Composite Materials Lecture Notes

Comprehensive vocabulary flashcards covering the mechanical properties, matrix materials, reinforcements, and processing methods of composite materials.

Strength

The ability of a material to withstand an applied load without failure or plastic deformation.

Stiffness (Young Modulus)

The resistance to deflection or the extent to which an object resists deformation in response to an applied force.

Toughness

The resistance of a material to being broken in two by a crack (fracture) and its ability to absorb energy and plastically deform without fracturing.

Hardness

The resistance of a material to deformation, indentation, or penetration by means such as abrasion, drilling, impact, scratching, or wear.

Processability

The ability to effectively produce products from industrial materials on standard equipment, determined by the viscosity of the polymer and controlled through molecular weight.

Density

Material mass per unit volume, designated by the symbol $\rho$. The SI unit is $kg\,m^{-3}$.

Composite

A material deliberately made from a combination of more than one constituent, where the combination gives advantages in properties.

Interface

the surface between the reinforcement and the matrix.

Interphase

The region of matrix close to the reinforcement which can have different levels of cure, crystallinity, microstructure, or stresses compared to the bulk matrix.

Thermosetting polymers

Polymers formed by step reaction polymerization that results in branching and crosslinking, such as epoxies, polyesters, and phenolics.

Polyester Resins

Low-cost resins involving ester links between acid and alcohol groups, typically cross-linked with styrene using a peroxide-based catalyst.

Polyvinyl esters

Resins similar to polyesters but with double bonds only at the ends, providing higher toughness and better chemical resistance.

Epoxy Resins

Resins with excellent mechanical properties and resistance to temperatures up to $180\,\text{°C}$, commonly used in aerospace.

PEEK (Poly-aryl-ethers)

A strong, stiff, tough, and chemically resistant thermoplastic with melting points up to $340\,\text{°C}$.

HDPE (High density polyethylene)

Polyethylene defined by a density $\ge 0.941\,g\,cm^{-3}$ with low branching and high tensile strength.

LDPE (Low density polyethylene)

Polyethylene with a density range of $0.910\text{--}0.940\,g\,cm^{-3}$, characterized by high branching, low tensile strength, and high ductility.

E-glass

A type of electrical glass fibre with the best processability and good properties for general-purpose use.

HM Carbon Fibre

High Modulus carbon fibre with greater stiffness (up to $380\,GPa$) but lower strength ($2.7\,GPa$) due to increased heat treatment.

HS Carbon Fibre

High Strength carbon fibre with higher strength ($4.5\,GPa$) but lower stiffness ($230\,GPa$).

Polyaramids (e.g. Kevlar)

Highly crystalline fibres with high toughness that do not melt but degrade in UV sunlight.

Carbon Nanotubes

Fylindrical structures with very high strength ($60\,GPa$) and stiffness ($1,000\,GPa$), available in single-walled (SWNT) or multiple-walled (MWNT) forms.

Graphene

A one-atom-thick planar sheet of $sp^2$-bonded carbon atoms packed in a honeycomb crystal lattice.

Diffusion Bonding

A solid-state processing method for metal matrix composites that relies on increased diffusion rates at high temperatures (below melting point) to join materials.

Resin Transfer Moulding (RTM)

A process where reinforcement is placed in a closed mould and liquid resin is injected under pressure or sucked through with vacuum.

Pultrusion

A continuous process where fibres pass through a resin tank and then a heated shaping die to produce constant cross-section profiles.

SMC (Sheet Moulding Compound)

A combination of chopped glass fibres, uncured resin, and fillers (like calcium carbonate) processed in a heated matched mould under high pressure.

Law of Mixtures (Equal Strain)

An upper bound model for composite stiffness defined as $E_c = V_r E_r + (1-V_r) E_m$.

Law of Mixtures (Equal Stress)

A lower bound model for composite stiffness defined as $E_c = \frac{1}{\frac{V_r}{E_r} + \frac{1-V_r}{E_m}}$.

Critical fibre length ($l_c$)

The length at which the tensile stress in a fibre reaches the fibre fracture strength ($\sigma_f^<em>$), calculated as $l_c = \frac{\sigma_f^</em> D}{2\tau_i^*}$.

Tsai-Hill Criterion

A failure criterion for unidirectional composites that accounts for interactions between stresses and provides a continuous uniform failure curve.

HDT (Heat Deflection Temperature)

The temperature at which a material loses a specific proportion of its short-term stiffness.

UL (Maximum Use Temperature)

The temperature at which a material would lose half its strength after 10 years of service.

Osmotic Blistering (Boat Pox)

A defect occurring when water collects at voids or debonds, dissolving degradation products and creating pressure that leads to cracks and blisters.