[brycelarson]

ok lefty - you seem to know this stuff - tell me this:

we have a sphere (imperfect though it might be). If you dent in one part of a sphere - does that force apply itself evenly around the entire surface of the sphere? - or is the outward force create directed in some direction related to where the sphere has dented in?

[Leftystrikesback]

It depends on what you're asking... I'll answer it in two different ways

First lets ignore the fill, if you dent a hollow sphere, the stresses are concentrated around the edges of the dent and won't affect the rest of the sphere at all

Now lets talk about a hollow sphere with fill.
We need to make some assumptions though; there is no air inside the ball, only paint, and the paint is incompressible.
Given that, which is probably true, the force will be constant around the surface of the sphere except for the spot where the dent is, where there is some additional stress due to the deformed dent.

Here's why; you dent the paintball in some spot, which decreases the volume on the inside of the paintball. But the paint is incompressible, so it can't change its volume, so to compensate it's pressure goes up and the paintball's shell slightly expands to correct the change in volume. Also, pressure in a static fluid is constant everywhere (ignoring gravity) so the original force used to dent the paintball is transformed into a constant pressure in the fluid, which is applied evenly everywhere. This is true for any shape, but pressure needs to be translated into a force, and in a perfect sphere this constant pressure turns into the same force at every point on the sphere.

but when you change the shape, so the ball is no longer spherical, things change drastically. That's what my point was in my last post, if you look at a cylindrical pressure vessel (any cylinder that holds pressure) the constant pressure on the inside translates into different forces (stresses) going in different directions across the surface. now I'm not suggesting a compressed paintball is cylindrical, but it's not spherical either so non-uniform stresses will pop up.

[cockerpunk]

lefty - great call.

the reason i was asking about void space and such is becuase i believe the seam is the site of most paintball failures, or the reason for them. polymers like gelatin with have some amount of crystalisity, not a true matrix like an alloy, but there mers will be aliened in a strong way.

this is because the seam is chaotic, and becuase of that the mers of the polymer are not properly set up to take loading. there is probably also holes and voids where the two sphere halves to not line up exactly right.

either way, i believe the initial cracking takes place at the seam due to voids caused by irregular formation and alignment of the mers multiply the stress. the intial cracking may happen even when loaded horizontally, but the true failure of the structure is when the crack propagates catastrophically though a large enough section of the shell. again, i see the seam as another source of voids, and stress multipliers along it as a a source for the initial cracking to propagate easily through.

for whatever reason though, the seam plays a critical role in terms of failure of the paintball. of that there is no doubt IMO.

and to those calling for more data points, understandable. but when you see the true scope of what this test was about, you will see how it all took something like 8 or 9 hours of work. the trends we are talking about are pretty large IMO so further data points, while nice, are not necessary.

you guys also hit the nail on the head. while we can understand that in an impact type situation, there will be alot less deformation then in this case. this test is primarily about stead state loading, and the LARGE deflections we saw was primarily becuase of the mers in the polymer shell creeping. and underbore portion of a paintball is much more stead state then an impact at 50 or more FPS. however, its easy to see that when we recommend a .003 to .005 underbore, thats like 5% tops perhaps of the total crush strength of the ball, and with a nice destrubted load like we tested too, we saw that percent even get lower. so an underbore again simply puts minimal stress on the paintball.

[Leftystrikesback]

In a polymer, the strength is influenced by both the length of the polymer chains (longer is stronger) and how they are mixed/tangled (more tangling is better overall). If polymer chains are lined up, the polymer will be strong in the direction of the lined up chains, but weak in all other directions. If the chains are not lined up, and the material is stretched in some direction, the chains move to line up in that direction strengthening the material. So more tangling leads to better strength overall, I think this is the key.

I think what happens in the seam is that only some of the chains have tangled between the two halves, so the bond of the polymer in the seam just isn't as strong as in the rest of the paintball. That probably happens because the halves aren't held together long enough or are cooled too much to maintain their shape or something like that in the manufacturing process.

This is kind of what you're saying I think, I just don't agree with the term void because it implies a missing atom somewhere. In a polymer, a missing atom in the chain just means a shorter chain... which I guess does have a similar effect on the material strength but is a completely different mechanism. Let's call it a gap?

This post has been edited by Leftystrikesback: 18 January 2009 - 09:27 PM

[Lord Odin]

If I may, I'd like to interject a question. If most of the breaks begin at the seam and the seam's strength is only as good as the bond between the two halves, then why are the thicker shells stronger than the thinner shells? Are the thinner shells cooling too rapidly to create a strong enough bond?

This post has been edited by Lord Odin: 18 January 2009 - 09:44 PM

[Leftystrikesback]

well a thicker shell should mean that the seam is thicker too (I assume). Lets say there are the same number of bonds/area due to the manufacturing process, a thicker seam would still mean more bonds overall, right? So the seam would get stronger in proportion to the thickness increasing (assuming seam thickness also increases), but still be weaker overall.
That's my justification, maybe there are others

This post has been edited by Leftystrikesback: 18 January 2009 - 09:48 PM

[Troy]

Leftystrikesback, on Jan 18 2009, 08:46 PM, said:

well a thicker shell should mean that the seam is thicker too (I assume). Lets say there are the same number of bonds/area due to the manufacturing process, a thicker seam would still mean more bonds overall, right? So the seam would get stronger in proportion to the thickness increasing (assuming seam thickness also increases), but still be weaker overall.
That's my justification, maybe there are others

Hydrogen bonds occur between the different chains, which overall, increases their strength, but you're pretty much right, the chain tangling is the greatest source of strength in a polymer chain. Polymer chains are crazy temperature specific (that's why you have different kinds of waxes for skis for different temps, and margerine doesn't melt at room temp), and the temps that they need to be heated up to to wrap each other up is probably a fairly specific one... I'm betting that the temp that they have to combine the two halves together doesn't allow enough molecular movement to let the two halves interlink very well. Just in like a weld in high strength aluminum, the two materials don't interlink very well, you end up adding in materials like scandium that form dendrites between the particle grains that increase interlinking between the two materials.

I'm going to deny the assumption that there isn't air in a paintball, as I've seen plenty of paintballs with very clear bubbles of air. I would imagine that paintballs that were properly filled would break after much less deformation then paintballs with air. I imagine the slight air pockets are a source of great elasticity.

Anyone that has changed out the hydraulic fluid on their mountain bike (or perhaps motorcycle), can tell you about all the damn air that can hide in a liquid. Based on the pressure of the fluid the air can easily dissolve in the fluid and make the paintball much more prone to compression then if it had a noncompressible fluid fill without any air introduced.

[Leftystrikesback]

I wish I had the money to put hydraulic disc brakes on my mountain bike... ah all the things I'll be able to do once I graduate...

But yeah you might be right about the assumption that there are no air bubbles in the fill. I can't think of a good way to test this... Poke a ball in the top with a pin and see how much you can squeeze it till paint comes out?

Edit: about the temperature dependence, I would imagine getting the gelatin to the right temp to create good bonds would also make them unable to hold their shape, so could be a trade off between well bonded halves and round balls with no dimples...

This post has been edited by Leftystrikesback: 18 January 2009 - 10:37 PM

[Lord Odin]

Leftystrikesback, on Jan 18 2009, 09:34 PM, said:

I wish I had the money to put hydraulic disc brakes on my mountain bike... ah all the things I'll be able to do once I graduate...

But yeah you might be right about the assumption that there are no air bubbles in the fill. I can't think of a good way to test this... Poke a ball in the top with a pin and see how much you can squeeze it till paint comes out?

What about a microwave? Place a ball inside a tupperware container that isn't completely closed and turn it on. If there is air in the ball, shouldn't it cause the ball to expand and/or pop? Or would this happen regardless if air was in there or not?

[Leftystrikesback]

Could be a good idea, time to microwave some paintballs? although it could boil the water in the fill and the vapor might blow up the paintball too. I still want to put one in the microwave haha.

[Lord Odin]

Leftystrikesback, on Jan 18 2009, 09:39 PM, said:

Could be a good idea, time to microwave some paintballs? although it could boil the water in the fill and the vapor might blow up the paintball too. I still want to put one in the microwave haha.

I thought about that. First thing that comes to mind are grapes. They don't expand; they simply explode because of that very reason.

What about oil-based paintballs? Do they still have water in them?

[Leftystrikesback]

well that was entertaining! put it in for 35 seconds, it expanded to about 3 times its size but did not explode. When the microwave stopped it was clear that inside the ball there was about 50% liquid and 50% gas. It deflated quickly, I couldn't tell if the gas was air or water vapor, the ball was hot and I didn't want to burn myself popping it. My guess is that most of the gas was water vapor since it expanded so much and so quickly. Still fun though, I recommend it. Didn't even make a mess. Careful though the ball is hot.

I don't own any paintballs with an oil fill... do those exist, I'd think they would stain? or do you mean the ones that are super waxy when they dry? If they were really oil based then they wouldn't have any water in them but the shell still would.

[Merc4Hire]

I thought the fill was supposed to be cornstarch, ground crayon wax and soybean oil, mostly, and that it is deliberately dried of water to avoid making the gelatin shell gooey.

[Jack Wood]

Merc4Hire, on Jan 19 2009, 04:59 AM, said:

I thought the fill was supposed to be cornstarch, ground crayon wax and soybean oil, mostly, and that it is deliberately dried of water to avoid making the gelatin shell gooey.

Generally paintballs do have a little air bubble/pocket in them.

This can be seen on many paints that have a clear shell.

You most certainly could not rely on every ball having zero air-pockets in them.

[TechPB-Mike]

that is an awesome rig

I think I'd have too much getting drunk with you and Bryce, and seeing who can put their hand under it and take the most pain

Science can quickly turn to torture when alcohol is introduced

[Troy]

Merc4Hire, on Jan 18 2009, 10:59 PM, said:

I thought the fill was supposed to be cornstarch, ground crayon wax and soybean oil, mostly, and that it is deliberately dried of water to avoid making the gelatin shell gooey.

Some of that may be true, but the major component is PEG (polyethylene glycol). It has the uncanny ability to absorb water VERY well... its fun stuff.

Leftystrikesback, on Jan 18 2009, 09:51 PM, said:

well that was entertaining! put it in for 35 seconds, it expanded to about 3 times its size but did not explode. When the microwave stopped it was clear that inside the ball there was about 50% liquid and 50% gas. It deflated quickly, I couldn't tell if the gas was air or water vapor, the ball was hot and I didn't want to burn myself popping it. My guess is that most of the gas was water vapor since it expanded so much and so quickly. Still fun though, I recommend it. Didn't even make a mess. Careful though the ball is hot.

This could be easily tested by taking a paintball's fill and microwaving it by itself and seeing if lots of water boils off.

[Troy]

Leftystrikesback, on Jan 18 2009, 09:34 PM, said:

I wish I had the money to put hydraulic disc brakes on my mountain bike... ah all the things I'll be able to do once I graduate...

I've worked at a bike shop for some time now... so I have a pretty sweet setup, that I got pretty cheap.

Leftystrikesback, on Jan 18 2009, 09:34 PM, said:

Edit: about the temperature dependence, I would imagine getting the gelatin to the right temp to create good bonds would also make them unable to hold their shape, so could be a trade off between well bonded halves and round balls with no dimples...

I think you are exactly right, by definition, at that point the material is pretty much at it's least rigid, because the molecules can move so much.

[sunshaker]

If you wanted to test a stack of paintballs to see the break value you could use a clear 10 round tube that has been cut down. You could easily set it up to test 1-9 or 10 paintballs (depending on if you cut the end off and the size of paintballs), you would be able to see which paintball(s) broke in the stack, they are cheap to buy (and you would probably want to have several set up as washing them out between tests would be a pain) and it would be fairly easy to set up (well getting the seem perfectly lined up might be a pain).

It might be interesting to see a comparison of oil-based vs water-based paintballs, or for that matter if you can find some of the old wax shelled ones.

[Jack Wood]

Question for Gordon:

When the balls were loaded with Vertical seam, and you say the fracture originated at the seam, do you mean that the fracture started at the base, where the ball was up against the slab, on the seam, or could the fracture eminate from a failure anywhere around the seam?

Did the fracture always start at the bottom of the ball, is what I'm asking?

Jack

PS, there is (I think) an error in the data for the Silver. The VMean/Hmean isn't the correct value using the Vmean and Hmean figures in the table. I make it 0.273

[cockerpunk]

Jack Wood, on Jan 19 2009, 12:01 PM, said:

Question for Gordon:

When the balls were loaded with Vertical seam, and you say the fracture originated at the seam, do you mean that the fracture started at the base, where the ball was up against the slab, on the seam, or could the fracture eminate from a failure anywhere around the seam?

Did the fracture always start at the bottom of the ball, is what I'm asking?

Jack

PS, there is (I think) an error in the data for the Silver. The VMean/Hmean isn't the correct value using the Vmean and Hmean figures in the table. I make it 0.273

ill check that math ...

without a camera watching the failures i dont know where it started, but in every break i remember (vertical or not) the failure was defiantly around or on the seam.

yup, fixed the value, when i added in the third data point i forgot to add those cells to the formula. thanks for the help there.

This post has been edited by cockerpunk: 19 January 2009 - 12:07 PM