Whether the disturbance to a structure is vibration on a ship's deck, a car crashing into a barrier, or the force of an explosion, the most cost-effective solution for additional strength is turning out to be two layers of steel on each side of a suitable filling.

Steel sheets offer strength and ductility, but fillings can provide greatly improved energy absorbency and/or flexural rigidity.

Steel encasing polyurethane seems to be a most advantageous method of constructing ship hulls and decks, halving construction costs in some instances, with a possible transfer to automotive. Steel in combination with polypropylene foam shows advantage in the ends of motorway crash barriers, but blast resistance in major constructions, seems to most effectively achieved by steel sheets linked by steel rods, but with the interstices filled with concrete.

The ship construction method, called the sandwich plate system, or SPS, was invented by Dr Stephen Kennedy, formerly a Professor of Structural Engineering at Carleton University in Ottawa. With his brother, Michael Kennedy, Dr Kennedy formed Intelligent Engineering, which now has a joint venture with BASF/Elastogran covering intellectual property, development and marketing. Current products are the fruit of no less than seven years work.

The idea is to form a rigid structure made from two steel skins, with occasional dividers but not stiffeners, and then fill the voids with polyurethane, formed in place. Elastrogran's Georg Knoblauch said that typical thicknesses of the PU 'meat' section in shipbuilding and repair panels are 30 to 60 or 70mm, but there is no reason, in principle, why the technique could not be applied to much thinner sections, as might be used in automotive construction. Advantages include an inherent ability to dampen vibration and the opportunity to do away with internal welded-on stiffeners. Welded-on ribs in any kind of construction greatly add to costs and provide golden opportunities for fatigue crack initiation where gaps exist where welds have failed to interpenetrate fully at the points where the ribs butt onto the skin. Eliminating them, where possible, is good, and reducing fatigue stresses on the remainder by reducing vibration is even better. In shipbuilding, they also provide additional sites for initiating corrosion.

Used in the renovation of 2,000 sq m of the main decks of two car ferries in January 2001, the structures were found to halve repair costs. They meet standards set down in guidelines for "Safety of Lives at Sea" (SOLAS) and fulfil the requirements of the highest A60 classification for ship's decks. A number of elements made from SPS for shipbuilding have now been approved by Lloyd's Register. Other recent uses include bridge deck construction. The sandwich material shows greatly improved resistance to low-speed impacts over un-reinforced sandwiches, but for higher speed impacts, different approaches are advantageous (More information at www.ie-sps.com and www.elastogran.de )

Foam/steel to reduce death on the roads

A German research institution has come to the conclusion that a combination of steel, and energy absorbing polypropylene foam is the best way to make the ends to crash barriers, especially the short ones used in Germany around the sides of bridge arches and in front of trees.

As we have commented on in this magazine before, in our September 2000 edition, the idea of ramping up the ends of crash barriers is liable to lead to vehicles turning over on striking them. 65km/h is apparently sufficient speed to result in a fatal accident, only slightly more than the 45 km/h sufficient to result in fatalities if a vehicle hits an unprotected tree or bridge pier.

The Center of Excellence for Plastics, in Schwarzheide, set up a joint project to solve the problem. The project involved Outimex Baytechnik, a major maker of crash barriers; Forster Montage, a company which installs and repairs barriers; BASF in its role as the plastics provider; and Febra-Kunststoffe, as the plastics processor.

The final design is described as 'System Primus'. The crucial part is an end loop, with crash barrier steel in front of and behind energy absorbing elements made of BASF's 'Neopolen' product. Steel cylinders behind the vehicle-side steel convert shearing forces into compression in the foam. Dr Gerhard Ramlow, head of the Competence Center, said that the same foam is already used successfully in automotive fender systems, and was preferred over other foams for its mechanical strength, energy absorbing performance, weathering resistance and lack of susceptibility to attack by rodents. It is also reusable and contains no blowing agents. Tests have been carried out at the Outimex crash facility, near Warsaw, and there are test installations in Brandenburg and Western Pomerania in the eastern part of Germany.

These show that impacting vehicles are progressively slowed down, without being bounced back into traffic. German federal approval is expected this Autumn, which is expected to lead to rapid acceptance by other European highway authorities. Research is continuing and it is quite possible that future designs may use foam exposed on the steel profile, instead of backing the steel. Plastic skinning of the foam blocks is also being considered although they seem to be pretty durable even when unprotected. (More information at www.outimex.de )

Concrete sandwiches take a blast

In instances where constructions may be expected to have to withstand even greater loads and shocks, there is growing interest in Bi-Steel, a product developed by Corus.

In Bi-Steel, pairs of steel plates are permanently connected by an array of transverse, bars, to form constructions, which may be filled with concrete. The panels are made by simultaneously friction welding both ends of the bars to the face plates.

For most applications, the face plates are made of S355 carbon steel, but the constructions can also be made of other materials, including stainless steel, if required by users such as those in water, process or energy industries.

Applications suggested by the Corus include: vaults; anti-terrorist protection; tunnels; reducing the thicknesses of bridge decks while allowing an increase in head room; offshore wind turbine foundations; structures for the offshore industry; and constructing tall buildings. The system is modular, and constructions using it can be rapidly assembled on site. It will also reduce the number of crane lifts compared to building with reinforced concrete. There is also much interest from the military to quickly build blast-resistant constructions. Corus provides a specialist blast design service as each structure has to be designed with the specific threat in mind. (more information at www.bi-steel.com ).

Constructions involving combinations of metal sheets and fibre-reinforced composites are available from a number of specialist suppliers for defence armour purposes. Metal offers ductility while composites absorb energy as fibres are pulled out of matrices. All kinds of combinations seem to have been made, including some incorporating ceramics.

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