From the 3-kilogram steel helmets used in World War II to today’s equipment that weighs only 1.2 kilograms yet can withstand armor-piercing bullets, bulletproof helmets have always protected the head—a critical part of the human body. Today, we take an in-depth look at the core of their protective principles and performance.

I. First, Understand: Where Does a Bullet’s Lethality Come From?

The harm bullets cause to the human body mainly stems from kinetic energy impact and penetrating damage. For example: A standard 9mm pistol bullet has an initial velocity of approximately 350 m/s, and the kinetic energy it delivers when hitting the head is equivalent to a 5kg stone falling from a 10-story building (about 30 meters high); a rifle bullet has an initial velocity exceeding 900 m/s, with kinetic energy 5 to 8 times that of a pistol bullet. Therefore, bulletproof helmets must achieve two key goals simultaneously: preventing bullet penetration and absorbing kinetic energy to reduce blunt trauma to the brain.

II. Three-Layer Protection: The Core Principle of Bulletproof Helmets

01 Outer Layer: The First Hard Barrier

It is mainly made of aramid fibers (Kevlar), ultra-high molecular weight polyethylene (UHMWPE), and some military models are also covered with an aluminum alloy or titanium alloy layer. When a bullet hits, the outer layer first squeezes the bullet to deform it (e.g., lead-core bullets are flattened), changes its flight path, and then disperses the impact force through partial fiber breakage—ensuring the helmet is not penetrated entirely.

02 Middle Layer: The Key to Absorbing Kinetic Energy

Made of foam plastic, honeycomb-structured materials, or advanced new materials like aerogels in high-end helmets. It acts like a "micro spring mattress": after a bullet penetrates the outer layer (or is blocked), the remaining kinetic energy pushes the middle layer to compress and deform—foam is compacted, and the honeycomb structure collapses. During this process, kinetic energy is converted into thermal energy or deformation energy, significantly reducing the impact force transmitted to the head.

03 Inner Layer: Close-Fitting Protection

The inner layer, which is in direct contact with the head, is mostly made of cotton or chemical fiber fabrics, and some are integrated with sweat-wicking and breathable materials. It not only improves wearing comfort but also further cushions residual vibrations and prevents debris from the middle layer from scratching the head. More importantly, the inner layer can be adjusted in size via Velcro or buckles to ensure the helmet fits the head tightly—if there are gaps, the helmet may slide when hit by a bullet, rendering the protection ineffective.

III. Three Indicators: Judging the Quality of a Helmet

01 Protection Level

The globally recognized NIJ (National Institute of Justice) standard classifies helmets into Levels IIA, II, IIIA, III, and IV:

• Level IIA: Protects against standard pistol bullets (e.g., 9mm Parabellum bullets);
• Level IIIA: The mainstream choice for police, capable of withstanding high-powered pistol bullets such as .357 Magnum and .44 Magnum;
• Level III and above: Primarily for military use, able to withstand 7.62×51mm NATO rifle bullets (Level IV can resist armor-piercing bullets).

China’s domestic standard GB 24409-2009 corresponds to Levels GA1, GA2, and GA3, with protective capabilities matching the NIJ levels.

02 V50 Value

Refers to the initial velocity of bullets at which 50% of the bullets can penetrate the helmet and 50% cannot. A higher V50 value indicates stronger impact resistance. According to standards, Level IIIA helmets require a V50 value of ≥488 m/s, while military Level III helmets require ≥610 m/s—this directly determines the helmet’s protective capability in extreme scenarios.

03 Blunt Trauma Depression Depth

Even if a bullet does not penetrate, the enormous impact force can still cause the inner side of the helmet to dent, leading to concussions or skull fractures. The NIJ standard clearly stipulates that the depression depth on the inner side of the helmet after impact must be ≤44mm (approximately the length of an egg)—this is key to avoiding "internal injuries."

IV. Future Trends: Lighter and Smarter

Military helmets are evolving toward "active protection + intelligent assistance": they use ultra-high molecular weight polyethylene (30% lighter than Kevlar), integrate micro-displays (to show real-time battlefield information), micro-radars (to warn of incoming bullets), and even have built-in sensors to monitor heart rate and concussion risks. Civilian security helmets will balance "riot protection + bulletproof protection" while optimizing ventilation design to improve comfort during long hours of duty.

The evolution of bulletproof helmets is the result of collaborative breakthroughs in multiple disciplines such as materials science, structural engineering, and mechanics—and it is also a "head armor" that guards lives. Understanding its principles and performance is not only a recognition of technology but also a tribute to guardians like soldiers, police officers, and security personnel.

Do you want to learn more about the specific differences between helmets made of different materials or the details of proper wearing? Feel free to leave a comment below!