why do fiber-wrapped tanks expire after 15 years?

[rawbutter]

I'm hoping someone with some technical knowledge can help me out here.

I recently had a rather disappointing experience with an SCBA tank that I acquired for free with the hopes of using it for paintball refills. When I was looking around at local places to get it hydro tested, I was sadly informed that, since it was fiber-wrapped, it's only good for fifteen years. I know that smaller paintball tanks have a similar life expectancy.

So, does anyone know why this is the case? If it passes hydro, why does it matter if the tank is five years or thirty years old?

I'm not asking because I'm bummed about my SCBA tank... I'm over that. I'm honestly just intellectually curious. What is it about fiber-wrapping a tank that makes manufacturers unwilling to give them a lifespan longer than 15 years? :confused:

[MedicDVG]

It is because the fibers in the tank expand time and time again with each fill. These constant expanions and contractions place wear on the fibers. 15 years is about how long the fibers can safely stretch and contract. Using the tank longer can lead to catastrophic failure supposedly.

You can check the DOT website for more information...

[Spider-TW]

Yeah, there's not a good way to keep track of how many times a bottle has been filled or how it was otherwise treated in storage. 15 years has to be a minimum estimated service life, so that no bottle fails in that time. You can bet there are plenty of bottles that would last past that, but you would probably have to destroy the bottle to prove it.

It would be nice if some DOT would take all the expired bottles on donation and test them to get more data, but the bottle manufacturers won't be paying for it. ;)

[cockerpunk]

the issue is that it is a composite.

so the fibers are in a matrix of basically glue. and the whole thing is only strong as long as the fibers and the glue stay together when loaded. even though both have very different properties when loaded independently. as long as they are stuck together, they will work fine.

after alot of cyclical loads you can get the fibers breaking away from the matrix on a micro scale. this means that the glue is taking more of the load then the fibers (because the fibers are stiffer then the glue). this makes it weaker.