Does CMF bulletproof body armor have any negative consequences for the wearer?

[jim mcnamara]

http://phys.org/news/2016-04-metal-foam-obliterates-bullets.html
The CMF armor on collision literally converts the projectile into small particles, spraying out.
This is interesting and definitely not my area.

Watch the attached video.
Questions:
1. the particles resulting from the hit have to have kinetic energy, if some particles are redirected to unprotected area (example: projectile hits high, just above the top of the sternum, redirect up into the lower jaw), are they a problem?

2. Impulse from the collision should push the armor back into the wearer of the armor.
Correct? There should be some negative consequences to the wearer at the point of impact.

Assumption:
There is no free lunch, if you ameliorate one issue, in this case severe or fatal wounds, then there there has to be some trade off. Please correct me.

 

[256bits]

#1. As a body armor suit, tests would have to be conducted. Best secondary means of protection might be of such as a type of flexible ski mask covering over the head, neck, upper chest of perhaps similar or same material but of lessor depth, to accept debris from a hit. I presume there is no way to ever certify a 100% full protection.
#2. Sure, an impact would hurt and bruise. I don't see any way around that , if there is to be mobility for the user. Maybe they could design an armor suit like the more rigid kind for a knight, so as to reduce localized trauma ( correct medical term escapes me right now ) - though that model might not be so user friendly.

Just throwing out some thoughts on the subject.

 

[Kevin McHugh]

I just posted this the other day.
https://www.physicsforums.com/threads/composite-metal-foams.866039/

 

[Ethan Borrou]

If you are using the most common hard armor that the military uses it is made of a ceramic plate that acts as a sacrificial layer. The plate breaks and the round stays mostly intact but it looses all of it's energy. Most metal plate armors have the same issue of bullets spalling upon impact. These fragments can cause injury and harm to the individual wearing the armor. Having a secondary armor patch attached to the face would be very uncomfortable for the wearer and would add another layer of cost to either the person, company, or government. Armor production companies like www.ar500armor.com usually add an anti-spalling coating to the strike face of the armor. This is usually a plastic coating similar in composition to Line-X http://www.linex.com/protective-coatings/security-and-defense this captures the bullet fragments so that you don't experience any secondary injuries.
As far as the impact felt when the bullet hits there are a few things that help out. Most military style body armors use a back layer of Kevlar like material, this is usually rated for Level III-A which would stop handgun rounds by itself. Soft armor stops rounds by dissipating the energy and since most handgun rounds are not pointed like rifle rounds you just need to slow them down enough to not penetrate your skin. This layer also acts as a shock absorber for the more high energy rounds that you need hard armor to stop. Some individuals also elect to add a third layer behind the soft armor and that would be what is known as a trauma pad. This is a pad that is made of layers of non-Newtonian fluid and padding. It further mitigates the shock of bullet impact on the wearer, more info here http://www.ar500armor.com/ar500-armor-trauma-pad-10-x-12-asc.html
I personally would cannibalize multiple different types of armor for my personal use (if I had the money to do this). I would use a combo of the following:
1-CMF plates with Line-X anti-spalling coating, front+back+2 sides
2-Carbon nanotube soft armor https://ctznarmor.com/product/shtf-tactical-xpc/
3-Trauma pad
4-Plate carrier with slots for all 4 plates http://www.blackhawk.com/Products/Protective-Gear/Protective-Gear/Armor/Carriers/S-T-R-I-K-E-Cutaway-Plate-Carrier.aspx

I hope this helps answer some of your questions.

 

[RogueOne]

Here is a video of some cheaper armor stopping the same bullet fired (Tungsten-Core M2AP) from the same cartridge (30-06). You can see behind the plate that there are significant impact forces from the bullet. However, it stopped the bullet.


Also, those metal fragments that you see coming from the bullet are not coming from the armor-penetrating component. The armor-penetrating component is actually a tungsten steel core that is contained inside of the bullet. What you see in the video is merely copper jacketing that will spall like that on any hard body armor. I'd like to see the condition of the tungsten steel core itself. I find that they rarely deform and I doubt that this one did either. I don't doubt that it stopped, but it probably did not deform.

H

 

[Everett Hunt]

Combining CMF with Hagfish slime may prove quite impressive.

 

[TeethWhitener]

Link to (possibly paywalled) paper
https://www.sciencedirect.com/science/article/pii/S0263822315000434?via=ihub

1. the particles resulting from the hit have to have kinetic energy, if some particles are redirected to unprotected area (example: projectile hits high, just above the top of the sternum, redirect up into the lower jaw), are they a problem?

From the paper:

Both types of composite armor systems successfully stopped the projectiles for single and multi-hit scenarios, for bullet speeds at or higher than those specified by NIJ 0101.06 standard. It is observed that the ceramic plate caused failure of the projectile upon impact. A post impact inspection of the CMF layer showed that it had successfully absorbed the kinetic energy of the projectile through compression and the aluminum and/or Kevlar™ backing plates supported the CMF layer as it was compressed, absorbing any residual tensile stresses and catching ejecta. Little or no fragments of projectile could be found after the NIJ-Type III tests on either one of the composite armor systems, which suggests a complete disintegration of the 7.62 × 51 M80 bullet for all samples tested.

2. Impulse from the collision should push the armor back into the wearer of the armor.
Correct? There should be some negative consequences to the wearer at the point of impact.

The paper gives a table with measurements of energy dissipation for each layer of the armor (the armor is a boron carbide ceramic/metal foam/backing laminate, where the backing is either aluminum or kevlar). In the type III cases (non-armor piercing), the armor absorbs ~85% of the bullet's kinetic energy, while in the type IV (armor piercing) cases, the armor absorbs ~70% of the kinetic energy.

Upon contact with the armor, the soft bullet is first flattened by the hard ceramic, which increases the surface area (decreases the pressure) over which the force is distributed. This ceramic is hard but brittle, and therefore fractures after absorbing ~5-15% of the bullet's energy. The ceramic/bullet system then plastically deforms the CMF, which absorbs ~50-70% of the energy of the bullet. The Kevlar or aluminum backing absorbs another 10% or so of the energy. The type III bullets have an initial kinetic energy of ~3.5 kJ, so after their encounter with the armor, there is a residual ~400J of energy, roughly the equivalent of getting kicked really really hard in the gut. So not a fun day, but not nearly as lethal as getting shot.

 

[scottdave]

There are tradeoffs. It is sort of like a seat belt might cause some broken ribs or collarbone, but keep your hadh from smacking into the windshield...

 

[Dr. Courtney]

Lots of variables. The M80 bullet is just a regular lead core in a copper alloy jacket. It is pretty easy to stop, since it is not an armor piercing bullet. The APM2 is not a tungsten core, but it does have a hardened steel core. Stopping this bullet has become the standard test for military armor.

The areal density of the metal foam that does the job is really the bigger issue than the thickness, as the push these days in armor is reducing the weight soldiers carry and providing the same protection. The ceramics widely used do not obliterate the steel core, they blunt the point and reduce the mass and velocity so that the layers behind it can completely stop it (usually a few layers of glass then an aramid or kevlar backing).

But to answer the OP - yes, redirected fragments can be a problem so most complete armor designs try and catch all the fragments rather than redirecting them. The rearward deflection of the armor in the stopping process is called backface deformation - BFD. It is measured in mm by shooting the armor with a special clay backing and then measuring the indentation of the clay. We've also measured it with high speed video using calibrated ballistic gelatin as backing. The bottom line is BFDs more than 44 mm can prove to cause serious injury or death in people and test animals comparable in size to people. The rapid acceleration of the chest wall sets up a shock wave in the lungs similar to an explosive blast, and one can estimate the energy transferred to the chest from the impact energy and the BFD for a given projectile.

If other factors are equal, stopping the same projectile with less BFD is better. But other factors are seldom really equal.

 

[FactChecker]

Yes, there is a danger from the fragments, but not nearly as much danger as the un-shattered projectile presented.

The momentum of the projectile is no more of a problem for the target as it is for the shooter -- so that is not the main issue. On the other hand, the energy of the gunpowder explosion is almost all contained in the velocity-squared of the projectile. By shattering the projectile into many fragments that can go in all directions, the energy is dissipated. Otherwise, the projectile takes all that energy into the target and "splatters" the target as though the gunpowder explosion happened inside the target. That is what the armor avoids. That is what kills. The armor keeps the energy from entering the body.

 

[256bits]

Lots of variables. The M80 bullet is just a regular lead core in a copper alloy jacket. It is pretty easy to stop, since it is not an armor piercing bullet. The APM2 is not a tungsten core, but it does have a hardened steel core. Stopping this bullet has become the standard test for military armor.

Yeah I wonder,
If he military has somewhat a different criteria than the police for vested armor wear.
A full metal jacket round would go right through the soft biological tissue and make a small hole, and continue on.
At least that is the theory for playing at war legally - rounds aren't supposed to have a small diameter entry and an 8-inch exit.
Not really the reason for the metal jacket initially, but it fulfills that requirement for soldier rounds.

Police, on the other hand, use I think mushrooming rounds, that are supposed to lodge in tissue ( less velocity? ), lessening both the possibility of a casualty behind the perp, along with ricochet. Of course, police do not know what they will be up against in an exchange of fire.

Military, swat team, regular policeman on the beat - armor vests the same, or different?

( Shooting game - one wants the penetration to stop within the animal, and cause excessive organ trauma bringing down the animal quickly. )

As said, lots of variables

 

[Dr. Courtney]

Military, swat team, regular policeman on the beat - armor vests the same, or different?

https://en.wikipedia.org/wiki/Bulletproof_vest#Performance_standards

The link above is fairly accurate. US military uses versions of Type IV, which stops armor piercing (hardened steel core) 162 grain 30-06 rounds at over 2800 feet per second. It has a hard ceramic plate and is heavy.

Most SWAT teams and other active engagement LEO teams use various versions of Type III and Type IV armor which is all hard and designed to stop rifle bullets, though Type III tends to be much lighter.

Typical patrol police officer armor is soft and a Type II or Type IIIA variant that is designed for pistol bullets. Most high velocity (>2000 ft per second) rifle bullets will go through it. Most departments are unwilling to bear the expense of military armor, and most police officers are unwilling to wear the heavy, bulky, and uncomfortable armor all the time on the job. Many departments also prefer not to give the appearance of expecting gun battles that tends to accompany military style body armor.

There are some Type IV armors that are lighter and less visible, but these are very expensive and usually reserved for VIPs like the POTUS.

 

[256bits]

Thanks. I appreciate that.
This is a very informative thread of posts.