2 - Fibre-reinforced plastics

Definition of composite material

What is it composed of?

What’s the purpose of each element?

Material that combines 2 distinct components to create a new material with superior performance

Fibers + matrix material

• Fibers

→ provide the structural strength & stiffness, bear most of the load

• matrix

→ usually a polymer that surrounds, protect & holds the fibres in place, distributes stress

What’s the most important advantage about composite materials?

→ combine strengths of their individual components

→ can withstand stresses that would break either component alone

Why do we use fibres?

• light weight

• Good tensile properties → excellent resistance to being pulled

• Strengthen the material in specific directions

What’s the matrix in fibre-reinforced materials?

What are its purposes?

Material that surrounds & binds the fibres together

• applies forces to the fibres

• Reinforces bonding between fibres

• stabilises / holds the fibres in place

• Protects the fibres from environmental damage

What kind of plastics matrices can be made from? (their basis)

• Thermosets (resins)

• Thermoplastics

Name different types of fibres

• Glass fibres

• Aramid fibres

• Carbon fibres

• Natural fibres

Properties of glass fibres

(+ 1 application)

+ high tensile strength

+ good stiffness (E-modulus)

+ less expensive than others

+ good chemical resistance

+ good thermal resistance

→ used in communication with light signals

Properties of aramid fibres

(+ 1 application)

Explain why they’re strong in certain directions

→ aromatic polyamides

• lightest reinforcing fibre (low density)

• highly anisotropic (properties vary significantly with orientation of fibres)

• UV-sensitive

• very strong

→ used for bulletproof vests

→very strong in fibre direction (covalent bonds)

→ weaker perpendicular to the fibres (hydrogen bonds)

Properties of carbon fibres

Explain why they’re strong in certain directions

• progressive stress-strain behaviour

→ deform gradually under stress, progressive increase in resistance as stress rises)

• corrosion resistant

• Highly anisotropic

• Sensitive to bending (must have no sharp points)

→ strong in the fibre direction (longitudinal) because covalent bonds take place in this direction

→ very weak in the transversal fibre direction because van der Waal’s bonds take place in this direction

What’s the difference between isotropic & anisotropic?

Explain with the example of aramid fibres

• isotropic

→ properties are constant in all directions

• anisotropic

→ properties depend on the direction

Aramid fibres:

→ very strong in fibre direction (covalent bonds)

→ weaker perpendicular to the fibres (hydrogen bonds)

What happens to a polymer when being processed into a fibre?

→ initially the internal molecules are unaligned, tangled, & randomly oriented

→ the material is physically pulled / stretched

→ the molecules rearrange & reorient in the same direction as the pull → significantly increases the strength of the fibre along its length

How does the diameter of a fibres affect its strength?

Why?

(Draw a graph, no values are needed)

Smaller diameter = stronger fibres

→ due to the orientation of the molecules

What’s the particularity about scrims (toiles)?

Fabric where fibres are arranged in multiple directions which provides good properties in all directions

What are the basic data required to determine characteristic values of composite materials?

• type of fibres

• Type of matrix

• Fibre content (percentage of fibres in the composite)

• Fibre alignement (orientation)

• Manufacturing process

What are the formulas to calculate the properties of fibre-reinforced plastics in the fibre direction?

Tensile strength =

(Fibre content * fibre tensile strength) + (matrix content * matrix tensile strength)

E-modulus =

(Fibre content * fibre E-modulus) + (matrix content * matrix E-modulus)

Compare briefly the properties of carbon fibre VS glass fibre reinforced polymers

Carbon fibres

→ very high stiffness

→ very high strength

→ low elongation, brittle

Glass fibres

→ lower stiffness

→ Lower strength

→ higher elongation, less brittle

List some typical applications of fibre-reinforced plastics

• aircraft constructions (wings, radars)

• Boats (easy to bring it to a complex shape)

• Vehicles

• Sport equipment (bikes)

• Plant construction (factories)

• Architecture

• Structures for constructions (bridges, reinforcement for concrete)