Lightweight materials and technology crucial to future of mass transit

Uploaded 15 Jun @ 14:27pm

That a fully laden Boeing 747, weighing in at anything up to 400 tons, upon achieving a groundspeed of 155 knots can take to the air and cruise at around 570 miles per hour for more than 8,000 miles before refuelling may seem little short of a miracle. However, just because such a feat is possible does not mean that it is desirable. Mel Dunkin reports.

The cost, both in fuel, and the long-term effects on the environment caused by the toxic pollutants from its combustion, cannot be sustained indefinitely. That 747 burns approximately 5 gallons of kerosene for every mile travelled and during the course of a 10-hour flight might consume up to 36,000 gallons, depending upon wind conditions.

In addition to the pressing need to develop cleaner fuels and more fuel-efficient engines, if the passenger jet is to remain an economical mass transit option, future planes will need to be faster and able to carry more people and cargo. To achieve this will require the industry to extend its use of lightweighting technology. In short, our much-loved Jumbos will need to lose some serious weight. Furthermore, what is true of planes, will apply equally to rail and road vehicles.

Although the term lightweighting owes its origins to a concept introduced by the automotive industry, its principles had already been applied to the manufacture of aircraft during the late 19th century when Count Ferdinand Zeppelin chose aluminium to construct the airframes for his gigantic lighter-than-air craft, including the ill-fated Hindenburg.
With the increasing demand for lighter and thus faster, winged aircraft during World War 1, aluminium soon replaced wood to become the material of choice for structural components as well as for a variety of engine components. More malleable and elastic than steel and 2.5 times lighter, it offered aircraft manufacturers a more versatile material that was resistant to corrosion and needed neither paint nor any alternative protective coating, prompting them to find other ways to extend its use. The supersonic Concorde featured an aluminium skin while the metal also accounts for 80% of the mass of the world’s top-selling passenger jet, the Boeing 737.

As early as 1915, manufacturers had begun looking for ways to improve on the performance characteristics of pure aluminium, looking for alternatives in its alloys with elements such as copper, magnesium and manganese. Lightweight alloys such as titanium aluminide and aluminium lithium developed in the ‘70s are half the weight of the nickel alloy but share its high thermal resistance and are easier to machine. Since the turn of the century, these heat resistant super alloys (HRSA) have become the materials of choice for both high- and low-pressure turbine blades as well as high-pressure compressor blades. At half the weight of nickel alloy blades, Boeing chose titanium aluminide for its 787 Dreamliner.

The Move to Composites
Today, the quest for even lighter materials continues and the search has been focusing on non-metallic compounds. Many of the uses for aluminium in aircraft construction have been superseded by composites, largely because they are not only lighter but also easier to form them into large and complex shapes that require no fasteners. Creating the same shape from metal requires machining and will often entail joining smaller sections in order to form the whole. This creates the need for fasteners or welds which add further to the overall mass of an aircraft and also become potential failure points.

Once seen as suitable only for non-critical structures such as cabin components, the demise of fibreglass and its replacement by carbon fibre reinforced polymers (CFRP) has led to an expanded role for composite materials in aerospace. Today they are to be found, increasingly, in functional components, including the engines, landing gear and the tough, lightweight skins that cover the wings and fuselage. Once again, Boeing has been quick to recognise the value of new materials, increasing its use in successive generations of aircraft. When, in 2007, the company’s 787 Dreamliner took to the air, advanced carbon fibre laminates and carbon sandwich composites accounted for 50% of the material used.
What is of immense value for flight within earth’s atmosphere can be even more so to those vehicles that venture beyond it. For example, the increased use of composites in the manufacture of the SpaceX Falcon rocket series has seen their payloads increase fourfold and the cost of each kilogram placed in orbit reduced by almost a third, between 2014 and 2016.

The future success of both aviation and space exploration seems inextricably tied to progress in lightweighting technology. However, the domination of CFRP which, together with honeycomb materials, have been the industry’s main contributors to weight reduction, may now be waning. Following years of extensive testing, a new generation of materials, known as ceramic-matrix composites (CRM), seems set to replace both these and HRSA in a number of their current applications. Refractory fibres such as silicon carbide reinforce the ceramic matrix, combining a low density to weight ratio and hardness with greatly improved thermal and chemical resistance. It is this combination of properties that is making this new breed of composites a preferred choice for use in hot-zone structures like exhaust and engine components.

Lightweighting trends in Road and Rail Transport
The constraints facing airborne vehicles are no less relevant to those restricted to surface travel. Though the fuel consumption of an individual car, lorry or locomotive and its contribution to pollution may be less than that of a jumbo, their collective impact both economically and environmentally is huge. To survive, like aeroplanes, automobiles and trains will need to accomplish far more with much less in the very near future.

Nothing improves fuel economy and, as a result, also reduces CO2 emissions more effectively than lightening the load. Since it is passengers and freight that generate revenue and heavy vehicles that erode it, an efficient mass transit sector must be able to transport more of the former while using less of the later. To achieve this, along with more fuel-efficient engines and cleaner-burning fuels, designing lighter vehicles with increased carrying capacity is crucial.

Driven as much by legislation detaining tough new EU emission targets as by economics, one manufacturer set itself a target of trimming its vehicles by between 250 and 750 lbs, depending on their size. In its largest model, the F-150 Raptor, Ford reduced the weight of the frame by 60 lbs with the use of high-strength steel, shedding even more weight by using high-strength aluminium panels for the box and front-end cab. The result is a pick-up truck that is tougher, faster and more fuel efficient than its predecessor while also being lighter by 700 lbs, just 50 lbs short of the proposed target.

In addition to employing lighter metallic components, automotive manufacturers have been investing in other lightweighting strategies and, among the solutions they have embraced, several employ products developed by 3M. Its structural adhesives are suitable for use in a wide range of multi-material structures. As well as being oil-tolerant, they are compatible with both the e-coating and induction curing processes.

For use around the door and boot openings, 3M also offers acrylic foam tapes as an alternative means to secure the rubber seals around these structures. By eliminating the need for the metal flanges that normally act to secure these seals, more weight is saved. In addition to its ultra-lightweight and water-resistant acoustic insulation product Thinsulate™, the company offers the means to reduce the weight of plastic components by up to 15% whilst retaining their strength and good looks. The 3M solution replaces traditional talc filler with glass bubbles of a high strength-to-weight ratio that is ideal for use in extrusion and injection moulding processes.

Lighter Trains essential to UK Rail Future
Despite predictions that the number of rail passenger journeys will double over the next 30 years, the figure actually declined by 2.7% last year, when compared with 2016. Price, overcrowding and excessive delays have all been cited as possible explanations, underlining the need for the improved efficiency of lighter and faster locomotives and rolling stock, and increased passenger capacity.

While there still seems to be more talk than action on the HS2 front, another rail project is already benefitting commuters on the route between Liverpool Street and Shenfield. Characterised by the widespread use of lightweight composite and an aluminium bodyshell, the new Crossrail trains are over 200 metres long, making them half as long again as the longest tube train in operation. Despite their greater capacity, these trains consume up to 30% less energy than others in current use.

The manufacturers of rolling stock have been quick to leverage the benefits of new materials such as magnesium alloys and aluminium foam. The latter consists of a 25mm-thick layer of a sponge-like composite of silicon, aluminium, copper and magnesium, sandwiched between 2 mm thick aluminium plates. The result is a panel that is highly resistant to impact and 20% lighter than its traditional fibreglass counterpart.

While the use of CFRP is now commonplace in non-critical structures and has even been applied in the manufacture of functional components, it might come as a surprise to learn that this material might be used to support the weight of a locomotive. Kawasaki’s new generation bogie design, known as the efWING (Environmentally Friendly Weight-saving Innovative New Generation), has put the composite to the ultimate test and it has passed with flying colours. Replacing conventional steel coil springs with CFRP leaf springs results in a bogie assembly that is not only 40% lighter but also offers a smoother ride and improved stability on the bends. Furthermore, the leaf-spring design makes it a viable candidate for 3D printing.

Materials for Tomorrow’s World
While the benefits to the environment that result from lightweighting are indisputable, there is a risk that the high production cost of many of the materials used could serve to discourage their adoption in some sectors. Recycling is, of course, one way to recoup some of the cost, as is the possibility of exploiting more sustainable sources for use in the manufacture of composites. Consequently, both of these considerations seem likely to be just as important to the development of future lightweight vehicles as their strength-to-weight ratio.

On the home front, the National Composites Centre and the Centre for Process Innovation have been collaborating on a project to develop enhanced structural composites. In consultation with the National Graphene Institute of the University of Manchester, they are developing and testing a number of new graphene composites. As well as aiming for products with improved functionality the project will also investigate ways in which to increase composite production and reduce its cost. While the main focus of the project is on the future needs of aviation, its success is likely to result in benefits throughout the mass transit sector.

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