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Adhesive is applied to "soft" ballistic fabrics (like kevlar or even fiberglass) in order to decrease bulge when being hit by a projectile.

What would be the effect of gluing layers of ballistic fabrics together (effectively, making it stiff), with a powerful adhesive, on the fabric's ability to resist penetration? Obviously, the adhesive itself will also resist the penetration, but that is not what I ask.

EDIT: Conecring Solar Mike comment about inertia, on howitworksdaily it said that "[kevlar] has a high tensile strength. This means it takes a huge amount of energy to make its fibres stretch even a little ... most will absorb the energy of the bullet by stretching a small amount."

So, does the stiffness from gluing will weaken the kevlar by not allowing it to strech, or it will actually make it stronger, because now it's harder to make them strech?

Example of kevlar properties from Fibermax:

Yarn Type 1111
Linear Density (nominal) [dtex] 1260
f 1000
Linear Density (effective) [dtex] 1350
Finish Content [%] 0.7
Elongation at Break [%] 3.10
Breaking Tenacity [MPa] 2987
Breaking Strength [N] 280
Modulus [GPa] 91
Tube Size [mm] 290x94
Number of Bobbins x Weight per Bobbin [kg] 6 x 4.5 / 90 x 4.5

Alaychem goes to Codidact
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  • If you stiffen the fabrics then inertia may allow the bullet to pass through. – Solar Mike Feb 28 '22 at 08:24
  • @SolarMike Fiberglass and kevlar are not elastic... does it matter? – Alaychem goes to Codidact Feb 28 '22 at 08:31
  • So if you take a bundle of kevlar fibres in your hand - can you squeeze them? – Solar Mike Feb 28 '22 at 09:04
  • Well, do some research hint: dyneema. – Solar Mike Feb 28 '22 at 10:35
  • @SolarMike Did some research, can you work with this data? – Alaychem goes to Codidact Mar 01 '22 at 18:49
  • With regards to your update: Stiffness is not the same as strength. it might be best to separate the two in your mind. Solar Mike is saying that the adhesive acts the same way the resin acts in carbon or glass composites: It spreads the load to multiple fibers even if the fibers aren't in direct contact with the load. Stiffness is a separate issue. – DKNguyen Mar 01 '22 at 18:51
  • @DKNguyen Of course. I suggest that maybe stiffen the fabric might make it weak... – Alaychem goes to Codidact Mar 01 '22 at 18:53
  • Note that though the overall material is stiffer, the individual fibers themselves aren't any stiffer or less elastic. Pliability of the fabric without adhesive is because the fibers can slide over each other and thin cross section so buckling which masks the compressive strength. The adhesive doesn't prevent the fibers from stretching since the adhesive is far more flexible than the fiber. What happens is that adhesive spreads the load out to more fibers so that more fibers are stretched a smaller amount for the same force. It also makes the fibers behave thicker so there is less buckling. – DKNguyen Mar 01 '22 at 19:03
  • @DKNguyen DId you mean Farris, rather then SolarMike? SolarMike said that _"If you stiffen the fabrics then inertia may allow the bullet to pass through"_ – Alaychem goes to Codidact Mar 01 '22 at 19:10
  • @AlaychemgoestoCodidact I'm just trying to address the edit in your post, wherever it comes form. But yeah, I guess it was Farris who brought up the adhesive acting as the binder in a composite material. – DKNguyen Mar 01 '22 at 19:12

2 Answers2

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A fast moving projectile has a large amount of kinetic energy. Of course, protecting someone or something from the projectile is all about dissipating as much of that energy as possible before the projectile would reach its target.

Kevlar becomes a good option for bullet resistant armor because of its high tensile strength in these scenarios specifically (as well as other qualities that is has). This is partly due to the material qualities it has on its own and partly due to the way in which it is woven.

Imagine a close-up view of a bullet passing through sheets of woven fabric. Some of the individual strands will break as the bullet passes through the first layer. Some of them would prefer to shift aside to allow the bullet to pass through. In the absence of any adhesive material, this first sheet is now useless once the bullet has passed through it. It has slowed the bullet somewhat but is completely ineffective once the bullet has passed through. With adhesive, though, it can still provide support to the material that has not yet allowed the bullet to pass through. These factors are finely tuned in armor products to allow for the best possible amount of bulge and overall bullet resistance with consideration to weight and other factors.

Farris
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  • Thank you for your answer, upvoted. To accept it, I need address @SolarMike concern about inertia – Alaychem goes to Codidact Mar 01 '22 at 07:52
  • I don't fully understand @SolarMike's statement. Perhaps you or he could clarify? Of course, the whole purpose of the armor is to distribute the kinetic energy away from the body/reduce the inertia of the bullet in a safe manner for the wearer. Though not entirely the same, these things are very closely related. – Farris Mar 01 '22 at 13:34
  • I've edited the question, does it clarify? – Alaychem goes to Codidact Mar 01 '22 at 18:50
  • Ah, I see. Like I had said, this would be a finely tuned balance. Too much stiffness from the adhesive would result in a less effective product because it would be too brittle. Too little adhesive and the product would likely "bulge" too much and not reduce inertia efficiently or be victim to some other mode of failure. – Farris Mar 01 '22 at 19:19
  • I should note though, that my experience personally is in bullet resistant glass products. While these function differently, the core concepts are largely the same so I feel it's possible to apply many of them to Kevlar armor as well. – Farris Mar 01 '22 at 19:21
  • Less effective means that it's better without the adhesive? anyway, can you merge those comments with your answer please? – Alaychem goes to Codidact Mar 01 '22 at 19:25
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    @AlaychemgoestoCodidact Less effective is probably in comparison to another adhesive or a different amount of adhesive. There's more degrees of freedom than just adhesive vs no adhesive. – DKNguyen Mar 01 '22 at 19:38
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Fabric based ballistic projection works by distributing load of it's force, kinetic energy perpendicular to it's direction.

Adhesives are already in use for bonding such fibers. Adhesives act to prevent the thread weaves from being undone, like poking a needle thru a shirt, often the hole displaces the fiber weave enter image description here

LazyReader
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