How do bulletproof vests work? - Max G. Levy

翻译人员: Xiang Zhou 校对人员: Gia Hwang

By 1975, Richard Davis had been shot in the chest at close range 192 times.

截至 1975 年，理查德·戴维斯胸部

已被近距离枪击 192 次。

But not only was he completely healthy,

但他非但安然无恙，

each of these bullets had been shot by Davis himself

而且每颗子弹都是戴维斯本人射出的，

as part of a demonstration to sell his new product:

从而通过演示向大家推销其新产品：

the bulletproof vest.

防弹背心。

Playing with firearms is always a bad idea,

摆弄枪支通常是个坏主意，

but after testing his design on empty vests,

但在对空防弹衣进行测试之后，

Davis became convinced that taking a bullet himself was the only way

戴维斯确信，朝自己开枪

是证明防弹背心有效的唯一途径。

to prove the vest’s efficacy.

And when people saw Davis walk away with just some stinging pain

人们见中枪之后的戴维斯轻松走开，

只有轻微刺痛和轻微皮外伤，

a minor cut,

they may have stopped questioning his sanity

他们不再怀疑戴维斯的精神状态，

and started wondering how such a light, flexible piece of clothing

而是开始想如此轻便、柔软的衣服

could stop a bullet.

是如何挡住子弹的？

The secret was in the material:

秘密在于防弹衣的材料：

a synthetic fiber invented a decade earlier

一种十年前发明的合成纤维，

by a material chemist named Stephanie Kwolek.

由材料化学家斯蒂芬妮·克沃勒克发明。

Her employers at DuPont had found huge success with nylon,

其雇主杜邦公司在尼龙上获得巨大成功，

the world's first synthetic fiber,

尼龙是首例合成纤维，

and they wanted Kwolek to create something even stronger

他们希望克沃勒克造出更坚固的材料，

they could use to mass produce durable, lightweight tires.

从而用来批量生产耐用、轻质的轮胎。

Like all synthetic fibers, nylon is a polymer:

像所有合成纤维一样，

尼龙是一种聚合物，

a long chain of repeating molecules, or monomers.

是由重复分子即单体组成的长链。

While some polymers repeat the same monomer over and over,

有些聚合物会重复使用相同的单体，

others chain multiple monomers in a steady pattern.

另一些则以稳定的模式连接多种单体。

It’s these two variables—

聚合物有两个变量：

which molecules are involved and how they bond to one another—

所包含的分子和分子间的连接方式，

that give each polymer its unique properties.

正是它们赋予每种聚合物独特的性质。

So, seeking to build on the strengths of nylon,

因此，为在尼龙的基础上继续改进，

Kwolek began a lengthy process of trial and error,

克沃勒克开始了漫长的反复试验，

combining various monomers in novel ways.

她用新的方式组合了各种单体。

And one of these resulting polymers was immediately very weird.

由此产生的一种聚合物立刻变得很奇怪。

Named Kevlar, this alternating blend of 1,4-phenylene-diamine

它被命门为凯芙拉，

是由 1,4-苯二胺和对苯二甲酰氯 交替混合而成的材料，

and terephthaloyl chloride combine at the molecular level

在分子水平上结合形成一系列平行链。

to form a series of parallel chains.

At rest, these chains align in strict rows,

静止时，这些分子链严格排列，

giving the polymer order and crystalline strength.

为聚合物提供有序结构和结晶强度；

But when pressure is applied, the chains wriggle around,

但当施加压力时，分子链会扭动，

allowing the material to flow like a liquid.

使材料像液体一样流动。

This so-called liquid crystal polymer was unprecedented,

这种所谓的液晶聚合物前所未有，

and when Kwolek’s team spun the viscous fluid into a fiber,

当克沃勒克团队将粘性流体纺成纤维时，

the results were better than they could have hoped.

结果比他们预期的更好。

Not only were the fibers flexible and resistant to heat, acid,

这些纤维不仅柔韧，

还耐热、耐酸、耐各种化学物质，

and various chemicals,

when woven together, they were also stronger than steel.

而且织在一起时，甚至比钢更坚固。

Metals are incredibly sturdy because of their unique atomic bonds.

金属由于其独特的原子键而非常坚固，

Where non-metal molecules are typically held together by the attraction

而非金属分子结合在一起通常是通过

原子核和一定数量的电子之间的吸引力。

between a nucleus and a set number of electrons,

metal nuclei are surrounded by a sea of shared electrons.

金属原子核则被包围在共用电子海中，

It takes a ton of energy to overcome the strength and resiliency

要克服这些海量金属键的强度和韧性

of these countless bonds.

需要大量的能量。

So when a bullet hits a steel plate,

因此，当子弹击中钢板时，

the material can usually absorb all the impact’s energy

金属材料通常可以在其被击穿之前

before the metal is pierced.

吸收子弹冲击的所有能量。

Compare this to a bullet hitting wood.

与子弹击中木头的过程相比，

The bonds holding wood together require much less energy to break,

要打破将木头粘在一起的化学键

所需能量比金属少得多，

which is why bullets can travel much further through wood than metal.

故子弹在木头中的穿行距离

比金属远很多。

Kevlar’s atomic bonds are also weaker than metal’s.

凯芙拉的原子结合键

也比金属的原子结合键弱，

But it compensates with a huge number of hydrogen bonds.

但是它用大量的氢键来补偿。

While not as strong as the atomic bonds within molecules,

虽不如分子内部的原子结合键那么强，

the attraction of hydrogen atoms and oxygen atoms between molecules

但分子间氢原子和氧原子的吸引力

also requires a huge amount of energy to overcome.

也需要大量能量才能克服。

And when threads of Kevlar’s polymer chains are woven into fabric,

且当凯芙拉聚合链形成的线织成织物时，

this strength is multiplied.

强度会成倍增加。

When a bullet hits Kevlar, the mesh of highly aligned, liquid-like chains

当子弹击中凯芙拉时，

网状的高度对齐的液体状分子链

absorb huge amounts of energy,

会吸收大量能量，

wiggling wildly while still clinging to their neighboring chains

疯狂地摆动的同时，

仍然通过氢键和邻近分子紧紧相连。

via hydrogen bonds.

And even if a bullet does have enough energy to penetrate the Kevlar,

且即使子弹确有足够能量穿透凯芙拉，

it would be moving considerably slower with much less destructive force.

子弹的移动速度也会慢得多，

破坏力自然要小得多。

Of course, Kevlar is not immune to everything.

当然，凯芙拉也不能防住所有伤害。

Strong forces can still be felt through the fabric,

人体仍能透过防弹衣感受到巨大冲击，

and its fibers gradually lose strength under ultraviolet light.

凯芙拉纤维在紫外线下会逐渐失去强度。

Additionally, new liquid crystal fibers hold up better against acid.

此外，新液晶纤维抗酸性比凯芙拉更好。

But Kwolek’s invention remains one of the most versatile

但是克沃勒克的发明仍是

地球上用途最多、使用最广的材料之一。

and widely used materials on Earth.

Today, companies rely on Kevlar’s lightweight impact resistance

如今，企业将凯芙拉的 轻质抗冲击性和耐久性

and durability in helmets, kayaks, spacecraft, and automobiles.

用于头盔、皮艇、航天器和汽车上。

Speakers sometimes use Kevlar because it can push air efficiently

扬声器中有时也会使用凯芙拉，

因为它可以高效地推动空气，

and quickly come to a dead stop when you pause your music.

并在你暂停音乐时迅速停止。

And yes, it also makes excellent tires.

而且没错，它还可以制造出很棒的轮胎。