Sandwich strength

A new composite material may soon be used as a replacement for steel or reinforced concrete in marine, civil and military structures. Jon Excell reports.

From the glass fibre-aluminium fuselage sections on the Airbus A380 to the body panels on MG’s X-Power SV sports car, composites are replacing traditional materials in an ever-growing range of automotive and aerospace applications.

But while lightweight composite engineering projects are fast becoming commonplace, engineers are only just beginning to explore the use of composite materials for ‘heavy-engineering’ and civil projects.

The growth of this area is perhaps best exemplified by the rise of SPS (Sandwich Plate System), a composite material originally formulated to protect oil platforms. Jointly developed by BASF and Anglo-Canadian engineering firm Intelligent Engineering, the material, which consists of steel plates bonded to a compact elastomer core, has already made a name for itself in the maritime industry, and is on the brink of replacing steel and concrete as the material of choice in a number of civil applications.

The story behind its development began in the mid-1980s, when structural engineer Stephen Kennedy was asked by the Canadian government to look into finding an improved material for the hulls of ice platforms used for oil drilling in Alaska’s Beaufort sea. Under the relentless battering of Alaskan ice floes, these hulls, made from stiffened steel, were becoming damaged. The reason was that despite the combined strength of a stiffened steel structure, the sections of steel that weren’t immediately backed by stiffeners were relatively easily penetrated.

Kennedy realised that the solution was to replace stiffened steel with a structure that instead of being locally stiffened was ‘globally stiff’ and turned his attention to composites being used on lightweight engineering projects. He developed a process for injecting an elastomer polyurethane core between two steel plates. And, courtesy of some clever chemical tweaking, this core was strong enough to handle the loads imposed on it by the steel.

The result was a material, which while offering an equivalent strength to stiffened steel, was both lighter and less prone to corrosion. Clearly Kennedy was on to something, but it was to be some years before his work began to have a wider impact.

Intelligent Engineering’s Guy Turner, commercial director of the company that Kennedy went on to found, explained that despite the huge promise of the material, its most likely areas of application — civil and maritime – meant that much of the past 10 years has been spent ensuring that it conforms to stringent safety regulations. ‘You don’t build a ship unless the regulators approve, so we spent about eight years developing the technology and ensuring that it was fully understood,’ he said.

It was during this proving process that the full range of the material’s advantages became apparent. ‘We found that something that had originally been developed for impact protection had a number of very interesting properties,’ said Turner. He explained that the elastomer core adds considerable benefits in terms of vibration, fire protection and blast resistance.

SPS’s tendency to resist vibration in particular makes it ideal for creating permanent building structures that are 70 per cent lighter but just as stable as the reinforced concrete alternative. ‘If you have a heavy concrete structure it won’t vibrate, and if you have an equally strong steel structure it will be a lot lighter but will have a vibration characteristic,’ said Turner. ‘But SPS will give you the dynamic performance of concrete at the weight of steel,’ he added.

This sentiment was echoed by SPS expert Georg Knoblauch from Elastogran, the BASF company involved with the material. Knoblauch explained that the material’s vibration characteristics offer potential benefits for the construction of sports stadiums and arenas. ‘SPS stands are 70 per cent lighter than conventional concrete structures and are very effective at absorbing the vibrations induced when thousands of fans jump to their feet to celebrate a goal,’ explained Knoblauch.

The safety appeal of the material is further enhanced by its fire performance and ability to resist ballistic impacts. Turner claimed that untreated SPS panels give at least equivalent fire protection to insulated steel, while tests carried out by Qinetiq and the US Navy have apparently shown the material to be a more effective barrier than steel against bullets and bomb blasts.

SPS made its commercial debut in the marine industry around three years ago, where it was initially used for repairing damaged steel plates. Using a process in which a plate is essentially bonded to the existing structure with elastomer, the composite has been used to repair a number of European ferries, a helicopter deck in Singapore and an oil production platform off the coast of Nigeria. Turner estimated that since its introduction SPS has been used to repair around 300,000ft2 of steel.

Intelligent Engineering is now keen to see the material take off in new-build applications and over the past few months has already met some success in this area. For instance, it recently made its first appearance as a fire barrier on the funnel casings for the DFDs Tor Magnolia, a ferry built by German shipyard Flensborg. Turner explained that in addition to its thermal and mechanical properties, the relatively slim profile of the material enabled the ship’s designers to create more space in which to accommodate the machinery contained within the funnel.

In one of the most impressive new applications Canadian SPS licensee Canam Manac Group recently completed the construction of a road bridge, where the use of SPS resulted in the development of a bridge deck 60 per cent lighter than its concrete equivalent.

The bridge, in Martin de Beaucem, Quebec, is apparently not only lighter than a traditional structure, but, according to Georg Knoblauch,|safer.

‘SPS is extremely stable and doesn’t become brittle with age, making an SPS structure more durable than conventional solutions,’ he commented.

A number of other bridge projects are currently under development around the world, with ThyssenKrupp evaluating the feasibility of the system for the construction of mobile bridge elements for roadwork sites. Meanwhile, Turner said that Intelligent Engineering is also looking at several large stadium projects, some of which are in the UK.

While the company is sensibly focusing on marine and civil projects, the material could potentially find applications beyond even these huge markets. For instance, although the structure currently uses steel, it would be possible, said Turner, to reduce the weight by replacing the steel with aluminium. Such a move could see SPS appearing on future generations of trucks and lorries, he said.

Perhaps the final question concerns the cost of SPS. Turner claimed that the extra expanse of the elastomer is soon offset by other factors – ‘the total project costs, the cost of other materials that you don’t need because you’ve dropped weight out, the speed of erection and the longevity of the structure all mean that you will have a very happy owner,’ he said.