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HURRICANES leave a trail of devastation in their wake. Now houses reinforced
with smart alloys will be better able to withstand them. Engineers are planning
to use an energy-absorbing alloy to stop roofs blowing away in gusting
winds.

“Hurricane destruction causes millions of dollars of damage each year,” says
Soheil Saadat at North Carolina State University in Raleigh. The worst damage
often happens when the wind tears the roof off a house. To hold it on, buildings
can be fitted with steel catches that strengthen the joints between the walls
and the roof.

But there’s a snag. “Catches made of steel can absorb energy from the wind by
deforming, but once they’ve yielded, they need to be replaced,” says Saadat.
“And if they break, the roof can go.” Along with colleagues at the University of
Puerto Rico in Mayagüez and the Kyoto Institute of Technology in Japan,
Saadat set about finding something better.

Their solution, soon to be published in the journal Smart Materials and
Structures, is to embed “tendons” made from nitinol, a shape memory alloy, into
the walls of buildings, and connect them to the roof. But unlike shape memory
alloys that return to their original shape when heated
(see “Sleeves up!”), these
tendons use the shape memory material’s ability to absorb lots of energy.

As the roof shakes during a moderate storm, the nitinol tendons stretch and
compress elastically, just like standard steel reinforcements. As winds get
stronger, and the roof begins to rock more violently, the tendons are stressed
more—until the stress gets so large that it forces a change in the
nitinol’s crystal structure.

This change absorbs large amounts of energy from the wind, which is stored in
the alloy. What’s more, the new crystal structure is better at absorbing energy
because its thin crystals can slide past each other as it is stressed
further.

Then, as the roof rocks back to its proper position, the stress in the
tendons is released. As they spring back, the alloy returns to its original
crystal structure, releasing the stored energy as heat.

“They’re doing the right thing,” says Cliff Friend, who studies shape memory
alloys at Cranfield University in Shrivenham, Wiltshire. “This is a pretty
effective way to absorb energy.” But embedding the tendons in concrete walls
could make them less effective, he says. The concrete would limit the amount of
stress the tendons feel, so only part of the alloy would undergo the
energy-absorbing phase change, he says.

Saadat plans to tackle this problem by pre-straining the tendons. He is
currently working on different tendon designs to optimise their energy
absorption.