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So, in my story, there is a fictional material called master cloth with about 10x the strength of Kevlar, and an average thread thickness of 1/8 inch diameter. The cloth is as light as most fabrics, and self-repairing of any holes that might come across it, making tailoring difficult. There will be an assassination attempt on the wealthy lord, who has managed to commission a ceremonial robe made out of this substance and which he is wearing during a city-blessing ceremony.

However, a man using a 200 to 250 lb crossbow shoots at the lord from from behind, at a range of about 40 yards. How thick would this fictional material have to be in order to let the crossbow bolt penetrate the master cloth robe and only barely scratch the skin?

This crossbow is bigger than typical and has a bolt that is also bigger than usual; the bolt is made mostly of metal, most likely a type of steel. The assassin's job is to kill the lord at any cost. The man was aiming for the base of the neck, which was not protected by the robe, but various factors contributed to the bolt's actually striking the lord down towards the small of his back.

If the crossbow can definitely puncture it, what weight or distance would it require to meet the specifications of barely puncturing but not wounding? Or multiple folds, like a wrinkle?

If this crossbow can't puncture this robe, at what draw weight or distance will it be possible to puncture, but not severely wound?

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    $\begingroup$ "average thickness in 1/8 an inch diameter per thread" Unless the fabric is tightly woven and at least 2 inches thick, arrows may be easily penetrating between the threads. $\endgroup$
    – Alexander
    Feb 28 '20 at 20:53
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    $\begingroup$ My advice would be to simply not mention any of these details. They don't obviously add anything to the narrative, other than an opportunity for you to be wrong. The crossbow is not quite powerful enough and/or the shooter was just too far away. The master cloth is just thick and tough enough. Bish bash bosh. Job done. Save your research efforts for where you'll get more reward ;-) $\endgroup$ Feb 28 '20 at 20:55
  • $\begingroup$ With a 250 pound-pull crossbow, the archer probably needs to brace that sucker when he fires. And the target probably needs more than piercing-resistant cloth to live through it since he will be smacked the equivalent of a 250-pound guy jumping on him. $\endgroup$
    – puppetsock
    Feb 28 '20 at 21:49
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    $\begingroup$ Also pounds, inches, and meters? Pick a unit system. $\endgroup$
    – puppetsock
    Feb 28 '20 at 21:50
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    $\begingroup$ @puppetsock the velocity of a bolt fired from a crossbow is a slightly tricky thing to compute... draw weight is only part of that equation. Suffice to say, the target will not be smacked by the equivalent of a 250-pound guy jumping on him. $\endgroup$ Feb 28 '20 at 22:36
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Kevlar is effective as a projectile stopper because of multiple layers. They dissipate the energy of the incoming projectile across each layer so that the impact of a pistol slug is absorbed over a large enough area so it won't penetrate. Modern body armor also has rigid plates to stop rifle rounds and maybe even knife thrusts. Thing is, a Kevlar Vest by itself won't keep a ne'er do well from puncturing you with an icepick.

The size of the thread (1/8 inch is huge for a thread) is going to be very hard to get a tight weave on. It'll be great against a sword slash, but from the perspective of a puncturing weapon it'll be just a series of interconnected holes. An incoming pointy thing may get stopped, but if the point comes in or gets deflected in to one of the gaps in the weave, it's not inconceivable that it will push aside the surrounding threads and penetrate into the skin beneath. As the pointy thing keeps going the threads may stop the movement, preventing it from creating a fatal wound. Look into Bodkin arrows. Those would be best against your fancy robe. A razor tipped broad head arrow is too wide to get far against the threads.

So, If your lord has a cloak that is only one layer thick of this miracle thread, it'll be a lot like wearing super light chainmail. Something thin and pointy will get through with less trouble than you might expect.

Other things to consider would be how flexible and stretchy this miracle thread is. If it has virtually no stretch, it will be rigid but more protective. If it's stretchy like a sweater from the land that natural fibers forgot, it may offer very little protection at all. The point of a bodkin arrow will just push the fibers aside like they sere barely there.

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The thing about Kevlar is that it's not designed to stop things with very small points like a crossbow bolt. Compared to crossbow bolts, bullets have a (relatively) large point and flatten out on impact, with most of their damage coming from the damage this effect causes to your internals. A bolt at that draw weight is more akin to a knife being launched at very, very high speeds, so even 'stab-proof' material is not guaranteed to stop it due to the amount of energy being concentrated on penetration alone. It would not, however, be able to penetrate any up-to-standard breastplate. Kevlar sucks against pointed impact penetration, like arrows and knives.The range on crossbows does determine their effectiveness, however.

In contrast, steel plate is good at grazing arrows and bolts due to it being far more resistant to such impacts and very static, but bullets tore right through it due to their flattening effect and distribution of energy.

So, when your material is stronger than Kevlar, I would recommend it sharing the more static, pointed impact-resistant traits of steel when hit with pointed weapons and ammunition.

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Make the thread thinner and use multiple layers of fabric.

In addition to the already provided answers, see also

Gambeson/padded jack - seems like even linen would have been sufficient, provided many layers are used

There are two distinctive designs of gambeson; those designed to be worn beneath another armor, and those designed to be worn as independent armor. The latter tend to be thicker and higher in the collar, and faced with other materials, such as leather, or heavy canvas. This variant is usually referred to as padded jack and made of several (some say around 18,[5] some even 30[6]) layers of cotton, linen or wool. These jacks were known to stop even heavy arrows

Myeonje baegab

Myeonje baegab (면제배갑, 綿製背甲) was a soft bulletproof vest invented in 1867 in the Joseon dynasty.[1][2]

It was invented following the French Campaign against Korea, 1866 and used in battle during the United States expedition to Korea in 1871. It was made of between 13 and 30 folds of cotton fabric and covered the upper torso.

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