SRS 4

[enginesrus]

Have airbags on the mind. And long before the Takata fiasco there where many people injured or killed by them. I would like to know why certain pressurized gas containers are required to have a pressure relief system or valve, as well as a fairly high strength container, where an airbag inflater can be made from almost a tin can thick steel with no relief system to protect from over pressure. I also curious why such dangerous pressure vessel is allowed to be so many inches from your face, eyes etc. while you are driving a vehicle? Weren't the first airbags deployed by CO2 gas cartridges, located away from the driver?

 

[BrianPetersen]

Airbags have saved a vastly larger number of injuries and deaths than they have caused ... even the Takata ones. The current automotive ones don't use a pressurised gas container, they use a small explosive charge. When they operate as intended, there is no issue.

 

[IRstuff]

Takata's inflator uses ammonium nitrate wafers that were manufactured without a drying agent, presumably, to save cost or simplify manufacturing. Gas cylinders, as exemplified on Mythbusters and other shows, cannot produce a sufficiently rapid inflation, since any connecting tube or hose will be limited by sonic velocity. An explosive initiator is the only practical way to an airbag to be fully inflated within the 20-30 ms necessary to prevent passenger injuries from the crash. The net result is that full inflation requires supersonic gas speeds, which can only be reliably obtained through explosions.

Interestingly, to some degree, that's how some otherwise wimpy tanks provide self-protection in the case where their armor is too thin; they have explosive reactive armor (ERA) which detonate to mess with whatever incoming anti-tank munition making contact with the ERA.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[3DDave]

It's not intended to be explosive. It is high burn rate. At least that's what the high-speed video of the ignition of the Takata gas generator is showing - an incremental, from one end to the other, combustion.

From what I looked at ammonium nitrate is hygroscopic and, with increased moisture, becomes resistant to burning. In the Takata devices the ammonium nitrate was in a small cylinder with periodic vent holes. As the fuel burns the gas was ejected through the incrementally exposed holes. It looked to me like, in the videos showing detonation, that contaminated/compromised fuel wasn't completely burning but instead was ending up plugging the holes - creating a tiny pipe bomb by increased pressure forcing an increase in heat causing bulk detonation.

Research into ammonium nitrate bombs was done by the US military to determine how long an IED based on ammonium nitrate would remain a threat when left exposed to the weather. They concluded that degradation from moisture would render them ineffective at some point.

In any case, the addition of a pressure relief might not be possible, but what is certainly possible is designing a reliable and safe rupture mode that doesn't result in generating shrapnel or a containment/deflection system to guide fragments away from the throats and faces of occupants of cars. I've seen enough explosive-forming of metal and explosive bolts to believe that this is possible. Not doing so is likely to save a dollar or two on the manufacturing cost of the airbag - the likely reason they don't. Note that using ammonium nitrate itself was to save a similar amount of money.

This video - notice the sputtering and then how gas venting slows.

https://www.youtube.com/watch?v=eBjxprHh3QI

 

[enginesrus]

I'm worried about the short women in my family. One has to sit with her stomach almost up against the steering wheel. What I read is the bags can create about 2k lbs of force. Like everything I think there is a better way, just not as cheap.

 

[EdStainless]

The airbag design is supposed to limit the force applied based on size, venting, and strength.
Small and/or light occupants are a real issue.
There are a number of newer implementations that actually alter the inflation based on the sensed weight in the seat (usually just high and low).
I have no idea how effective they are.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[3DDave]

While a small sample, a co-worker of mine, a fragile 70+ year old with really bad driving habits was in two crashes about a year apart. He was under 5 foot tall. In the first crash, no airbag in the car, the shoulder belt broke his collar bone and he wore a sling for a long time. In the second, the replacement car's airbag deployed and he was unhappy because the fabric gave his face a rug burn. I don't know the exact level of collision in either case, but it was an interesting contrast.

 

[enginesrus]

Great. Now what kind of propellant was used to replace the faulty propellant in the takata airbags? I thought I remember hearing it was the exact same stuff only newer? If so then its only a matter of time and the same ole thing will repeat itself. So now I ask how important are your eyes to you? Will Z87 safety glasses and shields stop the projectiles? I don't think so. There needs to be some serious engineering on these systems. The tin can pressure vessel has to go.

 

[BrianPetersen]

Blindness from flying shrapnel has never been a widespread issue, not even with Takata airbags.

If the Takata thing bugs you, just get one with airbags from a different supplier. Autoliv is another really common one.

 

[enginesrus]

What are the propellants used in the various manufacture airbags now? I suppose safety took a back seat because of some other perceived notion, because we sure didn't hear about inflaters blowing up in the early days of airbags.
So a particular car maker recalls for the 2 frontal airbags, and the other 4 are ignored? Does that mean they don't have the dangerous ammonium nitrate that the other 2 have or had?

The blindness issue, there have been many eyes ruined from them, more than deaths.

 

[BrianPetersen]

Appears that ammonium nitrate is what Takata uses. It appears that there are several other choices, the main one being a reaction between sodium azide and potassium nitrate.

 

[3DDave]

The greatest worry is areas with long periods of very high humidity. However, even with the very unfortunate deaths and injuries which should have been avoidable, Takata found itself in a classic squeeze.

The product, on delivery, was fine - cost effective and reliable. It was only after years of exposure, which might not have been well simulated in accelerated heat w/humidity tests, that the problem was spotted. The squeeze came in thusly - if Takata let on that this was a problem they would not only have to produce all the orders they currently had, get new orders to continue an income, but also spool up manufacturing to replace all inflators they ever made. I think few companies could survive that, plus they would have to somehow also develop an entirely new inflator that could not have any similar defect as both the replacement, the current production, and for future contracts in what would likely be a year before they got cut off by everyone.

Worse, major auto makers knew about these problems, which is what prompted the Takata inquiries, and those companies kept it quiet as well.

I see from

https://en.wikipedia.org/wiki/Takata_Corporation

that Takata had previously been involved with seatbelt failures and, again, the major makers knew about the problems and kept silent.

In the grand scheme of things, airbags do not guarantee survival; they simply significantly reduce the chances of death or serious injury. And even with the deaths from the shrapnel included, the overall reduction in deaths and other injuries was apparently a very large net positive. I would like that the airbag inflators had been designed to have a safe mode for over-pressure rupture rather than uncontrolled fragmentation.

In looking at some Chinese inflator manufacturer pages, they list a 15 year lifetime for their inflators. It would be nice to know what happens at the end of that time. The price of the inflator is around $20, but I expect that this is not replaceable individually in the airbag system.

 

[enginesrus]

So then for us that wish not to have rock blasting stuff inches from our faces, all auto manufactures should then offer a safer alternative propellant inflator, on request. Or all airbags should have a standard SAE inflator opening and bolt pattern just like an SAE transmission PTO opening and bolt pattern so we could shop around for a safer inflator.
Its just so hard to believe the ignorance of this whole issue. How often are these things removed from various year cars and tested? Some of the recalls are now many years ago. I personally don't care to gamble.

 

[BrianPetersen]

The design of those airbags is tailored to the design of the vehicle and its interior around them. Unless you are going to lock in the design of all vehicles so that they are all the same shape and size with identical interiors and seats, AND lock the whole industry out of all further developments, an SAE standard uniform airbag is not going to happen.

Vehicle designs are homologated with the airbags specified for them. You can't just swap to a "safer alternative". This is one of the things that trapped Takata and the manufacturers who used them. They knew the design flaw (which accelerated testing could not have found) but they couldn't change the design of the homologated airbags including their inflator without redoing the homologation of all of the affected vehicles, and redoing crash testing of dozens of 10 year old vehicle designs that are no longer in production is not going to happen. The recalls simply replaced like for like. Maybe minor changes to address moisture intrusion could be done but nothing could be changed that affected its performance.

If you don't like this then buy a vehicle with airbags from a different supplier.

 

[SwinnyGG]

I personally don't care to gamble.

You are more likely to hit the powerball than you are to be killed by airbag shrapnel.

 

[btrueblood]

The "original" TRW airbags used straight sodium azide (the nitrate is just used to initiate the decomposition reaction of the azide). Azide, in addition to being explosive, is a very toxic compound (inhalation of a few tenths of a gram will kill you). The risk to automobile passengers is fairly low (because normally most of the stuff is reacted before it gets out of the airbag), but the risk to the people assembling the bags wasn't. The major manufacturers have thus moved away from it, to "safer" propellants...which have issues of their own.

 

[enginesrus]

Powerball? Not so much as one number there, but with a dangerous airbag? Well this is the kind of gamble that I would win at.
So NHTSA must care more about the auto manufactures than the folks that get stuck with ill designed junk that may seriously injure or kill them, sort of shows what that organization is all about.

https://www.autoblog.com/2020/01/14/takata-replacement-airbags-recalled/

Supposedly Takata has not used Ammonium Nitrate in the inflators since 2019? And no one cares about what happens when the salt and rice congeal in the last transplanted non azide inflators that are posed in many vehicles on the road and ready to redesign your face.

 

[BrianPetersen]

Real world experience over the last couple of decades has been that in terms of occupant safety, the benefits outweigh the risks by an enormous margin.

 

[SwinnyGG]

Real world experience over the last couple of decades has been that in terms of occupant safety, the benefits outweigh the risks by an enormous margin.

Clearly your real world experience must be wrong, because engineers have not correctly designed anything in any industry since 1949

 

[EdStainless]

The original airbags, in addition to being hazardous to assemble were also very likely to break bones in your face (nose, cheek, orbital).
A friend's father experienced this, but he was glad since the accident would have been fatal otherwise.
But even the early fairly crude ones were significantly reducing fatalities.
As crumple zone designs improved and they were able to better dissipate the energy of impact and then use more reasonable inflation forces.
I have only been hit by an airbag once. Ironically it wasn't a serious accident. I won't say that it was fun, but it was no big deal.
More of a suprise than anything, it happens incredibly fast. No burns or scrapes, just a few bruises. Didn't even break my glasses.
Cravat, I am large and tall (6'1", 200lb) and I sit way back from the wheel.

Our reliance on a technical solution over behavioral ones in a sign of our times.
Virtually all collisions are the result of driver errors.We are so afraid to impose any restrictions on drivers because the privilege of driving has come to be seen as a right. Granted, we have built a society in US where driving is a necessity of life in most places which compound the problems.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed