In an advance that could lead to composite materials with virtually limitless performance capabilities, a University of Wisconsin-Madison scientist has dispelled a 50-year-old theoretical notion that composite materials must be made only of ‘stable’ individual materials to be stable overall.

Writing in the 2 February issue of the journal Physical Review Letters, engineering physics professor Walter Drugan proves that a composite material can be stable overall even if it contains a material having a negative stiffness, or one unstable by itself – as long as it is contained within another material that is sufficiently stable. “It’s saying you’re allowed to use a much wider range of properties for one of the two materials,” he says.

Comprising everything from golf clubs and bicycle frames to bridge beams and airplane wings, composite materials – or materials made by combining multiple distinct materials – deliver advantages over conventional materials including high stiffness, strength, lightness, hardness, fracture resistance or economy.

“The idea is that you have one material with some great properties, but it also has some disadvantages, so you combine it with another material to try to ameliorate the disadvantages and get the best of both,” says Drugan.

Until now, materials engineers adhered to proven mathematical limits on composite performance, he says. “For example, if you give me two materials and one has one stiffness and the other has another stiffness, there are rigorous mathematical bounds that show that with these two materials, you cannot make a material that has a stiffness greater than this upper bound,” says Drugan. “However, all these theoretical limits are based on the assumption that every material in the composite has a positive stiffness. In other words, that every material is stable by itself.”

When slightly disturbed, stable materials, like those with positive stiffness, return easily to their original state. A slightly compressed spring, for example, bounces back after the compression force is removed. Unstable materials, like those with negative stiffness, quickly collapse or undergo a large, rapid deformation at the slightest perturbation. In an example from the structures field, if a vertical column supports a load that becomes too great, even a slight disturbance can cause the column to buckle.

The idea of incorporating a material with negative stiffness into a composite designed to be highly stiff originated with UW-Madison Wisconsin distinguished professor of engineering physics Roderic Lakes, says Drugan.

Six years ago Lakes noticed that, in the mathematical formulas that predict how a composite will perform based on its component material properties, employing a material with a suitably chosen negative stiffness theoretically would yield an infinitely stiff composite.

Lakes took his ideas into the lab, where he created such a composite by embedding a material that behaved like one with negative stiffness in a matrix of a material with positive stiffness-somewhat like the shell of a golf ball surrounds its core. Through dynamic experiments, conducted under oscillatory loading, he showed that the composite stiffness was greater than the mathematical bounds indicated it could be, given the combination of materials.

Lakes and Drugan, who have had a continuing research collaboration on this topic, published a 2002 paper in the Journal of the Mechanics and Physics of Solids in which they showed that if a composite material containing a negative-stiffness phase could be stable, and if they tuned the negative stiffness the right way, the predicted composite property could be infinite stiffness for a broad range of composite materials.

Then Drugan set out to prove theoretically that such a material can be stable under static loading. “In general this is a very challenging problem, but I finally found a clean way to analyse it,” he says.

Drugan hopes his proof will awaken materials engineers to a new, broad range of possibilities for making composite materials.

Safety is a fireproof ceramic

Meanwhile, Ceram Polymerik has launched a novel material that transforms plastics and rubbers into a fireproof ceramic during a fire. In doing so, it acts as a fire barrier and flame retardant stable to above 1000ºC

The Australian company is hoping that its new product will have a big impact on the US$12bn global passive fire protection market. The market for flame retardants and fire barriers alone is estimated to reach US$6bn by 2015. Worldwide fire accidents cause more than 70000 deaths and US$115bn of property damage

a year.

Passive fire protection materials using this technology will offer added safety to the public and reduce damage to buildings and their contents. This enables rescue and emergency services to undertake their work in a more controlled environment.

In conventional polymer composites such as plastics and rubbers, inorganic components such as talc and calcium carbonate are widely used as fillers and for reinforcement. When burnt in a fire, these conventional polymers melt, adding fuel to the fire and leaving behind a powdery ash.

With Ceram Polymerik Ceramifiable Technology the resulting material is a hard self-supporting ceramic composition that provides a barrier to fire. This patented technology performs well in most types of plastics and rubbers, allowing products to be made which are relatively soft and flexible or quite rigid depending on the required application.