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?

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)

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

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)

TNO Centre for Lightweight Structures

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.

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).

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

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

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

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

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

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

Some typical polymer composite properties

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

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.

Examples of natural composites

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

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

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

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

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

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

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

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.

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

Approximate comparative materials data from J Quinn, Composites Design Manual