How could an object be impervious to high velocity piercing from arrows yet still be vulnerable to cutting from (extremely) razor sharp blades?

Question

I want to write about an object that won't be pierce-able even with the fastest of arrows (notice I didn't mention guns, only arrows) but that can somehow be crudely cut by an albeit very razor-sharp weapon (say something as sharp but as brittle as obsidian which is 3 times sharper than diamond and between 500-1000 times sharper than a razor or a surgeon's steel blade).

Is there anyway to make sense of this under real world physics or am I just going to have to handwave this as "something something magic" ?

*Also I would like to mention that (however misguided and uninformed it may be) I'm going off the assumption that making obsidian/diamond tipped arrows would be ineffective since they would be very likely to shatter on impact thus nullifying any cutting power they might have had, had they been used for simple stabbing or cutting for example.

Answer 1

I believe a non-Newtonian fluid may help out. Some of these (e.g. "oobleck") get harder when subjected to strong, sudden force. Conceivably, a fast arrow hitting this substance will cause it to almost-instantly "solidify", while a slow-moving razor blade can pass through it.

(BTW, it sounds like you want something that behaves like a Dune shield...)

The complication is I'm not sure if there are non-Newtonian semi-fluids... since it seems you don't want it to ever behave entirely as a liquid. That said, maybe there is a way to combine such a liquid with a porous substrate (think 'sponge'). An impact might slightly damage the substrate, but you'd have to keep hitting it in the exact same spot in order to penetrate.

Whether or not this is 100% hard science, it may at least be sufficiently plausible for your purposes.

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Edit: I see that you mention stabbing in some comments. If you want stabbing to work, you probably don't want a non-Newtonian fluid/sponge/whatever, since its ability to resist penetration is somewhat proportional to the force of the attempt. (Not sure if it's actually proportional, or if the resistance "plateaus" once you hit some threshold.) Just like penetrating a Dune shield, the way to damage this hypothetical armor is going to be gently. Slow slicing will work, vicious stabbing probably won't.

If you really just want something that resists arrows, but not being stabbed, boiled leather might actually be closer, or possibly some sort of paper armor or even something like Kevlar. Or, if you when you said you want to be able to "cut" it, you care about stabbing it with pointy things rather than slicing it with something scalpel-like, try chain mail.

Answer 2

Option A: Textile Armor

Gambison was a historical style of armor made from layers of linen sewn together. A good gambison can stop nearly all historical arrows fired even at relatively close range as well as duller bladed weapons. Most arrows just bounce off, but when they do penetrate (like when you fire an English war bow or crossbow at close range), it typically askews the path of the arrow as it penetrates each successive layer until it is turned to travel between the layers of cloth instead of through it.

That said, melee weapons are less prone to loose their alignment on a hit because they are braced in your hand so spears, war picks, thrusting swords, and particularly sharp curved swords are all fairly good at penetrating it.

Option B: A Hide and Wicker Laminate Shield

A shield does not need nearly as much stopping power as armor to save you from an arrow. Whereas armor needs to completely stop a weapon's penetration to keep you safe, shields are held away from the body; so, even when an arrow head goes through a shield, the friction with the shaft can still stop it before it harms the user.

The Persian Empire took advantage of this fact when they designed their shields. Mobility and archery were major aspects of their military doctrine; so, instead of making heavy shields that would slow them down but block almost anything like most other ancient civilizations, they made these very light weight tower shields that could screen their entire body, but just from arrows. The shields could be easily hacked through with a sharp sword or over penetrated with a spear thrust making them almost useless in melee (as the Greeks proved on more than one occasion), but against arrows, they were a very effective countermeasure for their weight.

This same principle can also be applied to situations where you just need a "thing" to block an arrow. So, a thin wall or door could also apply as long as you are not standing directly against it.

Option C: Plate Armor

If you slightly shift your thinking to just being about overcoming the armor in melee, but not with any arrow, then plate armor may be another solution. Tests of historical recreations of plate-mail against historical recreations of all sorts of weapons pretty consistently show that cutting or piercing it was practically impossible. Killing a plate armored warrior with a bow generally required a lucky shot to enter through an eye socket or exposed joint. By the 14th century, plate armor had become so well made that it was virtually impossible to get around with an arrow.

That said, a knight could still be killed through his armor by blunt force trauma. Front-heavy weapons like maces, axes, and war-hammers could kill a knight without actually penetrating the armor at all. This was even more true in the ancient era before the wide-scale use of arming jackets to cushion the blow. Daggers were also a popular option because their shorter blades gave one enough tip control to aim it into the very narrow eye slits which an arrow or sword would almost never be lucky enough to get into.

Answer 3

The problem would be EASIER with guns. If you shoot a 50 caliber bullet into a tank of water, the projectile either disintegrates or skews wildly. If you take a musket rifle with a mini ball, the projectile penetrates deeper and truer. They did this experiment on mythbusters. I can't quite picture the scenario, but I suspect your answer will involve a denser but amorphous substance LIKE water dispersing the force. A slow, sawing tool might be the best thing to cut your object.

Answer 4

The article for bulletproof vests in Wikipedia is in dire need of citations, but any site more aimed at explaining or selling such vests will confirm this:

Vests designed for bullets offer less protection against blows from sharp implements, such as knives, arrows or ice picks, or from bullets manufactured with hardened materials, e.g., those containing a steel core instead of lead. This is because the impact force of these objects stays concentrated in a relatively small area, allowing them a better likelihood of puncturing the fiber layers of most bullet-resistant fabrics used in soft armor. By contrast, stab vests provide better protection against sharp implements, but are generally less effective against bullets.

The reason stab vests are not great against bullets is because they handle impact differently. The bullet may not pierce you, but you still take the full impact at the contact point. Bulletproof vests on the other hand redistribute impact.

There is still the matter of arrows, which aren't properly stopped by either. You may use some handwavium for those.

Edit: thanks to Nosajimiki for this comment:

Just to clarify, this is true of textile vests like kevlar. Modern armor that is reinforced with ceramic plates will block a blade just as well as a bullet, the the plates ablate on impact so they don't take repeated blows to the same place very well; otherwise, they block it all.

Answer 5

A fine chain mail curtain.

This is an imperfect solution for a bunch of reasons, but meets the literal criteria and might suggest other solutions.

• Something that can move freely like a curtain would just be pushed backwards if hit at high speed, but if you grab it with one hand to put it under tension, you could cut it at close range. When free, the curtain can conduct kinetic energy away, like a non-Newtonian fluid does.
• Something slanted at the right angle (e.g. a curtain pushed back by an impact) could deflect something like an arrow, but be vulnerable to a well-aimed well-angled stab.
• Grain. Meats, fabrics, woods, and many other materials have a kind of grain that is very hard to cut across, but easy to cut along. Similarly, a chain mail curtain might be very hard to pierce, but a downwards cut that catches on one ring could create tension against the ceiling mount, and pull it straight and make it easier to cut.

Another questionable possibility: A giant block of very dense gelatin. (Or another substance)

• The substance should be a barrier that is thick and dense enough that one arrow could not pierce all the way through, as it loses velocity with distance traveled.
• A sword could get one through it, though, because you can land multiple sword strikes in the same location to progressively cut a hole through in a way you can't with arrows, both because you can't aim well enough and because arrows fill holes behind them with themselves.
• If the substance is gelatinous enough, though, it could collapse to fill in holes pierced by an arrow, and make piercing it that way even harder. Using a sword would get harder too, but still be doable with care and planning.
• Frankly, gelatin is too weak and wouldn't be a very good solution. Meat would work fine, though--a 10 foot cube of whale meat sitting in front of a doorway could not be pierced by arrows, but you could get through the doorway by hacking your way through the meat with a sword.

• Would take a while though.

• Also, kind of a disturbing image.