Lobzilla
Contributor
Bob is correct to be somewhat more conservative with Aluminum.
And, both Aluminum and Steel alloys used for common engineering applications, will experience first elastic and then plastic deformation, when subjected to increasing mechanical load. We try to avoid brittle fracture whenever possible and certainly in the case of scuba tanks that get banged around.
To understand the differences between these two materials, let's take a tension spring made out of Aluminum and one made from Steel and pull them. For each spring, there is a point at which the spring has been permanently stretched and will not 'spring' back to its original length. They yielded once we exceeded the yield strength of the material.
When we cram gas in our scuba tanks the tank will expand and hopefully spring back to its original size when emptied. We do not want tanks that have 'cheeks' (however small) after a fill.
The difference between Aluminum and Steel alloys used for scuba tanks is that the modulus of elasticity (spring rate) and yield strength is lower for Aluminum. That is the reason why an Aluminum tank has thicker walls, has more external volume, and is heavier for the same pressurized capacity. Aluminum is not a very efficient material for pressure cylinders (and springs) but it is cheaper to make tanks from it and the buoyancy characteristics may be more appealing, depending on our needs.
Once we pressurise a cylinder beyond the yield strength of the material, the wall of the cylinder will get thinner permanently. It should be clear that at this point the end is near. Either the pressure will continue to rise (filling or heating in the trunk of a car) until the cylinder blows or the next couple of fill cycles (again reducing the wall thickness) will cause catastrophic failure.
During hydrotesting, any plastic deformation beyond a 'grace' limit is considered reason for condemnation. However, hydrostatic testing is performed at 1.5 times the maximum operation pressure for most cylinders. This pressure is stamped into the cylinder after the letters TP.
Now we could hastily assume: If the hydro facility did not detect worrisome plastic deformation at 1.5 times the operating pressure then we should be fine with a cave fill to the same level. There is one little catch here - Fatigue. And this is where Aluminum looses hands down against Steel.
In the early days of pressurized airplanes, engineers where really surprised when fuselages failed catastrophically and blew the lid well below their elastic design limit. Back in the lab it was discovered that Aluminum will fatigue during repetitive load cycles at any(!) load, even if that load is well below yield stress. Another reason why we are not going to find many springs made from Aluminum.
Steel will fatigue too if stressed close to yield strength but in the case of scuba cylinders most of us may not get old enough to experience failure, UNLESS
a) the cylinders were of marginal design and had trouble passing even the first hydro
b) we exceed the elastic deformation limit through intentional or accidental (heat in car) overpressurization
c) corrosion reduced the wall thickness
d) pitting caused stress risers that aggravate fatigue
(The last two go typically hand in hand)
I am not suggesting what we do or not do with our tanks, just trying to shed some light on the factors involved.
Here is an interesting Navy study on scuba tanks. But please let's read the section about the destructive potential before getting any bad ideas while looking at the reported burst pressures.
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