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IT SOUNDS all wrong鈥攄rilling holes in a piece of wood to make it more
resistant to knocks. But it works because the energy from the blow gets
distributed throughout the wood rather than focusing on one weak spot. The
discovery should lead to more effective and lighter packaging materials.

Carpenters have known for centuries that some woods are tougher than others.
Hickory, for example, was turned into axe handles and cartwheel spokes because
it can absorb shocks without breaking. White oak, on the other hand, is much
more easily damaged, although it is almost as dense. Julian Vincent at Bath
University and his team were convinced the wood鈥檚 internal structure could
explain the differences.

Many trees have tubular vessels that run up the trunk and carry water to the
leaves. In oak they are large, and arranged in narrow bands, but in hickory they
are smaller, and more evenly distributed. The researchers reckoned this layout
might distribute a blow鈥檚 energy throughout the wood, soaking up a bigger hit.
To test the idea, they drilled holes 0.65 millimetres across into a block of
spruce, a wood with no vessels, and found that it withstood a harder knock. Only
when there were more than about 30 holes per square centimetre did the wood鈥檚
performance drop off.

A uniform substance doesn鈥檛 cope well with knocks because only a small
proportion of the material is actually affected. All the energy from the blow
goes towards breaking the material in one or two places, but often the pieces
left behind are pristine.

But instead of the energy being concentrated in one place, the holes provide
many weak spots that all absorb energy as they break, says Vincent. 鈥淵ou are
controlling the places where the wood breaks, and it can then absorb more
energy, more safely.鈥�

The researchers believe the principle could be applied to any
material鈥攆or example, to manufacture lighter and more protective
packaging. It could also be used in car bumpers, crash barriers and armour for
military vehicles, says Ulrike Wegst, at the Max Plank Institute for Metal
Research in Stuttgart. But she emphasises that you鈥檇 need to design the
substance with the direction of the force in mind. 鈥淭he direction of loading is
crucial,鈥� she says.