Destructive Air Tank Testing
 

*Disclaimer*
Don't try this at home. I have experience in destructive testing and I have a safe location to perform these tests.
Don't construe any results as approved methods for shrapnel mitigation.
Don't construe any results as suggestions for any action.
Don't construe any results as anything other than the results of an experiment.
Don't try this at home.
No students were present during these tests. Experienced adults only.
*/disclaimer*

[EDIT]
Video!

https://www.youtube.com/watch?v=Oea-...ature=youtu.be
[/EDIT]

We are experimentalists here on 95. Given the recent chatter on CD about failing plastic air tanks, and several proposed (or implemented) mitigation techniques, we decided to test some of these solutions. We have a video coming, but I thought I would share our preliminary results.

Most of this year's discussion, and several proposed or implemented solutions, is in this thread: http://www.chiefdelphi.com/forums/sh...d.php?t=127695

Some background information about using plastic to transport compressed gasses from The Plastic Pipe Institute: http://plasticpipe.org/pdf/recommend...ressed_gas.pdf

Safety during a destructive test is paramount, so we did a little research and found this paper detailing the radii for various threats at different amounts of stored energy. On Page 4.7 the thread radii are listed. We calculated that the energy stored in a single plastic storage tank at 120psi was around 370lb*ft of energy. To be safe we used the shrapnel radius for a 500lb*ft radius and gave ourselves a factor of safety of at least 10, and wore eye and hearing protection.

Paper: http://xpda.com/junkmail/junk215/PNNL-18696.pdf

Results summary:

No mitigation - we found shrapnel of appreciable size up to 45ft away lots of small shrapnel pieces, and some larger ones

2-3 layers of Gorilla tape - as some hypothesized the ends of the tank shot out at high velocity, we found one in a snow bank about 25ft away. Virtually all of the small shrapnel was eliminated, but the tank broke into 4 large pieces. However, the larger pieces didn't travel as far, 20-30ft or so.

Bumper fabric zip-tied around the tank - the zip-ties failed almost immediately and the fabric simply un-rolled from around the tank. Shrapnel was slightly mitigated, but large and small pieces were still found 20-30ft away.

Completely Assembled Bumper - we essentially replaced the pool noodles with an air tank. This actually contained virtually all of the shrapnel. It was a completely assembled bumper (3/4in plywood, screwed-in angle aluminum, stapled nylon fabric*). The angle aluminum ripped off of the plywood, and many of the staples failed, but it held together long enough to keep basically all of the shrapnel inside.

*This fabric is considerably stronger than your average bumper fabric. It is heavy-duty nylon, double rip-stop, and impossible to tear by hand along any direction.

Pictures!

Un-mitigated. Not bad for finding white plastic on a snow-covered driveway!


Gorilla Tape


Buper fabric and zip-ties.


Whole bumper.

Re: Destructive Air Tank Testing
 

A thick sock tie-wrapped or wired closed around the pneumatic lines.

Few of those methods (safe for the bumper) would "contain" anything. They merely redirect the energy. You want something that will expand and stretch to absorb the energy while containing the fragments and venting the gases.

When it comes to explosions, trying to contain them rigidly (with tie wraps and tape) rarely works.

Re: Destructive Air Tank Testing
 

Bumper material covers with reinforcements at the ends (double stitching and double material) and pleated material around the main cylinder of the bottle with sparse stitching that will break away, allow the bag to expand and catch the expanding pieces.

Re: Destructive Air Tank Testing
 

Well, if the sock solution has issues with pieces of tank cutting through it some kind of poly-carbonate mesh could be used.

Re: Destructive Air Tank Testing
 

Did you have any "before" pictures?

Re: Destructive Air Tank Testing
 

Just as a quick note: We had a fixed number of tanks to work with and so we tested the mitigation techniques we saw proposed most often and used materials we had on hand.

We would have tested additional containment methods but we sort of 'expended' all our plastic tanks. If anyone wants to send us more we'd be more than happy to do additional testing.

The aforementioned video will go into some depth about the hows and whats of the testing. For now we thought it was worth letting the community know that, at least in our tests, containment of these tanks failed more often then it succeeded, and that it requires substantially more robust solutions than many expect.

Re: Destructive Air Tank Testing
 

Here is an air tank ready to test.

Re: Destructive Air Tank Testing
 

What's the diameter on these tanks? I bet something like McMaster p/n 55545K96 would work wonders on containing shrapnel.

Re: Destructive Air Tank Testing
 

I was looking at that stuff too, I was wondering how to cap the ends. Maybe I could stitch it down with some heavy duty nylon string?

I am nervous about the location of my air tanks too.

Re: Destructive Air Tank Testing
 

What method are you using to cause the failure? Simple over-pressurization or are you intentionally over-tightening the end fittings? It would be brittle failure either way, but it may make a difference in how much energy is in the system at the time of failure. An over-tightened fitting would likely fail at a fair lower pressure (eg, potentially below 120psi) whereas a properly assembled tank would not.

On that same note, did you monitor the pressure that the tanks burst at, and if so how consistent was it?

Re: Destructive Air Tank Testing
 

Roughly 2.5" looking at the pictures and from first hand recollection

Re: Destructive Air Tank Testing
 

Instead of looking at more wonderful creative ways of containing shrapnel, I'm looking for ways to make sure all of these tanks get replaced with something more reliable at the regionals I attend....

Re: Destructive Air Tank Testing
 

I wish clippard made a metal air volume tank the same size as their biggest cylinder. I would love to have a 3" ID tank that is 6" long. hmmm I am gonna email them right now.

Re: Destructive Air Tank Testing
 

At what pressure did the tanks actually fail? I would expect them to have a safety factor of at least 3X.

Re: Destructive Air Tank Testing
 

I would have to agree with you.

These tanks were discontinued for a reason...

I would also like to know how you made them fail?

Threading the fitting in to far or some other method?

Re: Destructive Air Tank Testing
 

All the tanks were pressurised to 120psi (regulated output of a shop compressor) then struck with an air rifle pellet to induce the failure. It turned out to be remarkably consistent and reliable (well, as reliable as my aim!). The video of the testing will get into detail about the methods, but we think it's a reasonable simulation of a tank failing in FRC use due to a sudden impact

My subjective opinion is that each tank burst with the same sort of force. The shrapnel, noise and effect on containment all seem to back that up.

Re: Destructive Air Tank Testing
 

I agree entirely. 95 only uses metal tanks on our robots. Always have, always will.

As hinted at earlier, our goal is not to provide actual shrapnel containment solutions, but rather to demonstrate how difficult it would be to make plastic tanks as safe as metal tanks.

Re: Destructive Air Tank Testing
 

Broken laptop in homage to 1310's driver station? :P

Re: Destructive Air Tank Testing
 

Ok so Clippard can make air volume tanks the same dimensions as their cylinders. I went through their distributor and asked for a 3" bore 12" long storage tank (84 ish cubic inches). I will see what the quote is.

Re: Destructive Air Tank Testing
 

Maaaaybe...

Also it made the white tank stand out against the snow-covered driveway for Andy to aim at.

Re: Destructive Air Tank Testing
 

I'm thinking we're going to avoid them in future years. Unfortunately, we can't afford to swap the ones currently on our robot out for metal substitutes this year, but I'm not liking the explosive failures one bit.

If this continues, FRC should just go back to allowing only metal tanks.

Re: Destructive Air Tank Testing
 

These tests and all of the failures I know of are on a single brand and model of tank that is known by the manufacturer to have this problem. To my knowledge no one has tested the 2014 Clipboard tanks or any other manufacturer's tanks for these types of failures. We shouldn't talk in generalities so quickly, we still have very little information on this subject.

Re: Destructive Air Tank Testing
 

Along those lines, if anyone has a spare black tank or three they want us to test, please send it! Or any of the available plastic tanks.

Plastics are still plastics though... in the OP read the memo from the plastic pipe institute, it's only a page. A very carefully worded caution against using plastic to transport compressed gases. It's all about the cost of failure, and the simple fact of the matter is that plastics will generally shatter when they fail and metals will generally yield, crack, and vent all of the gas out without making shrapnel.

I think we might also try to make a metal tank fail, but that's a whole other ball of wax...

Re: Destructive Air Tank Testing
 

IIRC, people were having trouble with explosions last year, as well. Admittedly, those were due to threading the connectors in too far, which is no longer a problem with the built-in connectors

Fact is, though, this is a high-school competition and it's much easier for a high school student to inadvertently damage a plastic tank in a way that will cause a dangerous failure than a metal tank. I wouldn't be so concerned if these were only being used by adult engineers, but they're not. I certainly would rather be safe than save a pound or so of weight.

Re: Destructive Air Tank Testing
 

The failures noted so far this year have still been those 2013-era white plastic tanks to the best of my knowledge. Not the 2014-era black plastic tanks.

Re: Destructive Air Tank Testing
 

Yes the failures this year and last year were the 2013 Clippard White tanks. White tanks from Pneuaire, Andymark and others, along with the Black 2014 Clippard tanks have not had failures to my knowledge.

I also understand the risk of plastic tanks in general but there are also risks with many of the systems and tools that we use in competitive robotics. We need to mange the risks in the smartest ways we can to ensure the safety of the participants and spectators.

Re: Destructive Air Tank Testing
 

The clippard tanks we used were acquired through the AM FIRST Choice program. I believe AM simply re-sells Clippard tanks.

The risk of plastic tanks is obviously controversial. I'll relate a conversation I had with a lawyer when I was interested in selling potato cannons (another hobby of mine). He made a very good point that is very pertinent to this topic.

His professional opinion was that no matter the disclaimer I had customers sign that I could be held liable for any damages or injuries incurred though the use of a potato cannon I made and sold because there is NO NEED for a potato cannon, it's a toy. Car makers are generally safe from accidents and injuries incurred though the use of their cars because there is a genuine NEED to drive one.

In FRC we accept the risk of using a mill, late, welder, lead-acid battery, etc. because we NEED to assume those risks to build a robust and competitive robot. We do not have to accept the risk of creating shrapnel by using a plastic tank when metal tanks are available. The only functional difference is weight, and weight can be shed elsewhere in basically every robot design that I've ever seen.

Re: Destructive Air Tank Testing
 

I will see if I can get the ThunderChickens to send some of their old pneuaire ones.

PM me your address and I'll see what I can do. I have a few favors to trade with them still.

I would be very surprised if the Pneuaire tanks fail in the same manner as the white Clippard tanks, but it wouldn't be the first time I was surprise by the results of an experiment.

Re: Destructive Air Tank Testing
 

There are plenty of competitive teams that don't use a mill, lathe, or welder (I have never had a student weld anything on team I have been on in 12 years of FRC, doesn't mean I don't think other teams should do it). We needed to use light weight tanks just as much as we needed to use aluminum axles instead of steel. There are places to save weight but buying plastic air tanks is easier. Also AndyMark sales a variety of air tanks, some of them are from Clippard and others are not.

Re: Destructive Air Tank Testing
 

Paul- I'm sure James will be in touch about a shipping address.

Do you know any of the specifics about the differences between the black tanks and white tanks? We understand a fitting is pressed in but, besides that, they appear to differ only in color and perhaps size. They are advertised as 'polypropylene', but I haven't found any specifics beyond that.

Re: Destructive Air Tank Testing
 

The clear call for acton is to ban air rifles from competition! :rolleyes:

No, seriously: Thanks for doing this. The key take-aways are that shrapnel from a tank has a lot more force than most folks believe (and can imagine!), AND that a relatively minor impact can be catastrophic.

At MAR Clifton last weekend several teams with 'exposed' tanks (black or white) were asked to add some sort of protective covering to their tanks - not to contain shrapnel but to help avoid impacts that could damage the tank.

Re: Destructive Air Tank Testing
 

If you can spare one, do it, to demonstrate the difference in failure mode.

Re: Destructive Air Tank Testing
 

We've got metal tanks galore, in fact. We had meant to test one at the same time as the plastic tanks but ran out of daylight and, in any case, I'm fairly confident the air rifle we had wouldn't have caused anything more then a dinging noise at the range we were shooting at.

We're hoping to do a second round of tests in the near future and we'll include a metal tank or two while we're at it. Causing that to fail will probably require an actual firearm which, while entirely doable, is another level of complexity. We'll chew on that one.

Re: Destructive Air Tank Testing
 

First thank you for doing this type of testing. 1523 has always used metal tanks and are interested in seeing how they fail. For round 2 can you guys film it? I find it hard to visualize the amount of energy stored in a tiny plastic air tank causing the amount of damage documented in these cases. As for debris reduction methods:
1. a heavy sock zip tied around the tank
2. a layer of screen enclosure material wrapped around the tank (its tough and will let more air escape than bumper material)
3. electrical tape
4. a thin piece of sheet metal wrapped around the tank.
5. anything else quickly retrofitable for teams close to the weight limit and who have another event.

Re: Destructive Air Tank Testing
 

Please send tanks!

Also, a video of the tests will be out shortly... editing is a tedious process.

Re: Destructive Air Tank Testing
 

Those are just for-instances. Don't take it too literally. Any tool used in a shop has the potential to be dangerous, and yet every team uses them because they have to use tools to make a robot. No one has to use plastic air tanks if they choose not to.

It is easy to buy a different component to save weight, so I swap out motors. An RS775+CIM-U-Lator is 1lb lighter and more powerful than a mini-cim, for example. There are lots of places to save weight. I personally could not stomach the thought of a plastic tank on my robot causing anyone harm, so I choose not to use them on any robots that my team makes.

Re: Destructive Air Tank Testing
 

And that is perfectly fine but the important part is you shouldn't necessarily assume that every team feels the way that you do, I have never used a plastic clippard tank on a robot and probably won't, I however have and will continue to use the Pneaire tanks until I see evidence that the risk is too great. Andy Baker once said something about Ice Cream and it still being good even if it wasn't your favorite flavor and that it might be someone elses favorite flavor.

Re: Destructive Air Tank Testing
 

That's fine, as long as their favorite flavor of ice cream doesn't put shrapnel in a bystander's face, or my face. When someone else's choice puts an innocent bystander (in this case refs, human players, announcers, emcees, the audience, etc) at risk, that's when I start to have a problem.

Re: Destructive Air Tank Testing
 

A quickly-edited video!

https://www.youtube.com/watch?v=Oea-...ature=youtu.be

I also updated the OP with the video link.

Re: Destructive Air Tank Testing
 

That's why the uncalled G28 penalties are so upsetting. The risk with these tanks isn't that they will fail under normal circumstances. It's that they will fail when damaged by another robot. This is NEVER supposed to happen. It is against the rules to deliberately touch another robot inside their frame perimeter. If the tanks are mounted inside a robot's frame perimeter, they should be completely safe. Unfortunately, the style of game this year has resulted in a lot of people just shrugging their shoulders and accepting repeated G28 violations has "just part of the game", or "accidental, not deliberate".

I contend that these G28 violations are just as accidental as when a drunk driver gets behind the wheel and "accidentally" kills someone. When they get behind the wheel, the drunk driver assumes responsibility for the results of that action. Similarly, a team who drives a robot around the field with appendages hanging over their bumpers should be FULLY ACCOUNTABLE for the penalties which result. I knew we were heading down a slippery slope when the game design committee legitimized bad behavior by changing the rules to allow robots to extend outside the field. Rather than penalizing teams creating a safety hazard, they made it acceptable for teams to drive around with their appendages hanging out. This has led to the current culture of defenders choosing to extend their appendages, ram them into other robots, and cause damage, then saying "Whoops. It was an accident."

As we see from these tests, not enforcing the rules can have serious consequences. It's not just the air tanks that are a possible hazard. The batteries we use are just plastic boxes that contain acid. There are wires in the robot that when shorted together can cause fires. There are SAFETY reasons why G28 exists. Teams should be living in desperate fear of touching another robot inside their perimeters, and the penalties this would incur. This year, starting with the rule change allowing robots to extend outside the field, we've been more and more accepting of safety violations. I, for one, think this is a bad thing.

Re: Destructive Air Tank Testing
 

I'll abstain for the debate about how I'd like to see G28 called, but just as a note, your statement is not the rule: I've seen an entirely non-trivial number of alliance partners damage each other's robots. Moreover, legal bumper-bumper contact pushes many a robot into the side of low goal, the bars of which can you a number on many robots. Essentially, for teams to withstand expected legal damage, they still need to have their tanks relatively* well protected. Just something to remember.

*I do not know what that means specifically, which is why I'm reading this thread. Thanks 95!

Re: Destructive Air Tank Testing
 

I agree with everything you said Todd.

Though plastic air tanks are different than the battery and an electrical in that they have a much safer alternative, and a tank failure is arguably more dangerous than a fire or a battery rupture.

A ruptured battery will fizzle and spew acid all over the field, and then be contained with a spill kit. An electrical fire with burn a robot(s) and/or the field, and then get put out with a fire extinguisher. An exploding tank sends shrapnel flying.

Like I said, I agree that robots violating G28 could cause tank failures, and that's not okay. But, just like laws that discourage negative actions, laws do not prevent them, nor do laws prevent accidents. As the FIRST community we have a choice to try a multitude of fixes, or choose an intrinsically safe solution.

As an aside, in our second QF match at GSDE we were hit with a G28 technical foul when our bumper rode up on top of another robot's bumper. No contact inside either bumper zone, certainly no contact of any consequence. It's tough to ride that fine line in calling technical fouls...

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

I remember Clippard responding to the explosions last year by stating that when fittings were ***Properly*** installed, they were unable to get the tanks to burst as high as 600PSI

Properly being with correct teflon taping, and not overtightened until cracking

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

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? )

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

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.

^THIS^

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

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.

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.

Re: Destructive Air Tank Testing
 

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.

Re: Destructive Air Tank Testing
 

+rep for first correct answer!

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