What is a composite?
Materials composed of more than one base material usually matrix and reinforcement
What fibres ca reinforce a composite?
Long fibres - (i.e., carbon fibre glass fibre) spars/masts
Short fibres - injection moulded tennis rackets – mech properties aren’t as good as long fibres
Platelets - some types of car filler, resin ornaments etc.
What’s the difference between an Alloy and a Mixture
- Mixture doesn’t have to be 2
- Alloy is mixture of 2 different metals
What are the properties of polymer fibre reinforcing systems?
o Light, stiff, strong
o Absorb moisture
o Poor compressive strength
What are the properties of carbon fibre reinforcing systems
Lightweight, high strength, radiolucent
BUT: poor shear strength
Examples:
o Short fibre carbon reinforce UHMWPE in orthopaedic applications
Aimed to increase longevity
Aimed to increase longevity of bearing surfaces
Osteolysis and failure of tibial inserts in knee prostheses
Scaffolding device for ligament repair
o Performed poorly, permanent wear debris in joint
What are the properties of UHMWPE reinforcing systems?
- High modulus, strength, light weight
- Do not absorb water
- BUT adhere poorly to matrix, hence performance is not fully realised
- Applications: Dentistry (reinforcing acrylic resins), intervertebral disc prostheses, ligament augmentation
What makes biodegradable polymers PLA, PGA, and co-polymers good reinforcing fibres and what are some applications?
Variation in crystallinity and molecular weight
Ligament reconstruction (as fibres)
Scaffolds for tissue engineering (as fibres reinforcing tissues)
Biodegradable intramedullary pins and plates (as composites)
Biodegradable scaffolds for bone regeneration
What are the properties of S- and E-glass fibre reinforcing systems?
o High strength/weight ratio o Good dimensional stability o Resistant to heat, cold, moisture, corrosion o Low cost o Bio inert
What are the properties of bioglass reinforcing systems?
- Bioactive (take years to resorb)
- Composed of SiO2, Na2O, CaO and P2O5
- Advantages:
o Highly bioactive
o FDA approved and commercially available
o Tensile modulus similar to bone (30-35 GPa) - Disadvantages
o Mechanical weakness (UTS 40-60 MPa)
o Low fracture resistance
o Limited to low load applications - Also, S53P4 available and clinically used
Compare the strength of composites to metals.
- Fibre reinforced materials are very strong in the directions of the fibres
- They are weak in the transverse direction
- If the strength is divided by density then they are far stronger, per unit weight than steels or aluminium allows
Compare the modulus of composites vs metals.
- Unidirectional carbon has a higher modulus than steel (in fibre direction)
- CFRP has even more spectacular specific modulus
- GRP possesses only moderate stiffness
What is hand laminating?
Lay down the matrix (polymer) then lay down the reinforcement then more polymer and more reinforcement and build it up
What is Spray lay up?
- Gun chops the fibre and sprays it
- Though chopped fibres have poorer mechanical properties
What is Compression moulding?
- Stack up polymer fibre polymer fibre
- Could put all fibres in one direction or perpendicular etc
What is pultrusion?
- Fibre spores and they take a strain from each spore and then bring to one point and can make pultrusion rods
- Takes time and labour intensive
What are the open mould processes?
- Vacuum-bag
- Autoclave
- Filament
- Winding
What are the open mould processes?
- Vacuum-bag
- Autoclave
- Filament
- Winding
What are some close mould processes?
- Compression moulding
- Pre-peg (fibre and matrix) heated in closed two piece mould under pressure
- Injection moulding
- Fibre-matrix mix injected into mould at elevated temperature and pressure
- Continuous pultrusion
- Continuous fibres impregnated with resin in bath then drawn through heated die
Compare short and long fibres.
- ‘Short’ fibres are not as effective as ‘long’ or continuous fibres
o Load transfer mechanism results in end effects which may reduce the fibre stress
o Alignment is difficult to control
o Volume fraction is lower, particularly for randomly oriented short fibres - Short fibres are more often used with thermoplastic resins
- Injection moulding can lead to considerable fibre damage and reduction in length
Explain load transfer between matrix and fibres
- Under applied tension, load is transferred by shear at the matrix/fibre interface
- At fibre ends, the strain in the matrix is higher than in the fibre:
- Stiff fibres embedded in flexible matrix
- Shear stress and strain are a maximum at the fibre ends
- The tensile stress in the fibre is zero at the fibre end and increases towards the centre (sfmax = Ef e, where e is the strain applied to the composite)
- A fibre is said to be of critical length if it is just long enough for the tensile stress to reach its maximum value
Why is the fibre-matrix interface important?
- The interface between fibre and matrix is crucial to the performance of the composite - in particular fracture toughness; corrosion; moisture resistance
- Weak interfaces provide a good energy absorption mechanism - composites have low strength and stiffness, but high fracture toughness
- Strong interface results in a strong and stiff, but brittle composite
What are the fibre-matrix interface adhesion mechanisms?
- Absorption and wetting
- Interdiffusion - autoadhesion
- Electrostatic attraction
- Chemical bonding
- Mechanical adhesion
List the possible failure mechanisms.
Matrix Cracking Fibre fracture Debonding = interface failure Delamination - interlayer failure Fibre pull-out Micro-buckling Kink bands Cone of fracture
List the common metallic implant complications.
Stress shielding
Paediatric growth restrictions
Removal operations
Imaging artefacts
