Wood - stronger than steel

Some varieties of wood, such as oak and maple, are renowned for their strength. But scientists say a simple and inexpensive new process can transform any type of wood into a material stronger than steel, and even some high-tech titanium alloys. Besides taking a star turn in buildings and vehicles, the substance could even be used to make bullet-resistant armour plates.

Wood is abundant and relatively low-cost—it literally grows on trees. And although it has been used for millennia to build everything from furniture to homes and larger structures, untreated wood is rarely as strong as metals used in construction. Researchers have long tried to enhance its strength, especially by compressing and “densifying” it, says Liangbing Hu, a materials scientist at the University of Maryland, College Park. But densified wood tends to weaken and spring back toward its original size and shape, especially in humid conditions.

Now, Hu and his colleagues say they have come up with a better way to densify wood, which they report in the February 7 Nature. The results from their trials are impressive. The team’s compressed wood is three times as dense as the untreated substance, Hu says, adding that its resistance to being ripped apart is increased more than 10-fold. It also can become about 50 times more resistant to compression and almost 20 times as stiff.

The densified wood is also substantially harder, more scratch-resistant and more impact-resistant. It can be moulded into almost any shape. Perhaps most importantly, the densified wood is also moisture-resistant:

A five-layer, plywood-like sandwich of densified wood stopped simulated bullets fired into the material—a result Hu and his colleagues suggest could lead to low-cost armour. The material does not protect quite as well as a Kevlar sheet of the same thickness—but it only costs about 5 percent as much, he notes.

The team’s results “appear to open the door to a new class of lightweight materials,” says Ping Liu, a materials chemist at the University of California, San Diego, unaffiliated with the Nature study. Vehicle manufacturers have often tried to save weight by switching from regular steel to high-strength steel, aluminum alloys or carbon-fiber composites—but those materials are costly, and consumers “rarely make that money back in fuel savings,” Liu says. And densified wood has another leg up on carbon-fibre composites: It does not require expensive adhesives that also can make components difficult, if not impossible, to recycle.

Densified wood provides new design possibilities and uses for which natural wood is too weak, says Peter Fratzl, a materials scientist at the Max Planck Institute of Colloids and Interfaces in Germany who did not take part in the study. “Instead of creating a design for the material at hand, researchers can create a material to suit the design they want,” he says, alluding to a familiar process among aerospace engineers who have a long history of developing ever-stronger alloys to meet their needs.

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Source: scientificamerican.com

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