Bullet deformation
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Old 05-06-2009, 01:21 PM   #1
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Default Bullet deformation

Hi,

I've been working for a long time on a ballistics model to be used in a game. I have read quite a bit of material from material science to fluid dynamics to ballistics. I am by no means an expert, and have only rudimentary knowledge in most areas.

I am very close to finishing steady state and impact state ballistics, somewhat equivalent to what is more commonly known as external and terminal ballistics. However, I still have not finished the model for bullet deformation. The accuracy need not be great, and simple approximations are enough for my real-time purposes, so I've decided on the two following attributes:
  • deformation onset (m)
  • deformation rate (1/m/Pa)

Deformation onset is the neck of the permanent cavity, and deformation rate is simply the difference in length of a rod over a specific distance at a specific stress.

The follwing site gives me some ballistic cavitation profiles for various bullet (and there are probably plenty more sites), that I can use to approximate the behaviour of a bullet. However, this is in flesh, and does not account for how a bullet would behave in a different material, e.g. wood, concrete, or steel.

(1) Terminal Ballistics

The average shear strength of flesh is 0.4 MPa, and a specific structural steel 145 MPa, that's a huge difference. I would expect most soft bullet to shatter in a high velocity impact with steel, but how quick is the rate of change in deformation onset?

If a 5.45x39 mm Russian bullet has a deformation onset of 7 cm in flesh, as I increase the stress, how quickly would the deformation onset decrease? Would it decrease at all?

Deformation rate is how quickly the bullet deforms once it has reached the onset point.

Does anyone have any material that would help me with this issue? Does anyone have any practical experience with how bullets deform with different materials?

Any help will be very much appreciated!

EDIT: two early guesses is that it is either linear, or that it is mostly dependent on dynamic stress and thus sqared term.



Last edited by rika; 05-06-2009 at 03:33 PM.
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Old 05-06-2009, 01:55 PM   #2
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I am walking around the edge of not knowing what I'm talking about here, but my observations:

Bullet deformation (expansion) is the result of velocity and resistance. As expansion occurs, velocity slows. The more fragile the nose, the greater the expansion, velocity being equal. Further, density of a living creature, man or animal, will vary with depth of penetration.

With the variables in bullet design, construction and material, you've got a tough row to plow from the onset.

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Old 05-06-2009, 02:49 PM   #3
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Quote:
Originally Posted by Bob Wright View Post
Bullet deformation (expansion) is the result of velocity and resistance.
True, velocity is the most important factor, and "resistance" is actually strength and density.

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As expansion occurs, velocity slows.
As expansive deformation occurs the frontal area increases and thus the force exerted on the bullet increases, decelerating the bullet even faster. For fragmenting deformation, even though the bullet breaks apart and the area can be seen to decrease (or remain constant), the area of tissue affected is actually greater and causes even more damage. Fragmentation also causes even faster deceleration -- even though the area per fragment is smaller -- as the fragments are much lighter and lack the energy to penetrate as far as an unfragmented bullet.

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The more fragile the nose, the greater the expansion, velocity being equal.
I also have some basic knowledge of this. Various bullet designs and materials play an important role in this. Soft point, hollow point, hollow cavity, all expand more easily due to the lower strength of lead and the distribution of mass in the nose of the bullet.

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Further, density of a living creature, man or animal, will vary with depth of penetration.
This is not really a problem, as I can safely use an average density of flesh and skin. The simulations are realistic enough without a more detailed modelling of density.

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Originally Posted by Bob Wright View Post
With the variables in bullet design, construction and material, you've got a tough row to plow from the onset.
True, although I can simplify some aspects to make it simpler without losing too much realism. Although trying to create a somewhat comprehensive list of bullet designs has been somewhat exhausting. There are many aspects, construction, nose, point, etc., and I need to compile them into a shorter list that roughly corresponds to most designs.

Last edited by rika; 05-06-2009 at 02:54 PM.
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Old 05-06-2009, 03:28 PM   #4
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A lot of variables. Animals (and last time I looked, people are animals- except a couple in Congress that I think are in the vegetables class) are NOT of a uniform consistency. A bullet, such as a 30 cal FMJ, may pass completely thru a fleshy part of the body with minimal deformation, relatively small permanent wound cavity. But vary the track by 1 inch, hit a major bone, vast difference in effect. You may also experience where TEMPORARY wound channel is larger than the target. Example- a .220 Swift used on a prairie dog- small critter, LARGE temporary wound channel- they blow apart.

As far as metal to metal, in hitting a vehicle with AP rounds- not only deformation, but tendency of metals to FLOW under extreme pressures. Look at half inch metal hit by a 30-06 AP round- expect to find a RAISED rim around the hole.

Variables will include not only velocity/mass, but how hard bullet/target are, and if bullet is a shape designed to expand (and has enuff velocity to do that). Include relative STABILITY of the bullet- will it tend to yaw on impact- suddenly increasing it's cross section? Does bullet tend to DRAG clothing etc into the wound? Soft lead does- that why shot for ducks/geese may be copper plated lead- to get thru the feathers.
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Old 05-06-2009, 04:37 PM   #5
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A lot of variables. Animals (and last time I looked, people are animals- except a couple in Congress that I think are in the vegetables class) are NOT of a uniform consistency. A bullet, such as a 30 cal FMJ, may pass completely thru a fleshy part of the body with minimal deformation, relatively small permanent wound cavity. But vary the track by 1 inch, hit a major bone, vast difference in effect. You may also experience where TEMPORARY wound channel is larger than the target. Example- a .220 Swift used on a prairie dog- small critter, LARGE temporary wound channel- they blow apart.
True, animals (including humans) are not of a uniform consistency, but in my above posts I was talking specifically about flesh. Bones are also part of the model, and of course, bullets act completely different when impacting bone. Bone is roughly 175 times stronger than flesh! The part about temporary cavity was new though, I had never thought about it that way! So far I had outright dismissed it as it doesn't do any direct damage. However, if the animal is small enough that actually makes sense, it could blow apart. Thanks!

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As far as metal to metal, in hitting a vehicle with AP rounds- not only deformation, but tendency of metals to FLOW under extreme pressures. Look at half inch metal hit by a 30-06 AP round- expect to find a RAISED rim around the hole.
Yes, yes, nothing new there ^_^ All solids can actually be approximated pretty well using fluid dynamics when dealing with ballistics. If you want very accurate results you need to use more complicated models.

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Variables will include not only velocity/mass, but how hard bullet/target are, and if bullet is a shape designed to expand (and has enuff velocity to do that). Include relative STABILITY of the bullet- will it tend to yaw on impact- suddenly increasing it's cross section? Does bullet tend to DRAG clothing etc into the wound? Soft lead does- that why shot for ducks/geese may be copper plated lead- to get thru the feathers.
It's actually not hardness, but material strength, tensile strength of the bullet and shear strength of the target. Stability was another thing I hadn't really thought about, so far I had only dealt with straight wound cavities. Dragging cloth into the wound is not that important for my model, so I'll omit that one.
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Old 05-07-2009, 01:23 AM   #6
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Sounds like you have plenty of explanations for the answers that you're being given, so why the questions?

not to mention the 3 other threads (by my count) you've got about this game....

I hate to be a dick, but is this all you're here for?
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Old 05-07-2009, 01:25 AM   #7
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My thought is there has been more than a few saying they are developing a game and need info. For the most part the information is easy to get from the manufactures. So why ask the question here? Maybe to get some nice quotes for another use?
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Old 05-07-2009, 08:07 AM   #8
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Quote:
Originally Posted by falseharmonix View Post
Sounds like you have plenty of explanations for the answers that you're being given, so why the questions?

not to mention the 3 other threads (by my count) you've got about this game....

I hate to be a dick, but is this all you're here for?
Not being a dick at all, that's really what I'm here for. I've had trouble finding information for some things so I've tried to ask questions at various firearms forums, if maybe someone had more knowledge on ballistics than I do.

I didn't mean to be offensive or abusive, I've just run out of information, and thought I'd ask for some help/advise. Actually, 2 posts, the other one is someone else.

EDIT: and I'm really not trying to be dick, giving answers to the replies, I just want to show what I already knew and what was new. I really appreciate any information, even if I happened to already know it ^_^



Last edited by rika; 05-07-2009 at 08:11 AM.
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