Introduction to Composite Materials

Throughout the last four decades the exploitation of fibre-reinforced plastics (FRP) in engineering structures has been steadily diversifying from sports equipment and high performance racing cars, to helicopters and most recently commercial aeroplanes. Composite materials are essentially a combination of two or more dissimilar materials that are used together in order to combine best properties, or impart a new set of characteristics that neither of the constituent materials could achieve on their own. Engineering composites are typically built-up from individual plies that take the form of continuous, straight fibres (eg. carbon, glass, aramid etc.) embedded in a host polymer matrix (eg. phenolic, polyester, epoxy etc.), which are laminated layer-by-layer in order to built up the final material/structure.

An example of a composite laminate (1)

Cross-Section of Composite Laminate. The individual fibres and surrounding matrix are clearly discernible (2)

In the aerospace industry the benefits of exploiting the excellent specific strength and stiffness properties (strength and stiffness per unit weight) of composites in terms of lightweight structural design are immediately apparent. Furthermore, the laminated nature of high performance composite materials enables the designer to tailor optimum mechanical properties by orientating the fibre direction with the primary load paths. As a result, the first generation of commercial aircraft that contain large proportions of composite parts, such as the Boeing 787 Dreamliner and Airbus A350 XWB, are planned to enter service in the near future. Other advantages of fibre reinforced plastics, such as the relative ease to manufacture complex shapes, and their excellent fatigue and corrosion resistance, have made FRP composites increasingly attractive in the renewable energy sector.

Composite materials have actually been around for quite a long time. As early as 3000 B.C. the ancient Egyptians embedded straw in their mud bricks in order to control shrinkage cracks and improve the tensile strength. Furthermore, papyrus based cartonage and paper machéwere used to make mummy cases. In fact, manufacturing tubular shells using metals is quite difficult such that this ancient approach remains an important exploitation of composites today. Of course none of these materials would be suitable for the high performance requirements of the aerospace industry.

Mud and Straw Brick (3)

It was not until the invention of phenolic resin in 1909 that composites took-off in aircraft. The most famous example was the de Havilland Albatross transport aircraft manufactured from a ply-balsa-ply sandwich fuselage construction, which was later developed into thedeHavilland Mosquito multi-role combat aircraft for WWII. The large-scale wooden construction made the Mosquito extremely light, fast and agile. Furthermore, the Mosquito was cheaper than its metallic counterparts and allowed highly skilled carpenters from all over the UK to be contracted to help with the war effort. One disadvantage of early phenolic resins was their inability to cope with hot-wet conditions such that the Mosquito became notorious for disintegrating in mid-air in the Pacific War arena.

DeHavilland Mosquito Timber Fuselage (4)

Since the development of carbon and glass fibres in the 1950’s the aerospace industry is steadily moving towards “all-composite” civil aircraft. The most common fibre and resin types used today are:

Fibres
Glass	Carbon	Aramid – Kevlar™
Diameter ≈ 10 mm	Diameter ≈ 8 mm	Stiffness ≈ 125GPa in tension
Strength > 3GPa due to lack of defects on small diameter fibre	Strength > 5GPa due to highly aligned planes of graphite	Strength > 3GPa because of highly aligned linear polymer chains
Stiffness ≈ 70 GPa for cheaper E-glass and 85 GPafor more expensive R- or S – Glass	Stiffness ≈ 160-700 GPabut 230-400 GPa is the usual	Much weaker and less stiff in compression as linear polymer chains come apart
Susceptible to environmental attack and fatigue	Not susceptible to degradation by chemicals and good in fatigue	Susceptible to degradation by UV light and moisture
Fibres need silanetreatment to bond well to matrix	Fibres bond well with surface treatment	Fibres do not bond well at all leading to a weak fibre/matrix interface
Used in boats, wind turbine blades and other cost critical applications	Expensive material cost limits use to high performance applications were the higher mechanical properties are justified i.e. Racecars, aerospace etc.	Weak interface gives excellent energy absorption. Thus used for bullet-proof vests, helmets and impact protection on aircraft
Matrix
Phenolic	Polyester	Epoxy
First modern resin	Most commonly used matrix	Most common in aerospace
Tends to be brittle	Resin can be quite tough	Can be made quite tough
Wets out fibres badly	Wets out reinforcement very well	Wets out reinforcements very well
Good chemical, heat and fire resistance and don’t produce toxic gases in a fire	Poor chemical resistance and burns very easily	Good chemical resistance but will burn
Thus used in aircraft interiors	Very cheap resin used alongside glass fibres in boat hulls, wind turbine blades and other cost critical applications	Generally used in combination with carbon fibre for high performance, lightweight applications

The shift from metallic to composite construction has naturally induced a change in the design methodology of aircraft components. It has to be borne in mind that not only the mechanical properties of composites differ from those of metals, but that a whole range of physical and chemical properties are different.

– All composites have relatively low through-thickness thermal conductivities and thermal expansion coefficients in and out of plane may be widely different. Therefore thermal expansion mismatch stresses at attachment points can be a problem.

– Composites can be made with very high translucency to electromagnetic radiation eg. X-Ray.

– Electrical conductivity of composites is generally fairly low. Consequently, a copper mesh is often integrated in aerospace laminates to protect against lightning strike damage. However, this compromises a lot of the potential weight savings.

– Direct contact between carbon fibre reinforced plastics and aluminium components will corrode the aluminium over time. Therefore contact between carbon and aluminium at lug attachments and joints has to be prevented.

– All resins pick up water and their properties change as a result of this.

– Composites are not very resistant to mechanical wear effects. External surfaces may need treatment prior to painting.

– Composites tend to have relatively low stiffnesses on an absolute basis, from <10% to about 60% of steel.

– The failure modes in composites are very diverse and include fibre failure, resin failure, fibre/matrix debonding, delaminations etc., which generally increases the analytical workload. Often these failure modes are related such that it can be difficult to exactly predict the failure load.

– Composites will absorb impact energy by damage modes rather than local plastic deformation. This means failure is typically sudden and catastrophic without any prior warning that the structure has been overloaded.

– Fatigue, stress rupture and creep resistance varies from rather poor for glass FRP in wet conditions to excellent for many carbon FRP layups.

Especially due the uncertainty of correctly modelling the complicated failure modes, engineers have tended to revert to a “black” aluminium approach that has inhibited the full exploitation of composite materials in terms of potential weight savings. However, the ongoing research activities into advanced composites and increasing teaching in higher education will hope to resolve these issues in the near future.