| It sounds all wrong-drilling holes in a
piece of wood to make it more resistant to knock. 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 {{U}}(51) {{/U}} centuries that some woods are tougher than others. Hickory(山核桃木), for example, was turned into axe handles and cartwheel spokes (车轮辐条) because it can absorb shocks without breaking. White oak (橡木), for example, is much more easily damaged, {{U}}(52) {{/U}} it is almost as dense. Julian Vincent at Bathe University and his team were convinced the wood's internal structure could explain the differences. Many trees have tubular(管状的) vessels that run {{U}}(53) {{/U}}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 {{U}}(54) {{/U}} this layout might distribute a blow's energy throughout the wood, soaking up a bigger hit. To test the idea, they drilled holes 0.65 millimeters across into a block of spruce(云杉), a wood with {{U}}(55) {{/U}} vessels, and found that {{U}}(56) {{/U}}withstood a harder knock. {{U}}(57) {{/U}} when there were more than about 30 holes per square centimeter did the wood's performance drop off. A uniform substance doesn't cope well with knocks because only a small proportion of the material is actually {{U}}(58) {{/U}}. All the energy from the blow goes towards breaking the material in one or two places, but often the pieces left {{U}}(59) {{/U}} are pristine(未经破坏的). "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, "You are controlling the places{{U}} (60) {{/U}} the wood breaks, and it can then absorb more {{U}}(61) {{/U}}, more safely". The researchers believe the principle could be applied to any material-{{U}} (62) {{/U}} example, to manufacture lighter and more protective packaging. That could {{U}}(63) {{/U}}be used in car bumpers (保险杠), crash barriers and armor for military vehicles, says Ulrike Wegst, {{U}}(64) {{/U}}the Max Plank Institute for Mental Research in Stuttgart. But she emphasizes that you'd{{U}} (65) {{/U}} to design the substance with the direction of force in mind. "The direction of loading is crucial", she says. |