Composites- Types of Composite

Question 1

Three types of composite

Answer 1

Particulate-reinforced composites
Fibre-reinforced composites
Structural composites

Question 2

Subdivisions of particulate-reinforced

Answer 2

Large particles (micron scale) and nano particles

Question 3

Subdivisions of fibre-reinforced

Answer 3

Discontinuous (short) fibres which can be aligned or random.
Continuous (long) fibres which are aligned

Question 4

Subdivisions of structural composites

Answer 4

Laminates and sandwich panels

Question 5

What do the particles do?

Answer 5

Often stiffer than the matrix and act to restrict movement of matrix in vicinity of particle. Strong particle-matrix interface critical. Particles usually provide stiffness and hardness with matrix providing ductility and toughness.

Question 6

Aspects of particulates that affect properties of composite

Answer 6

Size, shape (aspect ratio), orientation, volume fraction, distribution and dispersion, surface treatment (for interface between reinforcement and matrix

Question 7

Particulate size types

Answer 7

Large (micron scale), treated by continuum mechanics (e.g shear lag theory).
Dispersion strengthened: strengthening metals with metal oxides (like precipitation hardening), treated at atomic or molecular level.
Nano-particles: at least one dimension is on the nano scale, nano-composites

Question 8

Particulate shape types

Answer 8

Spherical (3 similar dimensions)
Plate-like (2 long, 1 smaller)
Fibrous (1 long, 2 smaller)

Question 9

What is the reinforcement effect of particulates controlled by?

Answer 9

Aspect ratio. Can be length over diameter

Question 10

Aspect ratios of different particles and their effect

Answer 10

Spherical is about 1 so poorer reinforcement but uniform properties in all directions.
Plate- like or long and thin have greater or much greater than 1, better reinforcement in-plane or along long dimension, poorer reinforcement out-of-plane or transverse to long dimension

Question 11

Three types of particulate orientation

Answer 11

Aligned
Aligned in-plane
Random

Question 12

What does higher volume fraction of particulates lead to?

Answer 12

Higher composite stiffness

Question 13

What dispersion and distribution do you want from particulates?

Answer 13

Good distribution so are present in whole of material and good dispersion so particulates not clumped together

Question 14

Exfoliation example of layered silicate and polymer

Answer 14

Blocks of layered silicates surrounded by polymer to form a tactoid. Polymer gets between layers in intercalation. The layers then separated from each other in exfoliation with polymer in between

Question 15

Surface treatment and example

Answer 15

In most cases reinforcement and matrix phases incompatible. Surface of reinforcement often modified to improve reinforcement-matrix interface.
For nano-clay reinforced epoxy, reinforcement Clay is inorganic, matrix polymer is organic, modify clay surface with amphiphilic alkyl ammonium ions, curing reaction exfoliates clay

Question 16

Example of cemented carbide

Answer 16

Large tungsten carbide (or TiC) particles in cobalt matrix. Particles 10-100μm and approximately 90% by volume, provide stiffness and hardness. Cobalt matrix about 10% by volume and provides toughness and ductility. Gaps between particles (matrix) stop cracks in particles from traversing between particles so can’t keep propagating. Tougher matrix phase relives stress concentrations

Question 17

Example of rubber automobile tyres

Answer 17

Large carbon black particles (20-50μm) in vulcanised rubber matrix. Carbon black particles essentially spherical and 15-30% by volume. Vulcanised rubber is natural rubber crosslinked with sulphur and is 70-85% by volume. Composite has improved tensile strength, toughness, wear and abrasion resistance. Many other filled polymers which are essentially particulate composites.

Question 18

Why are air pockets bad in composites?

Answer 18

They are voids in the material and make properties unpredictable

Question 19

Difference between nano-scale and large particulates

Answer 19

Nano-scale particulates have very high surface area.
Nano-scale fibres can have very high aspect ratio like 150.
Property improvement therefore potentially significantly increased for nano-composites over more traditional particulate composites.

Question 20

Types of nano-scale reinforcement and examples of each

Answer 20

Spherical- ballotini and powdered silica
Plate-like- nanoclays, graphème and flaked silica
Fibrous- nanotubes/nanoropes and whiskers

Question 21

Example of tennis balls

Answer 21

10-50μm thick nano-composite layer significantly reduces loss of air pressure. This has vermiculite nano-clay platelets (1nm thick aspect ratio 10,000) in a butyl rubber matrix. The platelets are exfoliated so the layers are separated from each other. Layers are also aligned to provide very effective barrier against pressurised gases leaking out

Question 22

Example of flame retardant nano-composites

Answer 22

Addition of nano-clay to thermoplastic polymers significantly reduces their flammability.
Mechanism include creation of char layer on surface (barrier), recombination of radicals within clay layers (trap), reduced dripping (retention)

Question 23

Critical fibre length for effective reinforcement

Answer 23

L_c. At lengths below this the stress in fibre never reaches maximum possible. At lengths above this fibres can efficiently be used. Is about 1mm for most carbon and glass fibres

Question 24

Aspect ratios of short and long fibres

Answer 24

<15 are short fibres
>15 are long fibres

Question 25

What does shorter fibre length mean?

Answer 25

Lower composite modulus and strength

Question 26

Aligned or random alignment of fibres

Answer 26

Aligned gives good properties along fibre direction
Random gives equal properties in all directions

Question 27

Why are short fibres cheaper than long?

Answer 27

Created by chopping or cutting to length long fibres. Lower length or grade long fibres can be chopped. Short fibres can be processed using conventional polymer processing techniques (like injection moulding) or composite processing techniques (like compression moulding).

Question 28

Ideal length of fibres

Answer 28

Ideally they would continue from one end of the part to the other without breaks. Allows most efficient reinforcement as fermer fibre ends (where no stress can be maintained).

Question 29

Long forms of fibres

Answer 29

Tows (continuous aligned fibre bundles)
Woven, knitted, braided, stitched fabrics (bending of fibres induces crimp)

Question 30

Maximum and minimum modulus of forms of long fibre

Answer 30

Maximum modulus along fibres in tows
Minimum modulus across fibres in tows
All other between extremes

Question 31

What is fibre pull-out?

Answer 31

Where the fibre-matrix interface has failed and ends of fibres can stick out of matrix

Question 32

Structural composites: laminar structures

Answer 32

Stacked and bonded fibre-reinforced sheets (piles)
Fibres orientated in different directions to give relatively high strength and stiffness in number of directions in the plane

Question 33

Structural composites: sandwich panels

Answer 33

Face sheet, adhesive, honeycomb, adhesive, face sheet.
Outer face sheets are relatively strong and stiff (e.g carbon fibre composite, aluminium, titanium). Low density core material in honeycomb (e.g foams, honeycombs, lightweight woods). Gives lightweight composite structure with enhanced bending stiffness. Structural applications are like aircraft fuselage and construction materials