Materials

9-2001

Notebook

Steel makes lightweight fibre sandwich

A striking new material is being piloted for the automotive industry. Tom Shelley reports

A sandwich of stainless-steel fibres in stainless steel is offering great potential for weight savings in automotive construction. In its initial form, it comprises two sheets of stainless steel, 200 microns thick, separated by stainless-steel fibres a few tens of microns across. The choice of stainless as opposed to ordinary steel is to give it corrosion resistance.

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Drawn cups

Initial tests are oriented at spoilers on top of lorry cabs, but it looks to be a material of much promise with many possible derivatives. Other potential applications include roof assemblies, bonnets and tail gates.

In the UK, it is being studied by Dr Athina Markaki and Professor Bill Clyne at the Department of Materials Science and Metallurgy at the University of Cambridge. The base material has been developed by Volvo at its Technological Development Corporation in Gothenburg, Sweden, under the direction of Roland Gustafsson.

Different constructions are being tested in which the core fibres are either perpendicular to the two face plates, bonded to them by adhesive, or made in the form of woven or sintered mesh, joined by adhesive or brazing.

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The three configurations: perpendicular fibres, mesh and mat

We understand that the material with the perpendicular fibres is fabricated with the help of electrostatics that charge the fibres, with an opposite charge on the face plate to which they are to be bonded. This makes the fibres stand up before being fixed in position.

Total sandwich thickness can be as thin as 1mm, in which case the material can be handled and processed in the same way as conventional monolithic sheet. Initial trials indicate that the formability and weldability can be very good. Not only does the material have very high stiffness per unit weight, but it is also expected to exhibit good thermal insulation, and high acoustic and vibration damping capacity.

Studies are being made of its mechanical properties in service, resistance to being torn apart and electrical properties relevant to spot welding. Comparison is being made of measured properties with theoretical values derived from computer modelling. The stiffest constructions so far seem to be sandwich constructions using a brazed mat.

The general opinion is that the material is never going to be cheap, but it should be low enough in cost to be attractive to designers of commercial vehicles and performance cars. It should also be possible to fabricate similar constructions out of several other solid materials.

The research investigation is being financially supported by the Cambridge - MIT Institute. More information can be found at www.msm.cam.ac.uk/mmc .

Design Pointers

Thin sandwich constructions can be made using stainless-steel fibres between sheets of the same material

The constructions combine high stiffness per unit weight with resistance to corrosion and elevated temperatures

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