Destructive Air Tank Testing

[Siri]

Originally Posted by Oblarg

I'm absolutely certain that plastic tanks can be used in a manner which is entirely safe. But this is a high-school competition, not a professional engineering work environment, and I have serious reservations about hardware with such a dangerous failure mode in this context.

And it's not just a matter of over-tightening the fittings, though this does seem to be a common root cause of the starter flaw. There are plenty of other ways on a FIRST field to cause trauma to the tanks. As I mentioned, some of them are even completely legal (though not preferable). I've even seen people pass inspection with plastic tanks on their appendages. Just because it reportedly hasn't happened yet doesn't mean we should be ignoring other potential sources of damage.

[JamesCH95]

Originally Posted by Oblarg

I'm absolutely certain that plastic tanks can be used in a manner which is entirely safe. But this is a high-school competition, not a professional engineering work environment, and I have serious reservations about hardware with such a dangerous failure mode in this context.

This is exactly the concern. We have a product that has a very dangerous failure mode, being used in an environment where a-typical damage (major structural damage, G28 infractions, etc) is probably at an all-time high in FRC history, by people not necessarily aware of how to properly implement said product. More to the point, there are plenty of people near these tanks who are largely unprotected in the event of a failure.

After the testing we did, Andy A and myself feel very strongly about being proactive in reducing or eliminating this safety risk.

[DonRotolo]

Originally Posted by ToddF

It's not just the air tanks that are a possible hazard. The batteries we use are just plastic boxes that contain acid.

We generally do not pressurize our batteries.

Yes, there are hazards in this sport, but some are more reasonable than others, and some can be mitigated better than others.

A standard white gym sock will do little to contain shrapnel from an air tank. These tanks are a definite danger and we need to work harder to identify an effective mitigation of the danger they represent.

A standard testing technique for burst is the Hydro-test, where a gas cylinder is filled with liquid (instead of gas) to test whether it will burst or not. 120 PSI of air is dangerous; 600 PSI of water is far less dangerous.

(Kids: Why would that be? )

[Lil' Lavery]

Originally Posted by AllenGregoryIV

True, I didn't elaborate on the example nearly far enough. We all like ice cream, and about 4 years ago a few people started buying Company A's no fat vanilla ice cream. They could eat a lot of ice cream and not deal with the negative calories. Company B who had been selling us ice cream for a long time decided to also make a no fat vanilla ice cream. The problem is Company B's no fat vanilla ice cream has manufacturing problems that were quickly discovered and Company B has offered to replace all there no fat vanilla with there full fat vanilla free of charge to anyone that has bought it. Some teams never switched to company B's no fat vanilla they have been going out and buying Company A's ice cream the whole time, and it has never had any problems. Company B has just recently released a no fat chocolate ice cream. The chocolate ice cream so far seems to be pretty good it doesn't seem to have the manufacturing issues that plagued company B's vanilla ice cream. Lately people have been suggesting a ban on all no fat ice cream or at least on all vanilla no fat ice cream when really only one type of vanilla no fat ice cream has been shown to have issues.

You're still missing the point here. Risk analysis looks at two components, likelihood and severity. While you have repeatedly made claims that the likelihood of the failure of a Pneuaire tank is lower, the severity of the risk is the same. Plastics, by their nature, Untreated Polypropelene tend to fail in a brittle fashion. As a result, plastics as a pressure vessel tend to explode when they fail authorities such as the Plastic Pipe Institute and OSHA caution against using thermoplastics as pressure vessels unless they're buried or contained. The severity of a a failure of a pneuaire tank is still very high, as it would create high energy shrapnel.

What the people calling for examining a ban on plastic tanks are concerned about is the severity of the risk, not necessarily the frequency of failures. The argument that the reward of allowing teams to use a plastic tank does not outweigh the total risk factor, which is contrary to the reward of letting teams use tools. That's the argument you have to address, not that pneuaire tanks are less likely to fail.

[Daniel_LaFleur]

Originally Posted by Lil' Lavery

You're still missing the point here. Risk analysis looks at two components, likelihood and severity. While you have repeatedly made claims that the likelihood of the failure of a Pneuaire tank is lower, the severity of the risk is the same. Plastics, by their nature, tend to fail in a brittle fashion. As a result, plastics as a pressure vessel tend to explode when they fail. The severity of a a failure of a pneuaire tank is still very high, as it would create high energy shrapnel.

You're actually very close, but risk analysis (FMEA) rates 3 factors from a 1 to 10 rating)
1> likelihood -- the chance of a failure happening (in this case probably a 2)
2> Severity -- the damage a failure will cause (in this case with personal injury and severe trauma as well as innocent bystanders that have little to no idea of the danger [read: general public] I'd call this a 8)
3> Detection -- the chance to detect a failure BEFORE it happens ( almost no chance here so a 9)

That gives a FMEA rating of 144. Medical and automotive industry usually red flags at 60 and manufacturing usually at 90.

Originally Posted by Lil' Lavery

What the people calling for examining a ban on plastic tanks are concerned about is the severity of the risk, not necessarily the frequency of failures. The argument that the reward of allowing teams to use a plastic tank does not outweigh the total risk factor, which is contrary to the reward of letting teams use tools. That's the argument you have to address, not that pneuaire tanks are less likely to fail.

^THIS^

[Paul Copioli]

Originally Posted by Lil' Lavery

Plastics, by their nature, tend to fail in a brittle fashion.

OK, I let it go when the first few people said it, but I can't let it go any more.

Sean,
Your above statement is factually vague and incorrect when talking about plastics on Earth (I say on Earth only because of your last name).

Generalizing the material properties of all plastics is as appropriate as generalizing the materials of all metals. Better yet, even generalizing the material properties of a given plastic (like Nylon, for instance) is completely inappropriate. I don't expect many people are plastics experts, except for the chemical engineers that design the plastics; however, I have vast experience in the field of plastic material properties and their failure modes.

General use Polypropylene is very brittle. Color additives and clarifying agents can significantly change that behavior. For example, many companies use PP bags to hold small parts. They also may use clarifying agents that make the PP bag really clear. This makes the bags even more brittle.

It is possible to mix in additives to make PP very ductile. I am certain this is what Pneuaire has done. PP is desirable because it is used for many approved devices for use in food handling equipment. The Pneuaire tanks are also FDA approved.

Polyethylene is also a very ductile plastic.

PTFE (or Teflon) is extremely ductile, so much so that is it really malleable and hold a deformation if stressed too far.

The bottom line is that to make the claim that plastics are brittle is factually incorrect.

[JamesCH95]

Thank you for the correction Paul, it's sometimes very easy to thinking of one's own purview as 'everything' when that is often not the case. I know I've fallen victim to that mentality before.

Here is a MatWeb page with the range of various material properties for PP. Note the tremendous range for some properties.

What would be a good metric to determine a plastic's ductility by? I would assume elongation at break.

In any event it is variations like the aforementioned that would make it meaningful to test other types of tanks, like the Pneuaire or black Clippard tanks. If anyone has spares lying around we would love to test them! I will cover (reasonable) shipping costs.

[Trent B]

Elongation at failure would be the most accurate measurement of ductile failure.

Energy absorbed in impact testing would also show evidence of ductile vs brittle failure.

Finally, any evidence of necking is also generally indicative of significant deformation.

I believe the glass transition temperature of PP is generally significantly below freezing so the temperature of operation for FRC applications shouldn't cross that value.

[WIREDFTC]

Originally Posted by DonRotolo

A standard testing technique for burst is the Hydro-test, where a gas cylinder is filled with liquid (instead of gas) to test whether it will burst or not. 120 PSI of air is dangerous; 600 PSI of water is far less dangerous.

(Kids: Why would that be? )

Oo oo! I know! Water is an incompressible fluid, so if the tank fails it will break with MUCH less force, because the water can't expand when it leaves the tank.

[JamesCH95]

Originally Posted by WIREDFTC

Oo oo! I know! Water is an incompressible fluid, so if the tank fails it will break with MUCH less force, because the water can't expand when it leaves the tank.

+rep for first correct answer!

[Trent B]

Originally Posted by Paul Copioli

It is possible to mix in additives to make PP very ductile. I am certain this is what Pneuaire has done. PP is desirable because it is used for many approved devices for use in food handling equipment. The Pneuaire tanks are also FDA approved.

Out of curiosity, on what basis or with what information are you concluding that Pneuaire tanks have more additives / are made more ductile than the Clippers tanks?

Is it the lack of reports on cracks / failures developing from over tightening fittings?

I would be interested to see the difference in data from tensile specimens machined from all three tanks (white and black Clippards, and Pneuaire tanks).

Here are some Ashby Plots on a variety of unfilled PP polymers with different additions for UV resistance, flame resistance, clarity, etc. This doesn't include second phase additives like carbon fibers, glass fill, talc, etc.


Tensile Strength and % Elongation


Fracture Toughness and Young's Modulus