1. Composite Materials Fundamental questions
How do composite materials differ from other engineering materials?
What are the constituent materials, and how do their properties compare?
How do the properties of the composite depend on the type, amount and arrangement of the constituents?
How are composite products made, and why does manufacture affect quality?

2. Fibres have better stiffness and strength compared to bulk materials
Atomic or molecular alignment (carbon, aramid)
Removal of flaws and cracks (glass)
Strain hardening (metals)

10. Most reinforcing fibres are brittle (elastic to failure)
Hollaway (ed), Handbook of Polymer Composites for Engineers

12. Types of Natural Fibre
Bast fibres (flax, hemp, jute, kenaf…) - wood core surrounded by stem containing cellulose filaments
Leaf fibres (sisal, banana, palm)
Seed fibres (cotton, coconut (coir), kapok)

13. TNO Centre for Lightweight Structures

16. Advantages of Natural Fibres
High specific properties (low density).
A renewable resource; production requires relatively little energy
Crops are sink for CO2, returning oxygen to atmosphere.
Low investment and low cost production.
Low tooling wear.
Better working conditions, no skin irritation.
Thermal recycling possible.
Good thermal and acoustic insulating properties.

17. Disadvantages of Natural Fibres
Low strength, especially impact strength.
Variable quality (e.g. weather dependent).
Moisture absorption, which causes swelling of the fibres.
Limited maximum processing temperature.
Lower durability (potential for improvement through fibre treatments).
Poor fire resistance.
Price fluctuation (harvest results or agricultural politics).
Irregular fibre lengths (spinning is required to obtain continuous yarns).

18. Structures cannot be made from fibres alone - the high properties of fibres are not realisable in practice
A matrix is required to:
hold reinforcement in correct orientation
protect fibres from damage
transfer loads into and between fibres

19. COMPOSITES - A FORMAL DEFINITION (Hull, 1981)
1. Consist of two or more physically distinct and mechanically separable parts.

20. Polymer matrix composite combinations
epoxy polyimide polyester thermoplastics (PA, PS, PEEK…)
Fibre
E-glass S-glass carbon (graphite aramid (eg Kevlar) boron

21. Ceramic matrix composite combinations
SiC alumina glass-ceramic SiN
Fibre
SiC alumina SiN

22. Metal matrix composite combinations
aluminium magnesium titanium copper
Fibre
boron Borsic carbon (graphite) SiC alumina (Al2O3)

23. Composite property might be only 10% of the fibre property:

24. Some typical polymer composite properties

26. Examples of particulate composites
Concrete - hard particles (gravel) + cement (ceramic/ceramic composite). Properties determined by particle size distribution, quantity and matrix formulation
Additives and fillers in polymers: carbon black (conductivity, wear/heat resistance) aluminium trihydride (fire retardancy) glass or polymer microspheres (density reduction) chalk (cost reduction)
Cutting tool materials and abrasives (alumina, SiC, BN bonded by glass or polymer matrix; diamond/metal matrix)
Electrical contacts (silver/tungsten for conductivity and wear resistance)
Cast aluminium with SiC particles

27. COMPOSITES - A FORMAL DEFINITION (Hull, 1981)
1. Consist of two or more physically distinct and mechanically separable parts.
2. Constituents can be combined in a controlled way to achieve optimum properties.

28. Examples of natural composites

29. COMPOSITES - A FORMAL DEFINITION (Hull, 1981)
1. Consist of two or more physically distinct and mechanically separable parts.
2. Constituents can be combined in a controlled way to achieve optimum properties.
3. Properties are superior, and possibly unique, compared those of the individual components

30. Addition of properties:
GLASS POLYESTER = GRP
(strength) (chemical resistance) (strength and chemical resistance)
Unique properties:
GLASS POLYESTER = GRP
(brittle) (brittle) (tough!)

31. ADVANCED COMPOSITES vs REINFORCED PLASTICS
Aerospace, defence, F1…
Highly stressed
Glass, carbon, aramid fibres
Honeycomb cores
Epoxy, bismaleimide…
Prepregs
Vacuum bag/oven/autoclave
Highly tested and qualified materials
Marine, building…
Lightly stressed
Glass (random and woven)
Foam cores
Polyester, vinylester…
Wet resins
Hand lay up, room temperature cure
Limited range of lower performance materials

32. Why are composites used in engineering?
Weight saving (high specific properties)
Corrosion resistance
Fatigue properties
Manufacturing advantages: - reduced parts count - novel geometries - low cost tooling
Design freedoms - continuous property spectrum - anisotropic properties

33. Anisotropic properties - fibres can be aligned in load directions to make the most efficient use of the material

34. The ability to vary fibre content and orientation results in a spectrum of available properties

35. Why aren’t composites used more in engineering?
High cost of raw materials
Lack of design standards
Few ‘mass production’ processes available
Properties of laminated composites: - low through-thickness strength - low interlaminar shear strength
No ‘off the shelf’ properties - performance depends on quality of manufacture

36. Material quality depends on quality of manufacture.
There are no ‘off the shelf’ properties with composites. Both the structure and the material are made at the same time.
Material quality depends on quality of manufacture.

37. Poor quality - low fibre content, high void content
Good quality - high fibre content, ‘zero’ void content

38. Approximate comparative materials data from J Quinn, Composites Design Manual