STEM与日常科技·英语30篇（9）
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Why Bicycle Helmets Have Hidden Air Pockets
自行车头盔为何内置隐形气室
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Helmets use expanded polystyrene (EPS) foam with millions of tiny closed-cell air pockets.头盔采用发泡聚苯乙烯（EPS）泡沫，内含数百万个微小的闭孔气囊。
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On impact, these cells crush progressively to absorb kinetic energy over milliseconds.撞击时，这些气囊逐层压溃，在毫秒级时间内吸收动能。
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The foam’s density is tuned so softer zones compress first, delaying skull acceleration.泡沫密度经过优化，使较软区域率先压缩，延缓颅骨加速度。
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Ventilation channels double as structural ribs, guiding airflow while reinforcing shell strength.通风槽兼具结构加强筋功能，在引导气流的同时增强壳体强度。
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Testing involves dropping helmets onto angled anvils at speeds simulating real crashes.测试时将头盔以模拟真实事故的速度，坠落至倾斜的砧板上。
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Certified helmets reduce risk of brain injury by up to 85% compared to no helmet.经认证的头盔相比不戴头盔，可将脑损伤风险最多降低85%。
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Some newer models add MIPS layers that allow slight rotational movement during oblique hits.部分新型号增加MIPS层，可在斜向撞击时允许轻微旋转运动。
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After any crash—even if no damage is visible—the foam loses protective capacity and must be replaced.任何撞击后——即使外观无损——泡沫保护性能即已丧失，必须更换。