C). Poor adhesion with matrices
Natural fibres
Most brown. From agricultural plants. E.g hemp, flax, jute. Primarily cellulose fibres in a lignin matrix. Used for automotive interior panels
Advantages of natural fibres
Environmentally friendly. Biodegradable. Lower density and higher strength to weight ratio than E-glass. Acoustic damping. Cheap
Disadvantages of natural fibres
Low strength, modulus, melting point. Absorbs lots of moisture. Degrade above 200C
What are fibre treatments for?
Protection during manufacture and processing. Improve wettability. Improve bonding between fibre and matrix by removing weak layers and contaminants, adding chemically reactive groups and altering topography to promote mechanical interlocking
Fibre treatment of glass
Fibres given a sizing (often an organo-silane treatment). Uses Si bonded to R’ (organic) group and 3 OH groups. OH groups interact with surface of glass and organic group has good adhesion to organic matrix
Carbon fibre treatments
Oxidative: apply oxygenated functional groups to surface which are easier for a matrix to react with than pure carbon, can use O2, O3, CO2, HNO3.
Non-oxidative with reactive coatings has similar principle
Kevlar and UHMWPE fibre treatments
Similar idea to those for carbon fibres. Can oxidise surface. Can plasma treat either to etch surface and add functional groups
