Why not overfill Aluminum cylinders?

[Charles2]

In another thread, this comment was made:

Don't ever overfill an aluminum tank, aluminum isn't steel. I overfill steel regularly, but never aluminum.

Since both 3AL and 3AA tanks are both hydro tested to 5/3rds rated pressures, what is the problem with overfilling an aluminum cylinder to the same proportionate amount as a steel cylinder? If a cave filled LP cylinder is filled to 3500 psi, why not fill an AL80 to 4000 psi?

I apologize in advance for my ignorance.

 

[lowviz]

...//...I apologize in advance for my ignorance.

Never do that. That is why we are all here. None of us knows everything...

Aluminum WILL fail with repeated cycling. It doesn't have a really special property that steel has. Steel has an elastic limit, aluminum doesn't. See this: Fatigue limit - Wikipedia, the free encyclopedia

In short, you can cycle steel almost forever if you keep below its elastic limit. Aluminum will fail over time because it has to. Every time you fill an aluminum tank you hurt it a tiny bit. Overfilling hurts it a lot. It is cumulative.

 

[Charles2]

Thanks for the response. I love it when the answers are in picture form and that is a very nice graph. I always thought that aluminum had an elastic limit, but it's been a while since I studied and I could be wrong. Could you have meant endurance limit instead of elastic limit?

Based on the graph, let's select a design value of 20 ksi for the tank. Using that value, it does appear that the steel tank will last indefinitely; however, the aluminum tank will fail after approximately 100,000,000 cycles. Did I read that correctly?

Seems like I have some time. Or am I missing something?

 

[lowviz]

The alloy and wall thickness are chosen as a compromise to give you light weight, ability to contain pressure, and reasonable life. Just be nice to them and go with passing hydro. They will last a long time, no worries.

 

[KevinNM]

No, every flex of aluminum damages it. A naval architect mentioned he'd seen a boat where they made a hand rail out of aluminum instead of steel and it flexed a bit continually underway. It didn't last a week before it fractured.

 

[dumpsterDiver]

The fatigue limit of aluminum alloys... I often like to mention this when sitting in a window seat next to a wing- especially when they are clearly vibrating up and down at a very fast rate.

 

[lowviz]

And if you really want to mess with them, quote the design safety ratio that they all use...

 

[Wookie]

Think beer can. Holds beer just fine, as that's what it's designed to do. Bend it, and it fails almost immediately.

 

[Compressor]

Is the elastic limit of steel tanks defined? What is the limit for the LP Worthington tanks for example?

 

[lowviz]

This is a good place to start if you don't want to accept the enormous number of steel overfills that have been done over the years: Vintage Double Hose • View topic - Vintage tank hydros and "+" stamping

For the record, I won't ever ask that any of my 2250 steel tanks get pumped to over 3K.