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In my conworld, the inhabitants have futuristic technology and use armour that is made of primarily wood but will absorb bullet impacts and even armour-piercing rounds. It is cited as being very fire-resistant, though after enough time has elapsed it will burn through. What would be the conditions required to make wood this durable? Keep in mind, the planet these trees grow on is 1.5 times as massive as Earth, and the atmosphere is much richer in oxygen.

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    $\begingroup$ The kinetic energy transfer will still knock down the armor-wearing person. Experience with ceramic body armor absorbing 7.62mm rounds is that you'll live, but it's like a hard gut punch - wind knocked out of you, plenty of bruising, and you're worthless for a day or two while you heal. $\endgroup$
    – user535733
    Dec 2, 2019 at 18:31
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    $\begingroup$ The secret seems to be laminated layers of wood similar to how we make plywood. Don't know exactly what resin or glue they are using between the layers, but it seems to work. $\endgroup$ Dec 2, 2019 at 18:37
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    $\begingroup$ Bullet resistant. Bullet proof only acts as motivation for developing better armour piercing bullets $\endgroup$
    – nzaman
    Dec 2, 2019 at 19:21
  • $\begingroup$ chemistryworld.com/news/bullet-proof-wood-developed/… See this. The trees may do something like this naturally, or the people might do this process to the wood. You would want some padding behind the wood, at least. $\endgroup$ Dec 3, 2019 at 1:13

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In order for this to work, the composition of your trees would have to be VERY different from those on earth. Even dense wood is not particularly good at stopping bullets. A handgun round will generally penetrate wooden boards as much as 2-3 inches thick, and high-velocity armor piercing rounds from sniper rifles will go through as much as a foot or more of oak.

Those kind of thicknesses are pretty impractical for body armor, so you're going to need something pretty different.

Now, something that DOES grow naturally that IS bulletproof, or nearly so, comes from the animal, not vegetable kingdom. Some species of Mantis Shrimp form structures in their claws that have incredible levels of shock resistance, and would easily generate both the protective levels you're looking for as well as being practical for use as armor.

Instead of trees like we have on earth, imagine something much more like a coral forest where the corals form these kind of structures as they grow. Your inhabitants can harvest them and repurpose them as protection. There's all sorts of potential there.

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    $\begingroup$ Perhaps trees that use layered chitin instead of lignin? $\endgroup$ Dec 2, 2019 at 19:31
  • $\begingroup$ @ArkensteinXII basically, yeah. $\endgroup$ Dec 2, 2019 at 19:35
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This question made me smile, because I think there might actually be a tree on Earth that would stop a (lowish) caliber bullet.

I cut down a Black Gum tree (Nyssa sylvatica) that was shading too much of a garden (no great loss; my land had thousands of trees.) I cut in into lengths to split for firewood. It was dense and somewhat harder to cut than most other trees/wood.

But the real surprise came when I tried to split it. It completely overwhelmed my heavy duty hydraulic splitter. The grain was so twisted every which way that I had to just give up and haul the lengths into the woods.

Because of its twisted grain, black gum cannot be split so it has been somewhat spared by lumbermen. The resilient wood is good for making tool handles.

and

The wood of the black gum has fibers not only interwoven but twisted to boot. And while modern tools and cutting edges can surmount this obstacle, even the sharpest ax and the most expertly wielded froe [a tool for cleaving wood] of yesteryear made little headway splitting it.

Is an ax very much different from a bullet in shape?

Consider a tree that has highly twisting and interlocking fibers. A bullet can make some headway, but nothing close to what it can do to most woods with straighter grains.

If early settlers left it alone, it had to be formidable. Make your tree a bit more so.

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There are examples of biological systems depositing glass (diatom skeletons, for instance). Glass fibers encased in lignin in between the cellulose fibers would greatly reinforce a high-density wood (like teak or boxwood). The result wouldn't be "bullet proof" any more than modern body armor is "bulletproof" -- but it would be bullet resistant in much the same way. Layered scales of this wood, with padding between layers (made from the self-reinforced bark of the same tree?) could gradually absorb the energy of a bullet.

The tree that would have evolved to do this would have to grow in a silica-rich region (sandy soil, at a minimum) and would have an evolutionary need to grow tall, yet stand against strong winds. Think solitary redwood or sequoia -- instead of being protected by standing in a grove of similar trees, these are simply strong enough to keep standing and hold their branches in the most violent storm. The slow growth and long life cycle of these trees would, however, make the armor made from their wood relatively rare and expensive (especially after sustainability becomes a recognized concept).

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As an actual wood by itself with no special preparations, almost certainly not, cellulose is just too weak to do this by itself if we assume equivalent structure to real-world trees. Even the highest density woods (which are so dense they sink in water) aren't going to get you close to a proper anti-ballistic material like modern body armor provides.

However, if you do some form of multi-layer laminate, you might get acceptable performance against ball ammo (that is, ammo that doesn't have an armor piercing core). Essentially, you're making armor out of plywood, except you want a whole lot more layers, and probably want each layer thinner than typical plywood (you can pack more in, and better see any imperfections in the grain of each layer during assembly that would lead to weak points). You'll probably want hardwoods for this (more durable), but most likely softer hardwoods (if they're too dense, they won't do a good job of absorbing the kinetic energy of the bullet). Even then though, you're looking at maybe stopping handgun rounds and some light hunting rounds, probably not military-grade AP rounds, and definitely not anything larger than about 8mm coming from a rifle.

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Hardwood already is bulletproof. In fact, properly built and with enough thickness, it is proof against small naval artillery of the age of sail. It's actually rather impressive how little outside damage a 13-kilo cannonball does to the structure as it is just about at the needed power to penetrates a reconstruction of an 1864 frigate ship hull, as you might see here. Lighter and slower projectiles will bounce off, ones with more energy will do more damage to the structure. The problem is the needed thickness of the construction: That ship hull has about one foot of oak in three layers! Or you might want to listen to Drachinifel talking about the gunnery and construction of ships - and how they were damaged by the size of naval artillery. 6-pounders can't pierce such 12-inch hulls.

Ship hulls go in parts to 24 inches or more...

What are the problems and solutions to wooden armor then?

Wood splinters

When hit hard enough, the wood will splinter and aid in killing whatever is behind, adding shrapnel. To combat this, you need quite thick constructions, which are likewise bulky.

However, you can reduce the need for thickness by layering, and only needing to withstand smaller and slower projectiles, such as arrows on long-range.

Wood is inhomogenous

Well, that doesn't help against handguns, but the splintering is an effect of wood being inhomogeneous. There are fibers, all rather ordered, and they are not very interwoven. Now, we could fix part of this by layering the wood crosswise, but that was already what was done with the ship hulls. So we need to do something more.

Wood Ages

Did you know that wood ages and gets better at stopping projectiles? Old Ironsides, aka USS Constitution, was built from particularly old boards, and was as a result much harder to wound. What happened to her wood in aging - or as the shipbuilders call it seasoning? Simple, the resins in the wood cure and harden, making the wood fibers much harder to break apart from one another.

Laminated resin-impregnated wood

If our armor makers use a laminated setup of wood, possibly even quite thin boards or even strips woven into mats, and impregnate those with resin, layer and press them together, then you can end up with a structure very much akin to carbon fiber plates. Such plates are used in hard body armor at times, but generally, steel or ceramics are superior in performance in combination with flexible fibers such as Aramids.

Conclusion

Wood isn't good to make body armor past the age of arrows and even then is mediocre compared to a metal plate or ring armor.

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The simple answer is "No".

Wood is based on cellulose, and cellulose has very low tensile strength, compared to "strong" materials like kevlar or steel. Cellulose-based materials just can't be "bulletproof" in a traditional sense (ex. to make wearable armor).

However, plants in your world can potentially be made of other fibers. You can even have some kevlar-based vines, if you like.

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