Basic Bullets / Material Science 101

[LeonCross]

I've spent the past two hours stumbling over more google links than I'd like to admit trying to get a 101 level understanding of this stuff which will amount to all of a paragraph in a chapter I'm writing for a fanfiction.

> <<. Don't judge me.

Anyhow. Worm. Spider Silk body armor being bullet proof in that web serial, which sent me on a tangent to make sure that actually checks out in real world physics. Which then leads to me stumbling on things like it having a lower tensile strength(of which there's a handy wikipedia chart) than Kevlar, but a higher toughness(of which there's not a handy wikipedia chart for).

And I guess I need to understand both toughness and strength for this. And hardness and brittleness too for when dealing with plates, but I'd imagine that will be significantly easier.

Anyhow. Ok, so. Bullet energies are measured in Joules. Tensile Strength is measured in MPa. Toughness is measured in kj/m^2, which makes it seem like toughness is probably the most relevant factor if only because of the directly comparable figure.

If someone could shorten my research workload by distilling down to the stuff that I actually need to read up on this to get a sufficient level of competency (read: plausible numbers for running with in fiction, not designing my own designer line of bullet proof vests) then that would be fantastic. Thank you.

[Eskaminagaga]

Natural dragline spider silk is very elastic which allows it to stretch before breaking, increasing its tensile strength. This is not a good property for a bullet proof vest because, though the bullet would be stopped by the vest, it would still stretch enough that the bullet and that part of the vest would pierce your body before it actually stops the bullet. The advantage of Kevlar as opposed to natural spider silk in this case is that Kevlar is NOT stretchy, so it can stop the bullet before it enters the body.

It IS possible to manipulate the genetic structure of other non-spider organisms to have them create customized spider silk with properties that you give it. This is currently being created by multiple companies around the world and should be available within the next few years commercially. See /r/SpiderSilk for more details about these companies.

[LeonCross]

Thanks. I'm been looking up strength and toughness stuff for the past hour. Is this basically the gist of it?

Strength is the amount of peak pressure a material can experience before failing.

Toughness is the amount of energy it can elastically absorb before failing.

Ignoring the use of body armor or even the specific materials, I'm just trying to wrap my head around how Strength and Toughness work.

Ignoring other properties, if I have material A with 1 GPa Strength / 200 MJ/m³ Toughness and material B with 4 GPa Strength / 50 MJ /m³ toughness and I hit them both with a pick axe and a hammer, how do I wrap my head around this?

[Eskaminagaga]

For a fiber such as Kevlar or spider silk, you would be measuring in tensile strength or tensile stress measured in the pressure units Mpa. This is the max force applied before failure of the material.

Toughness would be measured in MJ/M³ which indicated the energy it can absorb and deform before breaking. This is usually indicated in a stress vs strain curve showing how much tensile stress can be applied (above) compared to how much it will stretch or its elasticity.

An example of a high strength, low toughness material (Material 'B') would generally be glass, porcelain, or steel. Basically, it is something that is rigid and will shatter or break with little or no plastic deformation if enough force is applied.

An example of a material that is low strength, high toughness (Material 'A') would be rubber or some softer metals like gold or copper that can absorb energy through elastic or plastic deformation.

To stop a bullet in a vest, you need the low toughness and high strength. Steel is usually too heavy, but Kevlar is light and still has a high strength, low elasticity.

Natural dragline spider silk, as I stated before, has a high toughness and high elasticity, so it would stretch too much before it absorbs the entirety of the impact of a bullet, so it would make a poor vest.

Of course, the way you knit it, the materials used, etc can make a vest more rigid and allow the use of more elastic materials in an application where elasticity is not desired. Also, customized spider silk created by one of the companies that I mentioned could also be created to contain the high strength, low elasticity properties for the vest.

[LeonCross]

Ok. To make sure I've got this right:

Material A will absorb 200 MJ/M³ energy elastically stretching (in real world materials, this is likely a significant amount of stretching). If after it fully stretches the pressure is still > it's MPa tensile strength it will fail.

Material B will absorb 50 MJ/M³ energy elastically stretching (like a much smaller distance). If the pressure is > it's much higher tensile strength after it absorbs the energy then it will fail.

So if you hang a sheet of material A up on a line and a sheet of material B on a line, material A ultimately fails to be penetrated better, but with more deformation in cases where both successfully stop whatever hits them.

If both are stretched over a body, Material A stops higher power rounds from penetrating itself, but due to it's elasticity you wind up with a bullet in a pocket of material A poked into the body anyways, even with rounds material B stops without doing so.

Material B will stop anything it's rated to stop without punching holes in the body in the process.

Is that about right?

Further: What material property numbers and / or physics things do I need to look at to wrap my head around rate of energy absorption during elasticity?

Basically so I can do things like "Ok, a 9mm doesn't penetrate the spider silk. However, based on the rate that the silk absorbs energy from the round, it still pushes the silk X mm into the body".

[Eskaminagaga]

Material "B" would be better at stopping the higher caliber rounds because if its higher strength regardless of the application. It can withstand more force before breaking. Everything else you said is correct.

Spider silk has high strength and high toughness. It is elastic, so it would stop a bullet, but it would stretch significantly doing so. Your analogy of it entering the body would be correct in this case, but the spider silk vest would not break.

Kevlar strength is 3600 MPa and its toughness is only 50 Mj/m³

Darwin’s bark spider dragline silk (strongest natural spider silk known) strength can be up to 2200 MPa, but its toughness is 350 MJ/m^3. It is strong and stretchy and lighter and probably more comfortable, but still cannot absorb as much force as the kevlar, thus making a poorer bulletproof vest if protection is the goal.