In mechanical terms, there are different types of ‘strengths’ (tensile, compressive, torsional, impact and so on), and materials are also measured according to the way in which forces are applied. Some materials perform exceptionally well under tension but poorly under compression (and vice versa). Additionally, some materials can withstand incredible forces, but only for short durations or for a few times before they fail, whilst others are more durable and can withstand repeated application of forces.
Also, strength-to-weight properties can vary with temperature – some plastics, for instance, have great strength-to-weight ratios at room temperature, but at elevated temperatures they soften and cannot support weight at all.
The shape and structure of an object is also of critical importance in terms of how strong it is. Hence, many so-called lightweight materials are really configurations of materials resulting in a lightweight structure. Honeycomb and foamed materials often fall into this category.
Lightweight materials are of interest to designers, engineers and manufacturers for many reasons.
They present great opportunities for energy saving, particularly when it comes to transportation. Lightweight materials are used wherever weight is a critical issue such as in the aerospace industry, but increasingly they are being utilised in other transportation sectors to save energy.
With rising fuel costs, light-weighting of vehicles has become increasingly important, so in the automotive industry the use of plastics as a replacement for some metal components has increased substantially.
Some cars utilise plastic body panels instead of pressed metal, whilst others will feature composites of polymer resins reinforced with glass, carbon or aramid fibres in order to produce complete car bodies that are substantially lighter (and therefore more fuel efficient) than those made of metal.
Lightweight materials are also used in bicycle design, with aluminium and carbon fibre replacing mild-steel tubing. Whilst the energy saving is perhaps less apparent, the benefits of the materials are not lost on their riders.
At the professional level of cycling, and in sports more generally, competitive advantage can be partly attributed to the use of lightweight high-performance materials. Globally, sporting goods is a multi-billion dollar industry and lightweight materials can be found in graphite tennis racquets, carbon-fibre super yachts and lightweight running shoes.
The energy saved by using lightweight materials in vehicles is one thing, but light-weighting the goods that are transported can have significant environmental and economic impact. The benefits arising from lightweight packaging materials, also, should not be underestimated.
In consumer electronics there is an unstoppable drive towards miniaturisation, and here lightweight materials have come to the fore. A great contributor to this progress has been the development of more advanced manufacturing processes in combination with the commercial viability of using more advanced materials.
Tracing the relatively young history of such products shows a move from injection-moulded commodity plastics to more advanced polymer blends of polycarbonate and acrylonitrile butadiene styrene (ABS) to the use of lightweight alloys that were typically cast, but now we also see the use of powder metallurgy, metal injection moulding and amorphous metals.
Lightweight metals are the heavyweights of the trend towards lightness in materials. Some of the metals are already in common use, while others are still in development.
Aluminium is probably the second most important metal after steel in terms of its economic impact and wide use across a wealth of industries such as aerospace (and transport more generally), packaging, electronics and construction (for architectural facades rather than for the building’s structure).
Magnesium is approximately seventy per cent of the weight of aluminium and twice the price, and is typically used in similar applications to aluminium, but it tarnishes and corrodes more readily, particularly in salty or acidic environments.
Titanium costs about ten times the price of aluminium, is very strong, has a high melting point and is corrosion resistant, making it highly suitable for advanced engineering applications, including implantation into the body to replace or repair bone. Increasingly it is used in high-end consumer electronics such as mobile phones and portable computers.
Most metals are crystalline in structure – that is, the atoms of the material are aligned in particular ordered patterns forming tiny crystal-like structures. Amorphous metals, on the other hand, do not have an ordered structure. This results in materials that can be significantly stronger than their crystalline forms.
These materials, also referred to as ‘metallic glasses’, are formed by rapidly cooling the metal from its molten state, thereby forming an amorphous structure. Metallic glasses can be made from a number of metal alloys including titanium, zirconium, iron and magnesium, but they are expensive.
We are all familiar with commonly used rigid and flexible foams such as foam rubber, polystyrene foam packaging and so on, but there are also metal foams that are incredibly lightweight and have great strength and stiffness – many metallic foams even float on water.
There are two principle types of foam structure: open cell and closed cell. A closed-cell metallic foam looks much like a chunk of expanded polystyrene that’s been painted silver, whereas an open-cell metallic foam has a very fine and porous bone-like structure.
These materials are used in a number of applications, including for impact absorption and insulation and in structural components, heat exchangers and filters. A few years ago there were only a handful of companies producing foamed metals, but now there are dozens of them, indicating a great growth in the use and application of these materials.
Because they have high strength-to-weight and stiffness-to-weight ratios, they’re of increasing interest to the transport industry.
Typically the foams are made by combining a gassing agent in the molten metal. However, research is under way to investigate how gravity affects the bubble formation in the foamed metals, and indeed NASA is conducting experiments in zero gravity on the International Space Station to explore this phenomenon.
Other foamed materials include: ceramic foams (silicon carbide foam), carbon foam and copper, zinc, nickel, tin, silicon, silver and gold foams.
Aerogels are the lightest solid material known to us. They weigh about three times as much as air (but can be made lighter than air if the air is removed), and are predominantly made of air and silicon dioxide.
They seem like an ultra-lightweight foam, and are commonly called ‘frozen smoke’ because of their misty-blue fog-like appearance (the blue colour is due to Rayleigh scattering – which also makes the sky look blue even though it’s clear). They are made by supercritical drying of a jelly-like material, which replaces the liquid with air (imagine removing all the liquid from a jelly without any change in its volume).
The material is brittle, and though it is argued that it can theoretically hold thousands of times its own weight, it is not typically used for its structural strength but for its incredible insulation properties – it is deemed the world’s best thermal insulator. In addition to these silica aerogels, there are also carbon aerogels.
Many lightweight materials are based on composite materials – materials that combine the benefits of two or more materials. Composites are used in a whole range of applications, such as car tyres (steel and rubber), aerospace (metals and ceramics), and construction (pre-tensile concrete combining steel and concrete, glass and other fibre reinforced polymers).
Fibre-reinforced polymers (FRP) have some of the highest strength-to-weight ratios. Typically FRPs consist of a thermosetting resin (usually polyester, vinyl ester or epoxy) and a reinforcing fibre such as e-glass, fibreglass, or Glass Fibre Reinforced Polymer (GFRP), carbon fibre (CFRP) or aramid fibre (Kevlar® by DuPontTM, for instance).
The fibres come in different fabric compositions, such as randomly oriented fibres (chopped strand mat), woven cloth (woven rovings), and unidirectional cloths and tapes.
These different compositions allow the composite material to be ‘designed’ specifically for its application, so, for instance, the orientation of the fibres can be placed where they are needed to appropriately distribute forces through the material. This ability to ‘design’ the material is really exciting as it opens up an extra dimension in the designers’ repertoire.
There are also natural fibres such as hemp (which has great strength) and plant-derived polymers such as cellulose-based resins, allowing for a completely organic composite material.
Fibreglass (GFRP) has very good strength-to-weight ratio, comparable with that of steel, but can outperform steel if the orientation of fibres is unidirectional. Carbon and aramid fibre-reinforced polymers have incredible strength-to-weight ratios – carbon fibre can have ten times the tensile strength of steel – and can exhibit great stiffness, making them ideal for lightweight structural applications.
The main failing of carbon fibre, beyond its high cost, is that when it fails, it does so catastrophically – the carbon-fibre hulls of the America’s Cup racing yachts that instantly broke in two and sank when pushed to their limit are a case in point.
Aramid fibres have greater impact resistance, so carbon/Kevlar® composite-weave cloths are readily used where lightweight stiffness and impact resistance are needed (in motor racing, for example).
Honeycomb structures are often used in combination with fibre-reinforced composite polymers to provide greater stiffness to large objects such as aircraft. The honeycomb core can be made of all manner of materials, including paper and card (often sandwiched between thin layers of plywood, wood veneer or MDF to make hollow core doors) and aluminium (Alucore used in architecture and transport).
While composite materials offer incredible properties, their full potential has not really been explored. Other fibres – fiber-optic fibres to transmit light and data, pressure-sensing piezoelectric fibres, or shape memory alloy wires that change the material’s shape – could be incorporated into a carbon/Kevlar matrix.
The weave and weft of the structure could also be further controlled in order to develop smarter composite materials. This technology makes all sorts of things possible and the possibility of a ‘knitted’ car is not as far fetched as it sounds.
By controlling the fibre orientation and the choice of materials ‘knitted’ to create the resulting ‘fabric’, it would be possible to determine where forces are distributed within the material, and also to generate interesting forms.
With the inclusion of piezoelectric materials or shape memory alloys, you could change the form by applying an electric current, and the material could be smart enough to ‘sense’ its surrounds and speed (the individual fibres could sense pressure, hence speed), and temperature, and adjust its form to adapt to these variables.
There has been a lot of interest in nano-technology, of which nano-materials are a significant area of research and development. There are two dominant categories of nano-materials. The first is based upon the atomic structure of carbon, of which carbon nanotubes are the most exciting.
Carbon nanotubes, structures that are perfectly ordered at the atomic level, were first discovered in 1991. They can have three configurations (referred to as ‘chirality’) – armchair, zigzag or chiral – and it is these perfect structures that give them incredible properties.
Carbon nanotubes are technically the strongest materials in the world – a whole order stronger than steel or carbon fibre, and theoretically over a thousand times stronger than steel. Their structure allows them to buckle, twist and flatten as they are deformed, and spring back without any permanent deformation when the force is released.
This property, if scalable, could result in extremely lightweight and strong materials that can return to their original form after impact (imagine cars that heal themselves after crashing, doing away with the need for panel beaters!).
The different chirality of their structures can give them different electrical properties, so they can be either conductors, semiconductors or insulators. This has great potential for the miniaturisation of computer technology.
Carbon nanotube film is being investigated for use in creating ultra-thin, flexible colour display screens. They’re also being used to develop strong, flexible solar cells that could be printed or spray coated onto various substrates (entire buildings perhaps), or fabrics (solar clothing, awnings and curtains for instance), or even to produce completely transparent solar panels on windows (as the nanotubes are too small to see when sandwiched between two panes of glass).
There is also discussion of carbon nanotubes being used to create extremely lightweight hydrogen fuel tanks, which could lead to some very interesting developments in the automotive industry.Carbon nanotubes have not ventured too far from the research lab as yet, and they are very expensive to make.
They have to be ‘grown’ in a number of expensive ways (chemical vapour deposition, high-pressure carbon monoxide deposition or by a plasma process) and controlling the chirality of the structures is difficult.
But there’s a lot of investment in developing cheaper and more predictable techniques for creating carbon nanotubes, and the race is on to commercialise these exciting new materials.
The second category of nano-materials is nano-particles and many of these are already in use today, in make-up, sunscreens, paints and plastics. Nano-clays can be added to polymers to cost-effectively reduce their weight and improve surface finishes in moulded parts without losing any structural integrity.
A lot of lightweight materials are expensive to produce at present (carbon nanotubes cost in the order of $250 per gram), but research is underway to develop cheaper manufacturing processes and carbon nanotubes are likely to come down in price to around $250 per kg in the next few years.
The potential for more economic uses of raw resources, particularly in terms of the energy savings that lightweight materials can bring, is exciting, but the cost (in energy as well as money) of manufacture can sometimes counteract this. To produce lightweight metals from raw ores, for instance, uses incredible amounts of energy.
Lightweight materials hold great promise for the future. They have much to offer the transport industry, and the prospect of creating ultra-lightweight vehicles is particularly appealing.
The idea of using less energy is particularly poignant in light of rising fuel prices and the pressure to reduce greenhouse gas emissions, but, additionally, we could see the development of a car so light that it could be carried by a couple of people, allowing it to be stowed or stacked on top of something to save space.
The prospects for miniaturisation of technologies has always been of interest to designers, but nano-materials could take this prospect to dizzying levels, where the fantastic becomes real and ‘magic’ is less impressive than our technological advances.
We would like to acknowledge the assistance of Jonathon Allen as an expert advisor for this story.