Researcher X said:
I don't see how that's possible. If you increase the force upon the same contact area, you will be increasing the pressure. A faster needle, knife, bullet will exert more pressure at its contact surface, than a slower version of the same mass and contact area.
But these variables all come back to varying the pressure, when the question is really about how much pressure the skin can take. There are lots of methods which can vary the amount of pressure you happen to apply, but 100psi of pressure is still 100psi of pressure no matter how you created it. A copper pin with the same contact area as a steel pin may deform more, but then it's producing less pressure, because it's producing less force on the same contact are (energy is being wasted through deformation).
That's true, though the small variation might not be a concern if you're just looking for a single example. At any rate, I'm thinking of the torso/upper body area.
No, someone just gave that figure to somebody else asking the question, without any other explanation.
You're right, but skin is elastic and vascular. Slow pressure first causes blood to leave the area (through brute force) and that can make piercing a region difficult. In practice, I'm sure that every region has a specific psi for a given time of day, and person, but if you're interested in ballistic armor here is the difference: A 130 grain bullet traveling at 1400 fps is going to be exerting orders of pressure above what is needed to penetrate skin. Once the round has entered flesh, temporary cavitation from a fast and especially super-sonic round further reduce drag and again, less rebound. If you never have a chance to experience that return force from the skin, it requires less (in practice) as read by a gauge on the receiving end, to penetrate.
Sharp, dull, is a matter of surface area, but in the case of Kevlar (for example) the issue of the weave also becomes a factor. You can sever the threads which provide protection with a slice, which weakens the fabric as the slice continues. In the end, you require less force exerted on the same material, because you're not engaging more of the fabric. The pressure required to pierce the full thickness of the vest is high, but a blade works from the outside, in, destroying that protection without challenging it. Skin has some of the same issues: pierce to what depth? If you slice or needle the skin, you destroy the ability of the dermis to spread the force of that blow and dissipate it. In other words, there's piercing an epidermal cell, which is different from piercing an array of cells and collagen, actin, fat, and more.
In PRACTICE, the 1" ball bearing is going to drag much of the skin around it thus presenting more of it's surface area than a 1" rod, which cannot be fully enveloped as a sphere might. This also goes back to material, which isn't an issue if we're talking about shrapnel and bullets. In that case, issues you should be concerned with are best thought of in terms of how the kinetic energy is going to be received: shrapnel tends to have a lot of mass compared to a bullet, and is often very hot. If you're wearing Kevlar, it could melt outer layers of fabric, or rocket through you and out the back.
As for the 100psi figure, if we're not talking about specific material, location, and more, it's a meaningless figure. I'm sure you could model this system given enough human volunteers and really impressive computers, but as of not it's still a real challenge.
