Ultra-light Stainless
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CARS BUILT with HSSA components could end up weighing 50 to 70 per cent less than conventional
vehicles.
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FIBRES OF S31603 STAINLESS steel are bonded to two thin plates of S30400 to create Volvo's new hybrid
stainless steel assembly (HSSA) material.
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MEASURING ENERGY ABSORPTION and impact fatigue of HSSA material at MIT. This will
help to evaluate the crash worthiness of components made from the new material.
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A stainless steel 'sandwich' material, developed by Volvo, promises lighter, safer and
environmentally sound cars, trucks, trains, aircraft and boats. By Dean Jobb
Nickel magazine, Feb. 2003 -- Roland Gustafsson sees himself behind the wheel of a sleek red Volvo, seated under a gleaming stainless steel hardtop that weighs a mere 8.5 kilograms. There would be a lot more to this car than good looks -- it would be safer, burn less gasoline and, when the car ended its days on the scrap heap, the shiny canopy would be 100 per cent recyclable.
Thanks to a composite material the Swedish scientist has invented, such cars -- as well as buses,
trucks, trains, aircraft and even ships containing ultra-lightweight stainless steel components -- could soon
be a reality. Gustafsson, project manager with Volvo Technology Centre's concept centre in Gothenburg, Sweden
has developed and patented a promising new material that is a hybrid stainless steel assembly
(HSSA) using nickel stainless steel. HSSAs are "sandwich" structures consisting of two sheets of thin
stainless steel bonded to a core of miniature stainless steel fibres (see top photo at left).
The result is a material as thin and easily shaped as conventional sheet metal, lighter and stiffer than
aluminum, and that offers built-in insulation against noise and vibration. The shiny hardtop on Gustafsson's
dream car would be eight times stiffer than one formed from sheet aluminum and would tip the scales at
one-quarter the weight of the same component fashioned from plastic composites.
"It's a construction material that is ultra-light and which has unique properties in strength, stiffness
and ductility," Gustafsson notes. "It's hard to believe it is that formable. We can deep-draw it even better
than solid steel."
Based on the material's superior performance during an initial round of tests, scientists in Britain and the
United States have launched an intensive research program to investigate the properties and potential uses of
HSSAs.
The idea of using a sandwich structure to strengthen materials is nothing new -- think of the corrugated core that reinforces an ordinary cardboard box. "It's a common way of creating stiff panels, to make them like a sandwich," says Gustafsson. "What I did was 'micronize' the idea and bring it down to a level where you have a core of stainless steel fibres which are, in this case, eight to twenty microns in diameter."
The idea grew out of work being conducted for Volvo's automobile division a half-dozen years ago. "We were looking for lightweight design in all different types of materials, and typically in those days it was very much about aluminum. I figured out that it could be interesting to try to use the structure to create stiffness and low weight, instead of just light material in itself."
He settled on stainless steel for its durability and corrosion resistance, using 0.2-millimetre-thick
sheets of S30400 for each of the faceplates
(sheets of S31603 have also been used). Thin
strands of drawn S31603 stainless -- commercially
available and used to make filters and electromagnetic shielding for computers -- were cut into
1-millimetre lengths to create fibres for the core. At diameters of less than 20 microns, Gustafsson says,
the fibres become an extremely good material.
To attach the fibres, Gustafsson worked with a German textile manufacturer to modify a process, known as
flocking, that is used to apply short fibres to cloth to create a velvety surface. The base plate is coated
with a thin layer of epoxy, and an electromagnetic field is switched on to accelerate the stainless steel
fibres on to the sheet metal, ensuring a firm bond and as much metal-to-metal contact as possible. The
magnetized fibres repel one another, keeping them for the most part perpendicular to the base plate. Varying
the current to alter the electromagnetic field makes it possible to control the surface density of the
fibres. The second plate is then attached with epoxy, and the sandwich sheet is pressed and baked under
pressure.
The prototype sheets have an overall thickness of 1.4 millimetres --comparable to the 1.2-mm-thick body aluminum used in the automotive industry -- but fibre thickness, length, angle and density can be varied to produce other forms of HSSAs, each with unique properties. "You can scale up or scale down the core by using different types of fibres," says Gustafsson. "We don't know the borderlines yet for different applications, for different combinations."
To understand how best to build and utilize HSSAs, the Cambridge-MIT Institute (CMI) -- a joint venture of Cambridge University and the Massachusetts Institute of Technology -- is funding a three-year research program. Work at MIT will focus on energy absorption, crashworthiness and the impact of fatigue. Researchers at Cambridge will experiment with different core components and arrangements of fibres, as well as examining the structure, durability and weldability of the material.
"What our research project is all about is optimizing the core structure, so it will give you the best combination of properties," says William Clyne, professor of mechanics and materials at Cambridge. "It's a concept, really, and exactly what properties you get does depend quite a lot on exactly what the core structure is."
Other grades of stainless can be used for both faceplates and fibres. The Cambridge team is exploring the option of using fibres formed from a higher-chromium stainless alloy such as S44600, to take advantage of its magnetic properties. The porous nature of the core -- empty space between the fibres can account for between 80 and 95 per cent of its volume -- demand the corrosion protection that stainless affords. "Simply painting the surface would obviously not eliminate the danger that moisture or fluids could get into the gap and cause it to corrode from the inside out," Clyne notes. Such permeability, however, may prove to be an asset, enabling sensors to be embedded or a cooling fluid to be circulated through the hollow core in applications where surface heat is a problem.
Clyne's research into the welding properties of HSSAs will be particularly important. Preliminary tests
conducted at Cambridge found it difficult to join the material using resistance welding techniques common in
the automobile industry. "The HSSA adhesively-bonded material could not be successfully welded in these
initial trials" and "weld flaws and faceplate damage occurred," states a paper published in February
2002 (click here to download a 1.7-MB
PDF file of that paper)
Clyne, however, is optimistic the problems can be overcome. "It certainly can be welded," he says of the
material, "but it does depend a lot on the core structure. What you need is a reasonably good conductant
through the thickness (of the sheet), so the steel fibres have to be in reasonably good contact with the two
faceplates."
The auto industry may hold the greatest potential for applying HSSAs, enabling car makers to design lighter,
more fuel-efficient vehicles without compromising safety. Besides their obvious potential as a
replacement for sheet metal in building fenders, hoods and roofs for cars, HSSAs can be formed into tubular
members and used as structural components. The material crushes in an accordion-like fashion (a property
Gustafsson says researchers have termed "sheer folding mechanism"), cushioning the impact of a collision (see
accompanying photo).
Preliminary studies of the material's crashworthiness found it absorbed 50 to 60 per cent more energy than
did solid sheet metal, says Professor Tomasz Wierzbicki, director of MIT's Impact and Crashworthiness
Laboratory. "You can see the future fleet of cars made of this material, which, though they'll have the same
weight, will be much stiffer and more crashworthy. Or you can say I'd like to maintain the stiffness and the
crash performance, but I'd rather decrease the weight."
Cars built with HSSA components could end up weighing 50 to 70 per cent less than conventional vehicles,
Gustafsson predicts. The material's hollow core makes it an excellent dampener, insulating against noise and
vibration. He has used HSSAs to design a one-piece firewall for cars, shielding occupants from the heat and
noise of the engine compartment without requiring a layer of thick rubber insulation.
The environmental benefits of HSSAs are extremely promising. Gustafsson has conducted a life-cycle assessment based on a material's environmental load -- a measure of the resources used to create a product and any emissions it creates -- for the hardtop of a car driven 200,000 kilometres over its lifetime. While a hardtop made of HSSAs has the highest environmental load at the manufacturing stage, fuel savings during use and recyclability make it more environmentally friendly than a hardtop built of steel, aluminum, or plastic composites. The stainless steel can be melted down and reused, Gustafsson says, with the adhesive layer burning off much like paint.
Industry partners involved in the CMI research program attest to the variety of potential applications for HSSAs. The list includes stainless steel producer AvestaPolarit, aircraft manufacturer Airbus UK, car makers BMW and Saab, and Volvo Truck Co. Besides cars, Wierzbicki foresees the material being used in a super-light laptop computer or to replace sheet metal in large appliances such as refrigerators and washers. Ballistic tests suggest HSSAs can stop and deform bullets, making it a candidate for use in making body armour. Gustafsson says the material could be used throughout the transportation industry to lighten components of trucks, buses, aircraft, shipping containers, and high-speed trains. He is already working with a university in Gothenburg to develop HSSAs made of copper-nickel alloys, which can withstand the corrosion of seawater, for use in high speed boats and ships.
Cost will be an obvious factor in bringing HSSAs to market. They will have to compete with materials with metal foam or honeycomb cores, but thinness alone gives them an edge over other sandwich composites, which are typically at least 10 millimetres thick. "It's not a winner in all situations, by no means, but it's a very interesting new material," Wierzbicki observes. Volvo has created a stand-alone company, HSSA Sweden AB, to kick-start the industrialization process.
Gustafsson believes the key to making his vision a reality is to bring nickel and stainless steel
producers on-side to help build demonstration vehicles with HSSA components. "It's a completely new design
philosophy, working with structure instead of mass, and it's hard for the engineering community to
understand that you really can design, in ultra-light material, structures that are stronger and stiffer
and more efficient than traditional design. I think that is the big challenge."
Dean Jobb is a Halifax-based freelance writer.
Photos: VOLVO, MIT and UNIVERSITY OF CAMBRIDGE
 Roland N-G Gustafsson Professor Tomasz Wierzbicki Professor Bill Clyne |
