What pressure would you fill a pair of AL80s to?

[Jay]

The poll is talking about not severely limiting the life of an AL tank.
There are folks who would fill lot's of al 80's to 3900 on a regular basis. How many hydro cycles did they get out of them?

[Nitrogenius]

The poll is talking about not severely limiting the life of an AL tank.

Good Point! Every single Fill to 3900 would definately Limit the lifespan of such tank..

[kotik]

Good Point! Every single Fill to 3900 would definately Limit the lifespan of such tank..

... and maybe the filler!

[Greenwood_60]

Aluminum fatigues with every fill. Period. It has a finite fatigue life. Steel does not. It requires a certain amount of stress before it fatigues. It has an infinite fatigue life below a certain threshold. I'd be surprised if anyone here know what those figures are for the tanks they dive. Even if they did, it would be for tanks in good condition, without accounted for stress raisers. Pitting and cracking will concentrate the stress. Filling a good AL80 to 3900 once is almost certainly never going to be a problem... but asking someone else to do it repeatedly on tanks they know little or nothing about is stupid.

I believe Faber once stated their tanks were designed to withstand 10,000 hydros. That tells me that cave filling steel tanks is fatiguing the tank, but within a considerable safety margin.

[Nitrogenius]

Steel does not. It requires a certain amount of stress before it fatigues. It has an infinite fatigue life below a certain threshold.

Well If I haven't been sleeping in my Steel Structure class back in 96-97 during my masters in Civil Engineering and my memory does not Play tricks on me, then there is no such thing like an infinite fatigue life for any material, also not for steel.
Repeated stress-no stress cycles are the most fatiguing element in structural Engineering. Steel can cope quite well with it in comparison to ther materials, but it fatigues with every cycle and eventually fails no matter the treshhold..

Filling a good AL80 to 3900 once is almost certainly never going to be a problem... but asking someone else to do it repeatedly on tanks they know little or nothing about is stupid..

This I concur with 100% !!

I believe Faber once stated their tanks were designed to withstand 10,000 hydros. That tells me that cave filling steel tanks is fatiguing the tank, but within a considerable safety margin.

Sounds plausible..

[Greenwood_60]

My understanding is a bit simplistic. From wikipedia: "Ferrous alloys and titanium alloys have a distinct limit, an amplitude below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminium and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes."

http://en.wikipedia.org/wiki/Fatigue_limit

[Nitrogenius]

Ok.. Looking to small scale stress as it seems to be indicated in the Wikipedia source (which by the way as everyone should know always maybe flawed since there is only proofreading by other users) there might be such "distinct Limit" which is so low, that it results in a Close to unmeasureable cycles until it would fail..
I still do not believe there is a complete "no failure" treshhold..
I also very much doubt that the stress Levels applied to a steel tank would fall under such threshhold that would seem an ineffective use of the material and cause unreasonable thinkness of walls and thus also unreasonable weight and cost..
However without looking deeper into the steel formulas I would am sure that even the test pressure during hydro would fall into a load Scenario being far away from the with steel critical elastic Limit and the reasonable cycles of a steel tank which certaintly would fall into a 4 Digit number over a reasonable lifetime with corrosion and potential outside damages affecting hull thinkness etc. would be far away from any cycle number getting critical below such elastic Limit. If memory serves well getting closer to such elastic Limit the cycles to failure a definately 5-7 Digits..
So conceptually one could say that load cycle fatigue for steel tanks should not Play a role in a steel dive tanks lifetime..

[Jay]

For years Faber had on thier site "....10,000 cycles to 4000 psi...."

[shellmoundal]

3000 Cold

[victorzamora]

Ok.. Looking to small scale stress as it seems to be indicated in the Wikipedia source (which by the way as everyone should know always maybe flawed since there is only proofreading by other users) there might be such "distinct Limit" which is so low, that it results in a Close to unmeasureable cycles until it would fail..
I still do not believe there is a complete "no failure" treshhold..
I also very much doubt that the stress Levels applied to a steel tank would fall under such threshhold that would seem an ineffective use of the material and cause unreasonable thinkness of walls and thus also unreasonable weight and cost..
However without looking deeper into the steel formulas I would am sure that even the test pressure during hydro would fall into a load Scenario being far away from the with steel critical elastic Limit and the reasonable cycles of a steel tank which certaintly would fall into a 4 Digit number over a reasonable lifetime with corrosion and potential outside damages affecting hull thinkness etc. would be far away from any cycle number getting critical below such elastic Limit. If memory serves well getting closer to such elastic Limit the cycles to failure a definately 5-7 Digits..
So conceptually one could say that load cycle fatigue for steel tanks should not Play a role in a steel dive tanks lifetime..

So, you're right....there IS no "no failure" threshold. The phenomenon by which this occurs is plastic deformation. Under a certain strain threshold, the deformation is "purely" elastic and therefore has no permanent crystalline structure changes. By "purely" elastic, I mean 99.999%+ elastic. Plastic deformation is a permanent crystalline structure shift. This is what "hardens" the tank. I'm not sure what the pressure is (tank mfgs won't release sufficient specs for my math, and I'm NOT cutting a tank apart for this forum )......but there's a point where plastic deformation actually becomes "relevant." The higher the pressure, the more lifespan reduction. From what I understand (and based off of math based off of a bunch of guesses).....our "normal" fill pressures will give our steel tanks long enough lifespans that rust and physical damage will take the tanks LONG before elastic deformation causes an issue. This is all under the assumption that we fill our tanks to "normal" pressures at "normal" speeds and treat them like "normal" people.