The term composite refers to a group of materials formed from at least two materials, which combine to give properties superior to those of the individual components. The many component material sand different processes that can be used make composites extremely versatile and efficient. They typically result in lighter, stronger, more durable solutions compared to traditional materials, and have found use in virtually every end-sector market with applications ranging from high performance aircraft parts to fishing rods to shower trays and a myriad of others.
Composites are generally used for one of 4 key reasons:
- Weight reduction
- Improved specific strength/stiffness enables reduced fuel use, range in transport, easier installation of structures, topside stability in vessels and offshore structures, and buoyancy for deep sea applications
- Durability and maintenance
- Composites don’t rust which is critical for certain applications; combine this with the excellent fatigue resistance and the materials can increase product lifespan by several times.
- Added functionality
- Composites offer thermal and electrical insulation, and sensors, electronics and cabling can be embedded.
- Design freedom
- Composites can be tailored to suit the application by choosing the constituent materials and embedding additional functionality.
There are many ways to manufacture composite parts but they fall under 3 main headings: open mould, closed mould and continuous processing. The method of processing must be considered at the design stage to endure the most appropriate one is selected for an application. The key types of processes for each category are given below:
Wet hand lay-up – a skilled manual process which requires low capital investment and is widely used for low-volume products such as boats. The component materials are placed in a mould and cured at ambient temperature.
Wet spray-up – fibre is fed from a bobbin and chopped into the resin stream as it is sprayed from a hand-held gun into the mould. This is then left to cure at ambient temperature. Applications include custom parts in low to medium volumes such as baths and storage tanks.
Pre-preg – fabric is pre-impregnated with resin, cut into layered pieces which are laid in a mould. This process is used for high performance components for aerospace and motorsport. The part can be cured in an autoclave, oven or hot press.
Vacuum infusion – dry fabric pieces are laid into the mould and covered with a vacuum bag, sealed at the edges. This is then compacted under pressure as resin is drawn through from a reservoir. This process is used for large components such as boat hulls, wind turbine blades or bridges.
Resin transfer moulding (RTM) – fry fabric is put into a 2-part mould which is closed in a press and resin is injected under pressure until the fabric is fully impregnated when the tool is heated to cure the resin. This process requires matched metal tooling and is used for mass production of 100-10000 parts/year.
RTM light – a combination of RTM and vacuum infusion which combines the benefits of low-cost tooling and equipment. Low production parts can be made this way.
Moulding compounds – SMC (sheet moulding compound) is compression moulded to make vehicle body parts, electrical cabinets and shower trays. BMC (bulk moulding compound) can be injection moulded or compression moulded to produce automotive parts, electrical equipment and household appliances.
Pultrusion – several fibres are pulled through a resin bath into a heated die where the resin hardens and the shape forms. The profile is pulled through and cut to the required length. This is used for structural sections, cable trays, bars and tubes.
Filament winding – fibre is pulled through a resin bath and wound onto a rotating mandrel. This process is used for pipes and tanks.
Continuous sheet – resin and chopped fibre are sprayed onto a moving carrier film, heated, shaped and cured in an oven. This is used to make rooflights and flat sheet products.
The matrix and the fibre are carefully chosen to maximise properties and ease of processing. The materials, along with the processing method, determine the properties of the finished part.
The most commonly used fibres are E-glass (made from silica sand and limestone), carbon (made from polyacrylonitrile), aramid (aromatic polyamide) and natural fibres such as flax and hemp. E-glass is the most widely used of these.
The resins used in composites are either thermoset or thermoplastic polymers. Thermoset resins are cured by a chemical reaction, they will not melt once formed and have much better mechanical properties and chemical resistance than most thermoplastic resins. The most commonly used thermoset resins are unsaturated polyester (UPR), vinyl ester, epoxy and phenolic. Thermoplastics solidify when cooled but will melt when heated. Commonly used thermoplastics include polypropylene (PP), nylon and Polyethylene terephthalate (PET).
Fillers are often included to reduce cost. Additives can be used to improve properties such as fire performance or UV stability.
Intermediate materials – these are fabrics woven or stitched from fibres, prepregs, SMC and BMC materials.
Core materials are used in a composite sandwich part to increase stiffness and reduce weight. Commonly used core materials are polymer foams, honeycomb structures and balsa wood.
Common tool materials for making composites are aluminium, steel, and invar (an iron/nickel alloy).
In recent years a range of composite materials have been developed to make tools to make components. These composite structures are epoxy based, bismaleimide, and graphite. These ‘exotic’ tooling composites are more expensive than the metallic options but offer advantages in accuracy and thermal characteristics. During the moulding and curing of composites the chemical and thermal reactions can be aggressive to the contact surfaces of the tooling, a major consideration in selecting the right tooling material.
Design with composites is often perceived as being complex, and for bespoke components design for manufacturing is critical, but several software packages are commonly used to integrate manufacturing process, surface engineering and produce detailed drawings of how plies are cut and laid up etc.
Composites are also produced as standard section/components with defined properties. These are used predominantly in sectors such as construction where off-the-shelf product selection is common place.
Aerospace One of the earliest application areas for composite materials. Carbon and glass fibre composite materials are widely used applications such as engine blades, brackets, interiors, nacelles, propellers/rotors, single aisle wings, wide body wings. Key properties are lightweight, high-strength, high-stiffness and good fatigue resistance.
Space Carbon fibre composites are most often used for space applications because of their high stiffness and excellent thermal stability. Applications include fairings, manipulator arms, antennae reflectors, solar array panels and optical platforms and benches. Glass fibre composites are used where thermal insulation is important e.g. local bracketry.
Automotive In the large-volume sector of the industry, glass fibre thermoplastic composites have found application in many internal component parts because of their lower cost, weight reduction and recyclability. In the specialist area, carbon fibre composites are used for chassis, specific structural components and complete vehicles.
Construction In the construction sector the materials are referred to as fibre polymer composites (FPC) to reflect the terminology being used in the developing Eurocodes. Carbon fibre plates are widely used for strengthening existing structures, but it is mainly glass fibre polyester composites that are used in construction due to their lower cost. The have found use in a wide range of applications from architectural cladding, to roofing, bridge decks, complete bridges, masts & towers, pipes, access covers as well as complete modular structures manufactured offsite. Key benefits include light weight leading to reduced installation costs, excellent durability, aesthetic flexibility, blast/fire resistance and low maintenance.
Defence Due to key properties such as lightweight, impact resistance, corrosion resistance and design for manufacture with the ability to embed functionality, composites are widely used in the defence sector for land vehicles, military aircraft, UAVs, naval vessels and weapons.
Marine This is a well-established sector for the use of composite materials. Glass fibre composites are widely used across most types of vessel, with carbon fibre composites used for racing yachts.
Medical Carbon fibre composites are finding application in the medical sector because of their lightweight, sigh stiffness and bio-compatibility.
Oil and Gas Composites have found extensive applications in the oil and gas industry for the last three decades in areas such as modules, protection, equipment, spoolable pipes and pressure vessels.
Rail Infrastructure applications of composites are becoming more established with trackbeds, gantries, lineside furniture and platform systems all now in place. Full composite bridge structures are also finding application offering reduced disruption to the rail network on installation as well as long life-cycle and low maintenance. Composite are also widely used in rolling stock interiors.
Renewables The use of composites for wind turbine blades has revolutionised this sector and enabled huge off-shore wind farms to be developed. Blades are predominantly glass fibre, with carbon fibre placed at critical points to increase blade stiffness and improve aerodynamics.
|Accelerator:||accelerates cure of a resin|
|Additives:||the term used for a large number of specialist chemicals which are added to resins/compounds to impart specific properties, for example, flame retardancy, and UV resistance|
|Adhesive:||substance applied to mating surfaces to bond them together by surface attachment. An adhesive can be in liquid, film or paste form.|
|Aramid:||high-strength, high-stiffness aromatic polyamide fibres|
|Blister, blistering:||undesirable raised areas in a moulded part caused by local internal pressure, due usually to rapped air, volatile reaction by-products or water entering by osmosis.|
|Bulk Moulding Compound (BMC):||polyester resin/glass fibre premix, for injection or transfer moulding, also known as dough moulding compound (DMC)|
|Carbon fibre:||reinforcing fibre known for its light weight, high strength and high stiffness.|
|Catalyst (also called hardener):||a chemical compound (usually an organic peroxide) which initiates polymerisation of a resin|
|Chopped strands:||short strands cut from continuous filament strands of reinforcing fibre, not held together by any means|
|Cleanroom:||A cleanroom is a controlled environment which uses filtration to remove airborne contaminants, creating a consistent and repeatable manufacturing environment.|
|Composite:||a material made up of resin and reinforcement (usually fibre)|
|Compression strength:||the crushing load at failure of a material, divided by cross-sectional area of the specimen|
|Core:||in sandwich construction, the central component to which inner and outer skins are attached. Foam, honeycomb and wood are all commonly used core materials.|
|Corrosion resistance:||the ability of a material to withstand contact with ambient natural factors without degradation or change in properties. For composites, corrosion can cause crazing.|
|Coupling agent:||a substance, which promotes or establishes a stronger bond at the resin matrix/reinforcement interface|
|Cracking:||actual separation of moulded material, visible on opposite surfaces of a part and extending through the thickness (fracture)|
|Cure:||the process of hardening of a thermosetting resin (by cross-linking of the molecular structure), under the influence of heat|
|Curing agents:||chemical compounds used to cure thermosetting resins|
|Curing time:||the time taken for a resin to cure to its full extent|
|Delamination:||splitting, physical separation or loss of bond along the plane of layers of a laminated material|
|Direct roving:||roving produced by winding a large and determined number of filaments direct from a bushing|
|Dough moulding compound (DMC):||polyester/resin fibre premix, for injection or transfer moulding, also known as bulk moulding compound (BMC)|
|Fibre:||a unit of matter of relatively short length, characterised by a high ratio of length to thickness or diameter|
|Filament:||a single textile element of small diameter and very long length considered as continuous|
|Filler:||material (usually low cost) added to a resin to extend it, or give special properties|
|Finishing:||application of coupling agent to textile reinforcements to improve the fibre/resin bond|
|Flexural strength:||the strength of a material in bending expressed as the stress if a bent test sample at the instant of failure.|
|Flow:||the movement of a resinous material, thermosetting or thermoplastic, under pressure, to fill all parts of a closed mould|
|Fracture:||cracks, crazing or delamination resulting from physical damage.|
|Gel:||the state of a resin, which has set to a jelly-like consistency|
|Gelcoat:||a thin layer of unreinforced resin on the outer surface of a reinforced resin moulding; it hides the fibre pattern of the reinforcement, protects the resin/reinforcement bond, gives smooth external finish and can also provide special properties; it is usually pigmented|
|Glass fibre:||reinforcing fibre made by drawing molten glass through bushings. The predominant reinforcement for polymer composites, it is known for its good strength, processability and low cost.|
|Honeycomb:||light weight cellular structure made from either metallic sheet materials or non-metallic materials and formed into hexagonal nested cells, similar in appearance to the cross-section of a beehive|
|Hybrid:||a resin or reinforcement made from two or more different polymers or reinforcement materials|
|Impact strength:||a material’s ability to withstand shock loading as measured by fracturing a specimen|
|Impregnation:||saturation of reinforcement with liquid resin|
|Laminate:||the structure resulting from bonding multiple plies of reinforcing fibre or fabric|
|Lay-up:||a resin-impregnated reinforcement in the mould, prior to polymerisation|
|Mat:||a widely used sheet-type reinforcement made up of filaments, staple fibres or strands, cut or uncut, oriented or random, lightly bonded together|
|Matrix:||resin or other material used to bind together fibres|
|Monomer:||a compound containing a reactive double bond, capable of polymerising|
|Ply:||layer which forms part of the laminated composite|
|Polyester:||usual term for an unsaturated polyester resin|
|Polymer:||a long-chain molecule, consisting of many repeat units|
|Porosity:||numerous air pockets or voids in a moulded product|
|Post-cure:||application of external heat to bring a resin to a stable state of cure in the shortest possible time|
|Preform:||reinforcement pre-shaped to the general geometry of the intended moulded part; it is used on more complex and deep-draw mouldings, to optimise distribution and orientation of fibres|
|Pre-preg:||a factory-made combination of reactive resins and reinforcing fibres, plus other necessary additive chemicals, ready to be moulded|
|Reactive resins:||liquid resins which can be cured by catalysts and hardeners to form solid materials|
|Release agent:||a substance which prevents a moulding from sticking to the mould surface; it may be a chemical compound or a solid material such as a cellulose or plastics film|
|Reinforcement:||key element added to resin (matrix) to provide the required properties; ranges from short fibres and continuous fibres through complex textile forms|
|Resin:||polymer with indefinite and often high molecular weight and a softening or melting range that exhibits a tendency to flow when subjected to stress. As composite matrices, resins bind together reinforcement fibres|
|Resin transfer moulding (RTM):||a moulding process in which catalysed resin is injected into a closed mould already containing the pre-formed reinforcement|
|Roving:||endless glass fibre bundles; a collection of parallel strands (assembled roving) or parallel filaments (direct roving) assembled without intentional twist|
|Sandwich structure:||composite composed of lightweight core material to which two relatively thin, dense, high strength, functional or decorative skins are adhered|
|Sheet moulding compound (SMC):||a flat pre-preg material, comprising thickened resin, glass fibre and fillers, covered on both sides with polyethylene or nylon film, ready for press-moulding|
|Size:||the coating on a fibre which helps bonding with the matrix and makes it easier to handle|
|Strand:||an assembly of parallel filaments simultaneously produced and lightly bonded|
|Thermoplastic:||a plastic, which softens each time it is heated|
|Thermoset:||a plastic which flows and then sets permanently on first heating, as a result of setting up a three-dimensional cross-linked molecular structure, and subsequently will not soften or dissolve|
|Tow:||a thick roving, defined by the number of filaments|
|Wet-out:||complete wetting/saturation of a fibrous surface with a liquid resin|