why can’t kevlar stop rifle bullets even though it’s strong than steel on an equal weight basis?

Question

Ar500 steel plate can stop rifle bullets. In my brain I’m thinking like “well if kevlar is stronger than steel, then slab of kevlar of the same size of an ar500 plate should stop rifle bullets too”. Clearly it doesn’t and I don’t understand why

In: Engineering

Answer 1

I used to make bulletproof windows, similar situation. You need to absorb a lot of energy in a very small area. Hard surfaces aren’t great at absorbing energy but are great at spreading it. Soft materials like vinyl (used in windows) deform and absorb energy, but don’t spread it well (and they puncture). Ideally you use both, and in a window you layer the hard and soft spreading and absorbing energy layer by layer.

I suspect kevlar would be better able to absorb energy, but steel better to spread it. Perhaps a combination of both would be better than either alone.