Matt Dawson discusses what engineers need to consider when choosing the right composite fibre types and properties
Composite materials offer a blend of properties that set them apart from traditional engineering materials. Characteristics such as high strength, lightweight, and ease of forming mean that engineers prefer composites for a wide variety of uses. Indeed, there are applications that would be impossible without them.
Once the decision to consider composites has been taken, one of the first questions to be addressed is that of fibre selection: Laminate performance depends strongly on the fibre properties, so it is worth investing effort into making the right choice. There are many fibre types from which to select, ranging from low-cost to highly exotic and offering a correspondingly wide range of properties. Understandably, the number of options can be a little daunting, so this article will introduce the more commonly available types of fibre, their properties, applications, and pros and cons.
Carbon Fibres
Carbon fibres offer outstanding specific strength and stiffness, making them ideal for high-performance applications such as aerospace and motorsport. Even in the risk-averse commercial aviation sector, carbon fibre composites are now supplanting aluminium alloys in airframe construction – new aircraft such as the Airbus A350 and Boeing 787 incorporate at least 50% composite structure by weight. Carbon fibres also offer excellent fatigue performance, environmental resistance and thermal stability. Their density is in the range of 1700-1900kg/m3.
Carbon fibres consist of graphene planes orientated parallel to the fibre direction. They are manufactured from either PolyAcryloNitrile (PAN) or mesophase pitch precursors and are available in a variety of grades, each offering a different balance of strength, stiffness and cost.
Despite their advantages, there are reasons why carbon fibres might not be the right choice: the most obvious being cost. If low weight isn’t critical, the use of carbon fibres may not be justified. It is also true that carbon fibre manufacture is an energy-intensive process with corresponding implications for CO2 emissions. In some cases, however, such as aviation, fuel savings resulting from a lighter structure will more than offset the emissions embodied in the material itself. More practically, carbon fibres are not transparent, so laminate defects may not be visually detectable.
Glass Fibres
When high strength and stiffness are less critical, glass fibres offer a cost-effective and less energy-intensive alternative to carbon. Often selected for marine, renewable energy and industrial applications, they exhibit good specific strength, fatigue performance and stiffness.
In structures that must interface with steel fabrications, their similar thermal expansion characteristics also help avoid thermally induced stresses. The transparency of glass fibres can be beneficial as it simplifies the detection of laminate flaws. At 2500-2600 kg/m3, their density is higher than that of carbon, but still only a third of that of steel.
It is important to note that the strength of glass fibres can be reduced by exposure to certain environments, for example marine, although this effect can be mitigated by careful matrix selection. Some applications, such as tidal energy blades, benefit from the use of both carbon and glass fibres: The widespread use of glass keeps overall cost down whilst the careful targeting of carbon reinforcement permits a slimmer structure than would otherwise be possible.
Natural Fibres
Of growing interest in recent years are natural fibres, which offer lower impact manufacturing. The most commonly used natural fibre is flax, however hemp and jute are also options. Natural fibres offer lightweight – flax fibre density is approximately 1450kg/m3 – but their strength is lower than other fibre types. Furthermore, the attraction of biodegradability confers the disadvantage of limited environmental resistance
Aramid Fibres
Aramid fibres, such as Kevlar, are employed in niche applications. Their main selling point is a combination of high strength and strain to failure and low density. Ideal for ballistics protection, they absorb very high levels of energy before rupture. At 1440kg/m3, the density of aramid fibres is similar to that of flax.
Composite materials aren’t the answer to every engineering problem, but with a wide range of fibres to choose from, it is no surprise that they are becoming ever more commonplace in the modern world. How could your next project benefit from composites?
Matt Dawson is the Director and Chief Engineer at Orthotropic Engineering